U.S. patent application number 13/100000 was filed with the patent office on 2011-08-25 for wavelength-specific optical switch.
This patent application is currently assigned to RavenBrick LLC. Invention is credited to Wil McCarthy, Richard M. Powers.
Application Number | 20110205650 13/100000 |
Document ID | / |
Family ID | 40407045 |
Filed Date | 2011-08-25 |
United States Patent
Application |
20110205650 |
Kind Code |
A1 |
Powers; Richard M. ; et
al. |
August 25, 2011 |
Wavelength-Specific Optical Switch
Abstract
A wavelength-specific optical switch combines one or more
tunable filters and bandblock reflectors such that the absorption
or reflection of selected wavelength bands in the optical spectrum
(visible, near infrared, or near ultraviolet) can be switched on
and off. The wavelength switch is programmable, multifunctional,
general-purpose, solid-state optical filter. The wavelength switch
may serve as a tunable notch or bandblock filter, a tunable
bandpass filter, a tunable highpass or lowpass filter, or a tunable
band reflector. The wavelength switch has particular, but not
exclusive, application in optics as a filter, band reflector, and
as a means of isolating particular wavelengths or wavelength bands
from a collimated light stream for transmission to, or rejection
from, a sensor.
Inventors: |
Powers; Richard M.;
(Lakewood, CO) ; McCarthy; Wil; (Lakewood,
CO) |
Assignee: |
RavenBrick LLC
Denver
CO
|
Family ID: |
40407045 |
Appl. No.: |
13/100000 |
Filed: |
May 3, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
12040570 |
Feb 29, 2008 |
7936500 |
|
|
13100000 |
|
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|
60892541 |
Mar 2, 2007 |
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Current U.S.
Class: |
359/889 |
Current CPC
Class: |
G02B 6/262 20130101;
G02F 2203/055 20130101; G02B 1/005 20130101; G02F 1/0147 20130101;
G02F 1/017 20130101; G02F 1/01783 20210101; G02F 1/01791 20210101;
G02B 6/0229 20130101; B82Y 20/00 20130101 |
Class at
Publication: |
359/889 |
International
Class: |
G02B 5/22 20060101
G02B005/22 |
Claims
1. A film for regulating reflection of incident radiant energy
comprising a first optical material layer; and a second optical
material layer; a temperature sensitive optical material positioned
between the first optical layer and the second optical layer.
2. The film of claim 1, wherein at a first temperature a first
percentage of the incident radiant energy is reflected from the
film and a second percentage of the incident radiant energy is
transmitted through the film; and at a second temperature a third
percentage of the incident radiant energy is reflected from the
film and a fourth percentage of the incident radiant energy is
transmitted through the film.
3. The film of claim 24, wherein the first optical material layer
reflects up to 50% of the incident radiant energy and transmits a
majority of non-reflected radiant energy; and the second optical
material layer reflects up to 100% of radiant energy transmitted by
the first optical material layer when the temperature sensitive
optical material is above the threshold temperature and transmits
up to 100% of radiant energy transmitted by the first optical
material layer when the temperature sensitive optical material is
below the threshold temperature.
4. The reflective film of claim 1, wherein the film is in the form
of a thin and flexible film.
5. The film of claim 1 further comprising a transparent substrate
that supports the first optical material layer, the second optical
material layer, and the temperature sensitive optical material.
6. The film of claim 5, wherein the transparent substrate is a
solid substrate.
7. The film of claim 1, wherein a range of wavelengths of radiant
energy regulated by the film comprises one or more of visual,
infrared, or near-infrared wavelengths.
8. The film of claim 1, wherein either or both of the first optical
material layer and the second optical material layer is spectrally
selective.
9. The film of claim 1, wherein either or both of the first optical
material layer and the second optical material layer comprises a
combination of multiple optical material layers.
10. The film of claim 1, where the temperature sensitive optical
material is designed or selected based upon frequency dependent
properties of the temperature sensitive optical material with
respect to incident light to affect one or more of aesthetic,
color, light or energy transmission, absorption, and reflection
properties of the film.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/040,570 filed 29 Feb. 2008 entitled
"Wavelength-specific optical switch", which claims the benefit of
priority pursuant to 35 U.S.C. .sctn.119(e) of U.S. provisional
application No. 60/892,541 filed 2 Mar. 2007 entitled
"Wavelength-specific optical switch," each of which is hereby
incorporated herein by reference in its entirety.
[0002] This application is related to U.S. Pat. No. 6,978,070 and
its divisional application Ser. Nos. 11/081,777 and 11/081,778 (now
U.S. Pat. No.7,276,432), and to U.S. patent application Ser. Nos.
11/144,326, 11/145,417, and 11/676,785, each of which is hereby
incorporated herein by reference. This application is also related
to U.S. provisional patent application Nos. 60/825,385 and
60/825,405, each of which is hereby incorporated herein by
reference.
BACKGROUND
[0003] This technology relates to optical switching devices
incorporating tunable filters and bandblock reflectors. The tunable
filters include devices (typically semiconductor devices) that
produce quantum effects. For the purposes of this document, the
term "optical" refers to visible, ultraviolet (UV), and infrared
(IR) light which obey the normal rules of optics. By this
definition, long-wavelength infrared, microwaves, radio waves,
extreme ultraviolet, x-ray, and gamma radiation are not optical
radiation. Optical filters and switches block light by absorbing or
reflecting certain frequencies while allowing others to pass
through. Short-pass and long-pass filters (specific to wavelength)
or high-pass and low-pass filters (specific to frequency) may be
used, or a narrow range of wavelengths/frequencies can be blocked
by a notch filter or bandblock filter, or transmitted by a bandpass
filter.
[0004] Semiconductors are capable of serving as filters in several
ways. The optical response of a semiconductor is a function of its
bandgap--a material-specific quantity. For photons with energies
below the bandgap, the semiconductor is generally transparent,
although material-specific absorption bands may also exist. Photons
with energies higher than the bandgap are capable of creating
electron-hole pairs within the semiconductor, and are therefore
generally absorbed or reflected. For example, a material like
gallium arsenide (GaAs) (bandgap .about.1.424 eV) is transparent to
infrared photons with a wavelength of 871 nm or greater, and opaque
to visible light, whereas silicon dioxide (SiO.sub.2) (bandgap
.about.9.0 eV) is transparent to visible and near-ultraviolet light
with a wavelength greater than 138 nm. Thus, semiconductor
materials are capable of serving as optical, infrared, or
ultraviolet longpass filters.
[0005] A semiconductor material will also generally show a strong
emission or luminescence peak at this bandgap energy or cutoff
energy, i.e., when stimulated with an electrical current, or with
absorbed photons of higher energy, the semiconductor material will
emit photons at the cutoff energy as a result of electron-hole
recombinations within the material. Photoluminescence (i.e.,
stimulating the material with high-frequency light and measuring
the resulting fluorescence or emission spectrum) is therefore
useful as a diagnostic tool to determine the quantum confinement
energy of a quantum well and thus predict its optical properties.
Strong absorption at and above the cutoff energy is also capable of
generating photoelectric effects within the semiconductor as large
numbers of electron-hole pairs are created.
[0006] The fabrication of very small structures to exploit the
quantum mechanical behavior of charge carriers, e.g., electrons or
electron "holes" is well established. Quantum confinement of a
carrier can be accomplished by a structure whose dimension is less
than the quantum mechanical wavelength of the carrier. Confinement
in a single dimension produces a "quantum well," and confinement in
two dimensions produces a "quantum wire." A "quantum dot" is a
structure capable of confining carriers in all three dimensions.
Some filters also incorporate quantum wells, quantum wires, or
quantum dot particles as dopants (much as leaded crystal
incorporates lead atoms or particles as dopants) to affect the
behavior of the filter. However, the optical properties of such
filters are fixed at the time of manufacture and are neither
multifunctional nor programmable.
[0007] The energy of an electron confined in a quantum well is not
only a function of bandgap, but of the quantum confinement energy,
which depends on the thickness of the well and the energy height of
the surrounding barriers (i.e., the difference in conduction band
energy between the well and barrier materials). This "bandgap plus
quantum confinement" energy moves the transparency of the material
into shorter wavelengths. Thus, while a bulk GaAs sample emits and
absorbs photons at approximately 870 nm, a 10 nm GaAs quantum well
surrounded by Al.sub.0.4Ga.sub.0.6As barriers has a 34 meV quantum
confinement energy and thus shows the equivalent cutoff at
approximately 850 nm. Therefore, for a given set of materials and a
given reference temperature, the cutoff energy can be fixed
precisely through the fabrication of a quantum well of known
thickness. It should be noted, however, that the bandgap is a
temperature-dependent quantity. As the temperature of a
semiconductor decreases, its bandgap increases slightly. When the
semiconductor is heated, the bandgap decreases.
[0008] Quantum dots can be formed as particles, with a dimension in
all three directions of less than the de Broglie wavelength of a
charge carrier. Quantum confinement effects may also be observed in
particles of dimensions less than the electron-hole Bohr diameter,
the carrier inelastic mean free path, and the ionization diameter,
i.e., the diameter at which the quantum confinement energy of the
charge carrier is equal to its thermal-kinetic energy. It is
postulated that the strongest confinement may be observed when all
of these criteria are met simultaneously. Such particles may be
composed of semiconductor materials (for example, Si, GaAs, AlGaAs,
InGaAs, InAlAs, InAs, and other materials) or of metals, and may or
may not possess an insulative coating. Such particles are referred
to in this document as "quantum dot particles."
[0009] A quantum dot can also be formed inside a semiconductor
substrate through electrostatic confinement of the charge carriers.
This is accomplished through the use of microelectronic devices of
various designs, e.g., an enclosed or nearly enclosed gate
electrode formed on top of a quantum well. Here, the term "micro"
means "very small" and usually expresses a dimension of or less
than the order of microns (thousandths of a millimeter). The term
"quantum dot device" refers to any apparatus capable of generating
a quantum dot in this manner. The generic term "quantum dot"
(abbreviated "QD" in certain of the drawings herein) refers to the
confinement region of any quantum dot particle or quantum dot
device.
[0010] The electrical, optical, thermal, magnetic, mechanical, and
chemical properties of a material depend on the structure and
excitation level of the electron clouds surrounding its atoms and
molecules. Doping is the process of embedding precise quantities of
carefully selected impurities in a material in order to alter the
electronic structure of the surrounding atoms for example, by
donating or borrowing electrons from them, and therefore altering
the electrical, optical, thermal, magnetic, mechanical, or chemical
properties of the material. Impurity levels as low as one dopant
atom per billion atoms of substrate can produce measurable
deviations from the expected behavior of a pure crystal, and
deliberate doping to levels as low as one dopant atom per million
atoms of substrate are commonplace in the semiconductor industry,
for example, to alter the conductivity of a semiconductor.
[0011] Quantum dots can have a greatly modified electronic
structure from the corresponding bulk material, and therefore
different properties. Quantum dots can also serve as dopants inside
other materials. Because of their unique properties, quantum dots
are used in a variety of electronic, optical, and electro-optical
devices. Quantum dots are currently used as near-monochromatic
fluorescent light sources, laser light sources, light detectors
including infra-red detectors, and highly miniaturized transistors,
including single-electron transistors. They can also serve as a
useful laboratory for exploring the quantum mechanical behavior of
confined carriers. Many researchers are exploring the use of
quantum dots in artificial materials, and as dopants to affect the
optical and electrical properties of semiconductor materials.
[0012] The embedding of metal and semiconductor nanoparticles
inside bulk materials (e.g., the lead particles in leaded crystal)
has occurred for centuries. However, an understanding of the
physics of these materials has only been achieved comparatively
recently. These nanoparticles are quantum dots with characteristics
determined by their size and composition. These nanoparticles serve
as dopants for the material in which they are embedded to alter
selected optical or electrical properties. The "artificial atoms"
represented by these quantum dots have properties which differ in
useful ways from those of natural atoms. However, it must be noted
that the doping characteristics of these quantum dots are fixed at
the time of manufacture and cannot be adjusted thereafter.
[0013] Tunable filters rely on various mechanical principles such
as the piezoelectric squashing of a crystal or the rotation or
deformation of a lens, prism, or mirror, in order to affect the
filter's optical properties. Most notable of these is the
Fabry-Perot interferometer, also known as an "etalon." Like any
mechanical device, such tunable filters are much more vulnerable to
shock, vibration, and other related failure modes than any
comparable solid-state device.
[0014] The addition of a mechanical shutter can turn an otherwise
transparent material--including a filter--into an optical switch.
When the shutter is open, light passes through easily. When the
shutter is closed, no light passes. If the mechanical shutter is
replaced with an electrodarkening material such as a liquid
crystal, then the switch is "nearly solid state", with no moving
parts except photons, electrons, and the liquid crystal molecules
themselves. This principle is used, for example, in liquid crystal
displays (LCDs), where the white light from a backdrop is passed
through colored filters and then selectively passed through or
blocked by liquid crystal materials controlled by a transistor. The
result is a two-dimensional array of colored lights which form the
pixels of a television or computer display.
[0015] A single-electron transistor (SET) is a type of switch that
relies on quantum confinement. The SET comprises a source (input)
path leading to a quantum dot particle or quantum dot device, and a
drain (output) path exiting, with a gate electrode controlling the
dot. With the passage of one electron through the gate path into
the device, the switch converts from a conducting or closed state
to a nonconducting or open state, or vice-versa. However, these
devices are not designed to control the flow of optical energy
(i.e., light).
[0016] Band reflectors may be constructed by a variety of different
means. In general, a band reflector is a filter that consists of
transparent materials of different indices of refraction or
different dielectric constants, such that certain frequencies or
frequency bands of light are strongly interfered with (reflected)
while other frequencies pass through with minimal reflection or
attenuation. Thus, a band reflector is highly transparent across a
broad range of frequencies, and highly reflective within a narrow
band of frequencies. Band reflectors are used, for example, as
cavity mirrors in certain types of lasers.
[0017] Each of these optical filters, switches, and combinations
described above are not programmable or multifunctional. That is,
they always pass or block the exact same wavelengths/frequencies of
light, which are determined at the time of manufacture and cannot
be altered thereafter.
[0018] Thermochromic materials change their color (i.e., their
absorption and reflection spectrum) in response to temperature.
Liquid crystal thermometers and liquid crystal tunable filters
(LCTFs) are based on this principle. Thermochromic plastics are
sometimes incorporated into baby bathtubs, bottles, or drinking
cups as a visual indicator of liquids that may be too hot or too
cold for safety or comfort. Thermochromic paints are sometimes used
to help regulate the temperature of objects or buildings under
heavy sunlight.
[0019] The information included in this Background section of the
specification, including any references cited herein and any
description or discussion thereof, is included for technical
reference purposes only and is not to be regarded as subject matter
by which the scope of the invention is to be bound.
SUMMARY
[0020] This technology is directed to a programmable,
multifunctional optical switch--hereinafter a "wavelength
switch"--incorporating semiconductor materials as tunable filters
and bandblock reflectors. Combinations of tunable filters and
bandblock reflectors in optical devices as described herein produce
the wavelength switch, which has particular, but not exclusive,
application in optics as a general purpose tunable filter, a
general purpose tunable band reflector, and as a means of singling
out particular wavelengths or wavelength bands for transmission to,
or rejection from, a sensor. Quantum-confined carriers serve as
dopants within the surrounding semiconductor material and the
functionality of the wavelength switch arises as a consequence of
the resulting changes in the optical filtering properties of the
semiconductor material. The specific optical functions described
herein should not be construed as limiting in scope, but rather as
explanatory examples to convey the nature and capabilities of the
wavelength switch, which is both multifunctional and programmable
and can therefore be used for a multiplicity of operations. This is
analogous to a digital computer, whose nature can be fully
understood without an exhaustive list of the calculations it can
perform.
[0021] For the purposes of this document, the term "switch"
includes both solid-state and mechanical optical devices for
selectively blocking or permitting the flow of energy, and includes
both digital switches (e.g., transistors and relays) and analog
switches (e.g., tubes and rheostats). Furthermore, a valve for
selectively blocking or regulating the flow of gases or fluids can
be considered analogous to a switch so that, in principle, the two
terms could be used interchangeably. It is also a feature of most
switch types that they can be run in reverse. In other words, while
a particular pathway may be identified as the source or input path,
and another as the drain or output path, there is not generally any
physical or operational barrier to reversing the roles of these two
paths, so that energy flows through the device in the opposite
direction.
[0022] In one implementation, the tunable filter may be a
solid-state, electrically or thermally tunable, quantum confinement
device composed of semiconductor materials. The tunable filter may
include a semiconductor quantum confinement layer (e.g., a quantum
well, an arrangement or layer of semiconductor quantum dot
particles, or an arrangement or layer of quantum wires) surrounded
by barrier materials, whether semiconducting, semi-insulating, or
insulating. The effective bandgap of the quantum well may then be
varied over an optical bandwidth by one of several methods, for
example, the application of a uniform electric field, the use of a
heater and/or thermoelectric cooler to alter the temperature of the
quantum well, or the application of a nonuniform (e.g.,
two-dimensional periodic) electric field to section the quantum
well into quantum dots. These variations in the effective bandgap
of the quantum well alter the optical properties of the quantum
well material, including the optical bandwidth, in predictable
ways, yielding a tunable optical filter. However, other types of
tunable filters could be used as well, including but not limited to
etalon filters, liquid crystal tunable filters, thermochromic dye
filters, or thermochromic semiconductor filters that do not rely on
quantum confinement for their operation. The basic functioning of
the wavelength switch is not affected by the exact form or
operating principles of the tunable filter.
[0023] In one implementation, the band reflector may be a
dielectric mirror, similar to the type employed in certain types of
lasers. In its simplest form, the dielectric mirror may be an
optical heterojunction, i.e., a film of transparent material with a
particular dielectric constant deposited on top of a transparent
substrate with a different dielectric constant, such that
particular frequencies of light encounter strong interference while
propagating through the interface, and thus are reflected. The
range of wavelengths (or frequencies) reflected by the band
reflector is known as the reflection band. In essence, this is the
exact opposite of an antireflection coating. However, in the more
general case a dielectric mirror may consist of multiple layers on
top of a substrate. Numerous other forms of band reflectors exist,
including Bragg mirrors, photonic crystals, and nano- or
micro-patterned gratings. In fact, many standard reflective
materials, for example, silvered and aluminized glass and chromed
metal, are band reflectors. However, in many cases the reflection
bands of such materials include the entire visible spectrum, as
well as portions of the infrared and ultraviolet spectrum, and in
general there are also absorption bands or reflection bands outside
the desired reflection band. In other words, these materials may
not be not transparent at all wavelengths (or in some cases, any
wavelengths) outside the reflection band. In any case, the design
of band reflectors is well understood in the prior art, and needs
no further elaboration here.
[0024] The function of the band reflector (or "mirror") is to pass
light with minimal attenuation, except within a particular
frequency range where the light is strongly reflected. For example,
a band reflector may be designed to reflect blue light (i.e., light
between the wavelengths of 424 and 491 nm) while remaining
transparent to the rest of the visible spectrum. Other band
reflectors may be designed to reflect infrared or ultraviolet
light, or to reflect the entire visible spectrum while remaining
transparent to near infrared and near ultraviolet wavelengths.
[0025] With the wavelength switch, light (e.g., white light) passes
through the tunable filter and then strikes the band reflector.
When the tunable filter is adjusted such that it strongly
attenuates all the light in the reflection band of a dielectric
mirror reflector, no reflection occurs at the mirror and thus all
light that strikes it passes through. Thus, the wavelength switch
(i.e., the device comprising the tunable filter and the band
reflector) is not observed to reflect any significant portion of
the light that strikes it. However, when the tunable filter is
adjusted to transmit light within the reflection band of the band
reflector, that portion of the light is reflected by the band
reflector and passes back through the tunable filter. Thus, the
wavelength switch is observed to reflect those particular
wavelengths, while transmitting all others.
[0026] The net effect is of a tunable optical band reflector or
notch filter. For example, a tunable filter capable of transmitting
or attenuating blue light, coupled with a band reflector designed
to reflect blue light, forms a solid-state wavelength switch that
either does or does not reflect the blue light incident upon it,
based on the state of the tunable filter. This is useful, for
example, in astronomy, to filter out unwanted wavelengths such as
those produced by streetlights. Alternatively, it can be employed
in protective optics for spacecraft sensors (e.g., star sensors),
to guard them against damage from laser light.
[0027] In some embodiments, only one tunable filter/band reflector
pair is employed in an optical device. In other embodiments,
multiple filter/band reflector pairs, operating on different parts
of the spectrum, may be arranged in a single optical device such
that the spectral range of the total wavelength switch is divided
up into bands, any one of which can be transmitted or reflected
back, on demand, through appropriate adjustment of the tunable
filters. This was previously achieved only through mechanical
means, e.g., by placing rotating mirrors in the path of a prism.
The present technology achieves the same effect in the solid state,
with no moving parts. In addition, the wavelength switch allows
selected frequencies to be separated from a stream of light without
disrupting the collimation of the light. For example, it is
possible to remove a single color of light from an image before it
reaches an imaging sensor, without affecting the clarity of the
image in other wavelengths. Finally, in some implementations optics
may not be aligned on a single axis so that, for example, light of
different colors can be separated out and directed to different
sensors. Again, this is accomplished in the solid state, without
moving parts.
[0028] The structure, composition, manufacture, and function of
quantum dot particles generally are taught in U.S. Patent
Application Publication No. 2003/0066998 by Lee et al., which is
hereby incorporated by reference as though fully set forth herein.
The structure, composition, manufacture, and function of exemplary
quantum dot devices are taught in U.S. Pat. No. 5,889,288 to
Futatsugi, which is hereby incorporated by reference as though
fully set forth herein. The structure, composition and manufacture
of addressable quantum dot arrays are taught in U.S. Pat. No.
6,978,070 to McCarthy et al. The wavelength switch reorganizes
these principles and devices into a device for removing select
wavelength bands from a stream of light, either to single them out
for sensing or analysis or to prevent them from reaching a sensor,
while allowing other wavelengths to pass normally. The quantum
confinement layers, particles, wires, devices, or arrays employed
by the wavelength switch may be of different design than those
described by Lee et al., Futatsugi, and McCarthy et al., but the
operating principles are essentially the same.
[0029] Other features, details, utilities, and advantages of the
present invention will be apparent from the following more
particular written description of various embodiments of the
invention as further illustrated in the accompanying drawings and
defined in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a schematic illustration of the natural optical
filtering effects of a block of semiconductor material, which is
generally transparent to photons at energies lower than the bandgap
of the semiconductor, and opaque to photons at energies higher than
the bandgap.
[0031] FIG. 2 is an illustration of the optical filtering effects
of a semiconductor quantum well, which differ from those of a
simple block of semiconductor.
[0032] FIG. 3 is an illustration of the optical filtering effects
of a layer of semiconductor quantum dots or quantum wires.
[0033] FIGS. 4A and 4B are schematic representations of one
embodiment of a wavelength switch including a tunable filter and a
band reflector.
[0034] FIG. 5 is a schematic representation of a second embodiment
of a wavelength switch including multiple tunable filter and band
reflector pairs.
[0035] FIGS. 6A and 6B are schematic representations of one
embodiment of a macroscopic, tunable, solid-state optical filter
for use in the wavelength switch of FIGS. 4A and 4B comprising a
multilayered microscopic fiber that includes a quantum well,
surface electrodes, and control wires, which form quantum dot
devices.
[0036] FIG. 7 is a schematic representation of another embodiment
of a of a tunable, solid-state optical filter for use in the
wavelength switch of FIGS. 4A and 4B comprising a quantum well to
confine charge carriers in a two-dimensional layer, and an
electrode to create an electric field across the quantum well to
alter its quantum confinement properties via the Stark effect.
[0037] FIG. 8 is a schematic representation of a further embodiment
of a tunable, solid-state optical filter for use in the wavelength
switch of FIGS. 4A and 4B comprising a thermochromic quantum well
surrounded by barrier layers, as well as a solid-state heater,
thermoelectric cooler, and thermostat for controlling the
temperature of the filter.
[0038] FIG. 9 is a schematic representation of yet another
embodiment of a of a tunable, solid-state optical filter for use in
the wavelength switch of FIGS. 4A and 4B, which uses a nonuniform
electric field to divide a quantum well into an array of quantum
dots by means of a charged, grid-shaped electrode.
[0039] FIG. 10 illustrates an exemplary application for the
wavelength switch of FIGS. 4A and 4B in which the wavelength switch
is used to protect a star sensor against laser light.
DETAILED DESCRIPTION
[0040] The present invention is directed to the use of tunable
optical filters and band reflectors to produce a
wavelength-specific optical switching device, or "wavelength
switch"--a device that is capable of switchably reflecting
particular wavelength bands within the optical (visible, near
infrared, and near ultraviolet) spectrum. A band reflector
naturally reflects light at all wavelengths that occur within its
wavelength reflection band. However, a tunable filter is used to
control which wavelengths of light are allowed to reach the band
reflector and thus which wavelengths are reflected.
[0041] FIG. 1 is an illustration of the natural optical filtering
effects of a block of semiconductor material 100. The block is
generally transparent to photons at energies lower than the bandgap
of the semiconductor material and opaque to photons at energies
higher than the bandgap due to the absorption of the higher energy
photons by electrons in the semiconductor material to excite the
electrons across the bandgap. Thus, the material serves as a
natural longpass filter. However, the bandgap is a
temperature-sensitive quantity, so that the exact cutoff wavelength
of the longpass filter changes as its temperature is varied, as
further described below with respect to the tunable optical filter
of FIG. 8.
[0042] FIG. 2 illustrates the same principle for a semiconductor
200 defining a quantum well. In this case, a quantum well layer 202
is surrounded by barrier layers 201 of a higher conduction energy,
such that charge carriers are preferentially drawn into, and
confined within, the quantum well layer 202 in the semiconductor
200. The effective bandgap of the quantum well layer 202 is equal
to the bandgap of the material forming the quantum well layer 202,
plus the charge carrier quantum confinement energy, which is a
function of the thickness and composition of the quantum well layer
202, and of the energy "height" of the surrounding barrier layers
201. This semiconductor 200 also acts as a longpass filter.
However, since the effective bandgap of a quantum well layer 202 is
higher than that of an ordinary semiconductor, the quantum well 202
is transparent to photons of higher energy, and thus allows more
wavelengths to pass through. Since the barrier layers 201 have an
even higher conduction energy than the quantum well layer 202, they
are transparent to still more wavelengths. Thus, filtering occurs
primarily in the quantum well layer 202 rather than in the barrier
layers 201, i.e., any wavelength capable of passing through the
quantum well layer 202 is also, by definition, capable of passing
through the barrier layers 201, whereas not all wavelengths that
pass through the barrier layers 201 will also pass through the
quantum well layer 202. Thus, in this case, the quantum well 202
itself is the filter while the barrier layers 201 serve as a
transparent substrate for the quantum well 202.
[0043] FIG. 3 illustrates the optical filtering properties of a
quantum confinement layer 302 in a semiconductor filter 300
composed of quantum dot particles, quantum wires, or
electrostatically confined quantum dots 303. Because the quantum
confinement energy in the quantum confinement layer 302 is higher
than for a simple quantum well, even more wavelengths are passed
through the filter 300 in this case. However, as in FIG. 2, since
the conduction energy of the barrier layers 301 is still higher
than that of the quantum confinement layer 302, filtering occurs at
the quantum confinement layer 302 and not in the barrier layers
301.
[0044] If the quantum confinement layer 302 (whether a quantum well
or a layer or arrangement of quantum dot particles or quantum
wires) is subjected to a uniform electric field (as described with
respect to FIG. 7), or is subjected to controlled variations in
temperature (as described with respect to FIG. 8), or is subjected
to nonuniform electric fields (as described with respect to FIGS.
4A, 4B, and 9), then the bandgap and quantum confinement energy can
be altered such that greater or fewer wavelengths of light are
transmitted by the confinement layer 302. Thus, the quantum
confinement layer 302 becomes capable of serving as a tunable
longpass filter with an exact cutoff wavelength controlled within a
range of possible cutoff wavelengths by external control
signals.
[0045] FIGS. 4A and 4B are schematic representations of a
wavelength switch 400 in which a tunable filter 403 is formed by a
quantum confinement layer 402 located on or within transparent
substrates or barrier layers 401, and the whole arrangement is
attached to, or placed adjacent to, a band reflector 404 which has
been designed such that its reflection band covers the same range
of wavelengths as the range of possible cutoff wavelengths for the
quantum confinement layer 402. Thus, when the tunable filter 403 is
in its ground state (see FIG. 4A), it blocks all wavelengths within
the reflection band of the band reflector 404 from reaching the
band reflector 404. As a result, the wavelength switch 400
transmits light within a particular range of wavelengths, and
reflects none.
[0046] However, when the tunable filter 403 is in an excited state
(see FIG. 4B), the cutoff wavelength is higher and thus the tunable
filter 403 begins to transmit light within the reflection band of
the band reflector 404. In this case, the wavelength switch 400
transmits light within one range of wavelengths and reflects light
within another (generally narrower) range. The wavelength switch
400 may be configured such that, for example, in the ground state
the wavelength switch 400 transmits red, orange, and yellow light,
while blocking green, blue, and violet light, and in the excited
state the wavelength switch 400 transmits red, orange, and yellow
light, reflects green light, and blocks blue and violet light.
Thus, through the application of external control signals, the
reflection of green light can be switched on and off. The optical
bandwidth of the tunable filter 403 is thus the range between the
bandgap in the ground state and in the excited state.
[0047] FIG. 5 illustrates a more complex embodiment of a wavelength
switch 500, wherein multiple tunable filters 503, 503', 503'' and
band reflector pairs 504, 504', 504'' have been employed to extend
the tunable range of the wavelength switch 500. From left to right,
each tunable filter 503, 503', 503'' has been designed to have a
tunable range at longer wavelengths and thus lower energies and
frequencies than the one before it. For example, the tunable range
for the filter 503 may be more toward the blue/UV end of the
spectrum than the range of filter 503', which occurs more toward
the blue than the range of filter 503''. In the preferred
embodiment the edges of these tunable ranges match up, so that the
tunable filters 503, 503, 503'' collectively cover a broad,
continuous region of the optical spectrum, although other
embodiments exist in which this is not the case.
[0048] Similarly, the band reflectors 504, 504', and 504'' are
arranged from left to right such that each has a reflection band
that occurs more toward the blue/UV end of the spectrum than its
neighbor to the right, and each is transparent to optical
wavelengths outside that reflection band. In one embodiment the
edges of these reflection bands are contiguous, so that the band
reflectors 504, 504', and 504'' collectively cover a broad,
continuous region of the optical spectrum, although other
embodiments exist in which this is not the case.
[0049] In the exemplary configuration shown in FIG. 5, the first
tunable filter 503 has a tunable range covering the blue portion of
the spectrum. It is shown in its most excited state (i.e.,
filtration of selected wavelengths is "off"), and is transmitting
blue, blue-green, green, yellow, orange, red, and infrared light
while blocking indigo, violet and ultraviolet. The first band
reflector 504 has been designed to reflect blue light while
transmitting ultraviolet, violet, green, yellow, orange, red, and
infrared light. However, indigo, violet and ultraviolet light are
blocked by the tunable filter 503 and do not reach the band
reflector 504 to be passed through.
[0050] The second tunable filter 503' in FIG. 5 has a tunable range
covering the blue-green band of the optical spectrum. The second
tunable filter 503' is shown in its ground state (i.e., filtration
of selected wavelengths is turned "on"), and is transmitting green,
yellow, orange, red, and infrared light while blocking blue-green,
blue, indigo, violet, and ultraviolet. However, blue, indigo,
violet, or ultraviolet light do not reach the filter 503' because
they were previously blocked by the first tunable filter 503. The
second band reflector 504' may be designed to reflect blue-green
light and to transmit all other wavelengths. However, in this
example no blue-green light reaches the second band reflector 504'
because it is blocked at the second tunable filter 503', and no
blue, indigo, violet, or ultraviolet light reach the reflector
because they were previously blocked by the first tunable filter
503. Thus, the band reflector 504' reflects no light and transmits
yellow, orange, red, and infrared light.
[0051] The third tunable filter 503'' may have a tunable range
covering the green portion of the spectrum. It is in its most
excited state (i.e., filtration of selected wavelengths is turned
"off") the third tunable filter 503'' may transmit green, yellow,
orange, red, and infrared light while blocking blue-green, blue,
indigo, violet, and ultraviolet light. However, no blue-green,
blue, indigo, violet, or ultraviolet light reaches the filter 503''
because they were blocked by the first and second tunable filters
503, 503'. The third band reflector 504'' may be designed to
reflect green light and transmit all other wavelengths. However, in
this example no wavelengths shorter than green will reach the third
band reflector 504'' because they have been blocked by the first,
second, and third tunable filters 503, 503', 503''. Thus, the third
band reflector 504'' reflects the green light, while transmitting
yellow, orange, red, and infrared.
[0052] The net behavior of this exemplary implementation of a
wavelength switch 500 is to reflect the blue and green light back
toward the source, while absorbing blue-green light. In this
exemplary embodiment, the wavelength switch 500 comprising all
three tunable filter 503, 503', 503'' and band reflector 504, 504',
504'' pairs will always transmit yellow, orange, red, and infrared
light, and will always block (i.e., absorb) indigo, violet, and
ultraviolet light. However, depending on the states of tunable
filters 503, 503' and 503'', the wavelength switch 500 can be
configured to reflect any or all of the wavelength bands comprising
blue, blue-green, and green. Through careful selection of the
ranges of the tunable filters 503, 503' and 503'' and the
reflection bands of the band reflectors 504, 504' and 504'', the
wavelength switch 500 can be designed to reflect other wavelengths
upon switch control. In fact, a sufficiently large stack of tunable
filter 503, 503' and 503'' and band reflector 504, 504' and 504''
pairs may be configured to reflect any band in the optical spectrum
upon switch control.
[0053] FIGS. 6A and 6B are schematic drawings of a macroscopic,
tunable, solid-state optical filter for use in the wavelength
switch of FIGS. 4A and 4B in the form of a multilayered microscopic
fiber 600. The fiber 600 includes a quantum well and surface
electrodes 608, which form quantum dot devices, and control wires
604 to carry electrical control signals to the electrodes. A
plurality of these fibers may act as the macroscopic, tunable,
solid-state optical filter. The control wires 604 may be contained
in an insulating medium 605, surrounded by a quantum well, plus an
optional memory layer 603. In one embodiment, the composition of
the insulator 605 is a semiconductor oxide, although a variety of
other materials could be used. The quantum well may be formed in a
central or transport layer 602 of a semiconductor (similar to the
negative layer of a P-N-P junction), for example, GaAs, surrounded
by barrier or supply layers 601 of a semiconductor with higher
conduction energy (similar to the positive layers of a P-N-P
junction). Because of the difference in conduction energies,
electrons "fall" preferentially into the lower energy of the
transport layer 602, where they are free to travel horizontally
(that is, within the layer) but are confined vertically
(perpendicular to the layer) by the higher conduction energy of the
barrier layers 601. However, the fiber 600 is not limited to this
particular configuration, and may include quantum wells made from
other materials and with other designs, as well as quantum wells
designed to trap "holes" or other charge carriers.
[0054] The transport layer 602 of the quantum well must be smaller
in thickness than the de Broglie wavelength of the charge carriers
for the charge carriers to be confined within it. For an electron
at room temperature, this would be approximately 20 nm. Thicker
quantum wells are possible, although they will only exhibit quantum
confinement of the charge carriers at temperatures colder than room
temperature. Thinner quantum wells will operate at room
temperature, and at higher temperatures so long as the de Broglie
wavelength of the carriers does not exceed the thickness of the
transport layer 602.
[0055] The surface of the fiber 600 may include conductors that
serve as the electrodes 608 of the quantum dot device. These
electrodes 608 confine charge carriers in the quantum well into a
small space or quantum dot (QD) when a reverse-bias voltage is
applied, since the negative charge on the electrodes 608 repels
electrons, preventing their horizontal escape through the transport
layer 602. The electrodes 608 may be powered by control wire
branches 606 reaching to the surface of the fiber 600 from the
control wires 604 in the center of the fiber 600. In one
embodiment, the electrodes 608, control wires 604, and control wire
branches 606 may be made of gold, although in principle they could
be made of other metals, or other materials, such as semiconductors
or superconductors.
[0056] Once the charge carriers are trapped in a quantum dot (QD),
they form an artificial atom that is capable of serving as a
dopant. Increasing the voltage on the electrodes 608 by a specific
amount forces a specific number of additional charge carriers into
the quantum dot (QD), altering the atomic number of the artificial
atom trapped inside. Conversely, decreasing the voltage by a
specific amount allows a specific number of carriers to escape to
regions of the transport layer 602 outside the quantum dot (QD). In
the embodiment of FIG. 6A, six electrodes 608 are provided for each
quantum dot (QD), although more or less could be used. By selecting
the voltages applied to these electrodes 608 it is possible to
alter the repulsive electric field, thus affecting size and shape
of the quantum dot (QD) confinement region. Changes to the
confinement region similarly alter the size and shape of the
artificial atom trapped inside the quantum dot (QD), either in
conjunction with changes to the "atomic number" of the artificial
atom or while holding the atomic number constant. Thus, the doping
properties of the artificial atom are adjusted in real time through
variations in the signal voltage of the control wires 604 at the
center of the fiber 600.
[0057] There are various possibilities for making the multilayered
microscopic fiber 600 of different materials, and in different
configurations. The most advantageous configurations are the
smallest, since smaller quantum dots can contain charge carriers at
higher energies (shorter de Broglie wavelengths) and thus display
atom-like behavior at higher temperatures. One exemplary fiber 600
would be similar in design to a single-electron transistor,
although molecules the size of benzene rings or smaller, if
employed as quantum dot particles, will be unable to hold large
numbers of excess charge carriers. This limits their usefulness in
generating artificial atoms. A somewhat larger but more practical
design is to employ electrically conductive nanotubes, such as
carbon nanotubes, as the control wire segments 604, and
fullerene-type molecules, such as carbon fullerenes, as the quantum
dot devices.
[0058] FIG. 7 illustrates tunable, solid-state optical filter for
use in the wavelength switch of FIGS. 4A and 4B in the form of a
quantum confinement device 700 that relies on the quantum-confined
Stark effect. The device 700 comprises an upper barrier layer 704,
a lower barrier layer 710, a transport layer 702, a surface
electrode 714 connected with a control path 718 for control by a
control unit 720, and a ground plane 709. Electrons or other
carriers are confined in the vertical dimension by the barrier
layers 704 and 710 of the quantum well, producing quantum
confinement carrier behavior in that dimension and thus altering
the effective bandgap. When the control path 718 is activated by an
external voltage source 716 within the control unit 720, the ground
plane 709 then drains to the negative side of the voltage source
through the control return path 712. The resulting potential across
the quantum well affects the quantum confinement energy of the
trapped carriers, via the quantum Stark effect. This affects the
optical properties of the transport layer 702, particularly in the
vertical direction, and thus allows the transport layer 702 to
serve as a tunable optical filter. Two possible paths 707, 707' are
shown for incoming light, along with two possible output paths 708,
708' for filtered light along respective axes.
[0059] FIG. 8 is a schematic representation of a tunable,
solid-state optical filter for use in the wavelength switch of
FIGS. 4A and 4B in the form of a thermochromic filter 800, for
example, incorporating a thermochromic quantum well as a longpass
filter along with a temperature-regulating unit 809 that controls
the temperature of the quantum well and therefore the bandgap. The
thermochromic filter 800 includes barrier layers 801 and 803
surrounding a well layer 802 with a transparent substrate layer 804
providing structural support. In addition, attached to the
thermochromic filter 800 are a heating device 805, a temperature
sensor 806, and a cooling device 807.
[0060] The heating device 805, cooling device 807, and temperature
sensor 806 are connected by wires 808 to a temperature-regulating
unit 809, which reads the temperature of the quantum well and
adjusts the output of the heating device 805 or cooling device 807
appropriately in order to keep the filter 800 at a particular
desired temperature, and thus a particular cutoff wavelength. In
one embodiment, the temperature-regulating unit 809 may be a
solid-state thermostat or thermal control circuit.
[0061] FIG. 9 is a schematic representation of a tunable,
solid-state optical filter for use in the wavelength switch of
FIGS. 4A and 4B in the form of an arbitrary number of quantum dots
912 in a layered composite film 900. The film 900 is composed of an
insulating layer 910, a transport layer 904, and a barrier layer
906. The transport layer 904 and the barrier layer 906 together
form a heterojunction 902. Two independent voltages 920, 926 are
controlled by a control unit 930 and four control wires 916, 918,
922, 924 connected thereto produce potentials across the entire
film 900 and the heterojunction 902, respectively. A metal film on
top of the insulating layer 910 may be fashioned into a grid
electrode 914 with multiple openings 928. If the openings 928 are
smaller than or comparable to the de Broglie wavelength of the
confined carriers, then quantum confinement effects will be
observed when the heterojunction 902 and the surface electrode 914
are charged. Specifically, one quantum dot 912 is formed in the gas
layer 908 between the transport layer 904 and the barrier layer 906
beneath each opening 928 in the grid electrode 914. Thus, a
plurality of artificial atoms are created in the layered composite
film 900 corresponding to each opening 928 in the grid electrode
914. Because this alters the effective bandgap of the material,
this device is once again capable of serving as a solid-state,
tunable optical longpass filter.
[0062] While solid-state quantum confinement devices offer a number
of advantages over other types of tunable filters, the wavelength
switch may also be configured to incorporate any other sort of
tunable filter, without altering the basic function of switching
reflection on and off for particular wavelengths of light.
[0063] FIG. 10 illustrates an exemplary implementation, wherein the
wavelength switch 1002 is placed in the optical path of a satellite
star sensor 1004 in order to protect the sensor from incoming laser
light 1010. The wavelength switch 1002 may be positioned between
the external optics 1006 and the star sensor 1004 by a
half-silvered mirror 1008 (transmission .about.50%, reflection
.about.50%). The star sensor 1004 may be protected by switching on
a reflection band in the wavelength switch 1002 for the wavelength
of the laser light 1010. The wavelength switch 1002 is actually a
stack of several pairs of tunable filters and band reflectors, each
pair acting over a different range of wavelengths, similar to the
configuration of tunable filters and band reflectors presented in
FIG. 5. The shortest wavelengths are reflected by the top tunable
filter/band reflector pair, and the longest wavelengths are
reflected by the bottom tunable filter/band reflector pair. The
particular wavelength of the laser may be notched out by a middle
tunable filter/band reflector pair, wherein the band reflector has
been adjusted to leave a window across the laser wavelength.
Numerous other uses exist for the wavelength switch 1002, and this
example should in no way be construed as limiting the scope of
possible applications.
[0064] From the description above, the wavelength-specific optical
switching device, or wavelength switch, may be understood to
provide a number of capabilities which were not previously
possible. First, the wavelength switch provides a solid-state means
of removing or isolating certain wavelengths from a stream of
incoming light, thus serving as a tunable optical notch filter,
bandblock filter, bandpass filter, longpass filter, shortpass
filter, or band reflector. Second, the wavelength switch may
provide a means of removing, attenuating, or manipulating
individual wavelength bands from a collimated stream of light
(e.g., an image) without destroying the collimation of either the
original stream or the separated wavelengths.
[0065] Also from the above description, several advantages of the
wavelength switch become evident. The wavelength switch provides a
solid-state, tunable filter that is capable of acting in both a
transmissive and a reflective mode. In addition, the wavelength
switch is useful in protective optics, e.g., to prevent laser light
from reaching a human eye or other delicate sensors. The wavelength
switch also offers a solid-state, purely optical means of
separating a light stream into multiple, independent streams (e.g.,
red, green, and blue images extracted from a full-color scene),
without the need for digital or analog signal processing. The
wavelength switch may also be useful in remote sensing, e.g., as a
way of breaking a scene into separate images for each wavelength
band in order to enhance subtle details that are not apparent in a
full-spectrum image.
[0066] The wavelength switch may also be combined with other
optical components (including lenses, mirrors, half-mirrors, light
sources, lasers, films, and gratings) to produce a wide variety of
switchable optical effects. Such effects may include, but are not
limited to, band-switchable amplification, attenuation,
transmission, diversion, rotation, acceleration, shifting,
reflection, absorption, delay, echo or repetition, inversion,
limiting or clipping, distortion, purification or filtering,
regulation, reshaping, reallocation, oscillation, identification or
characterization, and storage of optical signals.
[0067] The wavelength switch can be combined with other optical
components to produce desired optical effects that either were not
previously possible, could not be done in the solid state, or could
not be done as conveniently. The wavelength switch can be used as a
multifunctional, programmable, general-purpose, solid-state optical
filter and band reflector that combines in a single device the
capabilities of a wide variety of static optical components
available on demand. In other words, the wavelength switch becomes
an important new component in the tool kit of optical design
engineers.
[0068] Although the description above contains many specificities,
these should not be construed as limiting the scope of the
invention but rather construed as merely providing illustrations of
certain exemplary embodiments of this invention. There are various
possibilities for making the wavelength switch of different
materials, and in different configurations. A number of optional
components may also be added, including air gaps or vacuum gaps,
transparent substrates or spacer materials, adhesives, mounting
brackets, antireflection coatings, lenses, gratings, polarizers,
and static (i.e., non-tunable) optical filters or reflectors.
Numerous other variations exist which do not affect the core
principles of the operation of the wavelength switch. For example,
the band reflectors may be oriented at an angle to the incoming
light stream, or may be mounted such that they can be mechanically
reoriented, or may be fitted with adjustable gratings or other
components such that they behave optically as though they were
rotated.
[0069] Although various embodiments of this invention have been
described above with a certain degree of particularity, or with
reference to one or more individual embodiments, those skilled in
the art could make numerous alterations to the disclosed
embodiments without departing from the spirit or scope of this
invention. All directional references e.g., proximal, distal,
upper, lower, upward, downward, left, right, lateral, front, back,
top, bottom, above, below, vertical, horizontal, clockwise, and
counterclockwise are only used for identification purposes to aid
the reader's understanding of the present invention, and do not
create limitations, particularly as to the position, orientation,
or use of the invention. Connection references, e.g., attached,
coupled, connected, and joined are to be construed broadly and may
include intermediate members between a collection of elements and
relative movement between elements unless otherwise indicated. As
such, connection references do not necessarily imply that two
elements are directly connected and in fixed relation to each
other. It is intended that all matter contained in the above
description or shown in the accompanying drawings shall be
interpreted as illustrative only and not limiting. Changes in
detail or structure may be made without departing from the basic
elements of the invention as defined in the following claims.
* * * * *