U.S. patent application number 13/902447 was filed with the patent office on 2013-12-19 for light-absorbing structure and methods of making.
This patent application is currently assigned to Massachusetts Institute of Technology. The applicant listed for this patent is Massachusetts Institute of Technology. Invention is credited to Michael Scott Bradley, Vladimir Bulovic, Jonathan R. Tischler.
Application Number | 20130335826 13/902447 |
Document ID | / |
Family ID | 38372052 |
Filed Date | 2013-12-19 |
United States Patent
Application |
20130335826 |
Kind Code |
A1 |
Tischler; Jonathan R. ; et
al. |
December 19, 2013 |
Light-Absorbing Structure and Methods of Making
Abstract
A critically coupled optical resonator absorbs greater than 95%
of incident light of the critical wavelength with an absorber layer
less than 10 nm thick.
Inventors: |
Tischler; Jonathan R.;
(Sharon, MA) ; Bradley; Michael Scott; (Cambridge,
MA) ; Bulovic; Vladimir; (Lexington, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Massachusetts Institute of Technology |
Cambridge |
MA |
US |
|
|
Assignee: |
Massachusetts Institute of
Technology
Cambridge
MA
|
Family ID: |
38372052 |
Appl. No.: |
13/902447 |
Filed: |
May 24, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12162892 |
Feb 5, 2009 |
8449125 |
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PCT/US2007/003676 |
Feb 14, 2007 |
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13902447 |
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60772882 |
Feb 14, 2006 |
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Current U.S.
Class: |
359/614 |
Current CPC
Class: |
H01L 51/5275 20130101;
Y10T 29/49826 20150115; H01L 51/52 20130101; G02B 5/003
20130101 |
Class at
Publication: |
359/614 |
International
Class: |
G02B 5/00 20060101
G02B005/00 |
Goverment Interests
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0001] This invention was made with government support under Grant
Number MDA972-00-1-0023, awarded by DARPA. The government has
certain rights in the invention.
Claims
1.-50. (canceled)
51. A device including: a light emitting layer configured light of
a first wavelength; an absorber or emitter layer configured to
receive the first wavelength emitted from the light emitting layer
and transmit light of a second wavelength; and a dielectric Bragg
reflector configured to receive the light of the second wavelength
and transmit light out of the device.
52. The device of claim 50, wherein the light emitting layer is
patterned.
53. The device of claim 50, wherein the absorber or emitter layer
is patterned.
54. The device of claim 51, wherein the light emitting layer is
patterned and the absorber or emitter layer is patterned.
55.-57. (canceled)
Description
TECHNICAL FIELD
[0002] This invention relates to light-absorbing structures and
methods of making the structures.
BACKGROUND
[0003] Light-emitting devices can be used, for example, in displays
(e.g., flat-panel displays), screens (e.g., computer screens), and
other items that require illumination. Accordingly, the brightness
of the light-emitting device is one important feature of the
device. Also, low operating voltages and high efficiencies can
improve the viability of producing emissive devices.
[0004] Light-emitting devices can release photons in response to
excitation of an active component of the device. Emission can be
stimulated by applying a voltage across the active component (e.g.,
an electroluminescent component) of the device. The
electroluminescent component can be a polymer, such as a conjugated
organic polymer or a polymer containing electroluminescent moieties
or layers of organic molecules. Typically, the emission can occur
by radiative recombination of an excited charge between layers of a
device. The emitted light has an emission profile that includes a
maximum emission wavelength, and an emission intensity, measured in
luminance (candelas/square meter (cd/m.sup.2) or power flux
(W/m.sup.2)). The emission profile, and other physical
characteristics of the device, can be altered by the electronic
structure (e.g., energy gaps) of the material. For example, the
brightness, range of color, efficiency, operating voltage, and
operating half-lives of light-emitting devices can vary based on
the structure of the device.
SUMMARY
[0005] A high oscillator strength thin film can be applied to a
surface. The film can have an absorption coefficient greater than
10.sup.5 cm.sup.-1, for example, 10.sup.6 cm.sup.-1 or larger. The
films can be formed by adsorption into layered structures of
charged species with strong dipole-dipole interactions between
species. The films can be built by adsorption of species with
alternating charge on a solid substrate such as glass, silicon, a
polymer surface, or a previous polymer film disposed on a
substrate, etc.
[0006] The high absorption coefficient can arise from the
interaction of dipoles in a plane perpendicular to the probe
direction. The process used to form these films can allow for
strong dipole interactions within the adsorbed layer. Additionally,
the process can allow for precise deposition of a single physical
layer of the dipole-dipole-interacting absorbing species. As a
result, the dipole-dipole interactions in the plane of absorbing
species perpendicular to the probe direction can provide a high
absorption constant in the thin film.
[0007] The high oscillator strength film can be an element of a
critically coupled resonator (CCR). The CCR can include a
reflective element (i.e., a structure capable of reflecting light
of a desired wavelength, such as a mirror or dielectric Bragg
reflector) optically coupled to the film. Optical coupling of the
high oscillator strength film and the reflective element can give
rise to critical coupling, in which greater than 90% of light
having a critical wavelength is absorbed. The CCR can absorb
greater than 90%, greater than 95%, or greater than 97% of the
light at the critical wavelength. The critical coupling can be
characterized by a very low reflectance at a critical wavelength,
where the reflective element has a high reflectance in the absence
of the critically coupled absorber. The CCR can include a top
coat.
[0008] In one aspect, an optical device includes a light-absorbing
film having a thickness and separated a distance apart from a
reflective element. The light-absorbing film is critically coupled
to the reflective element. The light-absorbing film can be
critically coupled to the reflective element at a temperature above
77 Kelvin. The light-absorbing film can be critically-coupled at a
wavelength between 250 nm and 2000 nm, such as between 250 nm and
400 nm, between 400 nm and 700 nm, between 700 nm and 900 nm,
between 900 nm and 1200 nm, or between 1200 nm and 2000 nm.
[0009] The light-absorbing film can be separated a distance apart
from the reflective element by a light-transmitting material. The
thickness of the light-absorbing film can be less than 80 nm, less
than 50 nm, less than 25 nm, or less than 10 nm. The
light-absorbing film can include a light-absorbing material. The
light-absorbing film can include a multiply charged material. The
light-absorbing material can include an organic compound or an
inorganic compound. The light-absorbing material can include a
J-aggregate, which can include a cyanine dye. The multiply charged
material can include a polyelectrolyte. The light-absorbing film
can include an electrostatic bilayer which includes a first layer
including a polyelectrolyte and a second layer including a
light-absorbing material. The optical device can be arranged on a
substrate. The device can absorb at least 90% or at least 95% of
light at a critical wavelength.
[0010] The reflective element can be a dielectric reflector
including an insulator or semiconductor material or can include a
metallic mirror. The reflective element can be a dielectric Bragg
reflector, composed of insulator or semiconductor materials. The
reflective element can include a semiconductor material. A
dielectric reflector can derive its reflectivity from interference
phenomena associated with the real part of the index of refraction
of the reflecting elements. A very reflective mirror can be
constructed from insulating or semiconducting materials because the
reflectance is derived from a multitude of interference effects. A
dielectric Bragg reflector refers specifically to a mirror where
the thickness, d.sub.i, of the different materials is chosen to
satisfy the Bragg condition discussed in the text.
[0011] In another aspect, an optical device includes a
light-absorbing film having a thickness of less than 80 nm and an
extinction coefficient (K) of at least 1 at a critical wavelength,
and being separated a distance apart from a reflective element by a
light-transmitting material, wherein the light-absorbing film is
critically coupled to the reflective element. The light-absorbing
film can have an extinction coefficient (K) of at least 2, at least
3, at least 4, or at least 5 at a critical wavelength.
[0012] In another aspect, a method of making an optical device
includes arranging a light-absorbing film having a thickness a
distance apart from a reflective element. The distance is selected
to critically couple the light-absorbing film to the reflective
element.
[0013] The method can include arranging the reflective element on a
substrate. Arranging the reflective element on the substrate can
include applying a metallic mirror to the substrate, or forming a
dielectric reflector on the substrate. The method can include
disposing a light-transmitting material adjacent to the reflective
element. Disposing a light-transmitting material can include
forming a desired thickness of the light-transmitting material.
[0014] Arranging the light-absorbing film a distance apart from the
reflective element can include applying the light-absorbing film
adjacent to the light-transmitting material. Arranging the
light-absorbing film can include contacting a surface of the
light-transmitting material with a multiply charged material. The
multiply charged material can include a polyelectrolyte. Arranging
the light-absorbing film can include contacting a surface of the
light-transmitting material with a light-absorbing material.
[0015] In another aspect, a method of making an optical device
includes forming a thickness of a light-transmitting material
adjacent to a reflective element, and forming light-absorbing film
having a thickness of less than 80 nm and an extinction coefficient
(k) of at least 1 at a critical wavelength adjacent to the
thickness of the light-transmitting material. The light-absorbing
film is critically coupled to the reflective element.
[0016] In another aspect, a device includes a light emitting layer
configured light of a first wavelength, an absorber or emitter
layer configured to receive the first wavelength emitted from the
light emitting layer and transmit light of a second wavelength, and
a dielectric reflector configured to receive the light of the
second wavelength and transmit light out of the device. The light
emitting layer can be patterned, the absorber or emitter layer can
be patterned, or both.
[0017] In other aspects, a chemical sensor, a light harvesting
device, or an optical switch can include a light-absorbing film
having a thickness and separated a distance apart from a reflective
element.
[0018] Other features, objects, and advantages will be apparent
from the description and drawings, and from the claims.
BRIEF DESCRIPTION OF TEE DRAWINGS
[0019] FIGS. 1A-1B are a schematic depictions of a critically
coupled resonator.
[0020] FIG. 2 is a graph depicting results of optical measurements
on components of a critically coupled resonator and of an assembled
critically coupled resonator.
[0021] FIG. 3 is a graph depicting calculated optical properties of
a light absorbing film.
[0022] FIG. 4 is a graph compared measured and calculated optical
properties of optical devices.
[0023] FIG. 5 is a graph depicting calculated optical properties of
a critically coupled resonator.
[0024] FIGS. 6A-6C are schematic diagrams depicting a device
including a critically coupled resonator.
[0025] FIG. 7 is a schematic depicting increase of
photoluminescence using a critically coupled resonator.
DETAILED DESCRIPTION
[0026] Critical coupling occurs when (1) all of the incident
optical power is transferred through the front face of the CCR
absorber layer and (2) the Poynting vector in the dielectric layers
is purely imaginary. Consequently, two boundary conditions must be
simultaneously satisfied to achieve critical coupling: that is, the
magnitude of the reflection coefficient from air to the absorber
layer front face must be zero, and the magnitude of the reflection
coefficient from the absorber layer back face to the dielectric
spacer must be unity. The first condition is realized by impedance
matching the CCR with air, and the second by mismatching the
impedances across the absorber-spacer interface by a phase
difference of .+-. p/2. The second boundary condition also dictates
that the Poynting vector is purely real on the absorber layer side
of the interface.
[0027] Thin films having a high oscillator strength (i.e.,
absorption coefficient) can be made by alternately adsorbing two or
more materials capable of non-covalent interaction onto a support
or substrate from solution, where one material is a light absorbing
material. The non-covalent interaction can be, for example, an
electrostatic interaction or hydrogen bonding. Selection of
appropriate materials and assembly conditions can result in a film
where the light absorbing material participates in strong
dipole-dipole interactions, favoring a high absorption coefficient.
The light absorbing material can be a dye capable of forming a
1-aggregate.
[0028] J-aggregates are crystallites of dye in which the transition
dipoles of the constituent molecules strongly couple to form a
collective narrow linewidth optical transition possessing
oscillator strength derived from all the aggregated molecules. See,
e.g., M. Vanburgel, et al., J. Chem. Phys. 102, 20 (1995), which is
incorporated by reference in its entirety.
[0029] Layers of light absorbing material, which can be positively
or negatively charged, can be interspersed with layers of an
oppositely charged material. The oppositely charged material can
include a multiply charged species. A multiply charged species can
have a plurality of charge sites each bearing a partial, single, or
multiple charge; or a single charge site bearing a multiple charge.
A polyelectrolyte, for example, can have a plurality of charge
sites each bearing a partial, single, or multiple charge. A
polyelectrolyte has a backbone with a plurality of charged
functional groups attached to the backbone. The charged functional
groups attached to the backbone can be exclusively cationic (as in
a polycation), exclusively anionic (as in a polyanion), or be a
mixture of cationic groups and anionic groups. A copolymer of
cationic and anionic monomers is an example of a polyelectrolyte
having a mixture of cationic groups and anionic groups. Some
polyelectrolytes, such as copolymers, can include both polycationic
segments and polyanionic segments. The copolymer can be, for
example, a block copolymer, a random copolymer, or other copolymer.
A polycation has a backbone with a plurality of positively charged
functional groups attached to the backbone, for example
poly(allylamine hydrochloride) or poly(diallyldimethylammonium
chloride). A polyanion has a backbone with a plurality of
negatively charged functional groups attached to the backbone, such
as sulfonated polystyrene (SPS), polyacrylic acid, or a salt
thereof. Some polyelectrolytes can lose their charge (i.e., become
electrically neutral) depending on conditions such as pH. The
charge density of a polyelectrolyte in aqueous solution can be pH
insensitive (i.e., a strong polyelectrolyte) or pH sensitive (i.e.,
a weak polyelectrolyte). Without limitation, some exemplary
polyelectrolytes are poly diallyldimethylammonium chloride (PDAC, a
strong polycation), poly allylamine hydrochloride (PAH, a weak
polycation), sulfonated polystyrene (SPS, a strong polyanion), and
poly acrylic acid (PAA, a weak polyanion). Examples of a single
charge site bearing a multiple charge include multiply charged
metal ions, such as, without limitation, Mg.sup.2+, Ca.sup.2+,
Zn.sup.2+, Fe.sup.2+, Fe.sup.3+, Co.sup.2+, Cu.sup.2+, Cd.sup.2+,
Sn.sup.4+, Eu.sup.3+, Tb.sup.3+, and the like. Multiply charged
metal ions are available as salts, e.g. chloride salts such as
CoCl.sub.2, FeCl.sub.3, EuCl.sub.3, TbCl.sub.3, CdCl.sub.2, and
SnCl.sub.4.
[0030] The film can include hydrogen bonding polymers, such as, for
example, polyacrylamide (PAm), polyvinylpyrolidone (PVP), and
polyvinyl alcohol (PVA). The light absorbing film can include more
than two materials. One of these materials is the light absorbing
material and one of the other materials is either a multivalent
ionic species or hydrogen bonding polymer. Additional materials may
be included in the film to promote crosslinking, adhesion, or to
sensitize light emission or absorption.
[0031] The thin films can include one or several layers of a
polyelectrolyte and one or more charged species with strong
dipole-dipole interactions and any additional dopants. At least one
of the charged species used for strong dipole-dipole interactions
has a charge opposite that of the polyelectrolyte used for the
scaffold. When sequentially applied to a substrate, the oppositely
charged materials attract one another forming an electrostatic
bilayer. The polyelectrolyte provides a scaffold for the species
with strong dipole-dipole interactions to form a layered structure.
These films are compatible with other processes of building thin
films through alternate adsorption of charged species. The films
can be interspersed in a multifilm heterostructure with other thin
films.
[0032] The charged species with strong dipole-dipole interactions
can be a single type of species, such as a single type of
J-aggregating material (for example, a cyanine dye). Alternatively,
several charged species with strong dipole-dipole interactions
among the species could be used. The species used for the strong
dipole-dipole interacting layer can have individual dipoles that
can couple together to produce a coherent quantum mechanical state.
This allows for the buildup of coherence in two dimensions,
producing effects in the probe dimension perpendicular to the
interacting species.
[0033] J-aggregates of cyanine dyes have long been known for their
strong fluorescence. This strong fluorescence makes J-aggregates a
desirable candidate for use in organic light-emitting devices
(OLEDs), and such devices have been produced. The layer-by-layer
(LBL) technique for film growth, first developed by Decher et al.,
was extended to create thin films of J-aggregates, which have been
used to create an OLED with J-aggregates as emitters. See, for
example, E. E. Jelley, Nature 1936, 138, 1009; M. Era, C. Adachi,
T. Tsutsui, S. Saito, Chem. Phys. Lett. 1991, 178, 488; G. Decher,
J. D. Hong, J. Schmitt, Thin Solid Films 1992, 210, 831; H.
Fukumoto, Y. Yonezawa, Thin Solid Films 1998, 329, 748; S. Bourbon,
M. Y. Gao, S. Kirstein, Synthetic Metals 1999, 101, 152; Bradley,
M. S. et al., Advanced Materials 2005, 17, 1881; and U.S. patent
application Ser. No. 11/265,109, filed Nov. 3, 2005, each of which
is incorporated by reference in its entirety. J-aggregates (and
thin films including J-aggregates) can have a high oscillator
strength at a characteristic wavelength. In other words, the
J-aggregate strongly absorbs light of the characteristic
wavelength. The characteristic wavelength depends primarily on the
identity (i.e., the chemical structure) of the material forming the
J-aggregate, and to a lesser degree on other factors, such as the
chemical environment of the J-aggregate. For example, multilayer
films of polycation and anionic J-aggregate dye contain a high
density of J-aggregate and therefore have a high peak absorption
coefficient of .alpha.=1.0.times.10.sup.6 cm.sup.-1. See M. S.
Bradley, et al., Advanced Materials 17, 1881 (2005), which is
incorporated by reference in its entirety. Because the film has a
high absorption coefficient at its characteristic wavelength, a
very thin film (e.g., less than 50 nm thick, less than 25 nm thick,
less than 10 nm thick, or 5 nm thick or less) can absorb much of
the tight of the characteristic wavelength.
[0034] Layer-by-layer (LBL) processing of polyelectrolyte
multilayers can produce conformal thin film coatings with molecular
level control over film thickness and chemistry. Charged
polyelectrolytes can be assembled in a layer-by-layer fashion. In
other words, positively- and negatively-charged polyelectrolytes
can be alternately deposited on a substrate. One method of
depositing the polyelectrolytes is to contact the substrate with an
aqueous solution of polyelectrolyte at an appropriate pH. The pH
can be chosen such that the polyelectrolyte is partially or weakly
charged.
[0035] A substrate subjected to sequential immersions in cationic
and anionic solutions (i.e., solutions of polycation and
polyanion), or SICAS, can produce a multilayer including a number
of electrostatic bilayers on the substrate. An electrostatic
bilayer is the structure formed by the ordered application of a
multiply charged species (e.g., a polyelectrolyte or metal ion) and
an oppositely charged material (e.g., a light absorbing material,
polyelectrolyte or counterion). The properties of weakly charged
polyelectrolytes can be precisely controlled by changes in pH. See,
for example, G. Decher, Science 1997, 277, 1232; Mendelsohn et al.,
Langmuir 2000, 16, 5017; Fery et al., Langmuir 2001, 17, 3779;
Shiratori et al., Macromolecules 2000, 33, 4213, each of which is
incorporated by reference in its entirety.
[0036] The process conditions used in the deposition of the film
can be varied. Some process conditions that can be varied include
concentration, temperature, pH, salt concentration, co-solvent,
co-solvent concentration, and deposition time. The temperature can
be varied between, for example, 0.degree. C.. and 100.degree. C..,
or between 5.degree. C.. and 80.degree. C.. The pH can be varied
from 0.0 to 14.0, or from 3.0 to 13.0. The salt concentration can
range from deionized (i.e., no salt added) to 1 M. NaCl and KCl are
examples of salts used. Solutions can be prepared using water as
the sole solvent, or with water and a co-solvent, such as an
organic solvent. Some exemplary organic solvents include methanol,
ethanol, isopropanol, acetone, acetic acid, THF, dioxane, DMF, and
formamide. The deposition time can be 1 second or less; 30 seconds
or less; 1 minute or less; 5 minutes or less; 10 minutes or less; 1
hour or less; or several hours or more. In some circumstances,
deposition times will be in the range of 1 second to 10 minutes.
Deposition of the film can include a post-treatment of the film.
Post-treatment is any treatment applied to the film after the last
bilayer is applied. The post-treatment can include a heat
treatment, a pH treatment, a chemical modification, or other
treatment. The post-treatment can be selected to alter a desired
property of the film, such as its mechanical stability or
porosity.
[0037] The density of the film can be modified by repeatedly
immersing the substrate into solutions of the light absorbing
material prepared with different process conditions. As an example,
by cyclically immersing into a solution held at a temperature of
20.degree. C.. and then in a second solution held at 60.degree.
C.., the crystallinity of the resultant film can be enhanced and
dye density increased compared to films not treated in this
manner.
[0038] The film can include a plurality of bilayers, such as fewer
than 100, fewer than 50, fewer than 20, or fewer than 10 bilayers.
The film can include 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5,
5.0, 5.5., 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, or 10.0
bilayers. In some embodiments, the film can include bilayers
substantially free of light absorbing material, e.g., bilayers
where one layer includes a polycation and the other layer includes
a polyanion. Including bilayers that are substantially free of
light absorbing material can be advantageous, for example, in
altering the adhesion of the film to a substrate or in altering the
thickness of the film.
[0039] The light absorbing film can be deposited on a hydrophilic
or hydrophobic substrate. The film can be deposited onto conducting
(e.g., metallic), semiconducting, or insulating layers (including
glass and plastic); or bio-compatible materials, examples of which
are a polymer film that is hydrophilic or hydrophobic, an oxide
layer, a metal oxide layer, a metal layer, a DNA-coated surface,
and others. Examples of a hydrophilic polymer layer include
polyelectrolytes and hydrogen bonding polymers; amino acids;
proteins; and hydrophilic polymers. Examples of hydrophobic
polymers include PDMS, Poly-TPD, and MEH-PPV. Metal oxide layers
include, for example alumina, titania, and zinc oxide. Examples of
semiconducting layers are layers of Si, Ge, GaAs, GaN, AlGaAs,
GaAsP, CdSe, CdS, ZnS, and metal halides, such as AgCl, AgBr, and
AgI. Adhesion of the light absorbing film to the substrate can be
promoted by varying the process conditions described above.
[0040] The light absorbing film can be optically coupled to a
reflective element to form a resonator. The reflective element can
be a layer of semiconductor deposited on a glass substrate.
Alternatively, the substrate itself can be a semiconductor
substrate, for example, silicon as which is reflective at the
wavelength of 1550 nm. A CCR can be contructed on top of a silicon
substrate, possibly eliminating the need for additional mirror
layers.
[0041] An exemplary resonator structure that optically couples the
absorbing film to a reflective element is illustrated in FIG. 1A.
In FIG. 1A, an absorbing film 2 is a high oscillator strength film,
such as, for example, an electrostatic multilayer of a J-aggregate
forming dye and a polyelectrolyte. The absorbing film 2 has a
defined thickness, referred to as d.sub..alpha.. A surface of the
absorbing film 2 is in optical communication with a light
transmitting medium 1, e.g., air. Another surface of the absorbing
film 2 is in optical communication with a reflective element 4 via
an optically transmissive spacer layer 3. The spacer layer 3 has a
defined thickness, d.sub.s. The entire structure is arranged on a
substrate 5.
[0042] The resonator of FIG. 1A can achieve critically coupled
resonance when certain conditions are met. Critically coupled
resonance results in near-complete absorption of light of a
particular wavelength by the resonator. The CCR structure allows
near-complete absorption even when the absorbing film is very thin
(e.g., less than 50 nm, less than 25 nm, or less than 10 nm). The
CCR structure absorbs light (of the critical wavelength) to a
greater degree than the absorbing film does by itself (i.e., when
the absorbing film is not optically coupled to a reflective
element). The CCR can have an effective absorption coefficient at a
critical wavelength of greater than 10.sup.6 cm.sup.-1, greater
than 2.times.10.sup.6 cm.sup.-1, greater than 3.times.10.sup.6
cm.sup.-1, greater than 4.times.10.sup.6 cm.sup.-1, greater than
5.times.10.sup.6 cm.sup.-1, or greater than 6.times.10.sup.6
cm.sup.-1. The absorption coefficient a can be related to the
complex portion .kappa. of the complex index of refraction by the
formula .alpha.=4.pi..kappa.(.lamda.)/.lamda., where .lamda. is the
wavelength of light in centimeters. As such, the CCR can have an
extinction coefficient .kappa. of at least 1, at least 2, at least
3, at least 4, or at least 5.
[0043] The optical properties of the resonator illustrated in FIG.
1A can be described mathematically. The mathematical description
accounts for four regions of different refractive index: air (1),
absorbing film (2), spacer (3), and reflective element (4). For
simplicity in the mathematical description, reflective element 4
can be treated as a layer of silver mirror sufficiently thick to
neglect reflections from the mirror/substrate interface. The
mathematical description includes three interfaces between
materials of different refractive index: interface 12, between air
1 and the absorbing film 2; interface 23, between absorbing film 2
and spacer 3; and interface 34, between spacer 3 and reflective
element 4. The thickness of the absorber layer (d.sub..alpha.) is
represented by L.sub.2, and the thickness of the spacer is
represented by L.sub.3. For normal-incident light (i.e., light
approaching the absorbing film at 90.degree. to the surface of the
film) of wavelength .lamda., the reflection coefficient r of the
resonator is given by:
r = [ r 12 ( 1 + r 23 r 34 2 j .beta. 3 L 3 ) + 2 j .beta. 2 L 2 (
r 23 + r 34 2 j .beta. 3 L 3 ) ] [ ( 1 + r 23 r 34 2 j .beta. 3 L 3
) + r 12 2 j .beta. 2 L 2 ( r 23 + r 34 2 j .beta. 3 L 3 ) ]
##EQU00001##
where the Fresnel coefficient r.sub.ij for interface if is:
r ij = ( n ~ i - n ~ j ) ( n ~ i + n ~ j ) ##EQU00002##
[0044] and the wavevector .beta..sub.1 for the i.sup.th layer
is:
.beta. i = 2 .pi. n ~ i .lamda. ##EQU00003##
where n.sub.1 is the complex index of refraction for the i.sup.th
layer.
[0045] The reflection coeffecient of the resonator is related to
its percent reflectance (R) by R=|r|.sup.2. Critical coupling
occurs when the resonator parameters are selected such that R=0% at
.lamda..sub.e, since no light is transmitted through the CCR. In
actual devices, R can approach 0% but be greater than 0%. For
example the value of R at .lamda..sub.c can be 10% or less, 5% or
less, or 2% or less. The reduction in reflectance is due to
critical coupling, and not to an antireflective material. Although
antireflective materials are known, the CCR can be substantially
free of antireflective material. Critical coupling of the light
absorbing film and reflective element can occur at a wide range of
temperatures, such as 77 K or higher. The critical coupling can
occur at any temperature from 77 K and higher until the temperature
becomes so high that the material of the CCR begins to degrade.
Critical coupling can occur, in the range of 250 K to 350 K, or 200
K to 400 K.
[0046] FIG. 1B illustrates a CCR in which the reflective element is
a dielectric Bragg reflector (DBR). The DBR includes alternating
layers of material having different refractive indices, where the
thickness of each layer (d.sub.l) is chosen to meet the Bragg
condition, d.sub.i=.lamda./4n.sub.i, where .lamda. is wavelength
and n.sub.i is the refractive index of material i. The DBR can be
made by alternately sputtering two different materials of known
refractive index to deposit alternating layers of a desired
thickness on a substrate. The materials can be, for example, metal
oxides such as a titanium oxide or an aluminum oxide. The materials
can be, for example, metal oxides such as a titanium oxide, hafnium
oxide, silicon dioxide, zirconium oxide or an aluminum oxide. Other
materials that can be used in the DBR are conductive metal oxides
such as indium tin oxide, tin oxide, zinc oxide or indium zinc
oxide. The DBR can also be made of polymers and or chalcogenide
materials. The layers can be deposited using any one or a
combination of a variety of deposition techniques that include, for
example, sputter coating, thermal evaporation, chemical vapor
deposition, electron beam evaporation, spin casting, and
extruding.
[0047] A top coat can be included with the CCR. The CCR can include
a mirror element, spacer element, and absorber layer, where the
mirror layer was deposited on a substrate followed by deposition of
the spacer layer and absorber layer. In certain structures, these
three elements can be integrated with other layers of materials.
For instance, on top of the absorber layer a protective coating can
be placed on top of a transparent material, such as an organic
small molecule or polymer or metal-oxide like silica or alumina,
and critical coupling can be achieved. Furthermore, a CCR can be
constructed where the absorber layer can be deposited onto the
substrate, followed by spacer and mirror layers.
[0048] An optical switch can include a CCr. When light is incident
on the CCR from the absorber side of the device, almost none of the
light is reflected, and is instead absorbed within the absorber
layer. However, the absorptive tendencies of the absorber layer can
be momentarily switched off. During this brief moment, in which the
absorber layer is temporarily bleached, the CCR reflects the light
incident on it from the absorber layer side of the device. It is
possible that briefly the mirror layer can reflect light as
efficiently as if the absorber was physically not present. The
temporary bleaching of the absorber layer can be accomplished by
optically exciting the absorber layer with a high power burst of
light energy. The burst of light energy can be delivered at the
critical wavelength at which the CCR can act or at another
wavelength at which the absorber layer can absorb light. In the
case of J-aggregates of cyanine dyes, the extremely large
absorption non-linear response coefficient (.chi..sup.(3)) at the
J-aggregate resonance due to the cooperative coupling amongst the
individual dye molecules can enable optical switching to be
achieved with very low energy, arguably less energy than is
required to switch state-of-the-art silicon transistors. The
switching recovery time can occur within less than 20 ps, for
example, within 3 ps, which can result in a fast all-optical
room-temperature optical switch architecture which can find use in
short haul optical interconnects on silicon microchips.
[0049] A light harvesting device can include a CCR. The CCR
structure can include mirror element, spacer and absorber layer
with electrical contacts in order to extract photovoltaic energy
from the light absorbed within the absorber layer. The electrical
contacts can be placed on top of the absorber layer, underneath it,
or on the sides adjacent to it. The metallic mirror layer can act
as one of these contacts. A protective layer deposited on top of
the absorber layer, if made of an electrical conductive material,
can also be one of the electrical contacts. For example, the
contact can include indium tin oxide.
[0050] A chemical sensor can include a CCR. There are at least two
methods in which the CCR can be utilized for chemical sensing
applications. The first method can be a reflectance/absorbance
based sensor. The second method is a fluorescence based sensor. In
the first method, a component that is chemically sensitive to the
chemical to be sensed can be incorporated into the absorber layer
element of the CCR. When the chemical is present, this component
can alter the absorber layer's ability to respond to light at the
CCR critical wavelength. The reflectance of the CCR can thus be
modulated from nearly 0% (chemical absent) to nearly 100% (chemical
present). If the absorber layer material itself is sensitive to
this chemical, then adding a compounds to the absorber layer would
not be necessary although it could still enhance sensitivity. In
the second method, the absorber layer can absorb nearly all of the
light incident at the critical wavelength. This absorbed light
energy would actually be emitted as fluorescence from the absorber
material itself or from another component incorporated in the layer
to accept the absorbed energy and radiate it. However, in the
presence of the chemical to be sensed, one of these fluorescent
pathways can be quenched. Thus the presence or absence of
fluorescence would be indicative of the absence or presence of the
chemical in the environment. Another component can be incorporated
with the absorber layer that is sensitive to the chemical to be
sensed.
EXAMPLES
[0051] In the CCR of FIG. 1B, the absorbing layer was a thin film
consisting of layers of the cationic polyelectrolyte PDAC (poly
diallyldimethylammoniurn chloride) and J-aggregates of the anionic
cyanine dye TDBC
(5,6-dichloro-2-[3-[5,6-dichloro-1-ethyl-3-(3-sulfopropyl)-2(3H)-benzimid-
azolidene]-1-propenyl]-1-ethyl-3-(3-sulfopropyl)benzimidazolium
hydroxide, inner salt, sodium salt). Molecular structures of PDAC
and TDBC are shown in FIG. 1B. The DBR was 8.5 pairs of
sputter-coated TiO.sub.2 and Al.sub.2O.sub.3 layers, ending on
TiO.sub.2. The spacer layer was an additional sputter-coated layer
of Al.sub.2O.sub.3. The J-aggregate layer was prepared by
depositing PDAC and TDBC by sequential immersion into cationic and
anionic aqueous solutions (pH=5.5) utilizing the technique
described in M. S. Bradley, et al., Advanced Materials 17, 1881
(2005), which is incorporated by reference in its entirety.
Reflection and transmission measurements were made with light
incident from the I-aggregate side of the device.
[0052] The critically coupled resonator (CCR) structure can include
a dielectric Bragg reflector (DBR), a transparent spacer layer, and
a layer of J-aggregate cyanine dye. The DBR can include 8.5 pairs
of sputter coated TiO.sub.2 and Al.sub.2O.sub.3 layers, ending on
TiO.sub.2. The spacer layer can be an additional sputter coated
layer of Al.sub.2O.sub.3. The J-aggregate layer can include of the
cationic polyelectrolyte, PDAC, and the anionic cyanine dye, TDBC,
deposited via sequential immersion into cationic and anionic
aqueous solutions (pH=5.5) utilizing a previously described
technique (see, M. S. Bradley, J. R. Tischler, V. Bulovic, Adv.
Mater. 2005, 17, 1881, which is incorporated by reference in its
entirety).
[0053] When constructing a system for demonstrating strong
coupling, the natural tendency is to focus primarily on the optical
properties of the components, i.e. the excitonic layer and the
microcavity, and on the fully integrated composite system to check
for Rabi-splitting. Understandably, optical measurements of the
half cavity structure consisting of just one of the two mirrors
from the microcavity and the excitonic layer are routinely not
reported.
[0054] FIG. 2 presents reflectance and transmittance data for the
CCR, along with reflectance data for the neat PDAC/TDBC film and
for the dielectric stack of DBR with spacer layer without an
absorbing film applied. The vertical axis in FIG. 2 runs from 0.0
to 1.0, which corresponds to 0% to 100% in reflectance and
transmittance. When light of wavelength .lamda..sub.c=591 nm was
incident on the CCR of FIG. 1B from the absorbing layer side of the
device, the measured reflectance was R=2% (FIG. 2). In contrast,
for the DBR with spacer but without the absorbing layer, the
reflectivity at .lamda..sub.c=591 nm exceeded 95%, showing the
dramatic change in reflectance due to critical coupling. For the
same CCR, the transmittance at .lamda..sub.c is T=1%. Consequently,
97% of the incident light was absorbed within the 5.1.+-.0.5 rim
thick absorber layer, yielding a maximum effective absorption
coefficient of .alpha..sub.eff=6.9.times.10.sup.6 cm.sup.-1.
[0055] A wavelength resolved T-matrix simulation (FIGS. 3 and 4)
numerically confirmed the critical coupling phenomenon. To simulate
the CCR's reflectance, T-matrices corresponding to the PDAC/TDBC
film and the DBR were constructed. The film was modeled following
the procedure described in M. S. Bradley, et al., Advanced
Materials 17, 1881 (2005), wherein (n,.kappa.) are obtained through
a Kramers-Kronig regression based on reflectance data of the neat
film deposited on a SiO.sub.2 substrate (FIG. 3). FIG. 3 displays
spectrally resolved real and imaginary components of the refractive
index, (n,.kappa.), for 5.1.+-.0.5 nm thick PDAC/TDBC film
deposited on an SiO.sub.2 substrate.
[0056] In modeling the CCR, the DBR was modeled as 8.5 pairs of
TiO.sub.2 and Al.sub.2O.sub.3 layers, with refractive indices of
n=2.39 and n=1.62 respectively, with layer thickness adjusted to
satisfy the Bragg condition (d.sub.i=.lamda./4n.sub.t) for
.lamda.=565 nm, to reflect the results of the experiment. The
maximum value of .kappa. occurred at .lamda.=593 nm, while the peak
reflectance of the thin film occurs at .lamda.=595 nm. These models
were combined with a model of the spacer layer with n.sub.s=1.62
and thickness, d.sub.s, left as a free parameter. The simulation
reproduced critical coupling at .lamda..sub.c=591 nm (FIG. 4) for a
spacer layer thickness d.sub.s=90 nm and odd multiples thereof.
FIG. 4 compares measured and calculated reflectance for the CCR
device and DBR spacer stack. The calculated reflectance was based
on the T-matrix formalism. The calculated fit matched the
experimentally observed reflectance minimum at .lamda..sub.c=591
nm.
[0057] When d.sub.s was set to a value greater or less than 90 nm,
critical coupling did not occur at another wavelength, because once
the absorber layer thickness, d.sub.o, was set, critical coupling
can occur only at one specific wavelength, .lamda..sub.c. The
simulation also predicted that critical coupling was achieved at
.lamda..sub.c=591 nm with an Ag mirror replacing the DBR, and
d.sub.s=90 nm was still the critical thickness. With a non-ideal
metallic mirror (e.g. Ag), not all of the light absorption was
predicted to occur in the absorber layer, as would be the case with
an ideal metal mirror (.kappa..fwdarw..infin.), but the reflectance
from top surface of the CCR was still predicted to be zero, as a
result of critical coupling.
[0058] The critical coupling phenomenon observed for the 5.1 nm
thick film of PDAC/TDBC spaced 90 nm from the DBR of FIG. 1B is not
limited to these particular materials and thicknesses. Critical
coupling can be achieved with any thin film absorber layer of
sufficient oscillator strength (i.e., .kappa.), providing that
d.sub..alpha.and d.sub.s are set to the appropriate thicknesses. To
demonstrate this, a generalized formalism of critical coupling for
the CCR structure of FIG. 1B was constructed. As described above,
critical coupling occurs when the resonator parameters are selected
such that R=|r|.sup.2=0%, where
r = [ r 12 ( 1 + r 23 r 34 2 j .beta. 3 L 3 ) + 2 j .beta. 2 L 2 (
r 23 + r 34 2 j .beta. 3 L 3 ) ] [ ( 1 + r 23 r 34 2 j .beta. 3 L 3
) + r 12 2 j .beta. 2 L 2 ( r 23 + r 34 2 j .beta. 3 L 3 ) ]
##EQU00004##
where the Fresnel coefficient r.sub.ij for interface ij is:
r ij = ( n ~ i - n ~ j ) ( n ~ i + n ~ j ) ##EQU00005##
and the wavevector .beta..sub.i for the i.sup.th layer is:
.beta. i = 2 .pi. n ~ i .lamda. ##EQU00006##
[0059] FIG. 5 illustrates the generalized formalism for critically
coupling absorber layer of FIG. 1B as a function of absorber
.kappa.. Thicknesses for absorber and spacer layers were normalized
to the CCR wavelength, .lamda..sub.c. The reflectance plotted was
at .lamda..sub.c. FIG. 5 shows the absorber and spacer layer
thicknesses required to achieve critical coupling as a function of
the absorber layer oscillator strength .kappa.. The result was
plotted for three different values of the real part of the absorber
layer refractive index, n.sub..alpha..di-elect cons.(1.55, 1.75,
2.0.), n.sub.s=1.7 throughout, and the single mirror layer was
assumed to be Ag, with complex refractive index n=0.259.+-.j3.887
at .lamda.=591 rim (this value of n was derived from a fit of
published n values, see H. J. Hagemann, et al., J. Opt. Soc. Am.
65, 742 (1975), which is incorporated by reference in its
entirety). The thicknesses are normalized to .lamda..sub.c to
emphasize the generality of this model. The model showed that to
satisfy CCR conditions as .kappa. increases, the absorber layer
thickness must decrease, with a corresponding increase in the
spacer layer thickness. The model also showed that for a given
.kappa., as n.sub..alpha.increases, d.sub.s decreases, as expected,
while d.sub..alpha.stayed relatively constant. The model dictates
that in order to critically couple the d.sub..alpha.=5.1 nm thick
PDAC/TDBC film of FIG. 1 at .lamda..sub.c=584 nm
(d.sub..alpha./.lamda..sub.c=0.87%) the extinction coefficient of
the film must be .kappa.=4.2, which also sets
d.sub.s/.lamda..sub.c=15% or equivalently d.sub.s=88 nm for
n.sub..alpha.=2.0 and n.sub.s=1.7. These theoretical values agree
well with the experimentally measured .kappa.=4.2 and
n.sub..alpha.=2.1 at 591 nm (from FIGS. 3) and d.sub.s=90 nm for
the CCR structure in FIG. 2 with n.sub.s=1.62.
[0060] For the 5.1 nm thick film of PDAC/TDBC, the results in FIG.
5 were in close agreement with the experimental results and with
the full simulation for the CCR structure (FIG. 4). For wavelengths
in the range around .lamda..sub.c=584 nm and for d.sub..alpha.=5.1
nm, d.sub..alpha./.lamda..sub.c=0.0087, which dictates that
.kappa.=4.2. From the dispersion relation of FIG. 3, the value of
.kappa.=4.2 corresponds to n.sub..alpha.=2.1 and the observed
spacer layer thickness is d.sub.s=90.+-.1 nm. Consistent with these
observations, for n.sub..alpha.=2.0, FIG. 5 sets ds/.lamda.c=0.147,
or equivalently d.sub.s=90.3 nm for .lamda..sub.c=584 nm.
[0061] Table 1 presents calculated critical thicknesses of absorber
and spacer layers for critically coupling a thin film with
absorption coefficient .alpha.=4.0.times.10.sup.5 cm.sup.-1. Real
component of absorber layer refractive index n=1.75. Average
reflectance values were 0.3.+-.0.1%. The critical thickness in
absolute terms was relatively constant across wavelengths for fixed
.alpha..
TABLE-US-00001 TABLE 1 Wavelength .lamda..sub.c (nm) .kappa.
d.sub.a (nm) d.sub.a (%) d.sub.s (nm) d.sub.s (%) 350 1.11 12.4 3.5
27.6 7.9 410 1.31 12.1 3.0 36.4 8.9 450 1.43 12.2 2.7 42.1 9.4 525
1.67 12.1 2.3 53.2 10.1 600 1.91 12.0 2.0 65.0 10.8 700 2.15 12.0
1.8 77.4 11.5
[0062] Table 2 presents calculated critical thicknesses of absorber
and spacer layers for critically coupling a thin film of absorption
coefficient .kappa. to either of two wavelengths. The real
component of absorber layer refractive index was n=1.75. Average
reflectance value was 0.6.+-.0.3%, and the same reflectance was
produced at both critical wavelengths.
TABLE-US-00002 TABLE 2 Wavelength .lamda..sub.c (nm) .kappa.
d.sub.a (%) d.sub.a (nm) d.sub.s (%) d.sub.s (nm) 410 0.93 5.0 20.4
7.6 31.2 584 0.93 5.0 29.1 7.6 44.4 410 2.79 1.5 6.3 13.5 55.2 584
2.79 1.5 9.0 13.5 78.7 410 4.65 0.9 3.6 16.0 65.6 584 4.65 0.9 5.2
16.0 93.5
[0063] If the thin films that produce critical coupling in the CCR
were deposited as neat films on glass (n=1.48), T-matrix simulation
showed that they would absorb, on average, 32% of incident light at
.lamda..sub.c. The absorption increase due to the critical coupling
was therefore a factor of 3.1. Similarly, if such an absorber layer
were inserted into a symmetric .lamda./2 microcavity, the maximum
absorption would be 50% (with T=25% and R=25%). Thus, the CCR
structure was not only convenient (i.e., easier to make than a
symmetric microcavity), it was also essential for maximizing light
absorption. This suggests several practical device implementations.
For example, the CCR structure can be used in optically stimulated
chemical sensors where a thin luminescent chemosensitive film is
deposited on top of the CCR structure and excited by energy
transfer from the CCR absorber layer. See, for example, A. Rose, et
al., Nature 434, 876 (2005), which is incorporated by reference in
its entirety. Compared to existing structures, a factor of 6
reduction in pump power is expected, since the chemosensitive films
would effectively absorb 3.2 times more light, and the back mirror
would direct twice the photolumuniscence into the detector.
Application of the CCR phenomenon can also facilitate development
of single photon optics where it is desirable to absorb a photon
with 100% probability in the thinnest possible films.
[0064] When a thin film of PDAC/TDBC is placed at the antinode of
the optical field of a microcavity resonantly tuned to the
excitonic absorption peak, strong coupling was observed. See J. R.
Tischler, et al., Phys. Rev. Lett. 95, 036401 (2005), which is
incorporated by reference in its entirety. New polaritonic
resonances appeared in the linear optical measurements of the
composite structure corresponding to the superposition states of
the strongly coupled light/matter system. Strong coupling was
achieved even in low Q all metal microcavities, with Rabi splitting
of 265 meV, due to the high absorption coefficient of the PDAC/TDBC
films and their relatively narrow reflectance linewidth, FWHM=67
meV. The polaritonic band gap observed in these full microcavity
structures was manifested as a high reflectance at the uncoupled
excitonic resonance. The appearance of a polaritonic band gap
follows naturally from the realization that the strongly coupled
exciton-polariton device is a CCR plus a "top" DBR, separated
approximately .lamda./4n away from the CCR absorbing layer, where n
is the refractive index of the transparent spacer layer. With the
CCR in place, when the "top" DBR is added to complete the
microcavity, there is no optical feedback to cause a resonant dip
in reflectance and therefore the high reflectance of the "top" DBR
is observed as the polariton bandgap.
[0065] As alternatives to PDAC/TDBC and other J aggregates, CCR's
can also be constructed with a variety of highly absorptive
materials. Among non-epitaxially grown materials, CCR structures
could be implemented with organic polymers that are used in
biological assays and chemical sensors, with molecular materials
that are used in photodetectors and xerographic photoresistors, and
in the emerging uses of colloidally grown inorganic nanocrystal
quantum dots (QDs), with the QD continuum states providing the
necessary absorption. See, for example, C. A. Leatherdale, Woo, F.
V. Mikulec, M. G. Bawendi, J. Phys. Chem. B 106, 7619 (2002), which
is incorporated by reference in its entirety. A CCR can be used,
for example, in chemical sensors, nanoscale thin-film
photodetectors, or "Exciton-Polaritons" materials satisfying CCR
criteria.
[0066] Referring to FIGS. 6A-6C, a device including a critically
coupled resonator can include a blue- or ultraviolet-light emitting
structure, the light from which interacts with an absorber or
emitter layer which in turn transmits light at a plurality of
selected wavelengths through a critically coupled resonator. Any
combination of the absorber or emitter layer or the DBR layer can
be patterned.
[0067] Photoluminescence can be increased by a factor of about 6
when a critically coupled resonator structure is optically excited
near .lamda..sub.c. A factor of 3 increase can be attributed to the
critically coupled resonator film, which absorbs 32% when on glass
and nearly 100% on DBR, and a factor of 2 collection efficiency can
be attributed to a collection efficiency boost due to a back DBR
reflector. Using this type of structure, a sensor can be developed
that can be optically pumped with 1/6.sup.th power. See FIG. 7.
[0068] Other embodiments are within the scope of the following
claims.
* * * * *