U.S. patent application number 09/760398 was filed with the patent office on 2001-06-21 for resonant microcavity display.
This patent application is currently assigned to University of Georgia Research Foundation, Inc.. Invention is credited to Eilers, Hergen, Jacobsen, Stuart M., Jaffe, Steven M., Jones, Michieal L..
Application Number | 20010004188 09/760398 |
Document ID | / |
Family ID | 27767799 |
Filed Date | 2001-06-21 |
United States Patent
Application |
20010004188 |
Kind Code |
A1 |
Jacobsen, Stuart M. ; et
al. |
June 21, 2001 |
Resonant microcavity display
Abstract
A resonant microcavity display (20) having microcavity with a
substrate (25), a phosphor active region (50) and front and rear
reflectors (30 and 60). The front and rear reflectors may be spaced
to create either a standing or treaveling eledtromagnetic wave to
enhance the efificenty of the light transmission.
Inventors: |
Jacobsen, Stuart M.; (Powder
Springs, GA) ; Jaffe, Steven M.; (Palo Alto, CA)
; Eilers, Hergen; (Blacksburg, VA) ; Jones,
Michieal L.; (Athens, GA) |
Correspondence
Address: |
Sheldon R. Meyer
FLIESLER, DUBB, MEYER & LOVEJOY LLP
Suite 400
Four Embarcadero Center
San Francisco
CA
94111-4156
US
|
Assignee: |
University of Georgia Research
Foundation, Inc.
|
Family ID: |
27767799 |
Appl. No.: |
09/760398 |
Filed: |
January 12, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09760398 |
Jan 12, 2001 |
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09628490 |
Jul 31, 2000 |
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09628490 |
Jul 31, 2000 |
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09073711 |
May 6, 1998 |
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6198211 |
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09073711 |
May 6, 1998 |
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08581622 |
Jan 18, 1996 |
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5804919 |
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08581622 |
Jan 18, 1996 |
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08094767 |
Jul 20, 1993 |
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5469018 |
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08581622 |
Jan 18, 1996 |
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PCT/US94/08306 |
Jul 20, 1994 |
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Current U.S.
Class: |
313/461 ;
313/463; 313/509 |
Current CPC
Class: |
H01L 51/5262 20130101;
G02F 1/1336 20130101; Y10S 428/917 20130101; H01L 51/5265 20130101;
G02F 1/133521 20210101; G02F 1/133602 20130101; H01J 29/28
20130101; H05B 33/12 20130101; H05B 33/22 20130101; H01J 61/42
20130101; H01J 63/06 20130101; G02F 1/133621 20130101 |
Class at
Publication: |
313/461 ;
313/463; 313/509 |
International
Class: |
H01J 029/10; H01J
001/62 |
Claims
What is claimed is:
1. A luminescent display, comprising a resonant microcavity with an
active region, the active region having a phosphor disposed therein
for emitting light.
2. The luminescent display of claim 1, wherein said microcavity
comprises means for modifying a process selected from the group
consisting of spontaneous emission processes of the phosphor and
energy transfer processes of the phosphor.
3. The luminescent display of claim 1, wherein said microcavity
comprises means for modifying the spontaneous emission processes of
the phosphor.
4. The luminescent display of claim 1, wherein said microcavity
comprises means for modifying energy transfer processes of the
phosphor.
5. The microcavity of claim 2, wherein the resonant microcavity is
tunable.
6. The luminescent display of claim 5, and further comprising
electro-optic means for tuning the resonant frequency of said
microcavity.
7. The luminescent display of claim 5, and further comprising
piezo-electric means for tuning the resonant frequency of said
microcavity.
8. The luminescent display of claim 2, wherein said microcavity
comprises means for producing, in said phosphor, an electromagnetic
field having a substantially modified electric field amplitude when
said phosphor is excited by an energy source relative to the
amplitude of an electric field produced in said phosphor were said
phosphor disposed in free space and excited by the same energy
source.
9. The luminescent display of claim 8, wherein said electromagnetic
field having said substantially modified electric field amplitude
is a standing wave electromagnetic field.
10. The luminescent display of claim 9, wherein the standing wave
electromagnetic field has an antinode inside said microcavity, and
said phosphor is disposed in a region including said antinode.
11. The luminescent display of claim 9, wherein the standing wave
electromagnetic field has a plurality of antinodes inside said
microcavity, and said phosphor is disposed in a region including
said plurality of antinodes.
12. The luminescent display of claim 9, wherein the standing wave
electromagnetic field has a node inside said microcavity, and said
phosphor is disposed in a region including said node.
13. The luminescent display of claim 9, wherein the standing wave
electromagnetic field has a plurality of nodes inside said
microcavity, and said phosphor is disposed in a region including
said plurality of nodes.
14. The luminescent display of claim 9, wherein the phosphor
comprises a dopant within the microcavity disposed in a region of
the microcavity having a substantially modified electric field
amplitude.
15. The luminescent display of claim 8, wherein said microcavity is
dimensioned to produce a traveling electromagnetic wave having the
substantially modified electric field amplitude.
16. The luminescent display of claim 2, wherein the microcavity
comprises a structure selected from the group consisting of
coplanar microcavities, three dimensional microcavities, and
combinations thereof.
17. The luminescent display of claim 16, wherein the microcavity
comprises a structure selected from the group consisting of
confocal microcavities, hemispherical microcavities, and ring
cavities.
18. The luminescent display of claim 8, wherein said microcavity is
excitable to establish the substantially modified electric field
amplitude inside said microcavity.
19. The luminescent display of claim 18, wherein said display is
excitable by electrons.
20. The luminescent display of claim 19 further comprising a
cathode ray tube to generate exciting electrons for exciting said
active layer.
21. The luminescent display of claim 19 and further comprising a
filament cathode means disposed behind said microcavity in a vacuum
for emitting electrons to excite said active layer.
22. The luminescent display of claim 21 and further comprising a
control grid disposed between said filament cathod means and said
microcavity.
23. The luminescent display of claim 19 and further comprising a
high-voltage field emission device for exciting said active
layer.
24. The luminescent display of claim 19 and further comprising
field-emissive material disposed within said microcavity to cause
emitted electrons to tunnel from said emissive material into said
phosphor, thereby exciting said phosphor; and a conductive layer
disposed within said microcavity for conducting emitted electrons
to ground.
25. The luminescent display of claim 24 in which said active region
is disposed in a region of the substantially modified electric
field amplitude within said microcavity.
26. The luminescent display of claim 18 wherein said active region
is excitable by an electric field and further comprising means for
exciting said active region with an electric field.
27. The luminescent display of claim 18 wherein said active region
is excitable by electromagnetic radiation and further comprising
means for exciting said active region with electromagnetic
radiation.
28. The luminescent display of claim 27 wherein the means for
exciting said active region comprises a laser.
29. The luminescent display of claim 27 wherein the means for
exciting said active region comprises means for generating a plasma
discharge.
30. The luminescent display of claim 2 in which the resonant
microcavity comprises thin films.
31. The luminescent display of claim 2 wherein the microcavity
comprises: (a) a substrate; and (b) a structure disposed upon said
substrate comprising an active region and a plurality of reflective
regions.
32. The luminescent display of claim 31, wherein the luminescent
display comprises a plurality of said microcavities, each having a
resonant region therein, and said microcavities are operatively
coupled to form a larger resonant region.
33. The luminescent display of claim 31 wherein the plurality of
reflective regions comprise: (a) a front reflective region disposed
upon said substrate, and (b) a back reflective region; and the
active region is disposed between the front and the back reflective
regions.
34. The luminescent display of claim 33 in which the front
reflective region, the active region, and the back reflective
region comprise thin films.
35. The luminescent display of claim 31 wherein the substrate, the
active region, and plurality of reflective regions are each
comprised of inorganic materials.
36. The luminescent display of claim 31 wherein the substrate
comprises an organic material.
37. The luminescent display of claim 31 wherein the active region
comprises an organic material.
38. The luminescent display of claim 31 wherein the plurality of
reflective regions comprise an organic material.
39. The luminescent display of claim 31 wherein the substrate, the
active- region, and the plurality of reflective regions each
comprise organic materials.
40. The luminescent display of claim 31, in which said substrate
and said structure disposed thereon are flexible.
41. The luminescent display of claim 31 wherein at least one of
said plurality of reflective regions comprises a
wavelength-dependent reflector that is substantially reflective
within a narrow wavelength bandwidth and substantially transmissive
outside of said narrow wavelength bandwidth.
42. The luminescent display of claim 41 and further comprising an
opaque surface behind one of said wavelength-dependent reflectors
to increase display contrast.
43. The luminescent display of claim 31 wherein said reflective
regions comprise dielectric reflectors.
44. The luminescent display of claim 43 wherein said dielectric
reflectors further comprise a plurality of alternating parallel
layers, wherein layers comprising a material with a relatively low
index of refraction alternate with layers comprising a material
with a relatively high index of refraction. 45. The luminescent
display of claim 44 wherein said material with a relative low index
of refraction comprises a material selected from the group
consisting of fluorides and oxides.
46. The luminescent display of claim 44 wherein said material with
a relatively high index of refraction comprises a material selected
from the group consisting of sulfides, selenides, nitrides, and
oxides.
47. The luminescent display of claim 31 wherein at least one of
said plurality of reflective regions comprises a metallic
reflector.
48. The luminescent display of claim 31 wherein said active region
comprises a phosphor selected from the group consisting of
sulfides, oxides, silicates, oxysulfides, and aluminates.
49. The luminescent display of claim 48 wherein said phosphor
includes an activator comprising a material selected from the group
consisting of transition metals, rare earths, substances having
color centers, and combinations thereof.
50. The luminescent display of claim 31 wherein the thickness of
the active region is equal to a selected wavelength of light to be
emitted by the display multipled by an integer and divided by the
quantity 4 times the index of refraction for light of the selected
wavelength in a material comprising the active region.
51. The luminescent display of claim 31 wherein the microcavity
comprises a plurality of active regions and the thickness of the
plurality of active regions is equal to a selected wavelength of
light to be emitted by the display multipled by an integer and
divided by the quantity 4 times the index of refraction for light
of the selected wavelength in a material comprising the plurality
of active regions.
52. The luminescent display of claim 31 wherein the thickness of
the active region is equal to a selected wavelength of light to be
emitted by the display multipled by an integer and divided by the
quantity 2 times the index of refraction for light of the selected
wavelength in a material comprising the active region.
53. The luminescent display of claim 31 wherein the microcavity
comprises a plurality of active regions and the thickness of the
plurality of active regions is equal to a selected wavelength of
light to be emitted by the display multipled by an integer and
divided by the quantity 2 times the index of refraction for light
of the selected wavelength in a material comprising the plurality
of active regions.
54. The luminescent display of claim 2 wherein said resonant
microcavity comprises a photonic band gap material.
55. The luminescent display of claim 31, in which at least one of
the pluralty of reflective regions comprises a photonic band gap
crystal.
56. The luminescent display of claim 31, in which the active region
comprises a photonic band gap crystal.
57. The luminescent display of claim 31 and further comprising
means for generating a predetermined angular light distribution
from light emitted from said active region.
58. The luminescent display of claim 57 in which said means for
generating the predetermined angular light distribution comprises a
structure selected from the group consisting of lenses, diffusers,
holographic elements, gradient index elements, and combinations
thereof.
59. The luminescent display of claim 57 wherein said means for
generating a predetermined angular light distribution is disposed
within said substrate.
60. A method of generating a controlled color, directional light
beam comprising the step of exciting the luminescent display of
claim 1 to emit light from said microcavity.
61. A modulated light source, comprising: (a) the luminescent
display of claim 1; (b) means for exciting said phosphor in said
active region; and (c) light valve means in front of said
microcavity for modulating light emitted from said microcavity.
62. The modulated light source of claim 61, wherein said resonant
microcavity is adapted to emit polarized light.
63. A method of producing a luminescent display comprising the step
of growing a resonant microcavity containing an active region
inside said microcavity, in which a phosphor is grown in the active
region.
64. The method of claim 63 wherein the step of growing a resonant
microcavity comprises a growing process selected from the group
consisting of physical vapor deposition and chemical vapor
deposition processes.
65. The method of claim 63 and further comprising the additional
step of selecting the phosphor, active layer thickness, cavity
quality factor, and cavity type to control phosphor decay time.
66. The method of claim 63 and further comprising the additional
step of selecting the phosphor, active layer thickness, cavity
quality factor, and cavity type to control chromaticity of light
emitted from the display.
67. The method of claim 63 wherein the growth of the active region
is controlled so that the thickness of the active region is equal
to an integer number of quarter wavelengths of light corresponding
to a selected chromaticity.
68. The method of claim 63 wherein the width of the active region
is controlled so that the thickness of the active region is equal
to an integer number of half wavelengths of light corresponding to
a selected chromaticity.
69. The method of claim 63 comprising an additional step of
selecting the phosphor, active layer thickness, cavity quality
factor, and cavity type so that a resonance frequency of the
microcavity lies within a natural chromaticity of the phosphor in
the active region.
70. The method of claim 63 wherein the microcavity is grown on a
substrate selected to maximize the heat transfer efficiency of the
display.
71. The method of claim 63 and further comprising the additional
step of selecting the phosphor, active layer thickness, cavity
quality factor, and cavity type to control the directionality of
light emitted from the display.
72. A method of producing a luminescent display comprising the step
of using holographic photolithography to produce a resonant
microcavity containing an active region inside said
microcavity.
73. A communications device comprising: (a) a tunable resonant
microcavity; and (b) an active region disposed within said
microcavity and comprising a phosphor.
74. The communications device of claim 73, and further comprising
electro-optic means for tuning the resonant frequency of said
microcavity.
75. The communications device of claim 73, and further comprising
piezo-electric means for tuning the resonant frequency of said
microcavity.
76. An information recording device comprising: (a) a resonant
microcavity; and (b) an active region disposed within said
microcavity and comprising a photosensitive material.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a luminescent screen
comprising a resonant microcavity having a phosphor active
region.
[0003] 2. Description of the Prior Art
[0004] Conventional cathode ray tube (CRT) displays use electrons
emitted from an electron gun and accelerate them through an intense
electric field Projecting them onto a screen coated with a phosphor
material in the form of a powder. The high-energy electrons excite
luminescence centers in the phosphors which emit visible light
uniformly in all directions. CRT's are well established in the
prior art and are commonly found in television picture tubes,
computer monitors and many other devices.
[0005] Displays using powder phosphors suffer from several
significant limitations, including: low directional luminosity
(i.e., brightness in one direction) relative to the power consumed;
poor heat transfer and dissipation characteristics; and a limited
selection of phosphor chromaticities (i.e., the colors of the light
emanating from the excited phosphors).
[0006] The directional luminosity is an important feature of a
display because the directional properties influence the efficiency
with which it can be effectively coupled to other devices (e.g.,
lenses for projection CRT's). The normal light flux pattern
observed from a luminescent screen closely follows a "Lambertian
distribution"; i.e., light is emitted uniformly in all directions.
For direct viewing purposes this is desirable, as the picture can
be seen from all viewing angles. However, for certain applications
a Lambertian distribution of the light flux is inefficient. These
applications include projection displays and the transferring of
images to detectors for subsequent image processing.
[0007] Heat transfer and dissipation characteristics are important
because one of the limiting factors in obtaining bright CRT's
suitable for large screen projection is the heating of the phosphor
screen. As the incident electron beam density increases, the
phosphor temperature increases. When the phosphor reaches a certain
temperature, its luminosity decreases. This is known as thermal
quenching. With conventional powder-phosphor displays the
phosphor-to-screen heat transfer characteristics are relatively
poor, therefore heat dissipation is limited and thermal quenching
can occur at relatively low electron beam densities. Because
projection displays require high electron beam densities to produce
the brightness required to project an image, this inefficiency
makes conventional CRT's poorly suited for projection displays.
[0008] Chromaticity is important because the faithful reproduction
of colors in a display requires that the three primary-color
phosphors (red, green and blue) conform to industry chromaticity
standards (e.g., European Broadcasting Union specifications).
Finding phosphors for each of the three primary colors that exactly
match these specifications is one of the most troublesome aspects
of phosphor development.
[0009] The decay time of the activator (i.e., light emitting ion in
the phosphor) is also another important parameter for a phosphor.
In an ideal phosphor for high brightness applications, it is
desirable to control directly the decay time of the phosphor for
each display application. For example, in some applications,
shorter decay times allow rapid re-excitation of the activator with
a corresponding increase in the maximum light output. The decay
time is typically determined by the natural spontaneous transition
rate of the activator. In order to improve phosphor performance it
is therefore desirable to have control over this spontaneous
transition rate.
[0010] Another problem encountered in conventional phosphor
displays is that energy can transfer from one activator to another
nearby activator in the phosphor host matrix. This is a
nonradiative process where the efficiency of the phosphor is
reduced. The energy transfer increases with increasing activator
concentration and therefore it limits the density of activators
that can be incorporated in a display and thus the maximum light
output.
[0011] The use of a single-crystal, thin-film phosphor as a
faceplate for a CRT was first described in a British patent
application by M. W. Van Tol, et al., UK Pat. GB-2000173A (1980).
This patent taught the use of an yttrium aluminum garnet
Y.sub.3Al.sub.5O.sub.12 (YAG) film grown by liquid phase epitaxy
(LPE) on a single-crystal YAG substrate. The YAG film is doped with
a rare-earth ion which emits light when excited by electrons.
(Doping is the process wherein dopant ions are substituted for host
ions in the crystal lattice during crystal growth.) In this device,
the thickness of the thin-film layer is from one to six microns and
does not bear any relation to the wavelength of the light to be
emitted by the display.
[0012] This device exhibited several advantages over conventional
powder-phosphor displays. One such advantage was that heat was
transferred from the phosphor more efficiently because of the
perfect contact between the phosphor and the screen, and because of
the high thermal conductivity of the YAG substrate. The screen
could be loaded with a higher beam density without exhibiting
thermal quenching and, therefore, could produce more light.
[0013] Another advantage of single-crystal phosphor luminescent
screens versus powder deposited luminescent screens is concerned
with the resolution of a pixel (i.e., light producing spot). For
high resolution displays using powder phosphor, the limiting size
of a pixel--and hence the resolution of the screen--is determined
by the particle size of the phosphor powder. Single-crystal
phosphors, on the other hand, are not affected by this since they
do not contain discrete particles.
[0014] Powder phosphors further reduce resolution due to the light
scattering from the surface of the powder. Because of the lack of
discrete phosphor particles and the absence of light scattering,
thin-film displays have high image resolution, limited only by the
spot size of the exciting electron beam. The increasing demand for
higher resolution displays makes this a particularly attractive
advantage.
[0015] Yet another advantage is concerned with producing a vacuum
in a CRT. To allow the electron beam to travel between the electron
gun and the phosphor screen, a vacuum must be maintained within a
CRT. Conventional powder phosphors have a high total surface area
and, generally, organic compounds are used in their deposition.
Both the high surface area and the presence of residual organic
compounds cause problems in holding and maintaining a good vacuum
in the CRT. Using thin-film phosphors overcomes both of these
effects, as the total external surface area of the tube is
controlled by the area of the thin-film (which is much less than
the surface area of a powder phosphor display) and, furthermore,
there are no residual organic compounds present in thin-film
displays to reduce the vacuum in the sealed tube.
[0016] The thin-film phosphors of Van Tol, et al., exhibit one
prohibiting disadvantage, however, due to the phenomenon of "light
piping." Light piping is the trapping of light within the
thin-film, rendering it incapable of being emitted from the device.
This is caused by the total internal reflection of the light rays
generated within the thin-film. Since the index of refraction (n)
of most phosphors is around n=2, only those light rays whose
incident angles are less than the critical angle, .theta..sub.c
(where sin .theta..sub.c=1/n) will be emitted from the front of the
thin-film. The critical angle for an n=2 material is around
30.degree.. Therefore, the fraction of light that escapes from the
front of the thin-film is only about 6.7% of the total light. The
common design of placing a highly reflective aluminum layer behind
the film only doubles the output to about 13% of the light.
Moreover, this light is spread in a "Lambertian distribution" and
is not directional. As a result of light piping, the external
efficiency (i.e., the percentage of photons escaping from the
display relative to all photons created in the display) is less
than one-tenth that of powder phosphor displays. Therefore, in
spite of the unique advantages offered in terms of thermal
properties, resolution, and vacuum maintenance; the development of
commercial CRT devices based on thin-films is held back by their
poor efficiency due to "light piping".
[0017] Some schemes have been designed to reduce the "light piping"
problem. One scheme described by Bongers, et al., U.S. Pat. No.
4,298,820 (1981), uses a thin-film, deposited by LPE, with V-shaped
grooves etched into the surface to reflect light out of the
thin-film. This approach brought about an improvement in external
efficiency of around 11/2 to 21/2 times that of a thin-film display
without the V-shaped grooves. Given the previous external
efficiency of 13%, this would still only lead to a total external
efficiency of around 20% to 30%.
[0018] Another scheme, described by Huo and Hou, "Reticulated
Single-Crystal Luminescent Screen", 133 J. Electrochem. Soc. 1492
(1986), involves etching individual mesa shapes onto the thin-film
deposited by LPE. This led to a three times improvement in external
efficiency (still rendering only about a 30% external efficiency).
Furthermore, since the phosphor layer was no longer smooth, any
light rays that were internally reflected could find themselves
rescattered to areas far from their point of creation, thus
spoiling the resolution of the display.
[0019] Microcavity resonators, which can be incorporated in the
present invention, have existed for some time and have recently
been described by H. Yokoyama, "Physics and Device Applications of
Optical Microcavities" 256 Science 66 (1992) Microcavities are one
example of a general structure that has the unique ability to
control the decay rate, the directional characteristics and the
frequency characteristics of luminescence centers located within
them. The changes in the optical behavior of the luminescence
centers involve modification of the fundamental mechanisms of
spontaneous and stimulated emission. Physically, such structures as
microcavities are optical resonant cavities with dimensions ranging
from less than one wavelength of light up to tens of wavelengths.
These have been typically formed as one integrated structure using
thin-film technology. Microcavities involving planar, as well as
hemispherical, reflectors have been constructed for laser
applications.
[0020] Resonant microcavities with semiconductor active layers, for
example silicon or GaAs, have been developed as semiconductor
lasers and as light-emitting diodes (LEDs).
[0021] E. F. Schubert, et al, "Giant Enhancement of Luminescence
Intensity in Er-doped Si/SiO2 Resonant Microcavities" 61(12) Appl.
Phys. Lett. 1381 (1992), describes a resonant microcavity with an
Er doped SiO.sub.2 active layer. This device emits radiation in the
infrared region and is intended as a laser amplifier for
fiber-optic communications.
[0022] The Schubert device, the semiconductor lasers and the LEDs
are not as suitable for use in luminescent displays for several
reasons, They contain luminescent materials such as Si, GaAs, etc.,
in the active region which are suitable as laser media, but which
are typically inefficient emitters of visible light and require
excitation by the injection of electrons. They also are designed
with small planar surface areas that are inadequate for display
purposes. Moreover, because of the design of these devices and the
active materials used, they typically cannot be excited efficiently
with electron bombardment, an electric field, or ultraviolet
radiation. These excitation mechanisms are an essential part of the
current display technologies.
[0023] Furthermore, the laser microcavity devices work above the
laser threshold, with the result that their response is inherently
nonlinear near this threshold and their brightness is limited to a
narrow dynamic range. Displays, conversely, require a wide dynamic
range of brightness. Microcavity lasers utilize stimulated emission
and not spontaneous emission. As a result, these devices produce
highly coherent light making these devices less suitable for use in
displays. Highly coherent light exhibits a phenomenon called
speckle. When viewed by the eye, highly coherent light appears as a
pattern of alternating bright and dark regions of various sizes. To
produce clear, images, luminescent displays must produce incoherent
light.
[0024] In addition, it is important to distinguish the resonant
microcavity display from the laser CRT. This display is similar to
a CRT and scans an electron beam to write the information to the
luminescent screen. However, the light is not produced by the
spontaneous emission of the phosphor, but by stimulated emission.
The faceplate of the laser CRT is an electron beam pumped
semiconductor laser. The active medium, a semiconductor, is placed
between-two mirrors that form a laser cavity. The cavity structure
is contained within the faceplate. When pumped with a sufficiently
energetic electron beam, the device lases, producing a highly
energetic and directional light beam. Such a display is described
by A. S. Nasibov, et. al. in the article "Full Color TV projector
based on A.sub.2B.sub.6 electron-pumped semiconductor lasers", J.
Crystal Growth, 117, 1040 (1992).
SUMMARY OF THE INVENTION
[0025] The subject invention, the Resonant Microcavity Display
(RMD), is a luminescent display which offers the advantages of a
thin-film phosphor without exhibiting the light piping problem.
This is because it emits light in a highly directional manner as a
result of its geometry.
[0026] The resonant microcavity display is any structure that
modifies spontaneous emission properties of a phosphor contained
within the structure. The modification of spontaneous emission is
obtained by changing the optical mode amplitudes to the such a
degree that the phosphor favorably emits into a relatively few
optical modes. It is also possible to suppress emission in certain
optical modes. This modification of mode amplitudes can be created,
for example, by the formation of a standing wave electric field for
each favored mode within the structure and locating the phosphor at
the antinodes of these standing waves. It is essential that the
standing waves have substantially modified electric field
amplitudes relative to the the field amplitudes generated without a
cavity. Substantially modified refers to changes by a factor of two
or more in the field amplitudes.
[0027] In standing wave cavities, no enhancement can occur at the
node of the electric field. However, a ring cavity design 320 such
as that shown in the downward-looking view of FIG. 1 supports a
traveling wave 322 in which the electric field amplitude is
substantially modified throughout the entire cavity. As a result,
mode enhancement or suppression can occur throughout the cavity.
Compared to the standing wave cavity, more active medium 324 with
modified light emission can be utilized for the same cavity
volume.
[0028] One example of a resonant microcavity display is a
microcavity resonator comprising a phosphor sandwiched between two
reflectors, all of which are grown on a transparent rigid
substrate. The width of the active region is chosen such that a
resonant standing wave, of the wavelength to be emitted, is
produced between the two reflectors. In its simplest form, a single
coplanar microcavity, the two reflectors are parallel to each other
and the plane of the active region is parallel to the reflectors.
Other geometries which produce standing waves or traveling waves
with an increased electric field amplitude, such as combinations of
planar microcavities, three-dimensional microcavities, confocal
microcavities, hemispherical microcavities, or ring cavities are
also possible. These other geometries are well-known, in the art of
designing cavities.
[0029] Another structure that favorably alters the spontaneous
emission properties uses photonic band gap crystals. A photonic
band gap crystal can be formed from a monodispersed colloidal
suspension. The structures comprise periodic dielectric media to
create a band gap of energy for which light cannot propagate within
the structure. However, doping such a structure with a material
that has a resonance within the band gap will create a high Q
cavity. Such cavities can be one, two or three dimensional. The
cavity generates a standing wave with an enhanced electric field
amplitude in the region of the dopant. In order to create a
display, the photonic band gap crystals must be a phosphor. Henry
O. Everitt describes photonic band gap crystals in "Applications of
Photonic Band Gap Structures", Optics & Photonics News, 20,
(1992). FIG. 2 is a side view of a resonant microcavity display 350
on a substrate 352 using a photonic band gap crystal 354 as the
entire cavity structure.
[0030] Fabricating the RMD requires the use of a growth technique
capable of controlling layer thickness or the spatial resolution of
the refractive index to a precision of several nanometers. Such
techniques, for example, include, but are not limited to, chemical
vapor deposition (CVD) molecular beam epitaxy (MBE), atomic layer
epitaxy (ALE), electron beam evaporation, or sputtering.
Fabricating the RMD may also employ holographic photo-lithographic
techniques. In this case, the Bragg reflectors are created by
exposing a suitable material to a holographic pattern thereby
creating in the material alternating layers of high and low
refractive index regions. Such a technique is well known in the art
of fabricating holographic diffraction gratings.
[0031] The substrate can be either a crystalline, polymer, or an
amorphous solid. It can be made of any material that will allow the
other regions to be grown on it. Suitable substrate materials may
be chosen from a wide range of materials such as oxides, fluorides,
aluminates, and silicates. The substrate material can also be
fabricated using organic materials. The criteria involved in
selecting a substrate material include its thermal conductivity and
its compatibility (both physical and chemical) with other materials
forming the RMD.
[0032] The phosphor may be excited through several means,
including: bombardment by externally generated electrons
(cathodoluminescence), excitation by electrodes placed across the
active layer to create an electric field (electroluminescence), or
excitation using photons (photoluminescence).
[0033] The present invention is distinguished from other
microcavity devices in part by the placing of a phosphor in the
resonant microcavity. Phosphors are materials that exhibit superior
visible luminous efficiencies (where luminous efficiency, as used
herein, is defined as the ratio of light output in Watts over the
power input in Watts). Typically, the luminous efficiencies of
phosphors range between 1% and 20%. These high efficiency materials
are only classified as phosphors if the material efficiently
generates luminescence when excited by electrons, electric fields,
or light.
[0034] The active region may comprise a wide range of inorganic
phosphors (e.g., sulfides, oxides, silicates, oxysulfides, and
aluminates) most commonly activated with transition metals, rare
earths or color centers. In addition to inorganic phosphors, the
active region may employ an organic phosphor such as tris
(8-hydroxyquinoline) aluminum complex. The active region comprises
phosphors typically in the form of single crystal films,
polycrystalline films, amorphous films, thin powder layers,
liquids, or some combination of the above. A selection of phosphors
that have found commercial applications, and from which an
application dependent phosphor can typically be selected for use in
the present invention, is documented in "Optical Characteristics of
Cathode Ray Tube Screens," Electronic Industries Association
Publication TEP 116.
[0035] The reflectors forming the resonant cavity consist of either
metallic layers or Bragg reflectors. Bragg reflectors are
dielectric reflectors formed from alternating layers of materials
with differing indices of refraction. The simplest geometry for
dielectric reflectors consists of one-quarter wavelength thick
layers of a low refractive index material, such as a fluoride or
certain oxides, alternating with one-quarter wavelength thick
layers of a high refractive index material, such as a sulfide,
selenide, nitride, or certain oxides. The dielectric reflectors can
also be fabricated using organic materials. Mirrors can also be
formed using photonic band gap crystals. Any incident light with an
energy within the band gap will be reflected by the structure. FIG.
3 shows a side view of an illustrative embodiment of a resonant
microcavity display 340 on a substrate 342 in which an active layer
346 is sandwiched between two mirrors 344, 348 comprising photonic
band gap crystals.
[0036] In current display applications, only one side of the screen
is viewed. In the case of a microcavity, the design requires the
use of different reflectors in order for most of the light to be
projected towards the viewer. In the case of the simple coplanar
microcavity, this asymmetry is obtained by having one of the two
reflectors be substantially wholly reflective, meaning that it
reflects most of the light impinging on it. The other reflector
(opposite to the substantially wholly-reflective reflector) is
partially reflective, meaning that it does not reflect as high of a
percentage of impinging light as the wholly-reflective reflector
and allows some of the light to pass through it. Because of the
difference in reflectance of the two reflectors, virtually all of
the light produced in the active region escapes through the
partially-reflective reflector along the axis normal to the plane
of the device.
[0037] In the case of a microcavity structure, the dimensions
depend on the natural spontaneous emission spectrum of the phosphor
being used, as observed outside of a cavity. If the spectrum covers
a broad range of visible wavelengths it is possible to choose an
appropriate part of the spectrum (i.e., one that matches an
industry standard chromaticity) and construct the microcavity with
a matching resonance. The final chromaticity of the RMD will
correspond to the cavity resonance and will be different from the
natural chromaticity of the phosphor outside of the microcavity.
Conversely, if the phosphor's natural spontaneous emission spectrum
covers only a narrow range of visible wavelengths, the dimensions
would be chosen so that the cavity resonance would match one of the
phosphor's emission bands.
[0038] The RMD has a highly directional light output similar to
those of a projector or a flashlight and, as a result, RMDs can be
constructed to avoid light piping. This allows highly efficient
coupling to other devices. RMD's also have a high external
efficiency, approaching 100%. Since RMDs incorporate films, RMDs
permit the design of efficient thermal conduction of the heat
generated in the active layer. This feature combined with the
ability to reduce the phosphor decay time allow RMDs to utilize
intense excitation. As a result of the above, RMDs are especially
suitable for use in projection displays.
[0039] It is therefore an object of this invention to provide a
luminescent display that does not exhibit the problem of light
piping.
[0040] It is a further object of this invention to provide a
luminescent display with highly efficient heat transfer
properties.
[0041] It is a further object of this invention to provide a
luminescent display with a high external efficiency.
[0042] It is a further object of this invention to provide a
luminescent display capable of high resolution.
[0043] It is a further object of this invention to provide a
luminescent display which produces a highly directional output.
[0044] It is a further object of this invention to provide a
luminescent display in which the chromaticity of the emitted light
can be accurately controlled irrespective of the nature of the
phosphor used.
[0045] It is a further object of this invention to provide a
luminescent display wherein the phosphor used can be chosen to
optimize the display with respect to properties other than
chromaticity.
[0046] It is a further object of this invention to provide a
luminescent display wherein the decay time of the activator can be
tailored for the specific display application.
[0047] It is a further object of this invention to provide a
luminescent display which can be heavily loaded by the excitation
source without saturating the phosphor due to overheating.
[0048] Other objects and advantages of the present invention will
become apparent to those skilled in the art from the following
detailed description of the illustrated embodiments, when read in
light of the accompanying drawings.
[0049] Througout this specification, published articles are cited
for background purposes. These articles are hereby incorporated by
reference into this specification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] FIG. 1 is a top sectional view of a traveling wave cavity in
one illustrative embodiment of a resonant microcavity display in
accordance with the invention.
[0051] FIG. 2 is a side sectional view of an illustrative
embodiment of a resonant microcavity display in accordance with the
invention using a photonic band gap crystal as a resonant
microcavity display.
[0052] FIG. 3 is a side sectional view of an illustrative
embodiment of a resonant microcavity display in accordance with the
invention using photonic band gap crystals as mirrors.
[0053] FIG. 4 is a perspective illustration of an illustrative
embodiment of a resonant microcavity display in accordance with the
invention employing a planar mirror resonator.
[0054] FIG. 5 is a side sectional view of an illustrative
embodiment of a resonant microcavity display in accordance with the
invention employing a confocal resonator.
[0055] FIG. 6 is a side sectional view of an illustrative
embodiment of a resonant microcavity display in accordance with the
invention using multiple cavity structures.
[0056] FIG. 7 is a side sectional view of an illustrative
embodiment of a resonant microcavity display in accordance with the
invention incorporating an integral optical element.
[0057] FIG. 8 is a perspective view of one illustrative embodiment
of a resonant microcavity display in accordance with the invention
employing cathodoluminescent excitation.
[0058] FIG. 9 is a side sectional view of an illustrative
embodiment of a resonant microcavity display in accordance with the
invention as it would be used in a cathode ray tube.
[0059] FIG. 10 is a side sectional view of an illustrative
experimental embodiment of a resonant microcavity display in
accordance with the invention designed to emit light through its
front reflector with a wavelength of 530 nanometers.
[0060] FIG. 11 is a graph relating the reflectance of the resonant
microcavity display of FIG. 10 as a function of the wavelength of
the incident light.
[0061] FIG. 12 is a side sectional view of a direct view color
television employing a resonant microcavity display in accordance
with the invention.
[0062] FIG. 13a is a perspective illustration of an array of
pixel-sized microcavities as used in a color television in
accordance with the invention.
[0063] FIG. 13b is an illustration of a front view of an array of
pixel-sized microcavities as used in a color television in
accordance with the invention. The front view shown in FIG. 13b
corresponds to a view from the top of FIG. 13a.
[0064] FIG. 14 is a side sectional view of an illustrative
embodiment of a resonant microcavity display in accordance with the
invention incorporated in a vacuum fluorescent display.
[0065] FIG. 15 is a side sectional view of an illustrative
embodiment of a resonant microcavity display in accordance with the
invention using an array of high voltage field emission devices for
excitation of its active layer.
[0066] FIG. 16 is a side sectional view of an illustrative
embodiment of a resonant microcavity display in accordance with the
invention using a low voltage field emission material for
excitation of the active layer.
[0067] FIG. 17 is a schematic illustration of the standing wave
electric field in one illustrative embodiment of a resonant
microcavity display in accordance with the invention.
[0068] FIG. 18 is a perspective drawing of an illustrative
embodiment of a resonant microcavity display in accordance with the
invention excited by an electric field.
[0069] FIG. 19 is a perspective drawing of an illustrative
embodiment resonant microcavity display in accordance with the
invention excited with ultra-violet light.
[0070] FIG. 20 is a side sectional view of an illustrative
embodiment of a transparent resonant microcavity display in
accordance with the invention.
[0071] FIG. 21 is a schematic illustration of an illustrative
embodiment of a resonant microcavity display in accordance with the
invention employing a laser for excitation.
[0072] FIG. 22 is a schematic illustration of an illustrative
embodiment of a tunable resonant microcavity display in accordance
with the invention.
[0073] FIG. 23 is a schematic illustration of an illustrative
embodiment of a resonant microcavity display in accordance with the
invention used as a light source for a liquid crystal display light
valve application.
DETAILED DESCRIPTION OF THE INVENTION
[0074] The present invention employs quantum electrodynamic (QED)
theory to enhance the properties of the light emitted from phosphor
based luminescence displays. The performance of a given display
application depends on properties of the emitted light such as the
chromaticity, direction, and flux. These properties can be
optimized by employing the principles of QED theory in the design
of microcavities so as to control the spontaneous emission
characteristics of the phosphor activator for each specific display
application.
[0075] As seen in FIG. 4, one example of the present invention 10
comprises a phosphor embedded in a resonant microcavity 20 grown on
a substrate 25. The microcavity 20 further comprises a front
reflector 30, a phosphor-based active region 50, and a back
reflector 60. The active region 50 is disposed between two
reflectors 30 and 60. The structure may comprise a variety of
materials and may employ a variety of resonator designs. FIG. 4
illustrates a planar mirror design, whereas FIG. 5 illustrates the
present invention configured in a confocal mirror design. The
confocal design has the advantage of having an inherently higher
cavity quality factor (Q).
[0076] More complex cavity designs involve stacking multiple
microcavities. This design is similar to the standard method for
forming interference devices which typically consist of 2 or more
stacked cavities where each cavity is separated by a coupling
layer. Such structures are used in the fabrication of, for example,
bandpass optical filters, narrow band optical reflectors and long
wavelength or short wavelength cutoff filters.
[0077] The invention can only be completely understood by employing
quantum electrodynamic (QED) theory as applied to a cavity. Cavity
QED calculations allow one to determine the following parameters
for a given degree of activator excitation and activator
concentration: the amount of light emitted from the microcavity;
the angular spread of the light emitted; and the color of the light
emitted.
[0078] The calculation begins by determining the nature of the
electromagnetic field inside and outside of the cavity. This field
calculation uses Maxwell's equations with the boundary conditions
imposed by the microcavity. Applying Fourier analysis, the net
electromagnetic field is broken down into its fundamental
constituents, the optical modes.
[0079] An optical mode is a field with a characteristic frequency,
direction and polarization. The square of the field intensity
corresponds to the actual amount of light. One must select from
this field distribution those optical modes that correspond to
useful light. For a display, useful light is defined as any light
emitted from the cavity within a certain predetermined angular
spatial distribution and predetermined frequency spread.
[0080] The next step is to calculate the amount of light emitted by
each activator. This calculation begins by determining the
radiative decay rate of each activator for each possible optical
mode. The radiative decay consists of a spontaneous emission rate
and a stimulated emission rate. The resonant microcavity display,
however, only operates satisfactorily as a display when there is no
stimulated emission (i.e., constructing a microcavity to operate as
a laser would preclude using it as a display). The degree of
excitation, the type and concentration of the activators and the
resonator design determines when stimulated emission is an
issue.
[0081] The spontaneous emission rate is determined by using QED
theory to calculate the probability that a single excited activator
will decay into a specific optical mode. This calculation must use
the field strength appropriate for the location of the activator in
the cavity. The magnitude of the standing or traveling wave within
the cavity may have different values throughout the phosphor layer.
In addition, a certain probability exists that each excited
activator will decay without emitting light. To calculate this
non-radiative rate, one must consider cavity QED effects as they
apply to the physical mechanism responsible for the non-radiative
decay.
[0082] For a given excitation level, one can now calculate the
amount of spontaneous emitted light for each activator. The ratio
of the spontaneous rate to the sum of the radiative and
non-radiative rates yields the percentage of excitation that will
produce light. The amount of useful light is then determined by
calculating the amount of the spontaneous emission in the desired
optical modes. This calculation is performed for each activator.
Finally, the sum of all the activator contributions yields the
display intensity of the RMD.
[0083] The properties of the RMD that can be controlled include the
chromaticity, the directionality of the display, the luminous
efficiency and the maximum light output of the display. These
properties are tuned according to the requirements of the specific
luminescent screen application. The parameters that must be
considered for optimization are the microcavity Q, the microcavity
resonance frequency, the asymmetry of the reflectors, the resonator
design (i.e., planar, confocal, multiple cavity, etc.), the
phosphor, the thickness of the phosphor layer, the surface area of
the microcavity and the excitation source. These parameters cannot
be optimized separately; each affects the other adjustable
properties of the display.
[0084] The performance of the resonant microcavity can be described
by the Q of the cavity. The Q of the cavity is given by the
microcavity center frequency divided by the linewidth of the
microcavity resonance: 1 Q = v v
[0085] where .nu. is the microcavity resonance frequency and
.DELTA..nu. is the linewidth of the cavity resonance. The cavity Q
is determined primarily by the reflectance of the reflectors, the
resonator design, the asymmetry in the reflectance and any
imperfections in the cavity. These imperfections typically result
from defects in the structure of the resonant microcavity which
scatter light out of the cavity in a non-useful manner. The Q can
be measured empirically using an optical spectrometer.
[0086] As the cavity Q increases, the display brightness and
efficiency increases. In addition, the angular spread of the light
decreases and the linewidth shrinks altering the chromaticity. Note
that as the spatial distribution of the light narrows, the amount
of light in certain regions decreases. Depending on the display
application, this effect may or may not be desirable. For the range
of the current display applications, the engineered cavity Q will
typically vary between 10 and 10,000. The above effects can be
determined experimentally by measuring the light intensity as a
function of solid angle for resonant microcavities with different Q
values. Using this data, one can predict the required Q for a given
application.
[0087] For most current applications, only one side of the
luminescence screen is viewed. In these applications one should
choose reflectors with different reflectivities such that the
display preferentially forces the light out the cavity towards the
viewer.
[0088] The resonator design directly affects the Q and mode volume.
The latter term describes the actual volume of the activator layer
that is participating in producing useful light. This volume is
related to the spatial distribution of the electromagnetic field
within the activator layer. The design of the resonator will also
determine the spatial distribution of useful light. Due to the
relatively straightforward construction, the simplest design is a
planar resonator. However, other resonator structures which produce
standing waves or traveling waves with an enhanced electric field
intensity in a phosphor material may be useful. In particular,
multiple planar microcavities may be combined to allow for a larger
active region or to achieve greater control over the allowed
emission than can be achieved with a single cavity.
[0089] FIG. 6 provides one illustrative design for a 3 cavity
resonant microcavity 200. In this example, each cavity 201A, 201B,
201C comprises dielectric reflectors 202, 206. The dielectric
reflectors 202, 206 are separated by a half-wavelength coupling
layer 204 in cavities 201A and 201B, while adjacent cavities are
separated from each other by a half-wavelength spacer 208. The
phosphor material 209 is also a half-wavelength thick and is
located within the lowest cavity, taking the place of a
half-wavelength coupling layer in cavity 201C. The distances
specified are optical thicknesses, i.e. the index of refraction
multiplied by the physical thickness of the layer.
[0090] As already discussed in the case of the planar geometry,
there exists an entire set of parameters to consider including the
individual mirror reflectances and individual cavity Q's. In
addition, one must also determine the cavity spacing, coupling
layer, and the location of the phosphor material. The exact
specifications will depend on the specific display
requirements.
[0091] A primary design specification of the RMD is the
chromaticity of the emitted light. The center frequency and
linewidth of the cavity must be engineered so that the RMD displays
this color of light.
[0092] Once these parameters are selected, the phosphor must be
selected. The phosphor will need to have a natural luminescence
resonance that overlaps the cavity resonance. As the resonance
narrows and the overlap increases, the display efficiency and
brightness increase. A compromise between chromaticity and other
parameters may be required to optimize a display for a specific
application.
[0093] The intensity of light emitted by the phosphor is related to
the activator concentration: as the concentration increases, the
intensity of emitted light increases. The activator concentration,
however, is often limited by non-radiative energy transfer between
activators that quenches luminescence. These quenching effects are
concentration dependent. The quenching concentration varies from
phosphor to phosphor, depending on the magnitude of various energy
transfer parameters between activators. Cavity QED theory predicts
that there is an effect on these parameters since they relate to
spontaneous emission characteristics. Thus, another potential
advantage of the RMD is that energy transfer between activators may
be suppressed and phosphors could contain higher concentrations of
activators than was previously possible, without losing efficiency.
In addition, phosphors can simultaneously emit several wavelengths
corresponding to different optical transitions within the material.
However, only one of these transitions typically generates the
useful light of the display. A microcavity can be designed to
enhance this useful transition while inhibiting the non-useful
transition(s). This suppression will increase the efficiency of the
display. The ability of a structure to inhibit spontaneous emission
and energy transfer processes has been described by G. Kurizki and
A. Z. Genack in "Suppression of Molecular Interactions in Periodic
Dielectric Structures", Phy. Rev. Let. 61, 2269 (1988).
[0094] The display properties also depend upon the thickness of the
active region. Depending on the cavity design, there may be several
active region thicknesses that produce a predetermined frequency.
The range of thickness depends on the mirror construction. As the
thickness increases, the number of potentially excited activators
increases. With sufficient excitation energy, the total
luminescence can be increased with a wider active region. However,
the thickness may alter the spatial distribution in a highly
complex manner. In the case of a simple coplanar microcavity, the
angular spread of the light changes, with additional regions of
high intensity appearing at angles that are not normal to the plane
of the microcavity. More complex multiple cavity designs allow a
greater degree of control over the directionality of the
display.
[0095] Another key parameter in the resonant microcavity design is
the area of the emitting surface. Some applications will require
one large-area surface for the production of monochromatic light,
while other designs will need pixel-sized cavities capable of
producing red, green and blue light. The size of the pixel will be
determined by the resolution requirements of the display.
[0096] One other important parameter is the excitation source and
intensity. The display application will dictate the excitation
source. The decision process in selecting the phosphor must also
consider the efficiency of converting the excitation energy into
useful luminescence. This efficiency is well documented for the
registered phosphors, but can easily be experimentally determined.
The intensity of the source will primarily change the
brightness.
[0097] It should be noted that in considering the above design
parameters, the light properties of the display must not reach the
degree of coherence associated with a laser. To avoid this problem,
particular attention must be paid to the cavity Q, the activator
concentration and the excitation intensity.
[0098] The RMD design lends itself to the incorporation of an
optical element 382, such as a lens or a diffuser, fabricated
within or on top of the substrate 384 of a resonant microcavity
386, as shown in FIG. 7. For example, a lens would be useful to
modify the angular distribution of the light output produced by the
structure and thereby generate the required distribution. The lens
may be formed using photo-etching methods, which is well known in
the art of miniature semiconductor lasers. Another method would
employ the controlled placement of impurities to change the local
refractive index. This method is used to construct gradient
refractive index lenses which are commonly used in fiber
optics.
[0099] Using such a lens adds another parameter that must be
considered in the optimization of the display. However, such a lens
enables one to maximize the output of the resonant microcavity
without having to consider the required light distribution. For
example, such a lens would eliminate or reduce the demands for the
complex lens design currently required in the projection CRT
display applications.
[0100] Similarly, a diffuser can be used to precisely control the
angular spread of the light and thereby the field of view of the
display. With the ability to control the light distribution
independent of the microcavity, the spontaneous emission properties
of the phosphor can be maximized without having to consider the
required light distribution. A diffuser can be made using
holographic techniques, ruled grating techniquess, introduction of
internal scattering centers, or precisely controlled surface
roughening.
[0101] The RMD can be embodied using cathodoluminescence which
results from an electron beam bombardment of the phosphor. One
example of a device which employs cathodoluminescence is a
projection television. This application requires the highest
intensities possible because it requires a wide viewing area and
uses a light dispersing screen. In this application, the resonant
microcavity display is incorporated in a CRT.
[0102] Full color projection televisions require three separate
CRT's: one for each primary color. In this application, the RMD is
superior to conventional methods because it allows intense
excitation loading of the phosphor, highly directional output,
controlled chromaticity, and high external efficiency. Therefore
the RMD allows the use of relatively compact CRT's while
maintaining high luminescence.
[0103] In the case of a resonant microcavity display incorporated
in a CRT, the phosphor is excited by electrons emitted from the
electron gun, accelerated to a speed such that most of them will
penetrate the resonant microcavity to the depth of the phosphor.
The high energy electrons excite electrons in the phosphor from the
valence band into the conduction band. This additional energy is
trapped at the impurity. The impurity then relaxes by emitting
visible light.
[0104] In the case of a simple coplanar microcavity, the reflectors
can he either dielectric or metallic. The back reflector has a
higher reflectivity than the front reflector, so that light,
emitted by the phosphor, exits the cavity through the front
reflector, perpendicular to the plane of the thin film device. The
microcavity Q and the asymmetry in the reflectance determines the
percentage of light that exits the resonator through the front
reflector.
[0105] In the case of the simple coplanar microcavity, the width of
the active region affects the directionality of the light and is
chosen so that its optical path length, i.e., the product of the
distance between the back reflector and the front reflector and the
index of refraction of the phosphor material, equals an integer
multiple of the desired wavelength divided by 2 or 4 depending on
the index of the adjacent layers. These dimensions ensure that a
standing wave builds up between the back-reflector and the front
reflector. The wavelength of the emitted light is determined by the
resonant wavelength of the microcavity.
[0106] A dielectric, or Bragg, reflector consists of alternating
layers of material with high and low indices of refraction. The
number of layers determines the reflectivity of the reflector. The
reflectivity (R) of the reflectors can be calculated using the
following equation: 2 R = 1 ( n H n L ) N - 1 .times. n H 2 n 2 1 +
( n H n L ) N - 1 .times. n H 2 n 2
[0107] where n.sub.H and n.sub.L are the refractive index of the
high and low index of refraction materials, respectively; n.sub.s
is the index of refraction of the substrate and N is the total
number of layers in the stack. This equation is valid for normal
incidence. The width of each layer is equal to an odd integer
multiplied by the desired wavelength of light to be emitted divided
by the quantity 4 times the index of refraction of the material
used in tie layer. An alternate design uses holographic techniques
to form the reflectors. In this case, the mirror is formed from one
material with a continuously varying refractive index.
Photo-lithography would be used to fabricate the mirrors.
[0108] The Q of the cavity can be calculated once the reflectivity
is determined for the reflectors. In the case of the simple
coplanar microcavity, the equation that relates Q to reflectivity
is given by: 3 Q = 2 n v c ( - 1 l ( m R 1 R 2 )
[0109] where .nu. is the microcavity resonance frequency, n is the
index of refraction of the phosphor, .alpha. is the average
distributed loss constant, l is the width of the activator layer,
R.sub.1 is the reflectance of the front mirror and R.sub.2 is the
reflectance of the back mirror. The constant .alpha. is needed to
account for the non-ideal behavior of the cavity that results from
imperfections and spurious absorption.
[0110] The parameters chosen to optimize this display depend on the
required brightness of the display and the required directionality
of the light output. In the typical projection television
application, the display should be highly directional and bright.
For each color, the cavity Q can be optimized empirically by
measuring the total intensity emitted in the useful direction as a
function of the electron beam current. This efficiency measurement
is common in the television design art.
[0111] FIG. 8 shows one illustrative embodiment designed for
cathodoluminescence, the simple planar resonant microcavity, The
subject invention 10 comprises a resonant microcavity 20 grown on a
rigid transparent substrate 25. A layer of aluminum 80 is disposed
next to the microcavity 20 to channel off electrons deposited by
the electron beam and to provide an additional reflective surface.
The resonant microcavity 20 is grown onto the substrate 25 using
molecular beam epitaxy (MBE) or any suitable method of solid-state
fabrication. Some methods of growth known to the art (e.g., LPE at
its current level of development) are not suitable because they
cannot be controlled with the precision necessary to grow a
correctly sized microcavity. The active region 50 is excited by
electrons from an electron beam 54 entering through the aluminum
layer 80 and back reflector 60. The light 58 created in the active
region exits through the front reflector 30 and the substrate
25.
[0112] As seen in FIG. 9, this embodiment can be embodied in a
cathode ray tube (CRT) 100 comprising a glass vacuum tube 105
enclosing an electron gun (which is a means to generate an electron
beam) 110 aimed at a flat viewing surface 115 and distal from the
electron gun 110; and a phosphor-based resonant microcavity 20
disposed parallel to the flat viewing surface 115 inside the vacuum
tube 105. This embodiment is configured to produce monochromatic
light.
[0113] As shown in FIG. 10, an experimental embodiment designed to
emit light through the front reflector with a wavelength of 530
nanometers, the material used in the active region 50 is zinc
sulfide (ZnS) doped with manganese (Mn) at a dopant concentration
of 2%. The thickness of the active region 50 is 110 nanometers and
the phosphor has an index of refraction of n=2.4.
[0114] In the front reflector 30, the material used in the layers
with a relatively high index of refraction 32, 36, 40 and 44 is
ZnS, and the material used in layers with a relatively low index of
refraction 34, 38, 42 and 46 is calcium fluoride (CaF.sub.2). In
the back reflector 60, the material used in the layers with a
relatively low index of refraction 62, 66, 70, 74, 77, and 79 is
CaF.sub.2, and the material used in the layers with a relatively
high index of refraction 64, 68, 72, 76, and 78 is ZnS. All of the
high-index ZnS layers are 55 nanometers thick with an index of
refraction of n=2.4. All of the low-index CaF.sub.2 layers are 95
nanometers thick with an index of refraction of n=1.4.
[0115] The substrate 25 is made of CaF.sub.2. It is 2 millimeters
thick and has an index of refraction of n=1.4. The aluminum layer
80 is 50 nanometers thick.
[0116] The microcavity 20 is grown on the substrate 25 using MBE
and the aluminum layer 80 is deposited using vapor-phase
deposition.
[0117] The front reflector has a reflectivity of R=97.5% with 8
layers and the back reflector has a reflectivity R=99.9% with 12
layers including the aluminum layer. Because the back reflector is
more reflective than the front mirror almost all of light produced
in the cavity exits through the front reflector.
[0118] As shown in FIG. 11, the reflectance of the RMD is a
function of the wavelength of the incident light. At the resonance
wavelength of 530 nm, the reflectance dips to roughly
86%--indicating that the RMD will transmit this wavelength. At all
other wavelengths the reflectance is near 100%--indicating that the
RMD will not transmit light at non-resonance wavelengths. This
reflectance behavior is due to the fact that the cavity can only
support a standing wave of a wavelength equal to the resonance
wavelength of the cavity.
[0119] In another embodiment, the RMD can be used in a CRT as a
direct view television. FIG. 12 depicts a direct view color
television. The CRT 120 is similar to the one described in the
projection television embodiment, except that it has three electron
guns, 122, 124 and 126 one for each primary color. Each of the
electron guns produces a separate electron beam, 130, 132 and 134,
corresponding to the desired intensity of each color. The electron
beams excite a screen 140 on the viewing surface of the CRT.
[0120] As seen in FIG. 13a, the screen 140 comprises of an array of
pixel-sized microcavities 20. The array contains microcavities
designed to produce red light 142, green light 144 and blue light
146. The red-light pixels are excited by the "red" electron beam
130, the green-light pixels are excited by the "green" electron
beam 132, and the blue-light pixels are excited by the "blue"
electron beam 134. FIG. 13b shows a front view of the array of
pixels and the arrangement of colors. The design of color displays
with separate color pixels is well known in the art.
[0121] In this embodiment, the light emanating from the pixel
produces the required angular distribution. One could also envision
an embodiment in which a lens is used to achieve this display
requirement allowing for the maximum efficiency to be produced by
the resonant microcavity. The required angular distribution can
also be obtained using a diffuser such as a holographic optical
element.
[0122] The construction of the pixel is fundamentally the same as
that described in the embodiment for a projection television. The
primary difference is the size of the surface area and the angular
spread of light required. In this case, the surface area is
determined not by brightness, but by the resolution required by the
application. High definition television, medical and military
applications typically require the pixel size to be smaller than 25
microns. This requirement is difficult to achieve using current
technologies, but can be easily achieved using an RMD.
[0123] With the resolution and angular distribution specified, the
resonant microcavity display must be optimized for each color. This
optimization will use the above-described empirical method of
measuring the total light produced versus beam current. The
restrictions of the design due to the specification mean that
obtaining the maximum light output is primarily a function of the
phosphor activator. In the embodiment in which a lens is placed
outside the cavity, one has much more freedom in engineering the
cavity. Without the restriction on the angular distribution, the
cavity Q can be easily tailored.
[0124] In another embodiment using electrons that excite the active
layer, the resonant microcavity 217 can be incorporated in a vacuum
fluorescent display 210, as shown in FIG. 14. Display 210 comprises
individual pixels which are typically combined to form low
resolution, compact information displays and extremely large
displays.
[0125] A vacuum fluorescent display generally comprises an array of
cathodes 226, a control grid 224 and phosphor coated anodes,
corresponding to anodes such as anode 214 shown in FIG. 14. (Anode
214 shown in FIG. 14 differs from a conventional anode as described
below.) Electrons are first generated by the hot filaments that
form the cathode array 226. A positive voltage is applied between
the cathode array 226 and anodes 214. When the control grid voltage
is on, the electrons are accelerated by the positive potential
towards the phosphor layer which is deposited, in a conventional
vacuum fluorescent display, on top of the anodes. The remainder of
the display conventionally comprises a glass faceplate 212, glass
backplate 228, and a glass frit seal 222, containing a vacuum for
the control wire grid 224 and filament cathodes 226.
[0126] A resonant microcavity structure may be used to improve the
performance of this type of display. One possible illustrative
embodiment is depicted in FIG. 14. The resonant microcavity
structure 217 comprising an active layer 218 sandwiched between a
pair of dielectric mirrors 216, 220 disposed between the control
wire grid 224 and the anode 214 replaces the powder phosphor that
is conventionally deposited on anodes such as anode 214.
[0127] For small scale monochromatic displays, one resonant
microcavity 217 would be used and the pixels would be determined by
the control grid 224 and cathode 226 arrangement. If full color is
required, a resonant microcavity 217 would be required for each
primary color. An efficient layout would comprise alternating
stripes of microcavities 217 with separate stripes for each color.
In large screen applications, each pixel would incorporate one
resonant microcavity designed for a specific color. The array would
then comprise a triad of red, green and blue pixels.
[0128] As discussed above for the two CRT embodiments, the
parameters such as directionality, brightness, color and the
microcavity structure apply for the vacuum fluorescent display. The
design considerations and methodology required for optimizing the
display are also the same. For example, since this display is a
direct view type, with light emission directed towards a viewer in
the direction indicated by arrow A, the divergence of the emitted
light would be tailored for the viewer distance and required
viewing angle. Incorporation of lenses and diffusers must be
considered. The design of vacuum fluorescent displays for specific
applications is well known in the art.
[0129] In another embodiment using excitation by electrons, the
resonant microcavity can be incorporated into field emission
displays for both projection and direct view applications. This
display operates on the principle of electrons tunneling from a
microscopic tip or a microscopic region of a low work function
material. The electrons are then accelerated via a positive
potential and penetrate an adjacent phosphor layer. Typically there
is a evacuated region between the tips and the phosphor, but in
some applications the phosphor may be grown directly on top of the
emitting surface.
[0130] These displays may operate in both a high voltage and low
voltage mode. In the high voltage application, typically above 500
volts, an emitter array would be assembled behind each microcavity.
The display could consist of one microcavity that is the size of an
entire display to generate monochromatic light or the display could
consist of pixel size microcavities suitable for producing color
images. In these structures, the voltage must be sufficiently high
that the electrons can pass through the bottom mirror of the RMD
into the active layer to stimulate the phosphor.
[0131] FIG. 15 provides one illustrative embodiment of a
monochromatic field emission display 230 incorporating a resonant
microcavity 239 comprising an active layer 236 between mirrors 234
and 238. When a positive high voltage is applied between the anode
240 and cathode 246, the electrons are generated by the field
emissive material 244, which is sealed within an evacuated region
242 by seals 248. The electrons are then accelerated through the
evacuated region 242, penetrate the resonant microcavity 239 and
excite the active layer 236. The aluminum layer 240 is
approximately 50 nanometer thick and conducts the electrons to
ground.
[0132] In the low voltage application, the field emissive material
must be located inside the RMD due to the limited penetration depth
of the low energy electrons. Suitable emissive materials must have
a low work function so that a low voltage applied to the material
will induce sufficient numbers of electrons to be emitted. In this
application, a low voltage is applied across the resonant
microcavity and induces electrons to tunnel from the electron
emissive material into the phosphor and excite the activators.
Under the influence of the applied field, the electrons travel
through the phosphor and then into another material which conducts
the electrons to ground.
[0133] In one illustrative embodiment of a low voltage field
emission display 250 illustrated in FIG. 16, the resonant
microcavity 253 (which is deposited on substrate 252) would
comprise an oriented diamond film layer 256 that is deposited on
one side of the phosphor layer 258. Another conductive film layer
260 similar to diamond film layer 256 would be deposited on the
opposite side of phosphor layer 258 to conduct the emitted
electrons to ground. A low voltage potential would be applied
between conductive layers 260 and 256. Reflectors 254, 262 are
disposed outside the sandwich-like structure formed by conductive
layers 256, 260 and phosphor layer 258. This embodiment is depicted
in FIG. 16.
[0134] In the case of the simple coplanar microcavity, the key
design specification in all the display applications is locating
the active layer at or near the antinode of the electric field
inside the cavity. In the low voltage field emission display, this
specification is critical given the thickness of the active layer.
The basic structure of a phosphor layer sandwiched between two
emissive layers can be repeated, provided that the phosphor
material is located at or near an antinode. An illustration of a
standing wave field in one illustrative embodiment of a resonant
microcavity display 300 is shown in FIG. 17. Microcavity display
300 comprises a substrate 302, a pair of mirrors 304, and an active
layer 306. The electric field amplitude 310 in active layer 306 is
shown schematically. A node 311 and an antinode 312 are shown.
[0135] The design issues one must consider are fundamentally the
same as has been discussed in the other display applications.
However, the index of the electron emissive material must now be a
major factor in the design of the cavity. An additional concern is
the choice of material for the specific applied voltage range.
[0136] In addition, the RMD can be embodied in an
electroluminescent display. In this display application, a RMD is
sandwiched between two conductors. A voltage signal is applied to
the conductors and thereby induces what is termed thin film
electroluminescence (TFEL). An array of pixel-size elements is
constructed to form a luminescent screen creating a TFEL flat panel
display.
[0137] This embodiment would comprise an array of pixels, where
each pixel would be an electrically activated microcavity. FIG. 18
shows one pixel in the array 160. The pixel comprises a visibly
transparent substrate 162, a layer of Indium doped Tin Oxide (ITO)
164 (a transparent metal) acts as ground, and a resonant
microcavity 166. The resonant microcavity 166 comprises a front
reflector 168, a phosphor-based active region 170 and a back
reflector 172. Disposed next to the back reflector 172 is an
aluminum layer 174, which is deposited on each microcavity in such
manner that each cavity is electrically isolated.
[0138] This display would be excited by applying a voltage to the
aluminum layer 174 of the pixel microcavity 166. The addressing of
pixels is common in the art of flat panel display design.
[0139] This display would be optimized by measuring the amount of
useful light emitted versus the electric field intensity,
Particular attention must be paid to the phosphor selected since
(in this embodiment) the electroluminescence efficiency is
important.
[0140] Also, the RMD could be embodied as an array of pixels in a
flat panel display which uses ultra-violet light to excite the
phosphor. As seen in FIG. 19, each pixel 180 would comprise a
plasma discharge lamp 182 that generates ultra-violet light which
passes through a back reflector 184 and excites the active region
186 (i.e., the phosphor). The emitted light then passes out of the
display through the front reflector 183 and the substrate 190.
[0141] The RMD concept can also be used to fabricate a transparent
direct view flat panel display. This display is visibly transparent
except at the specific resonant wavelengths of the microcavities
that are used in the display. Both monochrome and full color
displays are possible. For example, to create a full color display,
one could chose the three wavelengths that correspond to the three
fully saturated colors specified by the international CIE color
standard for red, green and blue.
[0142] The transparent property is created by fabricating resonant
microcavities that use reflectors that only function as high
efficiency mirrors within a narrow wavelength bandwidth, typically
one nanometer or less. Outside this region, the reflectors transmit
nearly 100% and thus the RMD appears transparent to the eye. Such
narrow band reflectors can be best built using a multiple cavity
structure employing dielectric mirrors.
[0143] In one illustrative embodiment shown in FIG. 20, a flat
panel display would consist of an array of pixel size RMDs 500
excited by an electric field. Two transparent electrodes 504, 514
must be connected to either side of each microcavity 506 and could
be best fabricated using Indium Tin Oxide (ITO). Microcavity 506
itself would comprise an active layer 510 between mirrors 508,
512.
[0144] In addition to creating a transparent display, the same
reflector structure can be used to create a high contrast display.
In this embodiment, the rear surface is made opaque by another
opaque layer (not shown) or by replacing ITO layer 514 with an
opaque conductor. External ambient light would be transmitted
through the display and then absorbed by the rear layer. The
reflection from the front surface would be minimized because of the
high transmission properties of the reflectors outside the
resonance wavelengths. Such a display can be made to have very high
contrast ratios on the order of a 100 or greater. These direct view
displays can use any of the three excitation sources.
[0145] The use of organic material permits the construction of a
RMD out of flexible materials such as plastics.
[0146] The resonant microcavity display can also be excited using
laser light. Laser light results from stimulated emission processes
and is distinguished from spontaneous emitted light by the high
degree of spatial and/or temporal phase coherence. The laser light
would be chosen to have a wavelength that is absorbed by the
phosphor. The cavity structure must be designed to pass the laser
wavelength. In one embodiment shown in FIG. 21, a laser 412 would
be scanned horizontally and vertically across a luminescent screen
401 in a manner similar to the electron beam in a cathode ray tube.
The steering of beam 410 is typically accomplished by rotating
mirrors and acoustic optic modulators. The ability to write
sequential information with lasers is well known in the art.
Luminescent screen 401 itself comprises substrate 402 and
microcavity 403, including mirrors 404, 408, and active layer
406.
[0147] The RMD could also be used in a reverse configuration to
absorb light and generate an electric signal. The physics that
yields the enhanced emission of light demonstrated in the above
display also produces enhanced absorption. The light energy has to
be converted into electric energy.
[0148] Another application of the resonant microcavity using its
property of enhanced absorption is in field of photography. In this
application, the film would comprise resonant microcavities in
which the active layer includes a photosensitive material. As a
result, this film would absorb only at certain wavelengths
corresponding to the three primaries. Since the amount of
absorption can be precisely controlled, the film would be capable
of extremely accurate color reproduction. Information could also be
recorded by deriving an electrical signal from the photosensitive
material within the microcavity. The general design would be
similar to digital cameras employing charge coupled detectors.
[0149] The unique ability of an RMD to influence the emission
characteristics may also be used in memory storage devices. As
explained earlier, the confinement of an optical material in a
resonant microcavity affects the decay rate. Depending on whether
the cavity is in resonance with the transition energy of the
optical material or not, the lifetime is either decreased or
increased. It is therefore possible to significantly enhance the
lifetime of the material and to use this effect to store
information.
[0150] Another possible way to store information with a resonant
microcavity would be based on hole burning. This process and its
application for the storage of information is well known. By
putting the material in a resonant microcavity one could not only
use the enhanced absorption but also the earlier described effect
of increased lifetime to make the hole burning process more
efficient.
[0151] RMDs could also be used in the design of light valves. This
would require two RMD's. One RMD without a phosphor would be grown
on top of a RMD with a phosphor. The first RMD would modulate the
intensity of the light emanating from the second RMD. The modulator
would work by tuning the first RMD to its resonant frequency or
tune it away from its resonant frequency. The process of tuning the
first RMD (using the electro-optic or the piezo-electric effect)
would be achieved by applying a voltage to the first RMD. This
modulator could also be used as a switch by turning the light
completely on and completely off. A modulated RMD 421 grown on
substrate 422 is shown in FIG. 22. In this figure, RMD 421,
comprising mirrors 424, 428 with active layer 426 between, is
modulated by applying a voltage V to mirror layers 424 and 428.
This modulation can be accomplished using either electro-optic or
piezo-electric effects.
[0152] The ability to tune the cavity resonance using, for example,
electro-optic or piezo-electric effects, would allow the RMD to be
utilized in a variety of communication modes. Resonant
microcavities could be designed to emit light and receive light
over a range of frequencies and solid angles. These frequencies and
solid angles could be modified by applying electric signals. Thus
RMDs could be used to send and receive information. Friend or foe
identifiers used in military equipment would be one possible
use.
[0153] Using RMD's in a Plasma Display Panel could also be used to
build a fluorescence lamp. Compared to common fluorescence lamps
the RMD lamp has the advantage of strongly enhanced fluorescence
which results in a greater efficiency. A single RMD lamp would emit
light of a certain wavelength. This is useful for applications such
as stage-lamps. Common stage lamps emit over the UV, the visible
and the infrared region and use filters to select a certain
wavelength (color). This filter-process makes the lamp very
inefficient since most of the light is not allowed to exit the
lamp. In contrast, the RMD lamp creates only light of a certain
wavelength and does therefore not require a filter. The efficiency
is therefore much higher. The combination of a R, G and B device
would result in a white light source.
[0154] In general, any light source can in principle be substituted
by the resonant microcavity display. For example, incandescent
lights are typically filtered to produce colored lights for car
tail lights and traffic signal lights. Resonant microcavities can
replace these current light sources with highly efficient single
color and directional light sources. Excitation could use any of
the means already discussed.
[0155] In non-emissive displays, the light source and image
producing surface are separate. The image is typically formed using
a light valve which modulates the light produced by the light
source. A common light valve display uses a combination of one or
more liquid crystals and polarizers and forms what is called a
liquid crystal display (LCD). Light valves are used in both
reflection and transmission and find use in both projection and
direct view applications. The pixel size is determined solely by
the light modulator.
[0156] In each application, a sufficiently bright light source is
required. Often the display also requires full color capability.
Currently for flat panel application, a fluorescence lamp is used
as a backlight and creates the white light that is then modulated
by a LCD panel. To create a full color flat panel display, color
filters are inserted at each pixel to filter the white light and
generate the three primary colors.
[0157] The RMD can be incorporated in such flat panel display
applications and form the light source. For a monochromatic
display, the modulator would be attached to one large area resonant
microcavity. The microcavity can be excited by any one of the three
excitation means. Full color would be best generated by an array of
microcavities consisting of alternating stripes in which each
striped region is constructed to form one continuous resonant
microcavity designed to generate one color.
[0158] For projection devices, an arc lamp is used to generate a
white light source and the color is typically generated by using
dichroic filters to separate the three primary color components of
the white light. Instead, the three colors can be produced by three
independent resonant microcavities or by producing an array of
microcavities.
[0159] In addition, a LCD modulator requires the input light to be
initially polarized and uses a polarizer located in the input. It
is possible to eliminate this polarizer by designing the resonant
microcavity to generate polarized light. This can be accomplished
in a number of ways. For example, the region between the mirrors
can be fabricated using birefringent material in such a manner the
cavity will resonate at different frequencies depending on the
polarization of the light. The cavity can be designed so that only
one polarized light component will resonate at the desired
frequency.
[0160] The principal advantage of using resonant microcavities to
generate the light used for light modulators is the increased light
output efficiency. The RMD light source will produce high
brightness levels and is highly directional. The latter is
particularly useful for LCD applications since the input light must
be contained within a certain range of solid angle. In addition,
the elimination of color filters and dichroic beam splitters will
increase the overall throughput The other engineering advantage is
the compact nature of the RMD which is particularly useful for flat
panel applications.
[0161] In one illustrative embodiment shown in FIG. 23, a
monochromatic flat panel display 270 is depicted. In this example,
the resonant microcavity 275 is excited by UV light generated by a
plasma discharge 282 excited by a source 284 of AC power. Any
damaging UV that leaks out past microcavity 275 is absorbed by
substrate 274; another UV blocking substrate 286 may also be used
on the other side of plasma discharge 282. The light valve uses an
LCD 272 to modulate the light. LCD 272 can be addressed in a number
of modes and this specification is not affected by using the
resonant microcavity. The key design considerations for the
microcavity would involve the divergence of the light, the light
polarization, brightness and resonance wavelength.
[0162] The above embodiments are given as illustrative examples and
are not intended to impose any limitations on the invention.
* * * * *