U.S. patent application number 10/434941 was filed with the patent office on 2004-04-15 for feedback enhanced light emitting device.
This patent application is currently assigned to Zeolux Corporation. Invention is credited to Koch, Gene C., Magno, John N..
Application Number | 20040069995 10/434941 |
Document ID | / |
Family ID | 29420495 |
Filed Date | 2004-04-15 |
United States Patent
Application |
20040069995 |
Kind Code |
A1 |
Magno, John N. ; et
al. |
April 15, 2004 |
Feedback enhanced light emitting device
Abstract
A feedback-enhanced light emitting device is disclosed. A
feedback element coupled to an emissive element allows the emissive
element to emit collimated light by stimulated emission. Feedback
elements that provide this function may include but are not limited
to holographic reflectors with refractive index that varies at
least in part periodically and continuously.
Inventors: |
Magno, John N.; (Middletown,
NJ) ; Koch, Gene C.; (Bayville, NJ) |
Correspondence
Address: |
BAKER & MCKENZIE
805 THIRD AVENUE
NEW YORK
NY
10022
US
|
Assignee: |
Zeolux Corporation
Sammamish
WA
|
Family ID: |
29420495 |
Appl. No.: |
10/434941 |
Filed: |
May 8, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60379141 |
May 8, 2002 |
|
|
|
Current U.S.
Class: |
257/80 ;
257/E33.068 |
Current CPC
Class: |
G09G 3/2803 20130101;
H01S 3/1686 20130101; H01L 27/3244 20130101; H01L 51/56 20130101;
H01L 27/3281 20130101; H01L 51/5275 20130101; H01S 5/36 20130101;
H01L 51/5271 20130101; H01L 2251/5323 20130101; H01L 51/5265
20130101; H01L 51/5262 20130101; H01S 3/08059 20130101; H01L 33/465
20130101; H01L 27/3211 20130101; H01L 51/5234 20130101; H01L
27/3234 20130101; H01S 5/183 20130101; H01L 51/5246 20130101; H01S
3/1065 20130101; H01L 2227/323 20130101; H01L 51/5215 20130101;
G09G 3/32 20130101; H01L 27/3262 20130101; H01L 51/5281
20130101 |
Class at
Publication: |
257/080 |
International
Class: |
H01L 027/15 |
Claims
We claim:
1. A feedback enhanced light emitting device, comprising: a first
feedback layer adapted to receive and reflect light; a second
feedback layer adapted to receive and reflect light, at least one
of the first feedback layer and the second feedback layer having a
refractive index profile that at least in part varies periodically
and continuously along an axis normal or substantially normal to a
plane of a respective feedback layer; and an emissive layer
disposed between the first feedback layer and the second feedback
layer.
2. A feedback enhanced light emitting device, comprising: a first
feedback layer adapted to receive and reflect light; a second
feedback layer adapted to receive and reflect light, at least one
of the first feedback layer and the second feedback layer having a
refractive index profile that at least in part varies periodically
and continuously along an axis normal or substantially normal to a
plane of the first feedback layer, at least one of the first
feedback layer and the second feedback layer adapted to reflect one
or more predetermined wavelength bands of light at least along an
axis normal or substantially normal to the plane of a respective
feedback layer; and an emissive layer disposed between the first
feedback layer and the second feedback layer, the emissive layer
adapted to emit light, the emissive layer further adapted to
provide stimulated emission caused by the one or more predetermined
wavelength bands of light reflected from one or more of the first
feedback layer and the second feedback layer.
3. The device of claim 1, wherein the first feedback layer or the
second layer or both the first feedback layer and the second layer
comprises a hologram.
4. The device of claim 1, wherein the first feedback layer or the
second layer or both the first feedback layer and the second layer
comprises a hologram of a plane wave light source.
5. The device of claim 1, wherein the emissive layer comprises an
organic luminescent material.
6. The device of claim 1, wherein the emissive layer comprises at
least one or more of, a cross-linked organic luminescent material,
a cross-linked polymer luminescent material, a luminescent material
comprising molecules having molecular weight range between that of
a small molecule to a polymer, a small molecule luminescent
material dissolved in a polymer host, a fluorescent material, a
phosphorescent material, an organic and inorganic composite
luminescent material, an inorganic luminescent material, and a
liquid crystalline luminescent material.
7. The device of claim 1, further comprising: a first electrode
disposed between the first feedback layer and the emissive layer;
and a second electrode disposed between the second feedback layer
and the emissive layer.
8. The device of claim 7, wherein the first electrode is an anode
and the second electrode is a cathode.
9. The device of claim 7, wherein the first electrode is a cathode
and the second electrode is an anode.
10. The device of claim 7, further comprising: one or more buffer
layers disposed between one or both of the first and the second
feedback layers and one or both of the first electrode and the
second electrode.
11. The device of claim 10, wherein the one or more buffer layers
comprise at least transparent dielectric material.
12. The device of claim 10, wherein the one or more buffer layers
are used to hermetically isolate the device from atmospheric
contamination.
13. The device of claim 10, wherein the one or more buffer layers
are disposed to provide spacing between the first feedback layer
and the second feedback layer such that constructive interference
and stimulated emission occur at one or more selected
wavelengths.
14. The device of claim 1, further comprising a hole injection
layer disposed between the first feedback layer and the emissive
layer.
15. The device of claim 1, further comprising an electron injection
layer disposed between the second feedback layer and the emissive
layer.
16. The device of claim 1, further comprising a hole transport
layer disposed between the hole injection layer and the emissive
layer.
17. The device of claim 1, further comprising an electron transport
layer disposed between the electron injection layer and the layer
of light emissive organic material.
18. The device of claim 1, wherein at least one of first feedback
layer and the second feedback layer comprises at least one or more
of a plane mirror, a multilayer dielectric distributed Bragg
reflector, a specular surface of an electrode, and a non-photonic
crystal reflector.
19. The device of claim 1, wherein both the first feedback layer
and the second feedback layer transmit no light at a peak
wavelength of their spectral reflection bands and the light
emissive material radiates light into band-edge laser modes.
20. The device of claim 1, wherein a level of light fed back from
the first feedback layer and the second feedback layer is
sufficient to initiate laser action.
21. The device of claim 1, wherein one or both of the first
feedback layer and the second feedback layer comprises one or more
of tuned thickness and tuned refractive index contrast to optimize
an amount of light fed back into the emissive layer.
22. The device of claim 1, wherein one or both of the first
feedback layer and the second feedback layer comprises at least one
or more discontinuities in the continuously varying refractive
index profile.
23. The device of claim 1, wherein one or both of the first
feedback layer and the second feedback layer comprises a plurality
of individual feedback layer refractive index profiles.
24. The device of claim 1, wherein one or both of the first
feedback layer and the second feedback layer comprises refractive
index profiles that have superimposed refractive index profiles
with non-reflective functions.
25. The device of claim 1, wherein one or both of the first
feedback layer and the second feedback layer comprises refractive
index profiles having a dominant periodicity n/2 times a selected
wavelength of the feedback light, where n is an integer.
26. The device of claim 1, wherein one or both of the first
feedback layer and the second feedback layer is thinned in one or
both of physical thickness and optical thickness to enable light to
escape the device.
27. The device of claim 4, wherein optical thickness of one or both
of the first feedback layer and the second feedback layer is
thinned by varying a holographic exposure and a resulting
refractive index contrast of the hologram.
28. The device of claim 1, further including one or more layers
disposed between the first feedback layer and the second feedback
layer, the one or more layers formed with a structure that is
enabled to continue a refractive index alternation that comprises a
photonic crystal structure in one or both of the first feedback
layer and the second feedback layer.
29. The device of claim 28, wherein the one or more layers comprise
at least one or more of electrode, charge carrier injection layer,
charge carrier transport layer, carrier blocking layer, and the
emissive layer.
30. The device of claim 28, wherein the refractive index
alternation is produced holographically by fabricating the one or
more layers using one or more of a photopolymer and photosensitive
material and exposing the one or more of a photopolymer and
photosensitive material to light.
31. The device of claim 28, wherein the refractive index
alternation is produced by at least one or more of a cholesteric
and chiral liquid crystal structure in a material used to fabricate
the one or more layers.
32. The device of claim 1, wherein the first feedback layer or the
second feedback layer or both the first feedback layer and the
second feedback layer comprises at least a material having at least
a sinusoidally varying refractive index profile.
33. The device of claim 1, wherein the first feedback layer or the
second feedback layer or both the first feedback layer and the
second feedback layer comprises a plurality of regions adapted to
reflect a plurality of predetermined wavelength bands of light,
with at least one of the plurality of regions adapted to reflect a
predetermined spectral band of light centered on a wavelength
different from a predetermined spectral band of light centered on a
wavelength reflected by another one of the plurality of
regions.
34. The device of claim 33, wherein one of the first feedback layer
and the second feedback layer comprises at least a photonic crystal
structure and a plurality of regions adapted to reflect a plurality
of predetermined wavelength bands of light, with at least one of
the plurality of regions adapted to reflect a predetermined
spectral band of light centered on a wavelength different from a
spectral band of light centered on a predetermined wavelength
reflected by another one of the plurality regions, and with the
plurality of regions in the first feedback layer registered to a
corresponding plurality of regions in the second feedback layer
reflecting spectral bands centered on the same wavelengths of
light.
35. The device of claim 33, wherein the emissive layer comprises at
least a plurality of regions adapted to emit a plurality of
predetermined wavelength bands of light, with at least one of the
plurality of regions adapted to emit a predetermined wavelength
band of light different from a predetermined wavelength band of
light reflected by another one of the plurality regions, and with
each of the plurality of regions registered to corresponding
regions in at least one of the first feedback layer and the second
feedback layer with spectral reflection bands of light that at
least in part overlap the corresponding emitter spectral emission
bands.
36. The device of claim 35, wherein two or more of the plurality of
regions in the emissive layer registered to corresponding regions
in the feedback layers adapted to reflect a plurality of
predetermined spectral bands of light centered on different
wavelengths is adapted to emit predetermined different wavelength
bands of light from the same broad spectral band emissive material
that at least in part overlaps of all of the corresponding emitter
spectral emission bands.
37. The device of claim 1, wherein the first feedback layer or the
second feedback layer or both the first feedback and the second
feedback layer comprises a hologram having a plurality of
refractive index profiles superpositioned in the hologram.
38. The device of claim 1, wherein the first feedback layer or the
second feedback layer or both the first feedback and the second
feedback layer comprises a hologram having a plurality of
refractive index profiles corresponding through Bragg's law to a
plurality of wavelengths of light superpositioned in the
hologram.
39. The device of claim 1, wherein the first feedback layer or the
second feedback layer or both the first feedback and the second
feedback layer comprises one or more regions having constant
refractive index.
40. The device of claim 1, wherein the first feedback layer or the
second feedback layer or both the first feedback and the second
feedback layer comprises material having a photonic crystal
structure.
41. The device of claim 40 wherein both the first feedback layer
and the second feedback layer combine to form at least in part a
continuous photonic crystal structure.
42. The device of claim 41, wherein both the first feedback layer
and the second feedback layer transmit substantially no light
emitted by the emissive layer at the peak wavelengths of their
spectral reflection bands and the emissive layer radiates light
into band-edge light propagation modes of the photonic crystal.
43. The device of claim 42, wherein the emissive layer comprises an
organic luminescent material.
44. The device of claim 42, wherein the emissive layer comprises at
least one or more of, a cross-linked organic luminescent material,
a cross-linked polymer luminescent material, a luminescent material
comprising molecules having molecular weight range between that of
a small molecule to a polymer, a small molecule luminescent
material dissolved in a polymer host, a fluorescent ematerial, a
phosphorescent material, an organic and inorganic composite
luminescent material, an inorganic luminescent material and a
liquid crystalline luminescent material.
45. The device of claim 42, further comprising: a first electrode
disposed between the first feedback layer and the emissive layer;
and a second electrode disposed between the second feedback layer
and the emissive layer.
46. The device of claim 45, wherein the first electrode is an anode
and the second electrode is a cathode.
47. The device of claim 45, wherein the first electrode is a
cathode and the second electrode is an anode.
48. The device of claim 42, wherein a level of light fed back from
the first feedback layer and the second feedback layer is
sufficient to initiate laser action.
49. The device of claim 47, further comprising: one or more buffer
layers disposed between the a feedback layer and one or both of the
first electrode and the second electrode.
50. The device of claim 49, wherein the one or more buffer layers
comprises at least transparent dielectric material.
51. The device of claim 49, wherein the one or more buffer layers
is used to hermetically isolate the device from atmospheric
contamination.
52. The device of claim 49, wherein the one or more buffer layers
is disposed to provide spacing between the first feedback layer and
the second feedback layer such that constructive interference and
stimulated emission occur at a selected wavelength or
wavelengths.
53. The device of claim 42, further comprising a hole injection
layer disposed between the first feedback layer and the emissive
layer.
54. The device of claim 42, further comprising an electron
injection layer disposed between the second feedback layer and the
emissive layer.
55. The device of claim 42, further comprising a hole transport
layer disposed between the hole injection layer and the emissive
layer.
56. The device of claim 42, further comprising an electron
transport layer disposed between the electron injection layer and
the layer of light emissive organic material.
57. The device of claim 40, wherein the light emissive layer
comprises a defect in a continuous photonic crystal formed by the
first feedback layer and the second feedback layer.
58. The device of claim 57, wherein the defect comprises a
phase-slip in spatial phase along one dimension of the photonic
crystal of less than one wavelength.
59. The device of claim 57, wherein the light emitted from the
light emissive layer emanates into a defect mode.
60. The device of claim 59, wherein the emissive layer comprises an
organic luminescent material.
61. The device of claim 59, wherein the emissive layer comprises at
least one or more of, a cross-linked organic luminescent material,
a cross-linked polymer luminescent material, a luminescent material
comprising molecules having molecular weight range between that of
a small molecule to a polymer, a small molecule luminescent
material dissolved in a polymer host, a fluorescent material, a
phosphorescent material, an organic and inorganic composite
luminescent material, an inorganic luminescent material and a
liquid crystalline luminescent material.
62. The device of claim 59, further comprising: a first electrode
disposed between the first feedback layer and the emissive layer;
and a second electrode disposed between the second feedback layer
and the emissive layer.
63. The device of claim 62, wherein the first electrode is an anode
and the second electrode is a cathode.
64. The device of claim 62, wherein the first electrode is a
cathode and the second electrode is an anode.
65. The device of claim 59, wherein a level of light fed back from
the first feedback layer and the second feedback layer is
sufficient to initiate laser action.
66. The device of claim 62, further comprising: a buffer layer
disposed between one or both of the first and the second feedback
layers and one or both of the first electrode and the second
electrode.
67. The device of claim 66, wherein the buffer layer comprises at
least transparent dielectric material.
68. The device of claim 66, wherein the buffer layer is used to
hermetically isolate the device from atmospheric contamination.
69. The device of claim 66, wherein the buffer layer is disposed to
provide spacing between the first feedback layer and the second
feedback layer such that constructive interference and stimulated
emission occur at a selected wavelength or wavelengths.
70. The device of claim 59, further comprising a hole injection
layer disposed between the first feedback layer and the emissive
layer.
71. The device of claim 59, further comprising an electron
injection layer disposed between the second feedback layer and the
emissive layer.
72. The device of claim 59, further comprising a hole transport
layer disposed between the hole injection layer and the emissive
layer.
73. The device of claim 59, further comprising an electron
transport layer disposed between the electron injection layer and
the layer of light emissive organic material.
74. The device of claim 1, wherein the first feedback layer or the
second feedback layer or both the first feedback and the second
feedback layer partially transmits light received from the emissive
layer.
75. The device of claim 1, wherein the first feedback layer or the
second feedback layer or both the first feedback and the second
feedback layer comprises one or more of chiral and cholesteric
liquid crystals.
76. The device of claim 1, wherein the first feedback layer or the
second feedback layer or both the first feedback and the second
feedback layer comprises opals, particulate agglomerates having
structures akin to a crystalline lattice, middle phase lyotropic
liquid crystals in a fluid or polymerized state, and self-assembled
block copolymers.
77. The device of claim 1, wherein the first feedback layer or the
second feedback layer or both the first feedback and the second
feedback layer comprises a dielectric material of continuously
varying composition.
78. The device of claim 1, wherein the first feedback layer and the
second feedback layer are phase-locked or phase-registered.
79. The device of claim 78, wherein the phase-locking or
phase-registration is performed interferometrically.
80. The device of claim 1, wherein the first feedback layer and the
second feedback layer are formed holographically and simultaneously
using one simultaneous exposure to an array of interference fringes
formed by two beams of plane polarized light.
81. The device of claim 1, wherein the first feedback layer or the
second feedback layer or both the first feedback and the second
feedback layer comprises a hologram of plane waves written with
linearly, circularly, or elliptically polarized light.
82. The device of claim 1, wherein the first feedback layer or the
second feedback layer or both the first feedback and the second
feedback layer comprises a hologram fabricated by recording a
refractive index profile in one or more of dichromated gelatin,
dichromated emulsions, silver halide gelatine, silver halide
emulsions, photopolymer material, positive photosensitive material,
negative photosensitive material, and other photosensitive
material.
83. The device of claim 1, wherein the first feedback layer or the
second feedback layer or both the first feedback and the second
feedback layer is formed with a material with chiral centers.
84. The device of claim 1, wherein the emissive layer comprises
transparent electroluminescent material.
85. The device of claim 1, wherein the first feedback layer or the
second feedback layer or both the first feedback and the second
feedback layer comprise material patterned to reflect different
wavelength bands of light.
86. The device of claim 1, wherein the emissive layer is patterned
to emit different wavelength bands of light.
87. The device of claim 1, wherein the emissive layer comprises an
electroluminescent material having spectral emission band that
overlaps reflection bands of the first feedback layer and the
second feedback layer.
88. The device of claim 1, wherein all light emitted by the device
occupies a single light propagation mode.
89. The device of claim 88, wherein spacing between the first
feedback layer and the second feedback layer is equivalent to
.lambda./2 excluding phase shifts due to reflection, .lambda. being
a wavelength of the light in the single light propagation mode.
90. The device of claim 1, wherein light emitted by the device
occupies two or more light propagation modes.
91. The device of claim 90, wherein one or both of the device
substrate and cover are transparent and are used as spacers between
the two feedback layers providing proper spacing between the layers
to yield the desired laser mode spacing and spectral location.
92. The device of claim 1, further including: a substrate on which
one of the first feedback layer and the second feedback layer is
disposed.
93. The device of claim 92, wherein the substrate comprises at
least one or more of a flexible material, a rigid material, a
glass, a metal, and a semiconductor material.
94. The device of claim 93, wherein the flexible material comprises
one or more of a film of polyethylene terephthalate, (PET),
polyethylene naphthalate (PEN), bisphenol A polycarbonate, and
another engineering polymer.
95. The device of claim 1, further including: a cover disposed on
at least one of the first feedback layer and the second feedback
layer.
96. The device of claim 1, wherein the refractive index profile
comprises a profile intermediate between a square wave profile and
a sinusoidal profile.
97. The device of claim 1, wherein a spectral reflection band of
one or both the first feedback layer and the second feedback layer
are chosen so as to generate stimulated emission that substantially
overlaps neither a spectral excitation band nor a spectral
absorption band of the emissive layer.
98. The device of claim 1, wherein a reflection band of one or both
of the first feedback layer and the second feedback layer is
spectrally narrower than an emission band of the emissive
layer.
99. The device of claim 4, wherein the emissive layer comprises
inorganic semiconductor material.
100. A method of fabricating a feedback element coupled to an
emissive element in a device for emitting light, comprising:
disposing a layer of polymer on a substrate; and exposing the
polymer to light to record one or more interference patterns in the
polymer.
101. The method of claim 100, further comprising cross-linking the
polymer.
102. A method of fabricating a feedback layer, comprising:
successively exposing holographic patterns on holographic material
using photomasks in both image and reference beams of a holographic
set up to form a plurality of regions reflecting a plurality of
different wavelength bands of light.
103. A method of fabricating a feedback layer, comprising:
successively exposing holographic patterns on holographic material
using a single photomask in one of an image beam and a reference
beam to form a plurality of regions reflecting a plurality of
different wavelength bands of light, the holographic material
having a selected irradiance threshold of exposure such that energy
of an unpatterned beam alone does not cause refractive index change
in the material.
104. A method of fabricating a feedback enhanced light emitting
device, comprising: forming a substrate; forming a first feedback
layer; forming a first electrode on the first feedback layer;
forming an emissive layer on the first electrode; forming a second
electrode on the emissive layer; and forming a second feedback
layer on the second electrode.
105. The method of claim 104, wherein by the emissive layer is
formed by forming a layer of photo-cross-linkable material on the
first electrode and then exposing it to light to cross-link it.
106. The device of claim 7, wherein the cathode comprises a
transparent low work function material.
107. The device of claim 7, wherein the cathode comprises a first,
very thin metal layer disposed towards the emissive layer and a
second thicker layer comprising a transparent conductive material
such as indium-tin oxide.
108. The device of claim 7, wherein the anode comprises a
transparent high work function material.
109. The device of claim 79, wherein the second feedback layer is
formed holographically using an aerial interference fringe pattern
phase-locked or phase-registered to the first feedback layer
interferometrically.
110. The device of claim 91, wherein a transparent spacer is
fabricated between the first feedback layer and the second feedback
layer to provide selected spacing between the first feedback layer
and the second feedback layer to yield the selected laser mode
spacing and spectral location.
111. The device of claim 42, further including one or more layers
disposed between the first feedback layer and the second feedback
layer, the one or more layers formed with a structure that is
enabled to continue a refractive index alternation that comprises a
photonic crystal structure in one or both of the first feedback
layer and the second feedback layer.
112. The device of claim 111, wherein the one or more layers
comprise at least one or more of electrode, charge carrier
injection layer, charge carrier transport layer, carrier blocking
layer, and the emissive layer.
113. The device of claim 57, further including one or more layers
disposed between the first feedback layer and the second feedback
layer, the one or more layers formed with a structure that is
enabled to continue a refractive index alternation that comprises a
photonic crystal structure in one or both of the first feedback
layer and the second feedback layer.
114. The device of claim 113, wherein the one or more layers
comprise at least one or more of electrode, charge carrier
injection layer, charge carrier transport layer, carrier blocking
layer, and the emissive layer.
115. The device of claim 42, wherein a level of light fed back from
the first feedback layer and the second feedback layer is
sufficient to initiate laser action.
116. The device of claim 57, wherein a level of light fed back from
the first feedback layer and the second feedback layer is
sufficient to initiate laser action.
117. A feedback enhanced light emitting device, comprising: a first
means for reflecting light; a second means for reflecting light, at
least one of the first means and the second means having a
refractive index profile that at least in part varies periodically
and continuously along an axis normal or substantially to a plane
of a respective first and/or second means; and a third means for
emitting light at least as a result of receiving the light
reflected by at least one of the first means and the second
means.
118. A feedback enhanced light emitting device, comprising: a
hologram layer; a reflector layer; and at least a luminescent
material disposed between the hologram layer and the reflector
layer.
119. The device of claim 112, wherein the reflector layer comprises
at least a second hologram layer.
120. A feedback enhanced light emitting device, comprising: a
photonic crystal structure layer; a reflector layer; and at least a
luminescent material disposed between the photonic crystal
structure layer and the reflector layer.
121. A feedback enhanced light emitting device, comprising: a first
reflector; a second reflector; and at least a luminescent material
disposed between the first and the second reflector layers, wherein
the first reflector and the second reflector combine to form at
least in part a continuous photonic crystal structure.
122. A feedback enhanced light emitting device, comprising: a first
reflector; a second reflector; and at least a luminescent material
disposed between the first and the second reflector layers, the at
least a luminescent material comprising a defect in a continuous
photonic crystal formed by the first reflector and the second
reflector.
123. A feedback enhanced light emitting device, comprising: a first
feedback layer adapted to receive and reflect light; a second
feedback layer adapted to receive and reflect light, at least one
of the first feedback layer and the second feedback layer having a
refractive index profile that varies periodically and continuously
along an axis normal or substantially normal to a plane of a
respective feedback layer; and an emissive layer disposed between
the first feedback layer and the second feedback layer.
124. The device of claim 1, wherein a distance between the first
feedback layer and the second feedback layer is such that the space
between the feedback layers comprise a cavity in which light of one
or more desired wavelengths constructively interfere.
125. The device of claim 1, wherein light reflected by one or both
of the first feedback layer and the second feedback layer
stimulates emission of light from the emissive layer.
126. The device of claim 125, wherein the stimulated emission of
light results in substantial collimation of light emitted by the
device.
127. The device of claim 125, wherein the stimulated emission of
light results in laser action.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/379,141 filed May 8, 2002, incorporated herein
in its entirety by reference thereto. This application is related
to U.S. patent application Ser. No. ______, filed on May 8, 2003,
and entitled "LIGHTING DEVICES USING FEEDBACK ENHANCED LIGHT
EMITTING DIODE," and U.S. patent application Ser. No. ______, filed
on May 8, 2003, and entitled "DISPLAY DEVICES USING FEEDBACK
ENHANCED LIGHTING DIODE," which applications are incorporated
herein in their entirety by reference.
TECHNICAL FIELD
[0002] The present application relates to light source devices, and
particularly, to feedback enhanced light emitting devices.
BACKGROUND
[0003] Light emissive devices in the UV, visible and infrared
spectral regions have a vast number of applications but present an
even larger number of technical and economic challenges. One
challenge is in efficiently producing the desired output light
because the efficiency of producing output light may be reduced in
any number of ways. For example, light may be internally reflected
at refractive index boundaries; an excited energy state may be
converted into heat rather than light; an excited energy state may
be converted into light at a wavelength other than a desired
wavelength or wavelengths, for example, infrared, ultraviolet;
light may be absorbed by component layers of the device including
the light emitting material itself; and/or the emissive materials
may have poor charge carrier injection efficiency. Because of these
and other inefficiencies, the radiance level produced by a
presently known emissive device may be insufficient for a
particular application unless the emissive device is overdriven to
achieve the desired radiance. Overdriving a light emissive device,
however, may further reduce its efficiency and useful lifespan by
increasing the amount of heat generated.
[0004] Further, light emissive devices made with organic light
emitting diode (OLED) materials present difficulties in
fabrication. Typical OLED light emissive materials include polymers
or small molecules. Polymer OLED materials, however, are difficult
to produce because of solubility or chemical compatibility
problems. Further, although small molecules have sufficient vapor
pressure such that the molecules may be deposited onto substrates
by vapor deposition, small molecule OLEDs present different
problems in that they are mechanically and thermally fragile.
[0005] It is possible to overcome these disadvantages by
cross-linking these polymer OLED and small molecule OLED materials
to produce a cross-linked OLED material. Cross-linking OLED
phosphors, however, may reduce the conversion efficiency of
excitons to photons to the point where a useful light emissive
device does not result.
[0006] Similarly, light emissive devices made with inorganic light
emitting diode (LED) materials have low efficiency in the
conversion of energy to light. The low efficiency results from
several factors including high absorption in the inorganic light
emitting material and difficulty in coupling light out of the
inorganic light emissive material due to the high refractive index
of the light emissive material. The overall efficiency can be
increased when light emissive devices also include other elements
such as a multi-layer dielectric distributed Bragg reflector (DBR),
which are expensive to manufacture. The overall efficiency
increase, however, is limited by the light losses in the DBR.
Although the losses in the DBR can be improved through the
inclusion of index matching layers, the index matching layers
increase the cost and the number of layers in the device, further
complicating manufacturing of these devices.
[0007] Furthermore, although additional elements may be included in
emissive devices to improve a given characteristic of the devices,
the improvement of a given characteristic is often a tradeoff with
another characteristic and also complicates the emissive device by
requiring additional processing steps to form the additional
elements. For example, a multi-layer dielectric distributed Bragg
reflector (DBR) may be used to increase the amount of stimulated
emissions in the emissive layer at the cost of a substantial amount
of light being lost. Index matching layers may also be included to
reduce the amount of additional light loss. Such a method, however,
greatly increases the number of layers in the DBR. Such devices
also can be expensive and slow to produce, while only providing
marginal improvements to its characteristics.
[0008] In the case of a multi-color emissive device, the challenge
of producing efficient light emitting devices becomes greater. This
is because the structure of the device often includes a greater
number of more complex structures to handle multiple colors. The
additional complexity adds further processing steps, which increase
the cost and complexity of the manufacturing process while reducing
the yield and output. The added structures require more precise
positioning and are more difficult to fabricate with optically
smooth surfaces. Additionally, color emissive devices often have
poor color rendition because the use of broadband emissive
material. Accordingly, a need exists for a more efficient light
emitting device.
SUMMARY
[0009] A feedback enhanced light emitting device is disclosed. The
device in one aspect comprises an emissive layer disposed between
two feedback layers. The emissive layer is adapted to emit light
wherein the two feedback layers reflect at least some of the light
back to the emissive layer, thus stimulating emission. In one
aspect, at least one of the two feedback layers has at least in
part periodically and continuously varying refractive index profile
along an axis normal or substantially normal to a plane of a
respective layer.
[0010] In another aspect, the feedback layers may comprise a
hologram formed from a photopolymer having an optically written
sinusoidally varying refractive index profile. In another aspect,
the emissive layer may comprise cross-linked polymer.
[0011] In another aspect, a method of fabricating the feedback
layers comprises disposing a layer of polymer on a substrate and
exposing the polymer to light to record one or more interference
patterns on the polymer. In one aspect, the polymer is exposed to
light to cause cross-linking.
[0012] Further features as well as the structure and operation of
various embodiments are described in detail below with reference to
the accompanying drawings. In the drawings, like reference numbers
indicate identical or functionally similar elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 illustrates an emissive device in one embodiment.
[0014] FIG. 2 illustrates the critical angles at which light
undergoes total internal reflection.
[0015] FIG. 3 is a graph showing the efficiency, brightness, and
voltage of an OLED device.
[0016] FIG. 4 illustrates a device including a resonant cavity
equivalent.
[0017] FIG. 5 illustrates a pixelated device with a broadband
emissive layer and two pixelated feedback layers.
[0018] FIG. 6 illustrates a one-dimensional photonic refractive
index profile of a cholesteric liquid crystal.
[0019] FIG. 7 illustrates a feedback enhanced light emitting device
in another embodiment.
[0020] FIG. 8 illustrates a feedback enhanced light emitting device
having an OLED emissive element, two holographic feedback elements
and an optical spacer in another embodiment.
[0021] FIG. 9 illustrates a feedback-enhanced OLED structure in one
embodiment.
[0022] FIG. 10 illustrates an example of a double mask apparatus
for producing a patterned hologram in one embodiment.
[0023] FIG. 11 illustrates an example of a single mask apparatus in
for producing a patterned hologram in one embodiment.
[0024] FIG. 12 is a graph illustrating irradiance versus position
in the double mask apparatus of FIG. 10.
[0025] FIG. 13 is a graph illustrating irradiance versus position
in a single mask apparatus of FIG. 11.
[0026] FIG. 14 is a graph illustrating exposure rate versus
irradiance dose for a material used for hologram with irradiance
threshold=a.
[0027] FIG. 15 is a diagram that contrasts a structure of a
single-mode FE-OLED and a multi-mode FE-OLED.
DETAILED DESCRIPTION
[0028] FIG. 1 illustrates an emissive device in one embodiment. The
device 1 includes an emissive layer 2 and a feedback element 4. The
feedback element 4 may be a layer with a periodically and
continuously varying index of refraction that allows some light to
be transmitted through the feedback element 4. A second feedback
element 6 may also be included such that the emissive layer is
between the two feedback elements 4, 6. The second feedback element
6 may allow some light to be transmitted through the second
feedback element 6 or substantially reflect the light incident upon
it. In one embodiment, a structure with a periodic index of
refraction variation, a plane mirror, a distributed Bragg reflector
(DBR), or another reflector may be used as the second feedback
element 6.
[0029] In one embodiment, the device shown in FIG. 1 allows light
emanating from the emissive layer in a direction that is normal to
the planes of the two feedback structures to substantially reflect
back and forth between the two feedback structures. In passing
through the emissive layer multiple times, the emission of
additional light is stimulated by interacting with the excitons in
the emitter. Excitons are excited state pairs of electrons and
holes whose collapse leads to light emission in luminescent
materials. In this way the amount of light propagating normal to
the plane of the device is increased at the expense of light
propagating in the plane or at oblique angles. Since the in-plane
and obliquely propagating light does not normally escape the
luminescent device, this enhancement of normal to the plane
emission considerably increases device efficiency.
[0030] In one embodiment, if the feeding back of emitted light and
subsequent stimulated emission are very efficient, all light will
be emitted along the vertical axis and will be substantially
coherent. In this case, the device becomes a vertically emitting
laser or VCSEL (vertical cavity surface emitting laser). However,
laser action need not occur in order for the emitted light to be
substantially collimated and thus for the device to have
considerably enhanced energy efficiency. Because in these devices
the efficiency enhancement is due to feedback light, the devices
are referred to as feedback enhanced luminescent devices or
feedback enhanced light emitting devices.
[0031] The device in one aspect may also include other elements
such as an anode, a cathode, carrier injection and transport
layers, a transparent buffer layer lying between the feedback
layers and the emissive layer, or other elements. FIG. 7
illustrates an emissive device 700 having additional elements. An
integrated light diffuser may also be added outside of the feedback
layers.
[0032] In one embodiment, one or more methods for supplying
excitation energy to the emitter generally may include electrical
excitation or photo-excitation. If the excitation is electrical
current, it may be introduced into the emissive layer through a
pair of electrodes to induce electroluminescence. For example, a
pair of electrodes such as a cathode 102 and an anode 104 may be
placed between an emissive layer 2 and the feedback layers 4 and 6,
respectively.
[0033] The cathode 102 may include a transparent conductive
structure with a low work function surface adjacent to the emissive
layer 2 such that it is able to inject electrons into the emissive
layer 2. In one aspect, for the cathode 102 to have the desired
transparency, a two-layer cathode may be provided. The two-layer
cathode may include a very thin, for example, 5 nanometer (nm.)
metal cathode such that the metal is transparent or nearly
transparent. The metal may then be backed, for example, on the
feedback layer side, with a transparent conductor like indium-tin
oxide (ITO) to yield high enough conductivity to have a low
impedance device. The anode 104 may include a transparent
conductive material chosen to have a high work function such that
it is able to inject holes into the emissive layer 2.
[0034] The emissive layer 2 may include an electroluminescent
material whose spectral emission band overlaps the reflection bands
of the feedback layers 4 and 6. In one embodiment, the emissive
material is an organic luminescent material and the device is
referred to as a feedback enhanced organic light emitting diode
(FE-OLED). Alternatively, the emissive material may be an
organometallic luminescent material, a luminescent inorganic
semiconductor material such as GaAs, AlGaAs, or InGaN, or an
organic/inorganic composite luminescent material. In another
aspect, the emissive layer 2 may be a fluorescent or phosphorescent
emissive material.
[0035] The feedback layers 4 and 6 may include light non-absorbing
material with a periodically varying index of refraction. A way of
describing the function of these structures is that the light
entering the feedback layer material along the layer normal axis
suffers a small reflection each time it passes through one cycle of
the refractive index oscillation. When the feedback element is
thick enough, the feedback element may act as a nearly perfect
reflector at the resonant wavelength, 2d, where d is the pitch of
the refractive index spatial oscillation.
[0036] The feedback layers in one aspect may be fabricated from
plane wave holograms with peak reflectivity at the desired emission
wavelength.
[0037] In one embodiment, the device 700 shown in FIG. 7 may be
inverted. That is, the position of the cathode 102 and the anode
104 may be interchanged.
[0038] The device 700 also may include a substrate 106 placed
adjacent to a feedback layer, for example the feedback layer 6. The
substrate 106 is used as a layer on which the device 700 may be
built. In one aspect the substrate 106 may be comprised of a
transparent material. In one aspect, a material may be applied over
the device 700 to function as a cover 108. The cover 108, for
example, functions to hermetically seal out ambient water and
oxygen, or otherwise to protect the device 700 from chemical or
other degradation.
[0039] Other components of the device 700 may include a hole
transport layer between the anode 104 and the emissive layer 2. The
hole transport layer may be used to allow more electron/hole
recombination to occur at the emissive layer 2. For example, in
emissive layers having imbalance between electron and hole
mobilities, usually with low hole mobilities, the electron/hole
recombination tends to occur at the anode. Similarly, a device with
a direct anode/emitter interface tends to be inefficient because
many traps, that is, sites at which non-radiative de-excitation of
the emitter occurs, exist at the emitter/anode interface. Using
hole transport layers, for example, with high hole mobilities
minimizes the problem of the electron/hole recombination occurring
at the anode. The hole transport layer may also be chosen to have a
hole conduction band intermediate between those of the anode 104
and the emissive layer 2, thus providing more efficient hole
injection from the anode into the emitter.
[0040] A hole injection layer may also be provided between the
anode 104 and the hole transport layer. For example, if anode
materials like indium-tin oxide (ITO) having less than well defined
band structures that may lead to inefficient hole injection into
the device are used, hole injection layers like copper
phthalocyanine may be provided to better define band structure with
energy level intermediate between ITO and hole transport materials.
Providing the additional hole injection layers thus may assist hole
injection and produce a more efficient device. Hole injection
layers, however, are not required.
[0041] In another embodiment, additional hole transport layers may
be inserted between the hole injection layer and the emitter to
further smooth out band energy differences. If the hole transport
layer adjacent to the emitter has its electron conduction band at
an energy level nearly the same as the emitter, electrons may
"overshoot" the emitter with recombination occurring in the
transport layer rather than the emitter. This overshoot may be
eliminated by interposing an electron blocking layer that has a
high energy electron conduction band, but good hole conduction,
between the emitter and the transport layer.
[0042] In another embodiment, an electron transport layer may be
provided between the cathode 102 and the emissive layer 2. The
electron transport layer performs the similar function for
electrons that the hole transport layer performs for holes. As with
hole transport layers, additional electron transport layers may be
added to assist band energy matching.
[0043] In another embodiment, an electron injection layer may be
provided between the cathode 102 and the electron transport layer.
In one embodiment, a low work function material may be used for the
cathode 102. With a low work function material less energy is
expended injecting electrons into the device. For example, very low
work function metals such as calcium may be used, although calcium
may be chemically reactive and sensitive to moisture and oxygen.
Aluminum also may be used. For instance, overcoating the aluminum
with a very thin film of materials like lithium or magnesium
fluoride provides a "band bending" effect that helps relieve the
band energy mismatch.
[0044] In another embodiment, a hole blocking layer may be provided
between the emitter and hole transport layer to reduce hole
"overshoot" from the emitter. The above described carrier
transport, injection, and blocking layers are also typically used
in the conventional OLED devices. Accordingly, further details of
these elements will not be described herein.
[0045] In one embodiment, the device 700 may also include a buffer
layer, for example, a clear dielectric interposed between an
electrode and a feedback layer. When the buffer layer is placed
between the cathode 102 and the feedback layer 4, it may act as a
hermetic barrier between the cathode and the outside environment
especially during subsequent processing. In one embodiment, the
buffer layer also provides the right size spacing such that
destructive interference of light in the gap between the two
feedback layers does not occur. To achieve this function, the
buffer layer may be inserted between the feedback layer and the
electrode to adjust the optical thickness of the device. The buffer
layer may also be used to maintain the proper phase relationship
between the refractive index profiles in the two feedback layers.
In addition the buffer layer may be used to adjust the thickness of
the gap between the feedback layers thereby tuning the wavelengths
of the modes of the light that is resonating in the gap.
[0046] An example of a buffer layer arranged on a device in one
embodiment is shown in FIG. 8. FIG. 8 illustrates a device that
includes an emissive element 806, for example, an OLED layer, two
feedback elements 804 and 810, for example holographic layers, and
a buffer layer 808, for example, an optical spacer. As shown in
FIG. 8, the emissive element 806 may include a plurality of layers.
The device also may include a front glass 802 and a back glass
812.
[0047] The devices shown in FIGS. 1, 7, and 8 may substantially
reduce or eliminate the light losses, for example, due to total
internal reflections that may otherwise occur at the refractive
index mismatch at boundaries. This approximately doubles the amount
of light extracted from the device through, for example, a
substantial elimination of light absorption loss inside of the
device.
[0048] In one aspect, referring back to FIG. 1, the feedback
elements 4, 6 located on either side of the emissive layer 2 form a
resonant cavity. The feedback elements 4, 6 reflect light back into
the material of the emissive layer 2 and allow stimulated emission
to occur when sufficient light is reflected into the emissive layer
2. For example, the number of interactions between photons and
excitons regulate the rate of stimulated emission. Thus, for
example, by localizing light in the resonant cavity and thus
causing a high density of photons at the emissive layer 2, a very
rapid stimulated emission conversion may be produced.
[0049] Typically, without the induced stimulated emission,
spontaneous emission, which is a relatively slow and purely
statistical process, dominates the light generation process in an
emissive material. In one embodiment of the present disclosure, the
rapid conversion to stimulated emission leaves the spontaneous
emission process with little or no excited state energy to convert
to light. An even slower process, non-radiative de-excitation,
converts excited state energy to heat. Thus, stimulated emission in
one embodiment preempts conversion of excited state energy to heat
since the mechanism of heat formation is orders of magnitude slower
than that of stimulated emission. Thus in one embodiment, the
excited state energy of the device 1 may be converted predominantly
into light, not heat. The consequent reduction in heat generation
in this embodiment also may result in reduced temperature in the
device, which may allow for a longer life and more efficiency in
the device.
[0050] In conventional light emitting devices, the light absorption
loss occurs because light is emitted at oblique angles to the plane
of a device and reflected. For example, FIG. 2 illustrates the
critical angles at which light undergoes total internal reflection.
Light strikes the refractive index boundaries between a high
refractive index layer 8 and two adjacent layers 10 and 12, at
angles exceeding the critical angle at which light undergoes total
internal reflection. The reflected light is effectively trapped
within the high index layer 8 and does not contribute to the
radiance level of a device. In one embodiment of the method and
apparatus of the present disclosure, by allowing the light to emit
normal to the plane of a device and thus reducing the internal
reflection, higher radiance level may be achieved with lower
driving voltages. Lower driving voltages in turn may contribute to
extended lifetime of a device.
[0051] FIG. 3 is a graph illustrating the efficiency, brightness,
and voltage of an OLED device. Driving an OLED device at lower
voltages while achieving the desired radiance level has a great
impact on the efficiency and longevity of the device. This is due
to OLED material having peak efficiency soon after light generation
begins as drive voltage is increased and rapidly declines
thereafter with increasing voltage. Thus, achieving the desired
radiance levels at lower driving voltages through stimulated
emission has a great impact on the power conversion efficiency and
longevity of OLED devices.
[0052] In one embodiment, with the sufficient levels of light
feedback and resulting stimulated emission, the device 1 may be
used as a laser. Further, even if stimulated emission in the light
emissive layer 2 does not lead to lasing, the device 1 may exhibit
improved luminous efficiency. The improved luminous efficiency
occurs, for example, because in one embodiment the total amount of
light exiting the device 1 is substantially increased by the
increased production of light being emitted normal to the plane of
the device 1 as compared to light being emitted at angles that
result in total internal reflection. In one embodiment, the
emissive layer 2 may comprise cross-linked organic light emitting
diode (OLED) materials such as a small molecule or polymer OLED or
molecules of molecular weight intermediate between the two.
Although in known devices, cross-linking normally causes the
excited state energy to convert into heat, the feeding back of
emitted light into the emissive layer, in one embodiment of the
present disclosure, and the subsequent rapid stimulation of light
emission from the emissive layer that may occur, changes the
dominant mode of de-excitation in cross-linked emitter materials
from heat to the stimulated emission of light. Thus, in one
embodiment, the one or more feedback layers of the present
disclosure allows the OLED devices to include conventional
photo-induced and other cross-linked techniques. Accordingly, the
solubility problems of polymer OLED materials and thermal fragility
of small molecule OLED materials may be avoided.
[0053] In another embodiment, the emissive layer 2 may include a
small molecule emitter dissolved in a polymer host. Structures of
this type may be fabricated by solvent casting, for example, spin
coating a solution including a phosphor, a monomer, an initiator,
and a solvent. The polymer host dissolving may increase the thermal
and mechanical stability of the small molecule phosphor.
[0054] After the coating is dried to remove the solvent, the
coating may be cross-linked by exposure to ultraviolet or other
light resulting in a tenacious film that may be used as an emissive
layer 2. In one embodiment, both cross-linking the phosphor and
including a small molecule phosphor in solution in a polymer host
facilitate light emitting layer patterning by patterned exposure.
Both may also be used to fabricate highly efficient devices where
stimulated emission enabled by the presence of feedback layers is
the dominant de-excitation process.
[0055] In another embodiment, emissive layer 2 may comprise an
aligned liquid crystal material (for instance, in the nematic
phase) that has been rendered immobile by either cooling into a
glass phase or cross-linking as described above. The emitter
molecules may also be guests in a liquid crystalline host solvent.
Devices with emitters that have liquid crystalline order may emit
light with some level of plane polarization. They may be combined
with the polarized feedback layers described below.
[0056] In one embodiment, a feedback element is selected such that
it returns a desired percentage of emitted light back into the
emissive layer 2 with very high efficiency. Selecting the
appropriate feedback element may increase and maximize stimulated
emission of light from the emissive layer, thereby increasing the
total light output from the device. The light from the stimulated
emission process propagates normal to the plane of the device
ensuring nearly complete coupling of light out of the device. As
described above, having light emitted normal to the plane may
reduce light loss due to total internal reflection. Further, the
feedback element may be selected such that light absorption losses
in the feedback element are reduced.
[0057] In one embodiment, the feedback layer may be designed to
have a refractive index profile such that the light losses in the
feedback layer may be reduced or avoided. The reduction in light
absorption in the feedback layer may allow sufficient recirculation
of light to cause stimulated emission to dominate the light
generation process.
[0058] In one embodiment, for example, having continuous variation
in the refractive index of the feedback layer in contrast to abrupt
or discontinuous changes, for instance, as in DBR reflectors, may
minimize the light losses in the feedback layer. In one embodiment,
the continuous variation is periodic, that is regularly cyclically
varying in value. For example, a feedback layer may have index
variation meeting the Bragg condition: 2d sin .theta.=n.lambda.,
where d is the period of the cyclical variation in refractive index
in a volume grating, .theta. is the angle of incidence of light on
the grating, where .theta.=90.degree. and sin .theta.=1, n is an
integer representing the order of the reflection, and .lambda. is
the wavelength of the light that is desired to be reflected.
[0059] In another embodiment, all of the refractive index variation
need not be periodic with 2d=n.lambda.. Other purely random
variation may be superimposed on top of the desired periodic
variation. Yet in another embodiment, multiple periodic spatial
frequencies of variation may be superimposed.
[0060] Useful refractive profiles include, but are not limited to,
sinusoids, continuously varying sinusoidal like functions, and the
convolution of a sinusoid function offset so that sin(y) assumes no
negative values or a Gaussian function with a comb function.
[0061] In another embodiment, feedback layer elements may include a
refractive index profile designed to increase or maximize the
coupling of light out of the emissive layer. For example, the
refractive index profile may be one intermediate between a square
wave profile (discrete layers) and a sinusoidal profile.
[0062] Sinusoids and refractive index profiles intermediate between
sinusoids and square waves are producible holographically. A plane
wave hologram may be an example of an element that has a
continuously varying refractive index profile. For example, a
hologram with a sinusoidally varying refractive index profile may
be optically written into a feedback layer formed from a
holographic film.
[0063] In one embodiment, using holographic feedback layers may
allow the refractive profile of the feedback layer to be tuned from
a nearly sinusoidal to a nearly square wave profile by tuning the
contrast function of the holographic film. The result may be the
ability to tune the spectral width and shape of the feedback layer
reflection band.
[0064] The refractive index profile in a feedback layer is not
limited to reflecting a single wavelength, but may include in one
embodiment a superposition of two, three, or more profiles at
different pitches such that the reflection of multiple wavelength
bands may be performed by the same feedback layer. Alternatively,
the profile may have discontinuities or a region of constant
refractive index between regions of continuous variation, or may
include a plurality of the individual feedback layers as described
above.
[0065] In another embodiment, the refractive index profile of the
feedback element may be such that the amount of light produced by
stimulated emission is increased compared to light produced by
spontaneous emission or other processes, while simultaneously light
absorption losses in the feedback layer are reduced or avoided,
both factors contributing to increase in the light coupled out of
the device.
[0066] For example, the feedback layer may be designed to have a
proper ratio of light exiting the device to light reflected back
into the emissive region. This ratio is a function of the small
signal laser gain of an OLED structure. Thus, the ratio may vary
with both the emitter and other materials used in an OLED and the
exact structure of an OLED device. The proper ratio, for example,
may be generally determined by balancing between production of
additional light through stimulated emission and absorptive loss of
light in an OLED structure. Small signal gain is the net amount of
light gain per input photon flux extrapolated down zero photon
flux. The reason for this extrapolation is that each pass of light
through the device reduces the population of excitons due to
stimulated emission. Thus the gain in light output decreases for
each successive pass while the absorption losses remain constant.
Since different emitters have different intrinsic small signal
gains and since different emitters require different carrier
layers, layer thicknesses, etc. that result in different levels of
absorbance, each type device may have a different ratio. Having
this proper ratio in one embodiment may increase the amount of
light produced by stimulated emission as compared to light produced
by spontaneous emission. For instance, if too high a ratio of light
is reflected back into the emissive layer, more than an optimum
amount of light absorption may occur in the device as photons make
multiple passes through any light absorbing materials in a resonant
cavity of the device. On the other hand, if too high a ratio of
light is allowed to escape the device, insufficient light may be
reflected back into the emissive layer and inhibit maximizing of
stimulated emission.
[0067] In one embodiment, this proper ratio of light fed back to
light transmitted in the feedback layer may be achieved by properly
adjusting the physical thickness of the layer or the delta n
(maximum amplitude) of the refractive index variation in the layer.
These parameters may be varied to select the desired feedback to
transmission ratio. For example, a 7 micron thick layer of Slavich
PFG-03C holographic emulsion used as a feedback layer provides a
hologram with about 96% reflectance after optimum processing.
[0068] When this ratio is properly selected, an emissive layer that
normally emits light over a wide range of angles in at least one
dimension, if not isotropically in all directions, now may be
enabled, for example, with sufficient gain in the stimulated
emission process, to emit light in the form of stimulated emission
along the axis normal to the feedback elements.
[0069] In one embodiment, feedback layers with at least in part
periodically and continuously varying refractive index profiles may
be at one side of the device or may be at both sides. Where only
one feedback layer that is at least in part periodically and
continuously varying is used, a second layer of reflective
materials such as a mirror, a metallic surface, a layered
dielectric distributed Bragg reflector (DBR), or reflective anodes
or cathodes may be used as the feedback layer on the other side of
the device. A distributed Bragg reflector is a reflector composed
of a stack of layers of dielectric material of alternating
refractive index. Such a stack has discontinuities in the value of
the refractive index at the layer boundaries.
[0070] In another embodiment, feedback structures with periodic and
continuous variation in refractive index may act as photonic
crystal structures. A photonic crystal is a material that because
of a periodically varying refractive index along one or more axes
cannot support light propagation of particular frequencies along
those axes. In sufficient thickness it thus becomes a perfect
reflector over some spectral reflection band along those axes and
is said to have a photonic band gap of light energies it is
incapable of supporting. One-dimensional photonic crystal
structures that may be used for the feedback layers in one
embodiment may comprise layers of material in which the electron
density and therefore the refractive index has a uniform periodic
and continuous or other appropriate variation along the axis normal
to the plane of the layer. The variation need not be sinusoidal in
nature, but may include a dominant periodicity in the structure of
the material that is n/2 times the wavelength of the desired
stimulated light emission where n is some integer, for example 1.
Generally, n=1 is equivalent to the standard half-wave multilayer
reflector. Alternatively, reflective layers with higher values of n
may be made. Holography usually produces n=1 devices. For devices
in which the layers are produced discretely one at a time, the
layers may be produced with more thickness. To maintain
constructive interference in one embodiment, n may be an integer.
In another embodiment, two and three dimensional photonic crystal
structures may be used as feedback layers.
[0071] FIG. 4 illustrates a device 20 with photonic crystal
feedback elements that produce stimulated emission in one
embodiment. The device may be created by placing the feedback
elements or layers 22 on both sides of an emissive element or layer
24. If the device 24 is transparent to the desired wavelength of
light, for example, light emitted normal to the plane of the
emissive element or layer 24 and the surrounding feedback elements
or layers 22 may be reflected multiple times through the emissive
element or layer 24. When the emissive element or layer 24 is an
OLED, for example, the light may be reflected multiple times
through the exciton rich region of the OLED. This may produce the
stimulated emission of coherent light collimated normal to a plane
of the device in one embodiment of the present disclosure.
[0072] A feedback enhanced luminescent device in one embodiment
utilizes a photonic crystal behavior of dielectric materials with
at least in part periodically and continuously varying indices of
refraction to concentrate feedback light intensity in the exciton
rich zone of the emitter material. For example, in one embodiment,
this concentration of intensity may be enabled by using device
configurations, band-edge feedback enhanced luminescent devices or
defect-mode feedback enhanced luminescent devices, for instance,
band-edge FE-OLEDs and defect-mode FE-OLEDs.
[0073] In a band-edge embodiment, the feedback elements that define
both ends of the device resonant cavity may be completely
reflective at at least one wavelength in or near the emission band
of the emissive material. Both of the feedback elements in one
embodiment are photonic crystal feedback layers, as previously
described above, having the center wavelength of their reflective
bands slightly offset from the maximum of the fluorescence band of
the emissive layer. Stimulated emission occurs in this device at a
band edge of the reflective band of the feedback layers. In one
aspect, the band edge at which stimulated emission occurs is on the
side of the reflection band nearest the emissive material's
wavelength of maximum emission. Alternatively, stimulated emission
at both edges of the feedback layer reflection band may occur with
a broad band emissive material.
[0074] In another aspect, a band-edge emitting device may be
realized by having both reflectors that are completely reflective
holographically written feedback layers with their wavelengths of
maximum reflection, for example, identically offset in wavelength
from the emission maximum of the OLED or LED emissive material.
[0075] In a band-edge device in one embodiment, a photonic band gap
in the photonic crystal structure is created, thereby improving
performance of the band-edge device. In photonic crystal materials,
light with wavelengths in the spectral reflection band of the
material is not only reflected by the periodically varying
refractive index structure, it also cannot be propagated within the
structure. That is to say, the wave propagation modes that light
normally has in free state cannot exist in the photonic crystal.
Thus, the material has a photonic band gap in photon energies
analogous to the electronic bandgap in electron energies in some
crystalline materials. Band-edge lasing is enabled, for example,
because the wave propagation modes that are at forbidden
wavelengths in the photonic crystal structure are not destroyed,
but are expelled to the edges of the photonic band gap (in terms of
wavelength), yielding a high density of modes at wavelengths at the
reflection band edges. This is equivalent to saying that there is a
high density of optical states at the band-edge wavelengths in the
photonic crystal. A material embedded in the photonic crystal
structure sees this high density of states at those wavelengths.
The result is a very intensive interaction of light at the
band-edge wavelengths with the embedded material and the potential
for very intensive stimulation of light emission.
[0076] In another embodiment, the layer of luminescent material and
surrounding device structures (e.g. an OLED) may serve to offset
the phase of the refractive index alternation in the photonic
crystal layer on one side of the device from the phase of the
refractive index alternation in the photonic crystal layer on the
other side of the device. In this embodiment, the luminescent
material, for example, the OLED layers act to create a defect in
what would otherwise be a continuous, periodic variation in
refractive index from top to bottom in the device. In one
embodiment, the thickness of the OLED may be the equivalent of less
than one wavelength of light at the central wavelength of the
photonic crystal reflection band. In devices of this type, a
defect-mode of light propagation is induced that is highly
localized in the area of the OLED. This in turn results in
extremely efficient interaction of feedback photons with excitons
to produce stimulated emission. As in the case of the band-edge
lasing feedback enhanced device, the defect-mode device may have
very energy efficient light emission and low current threshold of
lasing.
[0077] In another embodiment, the feedback enhanced luminescent
devices may be single-mode or multi-mode devices. Single-mode
devices may be produced by fabricating devices with resonant
cavities (distances between feedback layers) with widths of
approximately the wavelength of light emitted by the emitter while
multi-mode devices have resonant cavities with widths at least
several times larger than the wavelength of light emitted by the
emitter. For example, FIG. 15 contrasts the structure of a
single-mode FE-OLED 1502 and a multi-mode FE-OLED 1504. The single
mode FE-OLED 1502 has the holographic feedback layers 1506, 1508
inside a glass package with a resonant cavity width of about 400
nm, a mode spacing of approximately 0.5 .mu.m and a spectral
linewidth of around 1.5 nm.
[0078] The multi-mode FE-OLED 1504 in one embodiment has the
holographic feedback layers 1510, 1512 outside the glass package.
For example, a mode spacing of approximately 0.2 nm occurs with the
feedback layers separated by 1 mm and using 500 nm wavelength
light. Spectral line width is determined by the reflective
bandwidth of the feedback layers 1510, 1512 and is around 100 nm.
In the multi-mode device 1504, the holograms may be applied after
the OLED is assembled.
[0079] In another embodiment, the multi-mode device may have the
feedback layers inside the glass package or one feedback layer
inside the glass package and one feedback layer outside the glass
package. Transparent spacers including relatively thick transparent
spacers may be used to fill space in between the emissive device
and the feedback layers thereby establishing the desired resonant
cavity thickness. In this approach, cavity thickness may be
established independently of mechanical considerations in device
packaging and may be used to provide a multi-mode device that may
be pixelated without parallax issues.
[0080] To a first order of approximation, in one embodiment, a
single-mode device with feedback layers has a resonant cavity
thickness of one-half the wavelength of the desired output light
and with the same phase of the periodic index variation at both
feedback layers inside surfaces. Other thicknesses of the same
order of magnitude and other phase relationships may be used.
[0081] To provide the desired light output from a feedback enhanced
device, the light generated in one embodiment may be coupled out of
the device in one or both directions normal to the plane of the
device. This may be achieved by manufacturing or thinning one or
both of the two feedback layers such that there is an insufficient
number of cycles of refractive index variation to completely
reflect the light produced and thus allowing light to be emitted
from that feedback layer.
[0082] Another approach to "thinning" one or both of the feedback
layers is to maintain their thickness at some standard value (e.g.,
7 microns) while reducing the delta n of the refractive index
variation in the feedback layer. In the case of holographic
feedback layers this may be done by reducing the total exposure
used to write the hologram.
[0083] In alternative embodiments, to form the above-described
photonic crystal feedback layers, the following materials may be
used as feedback layers, but are not limited to only such:
homogenously aligned monomeric and polymeric chiral liquid
crystals, opals, or other particulate agglomerates that have
structures akin to a crystal lattice; middle phase lyotropic liquid
crystals either in a fluid or a polymerized solid phase; or block
copolymers in phases with liquid crystalline structures in which
the oligomeric blocks form repeat units of the requisite length to
yield a structure with the desired periodic and continuous
variation in refractive index.
[0084] A block copolymer may be a self-assembled organic
photopolymer structure produced by consecutive linking of monomer
or oligomer elements into a self-assembled polymer structure. A
one-dimensional photonic crystal structure also may be created by
vacuum depositing a dielectric material of continuously varying
composition such that the resulting structure has the desired index
profile or an approximation of the desired index profile.
[0085] In one aspect, the desired index profile may depend on the
individual device. For instance, band-edge lasing devices may
require broader spectral reflection bands than other devices. In
this case a refractive index profile more closely approximating a
square wave profile may be used.
[0086] A more detailed description of the fabrication of plane wave
holograms is provided in the following description. In one aspect,
the feedback layer may contain a refractive index profile that is
recorded into a medium. For example, the refractive index profile
may be an interference pattern recorded into a photopolymer medium
by optical interference or a similar interference pattern recorded
into a photosensitive medium. The photosensitive medium may be a
silver halide sensitized gelatin emulsion such as Slavich PFG-03C,
which currently is available from UAB Geola, P.O. Box 343, Vilnius
2006, Lithuania. Details concerning fabrication of holograms from
this material may be found in J. M. Kim, et al.; "Holographic
optical elements recorded in silver halide sensitized gelatin
emulsions. Part 2. Reflection holographic optical elements" Applied
Optics 41, pp. 1522-33 (10 Mar. 2002) and J. M. Kim, et al.;
"Holographic optical elements recorded in silver halide sensitized
gelatin emulsions. Part 1. Transmission holographic optical
elements" Applied Optics 40, pp. 622-32(10 Feb. 2001), which are
incorporated herein by reference. A refractive index profile may be
recorded into a medium with a reverse contrast photosensitivity,
for example, by using positive or negative photosensitive
materials. Holograms may also be made by the use of dichromated
gelatin or other photosensitive materials.
[0087] Refractive index profiles may also be recorded by
non-optical means such as a self-assembled organic photopolymer
structure produced by consecutive linking of monomer or oligomer
elements consecutively into a self-assembled polymer structure or a
polymerized middle phase of a lyotropic liquid crystal. Refractive
index profiles may also be written using an electron beam
resist.
[0088] In another aspect, holograms may also be made from a film of
photopolymer material. The photopolymer material may be formed from
mixture of monomers such as an approximately 50:50 mixture of
ethoxylated bishpenol A diacrylate and trimethylol propane
triacrylate. These materials are available from Sartomer Corp.,
Exton, Pa. Upon the addition of a suitable photoinitiator, a
hologram may be recorded into the mixture.
[0089] In one aspect, a hologram may be produced at the desired
wavelength or may be produced at the double or another integer
multiple of the desired wavelength. This may be used in encoding
multiple colors in the same device. Alternatively, recording
methodologies may be combined.
[0090] In another aspect, a hologram feedback layer (for example, 4
FIG. 7) may be phase registered or phase locked to a hologram
feedback layer or other reflector or feedback layer (for example, 6
FIG. 7) using an interferometric alignment method. This results in
perfect or near perfect constructive interference that maximizes
the light intensity of an emissive layer. For example, light
reflected from the reflector or feedback layer (6 FIG. 7) is
interfered with the aerial fringe pattern used to record the
refractive index profile in the holographic feedback layer (4 FIG.
7). The vertical positioning of the aerial fringe pattern may be
adjusted to produce maximum constructive interference before the
holographic exposure of the feedback layer (4 FIG. 7) is
performed.
[0091] When holographic structures are used for both feedback
layers (4 and 6, FIG. 7), the recording exposure may be carried out
on both layers simultaneously with the same extended aerial fringe
pattern thereby phase-registering the two structures. This
phase-registrationn may be perfect or nearly perfect. The
registration and phase locking provide self-alignment of the
hologram in the feedback element.
[0092] A method of producing the holograms in one embodiment will
now be described in detail. In one embodiment, a glass plate with
Slavich PFG-03C silver halide sensitized gelatin (SHSG) holographic
emulsion on its surface is exposed to interfering plane wave beams
of, for example, the 458 nm. line of an argon ion laser. The
laser's output is beam split into "image" and "reference" plane
wave beams of light and these are expanded to be able to cover the
entire emulsion surface. The image beam then is made to impinge on
the emulsion from the front along an axis normal to the glass
substrate and the reference beam is made to impinge on the emulsion
through the glass substrate from the back along an axis normal to
the glass substrate. Thus, in one embodiment, the emulsion is
exposed to the aerial fringes created by the interference of the
two beams.
[0093] After the exposure, the emulsion is prehardened by immersing
the plates in formaldehyde solution (formalin) for about 6 minutes.
This cross-links the initially soft gelatin enough so as not to be
damaged by further processing and also makes the gelatin hard
enough so that air voids formed in it during the development
process do not collapse. However, the interaction with the silver
halide grains in the emulsion retards hardening of the gelatin
around the grains leaving it relatively soft.
[0094] In one embodiment, the formalin hardening solution may
comprise: 10 ml. 37% formaldehyde (formalin); 2 grams (g.)
potassium bromide; 5 g. anhydrous sodium carbonate; 1 liter
deionized water.
[0095] The previously exposed and prehardened plates are developed
using emersion into Agfa G282c photographic developer for about 3
minutes. This converts the silver halide grains that have been
exposed into silver, converting the latent image of the fringes
into a real one.
[0096] The plates are next bleached by immersion in PBU-metol
bleach for approximately 15 minutes. In one embodiment, the bleach
is prepared from the following ingredients: 1 g. cupric bromide; 10
g. potassium persulfate; 50 g. citric acid; 20 g. potassium
bromide; 30 g. borax; 1 liter deionized water. After mixing these
ingredients, 1 g. of p-methylaminophenol sulfate (metol) is
dissolved in the solution. The bleach is then buffered to pH 5
using borax and then 2% chromium (III) potassium sulfate is
added.
[0097] The bleach rehalogenates the silver grains in the exposed
area to silver bromide, but at the same time Cr(III) ions are
introduced into the gelatin immediately adjacent to the silver
grains and begin cross-linking it. In one embodiment, Cr(III) is
not introduced adjacent to the unexposed silver halide grains. The
plates are next immersed in 60 degree C. deionized water for about
10 minutes. During this time the Cr(III) finishes cross-linking the
gelatin adjacent to the reconstituted silver bromide grains.
[0098] The gelatin is dehydrated by immersion in a solution of 50%
industrial methylated spirit/50% water for about 3 minutes followed
by about 3 minutes immersion in undiluted industrial methylated
spirit. The plates are dried in a 45 degree Celsius oven for 5
minutes.
[0099] The emulsion is then further hardened by placing the plates
in a chamber with saturated formaldehyde vapor for 25 minutes. The
emulsion is next fixed by immersion in a fixing bath for about 2
minutes. In one embodiment, the fixing bath comprises of: 10 g.
anhydrous ammonium thiosulfate; 20 g. anhydrous sodium sulfate; 1
liter deionized water.
[0100] This fixing step removes all silver halide from the gelatin.
In the areas where the emulsion was exposed, the gelatin has been
cross-linked with Cr(III) immediately around the AgBr grains and
when the AgBr is removed voids are formed. In the areas where no
exposure occurred, the gelatin around the Ag halide grains is soft
and the voids formed immediately collapse leaving pure, homogeneous
gelatin. Thus, refractive index contrast is produced between areas
of pure gelatin and areas that are part gelatin/part air.
[0101] In one embodiment, the plates are washed and dehydrated by
immersion in: 50% water/50% isopropanol for 10 minutes; 100%
isopropanol at 20 degrees Celsius for 10 minutes; 100% isopropanol
at 45 degrees Celsius for 2 minutes. The dehydration expands the
size of the voids and leaves a hard, de-swelled gelatin matrix. The
plates are then dried in an oven at 45 degrees Celsius.
[0102] In one aspect, the plates may need to be sealed from
humidity in the air since water will re-swell the gelatin degrading
the holograms. This sealing may be done by coating a thin layer of
a sealant adhesive on the hologram surface. One material that may
be used is Pascofix, a cyanoacrylate material available from PASCO
Industrial Adhesives in Philadelphia, Pa. Photocurable epoxy
sealants may also be used.
[0103] In one embodiment, an OLED structure utilized in an FE-OLED
device may have the structure shown in FIG. 9, comprising the
following material. It is noted that the example shown in FIG. 9 is
not drawn to scale. The cathode backing 902 may comprise
approximately 150 nm thick indium-tin oxide material. The cathode
904 may comprise approximately 7 nm thick aluminum material. The
electron injection layer 906 comprise approximately 10 nm thick
lithium fluoride material. The electron transport layer 908 may
comprise approximately 35 nm thick aluminum triquinoline material.
The hole blocker 910 may comprise approximately 10 nm thick
bathocuproine material. The emissive layer 912 may comprise
approximately 50 nm thick H9680 material. The hole transport layer
914 may comprise approximately 75 nm thick
N,N'-di(3-methylphenyl)-N,N'-diphe- nylbenzidine material. The hole
injection layer 916 may comprise 10 nm thick copper phthalocyanine
material. The anode may comprise 150 nm thick indium-tin oxide
material. H9680 emissive layer may be obtained from Honeywell
Specialty Chemicals in Morristown, N.J.
[0104] The above OLED structure may be built by successive vacuum
depositions on a sealant coated surface on one of the SHSG
holographic plates 920 whose preparation was described above. Then
a second hologram 922 on which the sealant coat is not yet cured is
placed on top of the OLED structure 902-918 and precisely
positioned parallel to the bottom hologram using a piezoelectric
positioner so that when the sealant cures, the two holograms 920,
922 and the OLED 902-918 are all potted together with the
sealant.
[0105] In one aspect, the two sealant coatings on the two holograms
in the device are at least two microns thick. The device
accordingly may support multiple vertical modes. In one embodiment,
the lateral dimensions of the OLED layers may be 250 microns by 250
microns.
[0106] In the above-described fabrication approach, the ITO cathode
backing layer acts as a buffer between the hologram sealant and the
rest of the OLED structure. In another aspect, one or more
additional buffer layers may be used.
[0107] In one aspect, the one or more feedback layers in the device
allow even some light emissive materials with degraded function or
low efficiency to perform satisfactorily. This provides greater
flexibility in selecting the other layers and elements of the
device such as the emissive layer. Further, because the feedback
layers in one embodiment still allow satisfactory performance even
with some light emissive materials having degraded function or low
efficiency, a light emissive material may be chosen to optimize
other characteristics. For example, a device that has low
efficiency light emissive material because of overlap between the
light emissive material absorption band and the light emissive
material florescent emission band may be used if the feedback layer
reflection band and thus the device emission band does not overlap
the emitter absorption band.
[0108] Another example is a device with the electrodes, charge
carrier injection layers, and charge carrier transport layers
optimized or otherwise selected for their functions and with an
emissive layer selected for optimum carrier recombination to form
excitons, but which has poor quantum efficiency because of
non-radiative relaxation of exciton energy. Enabling stimulated
emission with feedback light from feedback layers in one embodiment
may allow almost all exciton energy to be emitted as light, thus
providing a highly energy efficient device.
[0109] A further example is a device with a light emissive layer
including an emitter molecule derivatized with cross-linking
functional groups. The cross-linking provides improved mechanical
properties and make the emissive layer less fragile under high
temperature conditions. With stimulated emission from feedback
light this device may also be made sufficiently energy
efficient.
[0110] In another aspect, the light emissive material may be chosen
for a high absorption cross-section or area of interaction with the
feedback light. For example, the emissive layer may be made with a
material that has large molecules or a high aspect ratio to the
reflected feedback light. The material may have a high
polarizability at the wavelength of the feedback light or the light
emissive layer may be thicker to provide a higher interaction
cross-section with the feedback light. The material may include a
light emissive material in which the molecules are aligned so that
their aspect ratio with the reflected feedback light has a large or
maximum value or a emissive layer in which the emissive material is
made more dense so that there are more molecules per unit of depth
that interact with the reflected feedback light. One such material
may include a metallophthalocyanine with its molecules oriented in
the device such that the molecules have a very large molecular size
and proper molecular aspect ratio. Orientation of the phosphor
molecules may be provided by adjusting the surface energy of the
underlying charged transport layer such that homogenous alignment
results or by photo aligning the underlying charge transport layer.
Molecules having a high cross-section of interaction with the
feedback light increase the likelihood of stimulated emission. This
reduces the amount of light that needs to be reflected by the
feedback element in order to make stimulated emission the dominant
light conversion process. In another embodiment, a patterned
feedback element or layer may be fabricated using a patterned
optical exposure. First, a layer of holographic photopolymer or
photopolymer precursor is cast on a substrate. Next, the
photopolymer or photopolymer precursor is exposed to a
cross-linking light radiation through a patterned photomask.
Alternatively, the patterned photomask may be omitted if the
cross-linking light radiation is provided as a modulated beam.
[0111] In another embodiment, a patterned, multi-color feedback
layer may be built using successive patterned exposures in the
patterned feedback layers to produce areas of feedback material
that reflect different colors. These patterned feedback layer areas
may be located so as to be registered with correspondingly
patterned emissive material areas. The emissive material areas
associated with patterned feedback layer areas reflecting a
particular color band may be selected to contain emissive material
that emit radiation in that color band.
[0112] In another aspect, a patterned feedback layer as described
above may be fabricated by using one photomask in the image beam
and one photomask in the reference beam of the holographic set-up.
FIG. 10 illustrates an example of a double mask apparatus 100 that
produces a patterned hologram in one embodiment. A material such as
holographic emulsion 20 is exposed to the reference beam 14 and the
image beam 12 via the two photomasks 18, thus exposing patterned
light beams 22 to the holographic emulsion 20. In this set-up, a
light source such as a laser 2 directs the laser light 4 to beam
expanding optics 6. Light emitted from the beam expanding optics 6,
that is expanded laser beam 8 is deflected partly and transmitted
partly by a beam splitter 10. A beam splitter 8 may be a simple
mirror made with a thinner coating of silver than a conventional
mirror so that it does not reflect all of the light that is
incident upon it, some of it being transmitted. The light
transmitted is an image beam 12 which is exposed on the holographic
emulsion 20 via the photomask 18. The light deflected 14 is a
reference beam and is reflected by mirrors 16 and is also exposed
on the holographic emulsion 20 via the other photomask 18. The
resulting interference pattern recorded by the holographic emulsion
20 is the hologram.
[0113] In another aspect, one of the two photomasks 18 may be
eliminated by using a high gamma photopolymer as a holographic
medium 20 that has an exposure intensity threshold. FIG. 11
illustrates an example of a single mask apparatus 200 that produces
a hologram in one embodiment. In this embodiment, one photomask 18
may be used in the image beam 12. In this single photomask method,
the unmasked beam (the reference beam) 14 may not have sufficient
energy to exceed the exposure intensity threshold and thus to cause
cross-linking in the photopolymer. The masked beam (the image beam)
12 also may not have sufficient energy to cause cross-linking in
the photopolymer, although this is not required. The combination of
the two beams, however, may be sufficient to cause
cross-linking.
[0114] FIG. 12 is a graph 300 illustrating irradiance versus
position in the double mask apparatus of FIG. 10. The graph
illustrates the modulation of the exposing light from zero
irradiance when optically dense features on photomasks are in place
in both the image and reference beams to full irradiance when both
beams are passing through apertures in the photomasks. In this set
up, both photomasks are aligned to each other and to the
holographic emulsion. FIG. 13 is a graph 400 illustrating
irradiance versus position in a single mask apparatus of FIG. 11.
This graph illustrates that in this case the combined irradiance of
the exposure beams is modulated between full irradiance and some
lower, non-zero value of irradiance, depending on the relative
intensities of the image and reference beams. If, for example, in
one embodiment the irradiance threshold of the holographic
emulsion, portrayed in graph 500 in FIG. 14, is higher in
irradiance than the low value of irradiance modulation in FIG. 13,
then patterned holograms may be successfully recorded. This set-up
does not require the registration of two masks.
[0115] A holographic feedback layer may be produced by coating a
layer of photosensitive holographic recording material onto a
device substrate. The material is then exposed to light so as to
produce a pattern, such as a plane wave interference pattern, down
into the depth of the layer such that the level of exposure of the
recording material is uniform in all directions in the plane, but
varies in uniformity sinusoidally along the axis normal to the
layer plane. A plane wave interference pattern may be produced by
generating a plane wave of the desired wavelength, splitting it
into two components, phase delaying one component relative to the
other, then simultaneously exposing the recording film to both
components. This process records the hologram of a plane wave
source. The recording film or material is sensitized so that when
it is exposed to light a cross-linking reaction occurs causing the
refractive index to change. For instance, the Slavich material
contains sensitizing dyes that render it panchromatic. This in one
embodiment may be one way of recording the desired cyclic variation
of refractive index in the feedback layer.
[0116] In another embodiment, an emissive material with broader
fluorescent emission bands may be combined with a feedback element
optimized to allow single mode emission of a desired wavelength to
constrain the emission bandwidth of the emissive material. As
previously described, feedback elements may cause stimulated
emission in the emissive element. The stimulated emission may be
limited to the same wavelength band or bands as the reflection band
or bands of light of the feedback element. Thus, a broadband
emissive material may be used with a narrow band reflective
feedback element to produce a narrow wavelength band of light, for
example, with substantially no loss of energy conversion
efficiency. In one aspect, this provides a great deal of design
freedom in emissive material selection. For example, in devices
with emissive materials in which the spectral emission band
overlaps the absorption band such that self-absorption normally
occurs, the narrow band reflective feedback element may be chosen
to have a peak wavelength that does not overlap the emissive
material absorption band. In this way, for example, emissive
material self-absorption may be eliminated and a more energy
efficient device may result. Similarly, in another embodiment, a
precise placement of the feedback layer reflection band may be used
to avoid absorption bands of other device layers.
[0117] In one aspect, an ability to tune the device output away
from a material absorption band implies that otherwise highly
useful emissive materials and other device materials that are
normally rejected because of self-absorption or incompatibility
with absorption bands of other device layers may now be used in
devices with feedback layers. Another use of this effect is to have
the feedback element have two or more separate spectral reflection
bands. Where the separate reflection bands each overlap a portion
of the broad emission spectrum of an emitter material, multiple
wavelength bands of light may be emitted from the same region of
the emissive material.
[0118] Another use may be one in which separate reflection bands
are patterned in different areas of the feedback layers. For
example, FIG. 5 illustrates a pixelated device 30 with a broadband
emissive layer 32 and two pixelated feedback layers 34 in one
embodiment. For instance, for an emitter that radiated light of
reasonable intensity between 520 nm. and 700 nm., a checker board
pattern in the feedback layers is patterned in which the refractive
index alternation in the layers is equivalent to a wavelength of
520 nm. Then the complementary checkerboard pattern is patterned so
that the index profile corresponds to a wavelength of 650 nm. In
this way, the device emits a checker board pattern of light with
the alternating squares (pixels) colored red and green.
[0119] In one embodiment, the pixelated feedback layers 34 include
first areas 36 and second areas 38. The first areas 36 reflect
light of a first wavelength while the second areas 38 reflect light
of a second wavelength. The first areas 36 of the pixelated
feedback layers 34 cause stimulated emission in the broadband
emissive layer 32 only or substantially only at the first
wavelength. Similarly, the second areas 38 of the pixelated
feedback layers 34 cause stimulated emissions in the broadband
emissive layer 32 only or substantially only at the second
wavelength.
[0120] Thus in one embodiment, a multi-colored device may be made
without providing separate emissive materials for each of the first
and second wavelengths. This may reduce the processing steps
required for fabrication of the broadband emissive layer 32. In one
embodiment, if refractive index profiles are recorded into the
pixelated feedback layers 34, the first and second areas 36, 38 may
be formed at the same time to further reduce the number of
fabrication steps and complexity of the pixelated device 30.
Further, although FIG. 5 illustrates two wavelengths, three of more
wavelengths of light may be stimulated from the broadband emissive
layer 32. Such a pixelated device 30 may be used to display
digital, alphanumeric, or graphic information. For example, the
pixelated device 30 may be used to create a wide color gamut in
graphic displays without emitting unwanted infrared and ultraviolet
wavelengths of light.
[0121] In another embodiment, both the feedback layers and the
emitters may be patterned. For instance, red, green, and blue
emitters may be patterned in pixels with overlaid feedback layer
patterned red, green, and blue respectively. In another embodiment,
one or more feedback layers may be formed from a photosensitive
material having a hologram of a plane wave written on the material
using polarized light. The light may be plane polarized along any
axis perpendicular to its direction of propagation or alternatively
the light may be right circularly, left circularly, or elliptically
polarized light. In these embodiments, the feedback layer returns
polarized feedback light into the resonant cavity of the OLED or
LED device. In one embodiment, this polarized feedback light
stimulates emission of light from the emissive layer that has an
identical polarization state. Thus, a substantial portion or all of
the light produced by the OLED or LED device may have a
polarization state identical or substantially identical to the
original light used to write the hologram in the feedback
layer.
[0122] In one embodiment, one or more polarized holographic
feedback layers may be written with a patterned exposure through a
photomask. In one aspect, separate areas of an emissive device may
be constrained to emit light having different polarization states
by using a series of patterned exposures with light of differing
polarization states. For example, feedback layers patterned in a
pixelated rectangular matrix may have alternating pixels with
orthogonal linear axes of polarization.
[0123] In another embodiment, one or more feedback layers may be
formed with materials with chiral centers and used as photonic
crystal feedback layers, for example, if the materials have
one-dimensional photonic crystal refractive index profile. For
instance, homogenously aligned cholesteric liquid crystals have
chiral centers that induce helical structure such that the
one-dimensional photonic crystal refractive index profile results.
This is illustrated in FIG. 6. The polarization direction of plane
polarized light is not affected by passage through the medium when
the pitch of the helical structure of the material is sufficiently
short. In this case, light with an axis of propagation parallel to
the helical axis sees a sinusoidally varying refractive index. This
structure may be formed from a polymer film produced by
polymerizing cholesteric reactive mesogen monomers.
[0124] In another embodiment, one or more conventional OLED
conductive and semiconductive layers may be fabricated so as to
allow them to also act as a photonic crystal structure, for
example, in addition to their conventional functionalities in an
OLED. For example, the photonic crystal structure of the feedback
layers may extend into the OLED structure itself. For instance, the
materials used to build up the hole injection, hole transport,
emission, electron transport, and electron injection layers in an
OLED may have chiral liquid crystalline phases or structures of the
same pitch as the refractive index alternation in the feedback
layers. These materials may be derivatized with cross-linking
moieties and sensitized such that interference patterns may be
recorded in them. In this way feedback layer holograms may be
extended into the OLED.
[0125] A useful aspect of modifying the optical properties of OLED
component layers so as to extend the photonic crystal structure
into the OLED structure is that, as was described above, to produce
a defect-mode laser, the defect zone in which the OLED emitter is
located may have a thickness of less than the equivalent of one
wavelength of light. For example, this aspect is useful in the case
of blue FE-OLEDs because for blue light a thickness equivalent to
one wavelength is smaller than the combined thicknesses of the
various OLED functional layers. Thus, in one embodiment, a way of
designing a defect-mode device may involve extending the photonic
crystal structures into OLED as described above.
[0126] The embodiments described above are illustrative examples
and it should not be construed that the present invention is
limited to these particular embodiments. For example, although OLED
devices were used as examples of emissive devices, other
luminescent material or structures may be used, not limited to
OLEDs. Further, although refractive index profiles, direction of
light, etc. were described as being "normal" to a plane, it should
be understood that they need not be exactly normal, rather in a
close range of being normal or substantially normal. Accordingly,
the embodiments described in this application also may include
cases in which they are about normal or substantially normal to a
plane. Further, various components and aspects described with
reference to different embodiments are interchangeable among
different embodiments, and are not limited to one particular
embodiment. Thus, various changes and modifications may be effected
by one skilled in the art without departing from the spirit or
scope of the invention as defined in the appended claims.
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