U.S. patent application number 09/918398 was filed with the patent office on 2004-10-28 for configurable photonic device.
Invention is credited to Matsuura, Naomi, Ruda, Harry E., Yacobi, Ben G..
Application Number | 20040213534 09/918398 |
Document ID | / |
Family ID | 22832397 |
Filed Date | 2004-10-28 |
United States Patent
Application |
20040213534 |
Kind Code |
A9 |
Matsuura, Naomi ; et
al. |
October 28, 2004 |
Configurable photonic device
Abstract
A photonic crystal, and a photonic device having such a photonic
crystal, configured by changing its physical geometry in at feast
one region to alter light propagation and/or confinement, The
configuring means may include electrostrictive, piezoelectric or
magnetostrictive components of the photonic crystal, or an
actuation device affixed to the photonic crystal.
Inventors: |
Matsuura, Naomi; (Toronto,
CA) ; Ruda, Harry E.; (Toronto, CA) ; Yacobi,
Ben G.; (Mississauga, CA) |
Correspondence
Address: |
Robert B. Storey
Bereskin & Parr
Box 401
40 King Street West
Toronto
ON
M5H 3Y2
CA
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 0026570 A1 |
February 6, 2003 |
|
|
Family ID: |
22832397 |
Appl. No.: |
09/918398 |
Filed: |
July 31, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60222481 |
Jul 31, 2000 |
|
|
|
Current U.S.
Class: |
385/129 ; 385/15;
385/39 |
Current CPC
Class: |
G02B 6/3572 20130101;
G02B 6/3578 20130101; G02B 6/43 20130101; G02F 2202/32 20130101;
G02B 6/357 20130101; G02B 6/1225 20130101; G02B 6/3556 20130101;
B82Y 20/00 20130101; G02F 1/0128 20130101; G02B 6/351 20130101;
G02B 6/3546 20130101; G02B 6/3534 20130101 |
Class at
Publication: |
385/129 ;
385/015; 385/039 |
International
Class: |
G02B 006/10; G02B
006/26 |
Claims
We claim:
1. A photonic device comprising a photonic crystal having
configuring means for effecting a change to the physical geometry
in at least one region of said photonic crystal such that the
propagation of light therethrough or the confinement of light
therein is thereby altered.
2. The photonic device of claim 1, wherein said configuring means
includes an electrostrictive component of said photonic
crystal.
3. The photonic device of claim 1, wherein said configuring means
includes a piezoelectric component of said photons crystal.
4. The photonic device of claim 1, wherein said configuring means
includes a magnetostrictive component of said photonic crystal.
5. The photonic device of claim 1, wherein said configuring means
includes an actuation device affixed to said photonic crystal.
6. A photonic device as recited in one of claims 1 to 5, wherein
said configuring means provides at least one change in the
respective direction of propagation of one or more beams of light
of fixed frequency.
7. A photonic device as recited in one of claims 1 to 5, wherein
said configuring means provides at least one change in the
respective electromagnetic field pattern of one or more modes of
confined light.
8. A photonic device as recited in one of claims 1 to 5, wherein
said configuring means provides at least one change in the
respective frequency of one or more beams of light propagating
through said device.
9. A photonic device as recited in one of claims 1 to 5, wherein
said configuring means provides at least one change in the
respective frequency of one or more modes of light confined in said
device.
10. A photonic device as recited in one of claims 1 to 5, wherein
said configuring means provides at least one change in the
non-linear response in said device, for light propagating
therethrough or confined therein.
11. A photonic device as recited in one of claims 1 to 5, wherein
photonic device in claim 1, wherein said configuring means provides
at least one change in each of the respective direction and
frequency of one or more beams of light propagating through said
device.
12. A photonic device as recited in one of claims 1 to 5, wherein
said configuring means provides at least one change in each of in
the respective electromagnetic field pattern and frequency of one
or more modes of light confined in said device.
13. A photonic device as recited in one of claims 1 to 5, wherein
said configuring means provides at least one change in each of the
respective spatio-temporal electric and magnetic field intensities
associated with one or more beams of light propagating through said
device, or confined therein.
14. A photonic device as recited in one of claims 1 to 5, wherein
said configuring means provides one or more changes in the
respective direction, frequency, electric and magnetic field
intensity, or combinations thereof, associated with one or more
beams of light propagating through said device as a function of
time.
15. A photonic device as recited in one of claims 1 to 5, wherein
said configuring means provides one or more changes in the
respective electromagnetic field pattern, frequency, electric and
magnetic field intensity, or combinations thereof associated with
one or more modes of light confined within said device as a
function of time.
16. A photonic device as recited in one of claims 1 to 5, wherein
said configuring means provides at least one change in the symmetry
of one or more modes of light confined in said device, or
propagating therethrough.
17. A photonic device as recited in one of claims 1 to 5, wherein
said configuring means is adaptive for configuration in part or in
whole of said device, and includes compensation in part or in whole
of said device, and includes compensation in part or in whole of
the physical geometry of said photonic crystal.
18. A photonic device as recited in one of claims 1 to 5, wherein
said configuring means comprises at least one measured output
signal to said device and at least one applied input signal to
change the physical geometry of said device, so as to provide for
either closed loop control or open loop control.
19. A photonic crystal for use in a photonic device, said photonic
crystal comprising configuring means for effecting a change to the
physical geometry in at least one region of said photonic crystal
such that the propagation of light therethrough or the confinement
of light therein is thereby altered.
20. The photonic crystal of claim 19, wherein said configuring
means includes an electrostrictive component of said photonic
crystal.
21. The photonic crystal of claim 19, wherein said configuring
means includes a piezoelectric component of said photonic
crystal.
22. The photonic crystal of claim 19, wherein said configuring
means includes a magnetostrictive component of said photonic
crystal.
23. The photonic crystal of claim 19, wherein said configuring
means includes an actuation device affixed to said photonic
crystal.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to photonic devices, and
more particularly to man-made photonic crystals suitable for use in
controlling light propagation in photonic devices.
BACKGROUND OF THE INVENTION
[0002] Photonic crystals are periodic, dielectric, composite
structures in which the interfaces between the dielectric media
behave as scattering centres for light. Photonic crystals consist
of at least two component materials (one of which may be air)
having different refractive indices. The materials are arranged
alternatingly in a periodic manner that is scaled so as to
interfere with the propagation of light in a particular wavelength
range. Light is scattered at the interfaces between the materials
due to differences in the refractive index (or refractive index
contrast) of the two materials. The periodic arrangement of the
scattering interfaces prevents light with wavelengths comparable to
the periodicity dimension of the photonic crystal from propagating
through the structure. The band of blocked or forbidden wavelengths
is commonly referred to as a photonic bandgap.
[0003] Practical applications for photonic crystals generally
require manmade structures. Photonic devices are designed primarily
for light frequencies ranging from the ultraviolet to the microwave
regime (corresponding to wavelengths from 10 nanometres to 10
centimetres, respectively). Photonic crystals having these
corresponding periodicities are not readily available in
nature.
[0004] The simplest photonic crystal structure is a multilayer
stack, consisting of alternating layers of dielectric materials
with different refractive indices. The period of the structure (or
unit cell dimension) is the combined thickness of a single layer of
each of the dielectric materials. Such structures offer periodic
refractive index contrast in one direction only and are known as
one dimensional (1D) photonic crystals.
[0005] A 1D photonic crystal theoretically can act as a perfect
mirror (i.e., having 100% reflectivity) for light with wavelengths
within its photonic bandgap, and incident normal to the multilayer
surface. Such 1D photonic crystals can be used in a variety of
optical devices, including dielectric mirrors and optical
filters.
[0006] When the periodic refractive index contrast is extended to
two (or three) directions, the structures are known as 2D (or 3D)
photonic crystals. In 2D photonic crystals, light may be reflected
from any angle incident in the plane of periodicity (within its
photonic bandgap), while for 3D photonic crystals, light may be
reflected from any angle of incidence (within its photonic bandgap)
3D photonic crystal structures exhibiting this property have full
photonic bandgaps.
[0007] Practical applications of photonic crystals generally depend
on intentionally introducing defects into the periodic structure so
that the propagation and/or confinement of light with wavelengths
that would otherwise be forbidden can occur, that is, through
so-called defect states located within the photonic bandgap.
[0008] Defects are defined as regions of the photonic crystal
having a different geometry (i.e., spacing and/or symmetry) and/or
a different refractive index contrast from that of the general
periodic structure. For example, in a photonic crystal comprised of
a periodic array of air-holes within a dielectric sheet, a possible
defect would include leaving an array position with the dielectric
material intact and not having an air-hole at that location. Given
appropriate characteristics of the dielectric sheet and lattice
symmetry, an optical cavity in the vicinity of the defect can form,
suitable for confining at least one mode of light.
[0009] As another example, in the case of an array of dielectric
columns separated by airspaces, removal of a series of columns (in
a line), would create a defect through which specific wavelengths
of light otherwise forbidden would be able to propagate. By
appropriately eliminating further columns, light may be directed to
form all-optical circuits (i.e., so-called planar lightwave
circuits). Such circuits benefit from extremely tight bend radii to
furnish compact optical circuits.
[0010] It has been recognized that photonic crystals may also
provide imperfect reflectivity (i.e., reflectivity less than unity)
due to imperfections in the periodic structure, Such imperfections
can act in a manner similar to defect states, but occur through
inadvertence and often result from limitations in the fabrication
process. Although fabrication techniques have improved, unintended
imperfections continue to occur and no practical means has been
proposed to remove or correct them.
[0011] Defect states in 1D, 2D, and 3D photonic crystals result in
planar, linear, and point localization, respectively. Since the
presence of defect states in photonic crystal structures can
precisely control light propagation or confinement, the design of
photonic crystal-based optical devices has been extensively
explored. Practical applications of photonic crystals to date
include waveguides, light cavities, high a filters, channel drop
filters and mirrors.
[0012] There has recently also been great interest in exploring the
use of photonic crystals for applications such as planar lightwave
circuits, wavelength division multiplexing applications, optical
switches, optical computing, tunable gates, interconnects, and so
forth. In such applications, the defect state in the photonic
crystal would have to be altered in a controlled fashion to create
a tunable wavelength band that can propagate through or be confined
in the device. The main limitation of traditional photonic crystals
is that control over the propagation or confinement of light is
determined and fixed by its physical structure, as the defect state
is permanently fixed in the photonic crystal, Although fixed
defects in photonic crystals offer an ability to control light
propagation or confinement, once such defects are introduced, the
propagation or confinement of light in the crystal is determined.
Thus, discretionary switching of light, for example, or re-routing
of optical signals, is not available with fixed defects in a
photonic crystal.
[0013] In order to configure the propagation or confinement of
light through a photons crystal, the main approach that has been
investigated is the modulation of refractive index contrast,
principally using an applied electric field, and the uniform
adjustment of the arrangements of the dielectric elements in the
whole photonic crystal.
[0014] For example, U.S. Pat. No. 6,058,127 (Joannopoulos et al)
discloses a technique for refractive index contrast modulation by
applying an electric field to the dielectric. However, known
materials offer only low electro-optic coefficient values for
applied electric fields below the breakdown limit, and thus yield
only small changes in refractive index.
[0015] In U.S. Pat. No. 5,973,823 (Koops et al.) and U.S. Pat. No.
6,064,506 (Koops), the photonic crystal cavities were filled with a
material adjustable by an electric field. In this case, the
refractive index contrast between the two media is significantly
lowered, limiting the possibility of obtaining a full photonic
bandgap.
[0016] An example of adjusting the photonic crystal periodicity is
through the use of temperature, pressure or field excitation to the
photonic crystal as described in U.S. Pat. No. 5,688,318 (Milstein
et al.), but the proposed method does not provide discretionary
modification of selected elements of the photonic device. No
control of the defect states is provided.
[0017] None of the previously disclosed approaches for tuning
photonic crystals has been successful in practical applications
because they either provide only negligible changes in the ability
of the structures to guide or confine light, or they significantly
reduce the size of the photonic band gap, or they produce only
non-local changes that are not useful for re-routing or confining
light in a discretionary fashion.
[0018] It is an object of the present invention therefore to
obviate or mitigate the shortcomings of known photonic crystal
structures, and particularly to provide photonic crystal structures
through which it is possible to change the propagation and/or
confinement of light in a discretionary fashion,
SUMMARY OF THE INVENTION
[0019] According to the present invention, there is provided a
photonic crystal, and a photonic device comprising a photonic
crystal, having configuring means for effecting a change to the
physical geometry in at least one region of said photonic crystal
such that the propagation of light therethrough or the confinement
of light therein is thereby altered, or such that both the
propagation and confinement of light are altered. The configuring
means for effecting such a change may include an electrostrictive,
piezoelectric or magnetostrictive component of the photonic
crystal, or a microactuation device affixed to the photonic
crystal.
[0020] It has surprisingly been found that such effected changes in
the physical geometry of region(s) of a photonic crystal can be
used to control propagation and confinement of light in a
discretionary fashion. The invention therefore offers the advantage
of the hitherto unavailable combination of precise control of light
propagation or confinement, or both, with the ability to route
light propagation in a discretionary fashion based on geometrical
configuration in a region.
[0021] Preferably, the photonic device of the present invention
comprises a photonic crystal and a support in which the physical
geometry of the photonic device elements (photonic crystal units
and its supports) can be reversibly or irreversibly changed in any
given region, at particular times, or continuously at a chosen
frequency.
[0022] Changes in physical geometry may include changes in the
dimension of the photonic device elements and changes in their
location, and may occur for a single element or a plurality of
elements. Such changes may be accomplished by configuration of
either the photonic crystal itself or its support (i.e., changing
the physical geometry of the elements based on photonic crystal
element configuration, or support configuration).
[0023] The photonic crystal configuration may occur based on its
material properties, e.g., through the piezoelectric (or
electrostrictive) effect or through actuator movement, e.g., a
micro-actuation device, including so-called
micro-electro-mechanical-systems (MEMS) technology.
[0024] Configuration of both the photonic crystal and its support
may be accomplished using the intrinsic properties of a particular
class of dielectric materials that exhibit the piezoelectric
effect. The piezoelectric effect refers to the application of an
electric field resulting in the physical dimensions of a given
piezoelectric element (including the photonic crystal support)
being changed in a predetermined manner. Selectively applying an
electric field to one or more given piezoelectric elements (e.g.,
by using electrical contacts) of a photonic crystal and/or its
support (i.e., in which the crystal and/or its support is composed
of dielectric material exhibiting a piezoelectric response), will
significantly deform (e.g., deflecting, elongating, compressing or
otherwise) such elements leading to the selective introduction or
removal of defects in the photonic crystal.
[0025] Such changes are significant in that they permit the
propagation, confinement, or both of selected wavelengths and modes
of light in the structure to be changed. Since changes in dimension
are proportional to applied electric field for an appropriate
choice of applied field, the allowed wavelength for propagating
and/or confined light in the vicinity of the introduced defect may
be changed or tuned continuously. Moreover, defects so created for
light propagation or confinement may be opened and closed at a
predetermined rate by applying an appropriate alternating electric
field.
[0026] The present invention encompasses devices comprising
photonic crystals having intentionally introduced defect states and
devices comprising photonic crystals having no pre-existing
intentional defects in their periodic structure.
[0027] Configuration also provides a mechanism for adaptively
removing imperfections in a photonic crystal. For example, the
imperfections characteristic of real structures may be compensated
by using electric fields applied in a discretionary fashion to
elements of the structure, throughout the structure. Also, this
principle may be used to configure the photonic device by measuring
the output signal(s) of the photonic device and applying input
signal(s) to change the physical geometry of the device so as to
provide for closed loop control. This approach, for example, can be
used to improve yields in the fabrication of complex, integrated,
light circuits. Such devices are also amenable to open loop
control.
[0028] Configuration of the support or substrate allows a photonic
crystal of arbitrary dielectric material to be selected. The
advantage in this case is that the constraint of using a
piezoelectric material is relaxed, and the opportunity to select
materials with superior refractive index contrast is available.
Using standard bonding technology, the photonic crystal can be
mated to an array of actuators that change the geometry of the
photonic crystal permitting defects to be introduced arbitrarily.
MEMS technology is well suited for fabricating such arrays and
readily offers the possibility of producing actuators capable of
yielding displacements with sub-nanometer resolution.
[0029] In addition to the piezoelectric effect, other means of
movement that may be used to configure the photonic element(s)
include configuration due to a photonic element's interaction with
electrostatic forces or pneumatic forces, and configuration due to
the photonic element's intrinsic properties (e.g., electromagnetic
effect, thermal effect, magnetostrictive effect, electrostrictive
effect, or electro-thermal effect).
[0030] The invention permits heretofore unattainable control of
light propagation, confinement, or both in a photonic crystal,
allowing appreciable modulation of tho propagating light (in free
space or otherwise), for any dimensionality (1D, 2D, and 3D
photonic crystals), selectively in space and in time.
[0031] Propagation and confinement of light in a photonic device
may include rectilinear propagation, reflection, refraction,
focusing, dispersion, diffraction, interference, polarization,
absorption, emission, and amplification. Also included are
perturbation(s) of the photonic device by, for example, light and
electric field, that alter the propagation and confinement
characteristics of light in the photonic device such as,
second-harmonic generation, frequency conversion, parametric
amplification, parametric oscillation, third-harmonic generation,
self-phase modulation, self-focusing, four-wave mixing, optical
amplification, optical phase conjugation, optical solutions, and
optical bistability.
[0032] The invention therefore entails (a) providing changes in
dielectric properties by adjusting the periodicity of the
dielectric in the photonic crystal, (b) affecting such changes in a
given location of the photonic crystal, (c) being able to realize
such changes in a discretionary fashion, and (d) being able to
modulate as a function of time such changes, so as to render a
modulation model for configuring the propagation of selected
wavelengths of light in such photonic crystals, in both space and
time.
[0033] The invention encompasses the configuring (or tuning) of a
photonic crystal, in whole or in part, in space and in time, using
photonic crystal element configuration, support element
configuration, or both photonic crystal element configuration and
support element configuration. Such configuring and re-configuring
may be used to alter propagation direction or wavelength, affected
in space or time, and may be applied to both existing defect states
and perfectly periodic photonic crystal structures. The invention
may also be used to compensate for an imperfect photonic crystal
structure, and to tune signals confined within or propagating
through the photonic device, with or without feedback for closed or
open loop control.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] In order that the invention may be more clearly understood,
reference will now be made to the following drawings in which:
[0035] FIG. 1 is a schematic representation (elevation view)
showing piezoelectric-induced elongation/compression of a photonic
crystal element of the present invention;
[0036] FIG. 2 is a schematic representation (elevation view)
showing piezoelectric-induced bimorph deflection of a photonic
crystal element of the present invention;
[0037] FIG. 3 is a schematic representation (elevation view)
showing piezoelectric-induced elongation/compression of the support
of the present invention;
[0038] FIG. 4 is a schematic representation (elevation view)
showing piezoelectric-induced bimorph deflection of the support of
the present invention;
[0039] FIG. 5 is a schematic representation (elevation view)
showing microactuator-based lateral translation of a photonic
crystal element of the present invention;
[0040] FIG. 6 is a schematic representation (elevation view)
showing microactuator-based vertical translation of a photonic
crystal element of the present invention;
[0041] FIG. 7 is a schematic representation (elevation view)
showing microactuator-based rotation/displacement of a photonic
crystal element of the present invention;
[0042] FIG. 8 is a schematic representation (elevation view)
showing microactuator-based rotation of a photonic crystal element
of the present invention:
[0043] FIG. 9 is a schematic representation (elevation view)
showing microactuator-based lateral translation of a group of
photonic crystal elements of the present invention;
[0044] FIG. 10 is a schematic representation (elevation view)
showing microactuator-based vertical translation of a group of
photonic crystal elements of the present invention;
[0045] FIG. 11 is a schematic representation (elevation view)
showing microactuator-based rotation/displacement of a group of
photonic crystal elements of the present invention;
[0046] FIG. 12 is a schematic representation (elevation view)
showing microactuator-based rotation of a group of photonic crystal
elements of the present invention;
[0047] FIG. 13 is a schematic representation (plan view) showing
the movement of a photonic crystal element of the present
invention;
[0048] FIG. 14 is a schematic representation (plan view) showing
the change in size of a photonic crystal element of the present
invention;
[0049] FIG. 15 is a schematic representation (plan view) showing
the change in shape of a photonic crystal element of the present
invention;
[0050] FIG. 16 is a schematic representation (plan view) showing a
configurable waveguide as an example of an embodiment of the
present invention;
[0051] FIG. 17 is a three-dimensional schematic representation
showing a configurable spatial light modulator as an example of an
embodiment of the present invention;
[0052] FIG. 18 is schematic representation (plan view) showing an
optical filter as an example of an embodiment of the present
invention;
[0053] FIG. 19 is schematic representation (plan view) showing an
optical switch as an example of an embodiment of the present
invention;
[0054] FIG. 20 is schematic representation (plan view) showing a
tunable wavelength-division multiplexing/demultiplexing (WDM)
system as an example of an embodiment of the present invention;
[0055] FIG. 21 is schematic representation (plan view) showing a
tunable phase modulator as an example of an embodiment of the
present invention;
[0056] FIG. 22 is schematic representation (plan view) showing a
tunable channel drop filter as an example of an embodiment of the
present invention;
[0057] FIG. 23 is schematic representation (plan view) showing a
real, imperfect photonic crystal structure;
[0058] FIG. 24 is schematic representation (plan view) showing a
compensated imperfect photonic crystal structure of the present
invention.
DETAILED DESCRIPTION THE INVENTION
[0059] A first preferred embodiment of the invention, in which the
physical geometry of a photonic crystal may be configured due its
intrinsic (or material) properties, is shown schematically in FIG.
1. The photonic device comprises a photonic crystal 30 which
includes pillars of a piezoelectric material 31 separated by air 32
on a support 33. A voltage applied across the piezoelectric pillar
31 will configure the pillar such that the dimensions (size or
shape) of the pillar will be changed (from the dotted line to the
solid line). The resulting change in periodicity will alter the
optical properties of the photonic crystal.
[0060] An advantage of configuration of the photonic device element
by directly changing the geometry of the photonic crystal itself is
that it leads directly to changes in the optical properties of the
photonic device. The benefit of configuration of the photonic
device element using the support is that there is no restriction on
the selected photonic crystal that may be physically affixed to
it.
[0061] The photonic crystal or the support may be configured using
the well-known piezoelectric or electrostrictive effects.
Piezoelectric materials have been used to permit transducer
applications to be realized for almost forty years) and as thin
film micro-sensors and micro-actuators for over a decade.
[0062] Several material classes are suitable for directly altering
the physical geometry of the photonic device element. These include
high refractive index piezoelectric and electrostrictive materials
such as barium titanate and lead zirconate titanate, which
typically have refractive indices greater than 2 in the visible to
near-infrared wavelength range (e.g., n.about.2.4 for BaTiO.sub.3
and n.about.2.6 for PZT/PLZT) in the former class, and lead
magnesium niobate, PMN, (i.e,, n.about.2.5) in the latter
class.
[0063] Application of an electric field on the configurable
element(s) of a photonic device will cause the element to
significantly deform (e.g., deflecting, elongating/compressing or
otherwise) leading to the selective introduction and removal of
defects in the photonic crystal. This deformation is caused by the
application of an electric field, and may be in direct proportion
to the applied field (i.e., for an appropriate intensity of applied
electric field), allowing the defect to be tuned continuously.
Defects may also be introduced or removed at a predetermined rate
by applying an appropriate alternating electric field. In addition,
this modulation model also provides a mechanism for adaptively
perfecting a photonic crystal, by compensating for imperfections
arising naturally from the nature of practical fabrication
processes. That is, electric fields may be applied in a
discretionary fashion to selected elements of the structure,
throughout the structure, to compensate for the imperfections
characteristic of real structures.
[0064] One example of how the electric field may be applied to the
photonic crystal in this invention is through the use of a
pre-patterned substrate as the bottom electrode and a metallized
membrane serving as a continuous (i.e., sheet.) top electrode,
Known lithographic techniques may be used to fabricate
pre-patterned bottom electrodes on the substrate (typically chosen
from glass or silicon). Suitable bottom electrode materials may
include gold, platinum and conducting oxides (e.g., RuO.sub.2), For
optical applications, the pre-patterned area may range from several
micrometers in size (e.g., for a switch) to several centimetres in
size (e.g., for dense wavelength-division multiplexing, or DWDM,
applications).
[0065] The advantages of using silicon-based substrates include the
ability to micromachine v-grooves (i.e., MEMS technology) for
aligning optical fibres and/or light sources with the photonic
crystal. A piezoelectric film (e.g., PZT) may be deposited on top
of the pre-patterned substrate using a thin film deposition process
(e.g., sol-gel processing, sputtering, and MOCVD). A resist may be
deposited on the piezoelectric thin film, patterned using known
lithography to reveal apertures for etching. A suitable process for
etching is dual frequency reactive ion etching, a standard
commercial process.
[0066] In order to align the photonic crystal elements of the
piezoelectric material, coarse alignment may be accomplished first
by using the silicon wafer flats, and then by using backside
infra-red alignment for fine adjustment. The size of the electrodes
may be anywhere from about 50 nm up to about 1 mm scale, depending
on the desired size of the photonic crystal elements (e.g.,
electron-beam or x-ray lithography is suitable for nanometer-scale
lithography, while optical lithography is suitable for
micrometer-scale lithography).
[0067] The top electrode may be made of a suitable flexible,
conducting material (e.g., metal-coated membrane). The membrane may
be coated with a thin (micrometer-scale) coating of metal (e.g.,
aluminum, gold, or a eutectic such as Al--Au or In--Sn), which may
be bonded to the top of the photonic crystal. Bonding may be
accomplished using localized heating (e.g., rapid thermal
processing, or optical radiation from micromachined polysilicon
resistor formed on the membrane). A electric field may be applied
selectively to a single photonic crystal element or to groups of
photonic crystal elements through the bottom contact while
grounding the top contact, resulting in reconfiguration of the
photonic crystal element(s).
[0068] It is also possible to use other types of piezoelectric
elements, such as bimorphs, or other flex-tensional devices,
Bimorph deflection results in very large displacements of the
photonic crystal elements, depending on the height of the element
(for example, proportional to 3(d.sub.31(E(t.sup.2/(2(D), where E
is the field applied to the bimorph, t is the height of the
element, and D is the thickness of the element).
[0069] Turning to FIG. 2, another embodiment of the invention is
shown in which the photonic crystal element(s) may be configured
through bimorph deflection due its intrinsic (or material)
properties. The photonic crystal 30 includes pillars of a
piezoelectric material 31 separated by air 32 on a support 33. A
voltage applied across the base of piezoelectric pillar 31 will
configure the pillar such that the size and/or shape of the pillar
will be changed (from the dotted line to the solid line). The
resulting change in periodicity will alter the optical properties
of the photonic crystal.
[0070] The photonic crystal may be indirectly configured through
alternations of the physical geometry of the support. This may be
accomplished using the intrinsic properties of the support (again,
through an effect such as piezoelectricity) or using a form of
actuator technology (e.g., MEMS). In this reconfiguration model,
the photonic crystal may be of any suitable material, allowing the
selection of a dielectric material with a high refractive index.
Using standard bonding technology, the photonic crystal can be
mated to an array of actuators that cause a local change in
geometry of the photonic crystal, permitting defects to be
introduced/removed arbitrarily. In addition to using substrates
exhibiting the piezoelectric and electrostrictive effect,
micro-electro-mechanical-systems (MEMS) may be used to mechanically
configure the defects within the photonic crystal, The degree of
dimensional change of the substrate depends on the actuation
mechanism.
[0071] An advantage of using intrinsic properties to reconfigure
the support is that the transducer material technology is well
known to those skilled in the art, with many inexpensive, high
piezoelectric coefficient piezoelectric/electrostrictive components
commercially available (e.g. from Kistler instrument Corp.; TRS
Ceramics, Inc.; Physik Instruments; APC International Ltd.). Also,
transducer characteristics have been continually improving, with
very large piezoelectric responses recently reported in the
literature (for example, d.sub.33.about.2,500 pC/N for PZT, Penn
State Annual Report, 1998).
[0072] There are a very large number of piezoelectric/transducer
materials, in the categories of crystals (e.g., rochelle salt,
quartz, ammonium dihydrogen phosphate (ADP), potassium dihydrogen
phosphate (KDP), tourmaline, zinc blende (ZnS), barium titanate
(BaTiO.sub.3), lithium niobate (LiNbO.sub.3), lithium tantalate,
bismuth germanium oxide), ceramics (e.g., lead titanate
(PbTiO.sub.3), lead zirconate (PbZrO.sub.3), lead metanicbate
(PbNb.sub.2O.sub.0), load zirconate titanate
(Pb(Zr.sub.1Ti)O.sub.3), barium titanate (BaTiO.sub.3)), polymers
(e.g., polymer polyvinylidene fluoride (PVDF), polyvinylidene
fluoride-trifluoroethylene (VF.sub.2-VF.sub.3)), composites,
magnetostrictive alloys (e.g., terbium dysprosium
iron(Tb.sub.0.3Dy.sub.0- .7Fe.sub.1.93), terbium dysprosium
(Tb.sub.0.6Dy.sub.0.4)), and electrostrictive ceramics (e.g., lead
magnesium niobate-lead titanate (PMN-PT)).
[0073] Very large strains have been observed--for example,
piezoelectric PZN:PT with strains of 1.7%, and electrostrictive
PVDF:TrFe copolymers with strains of 4% (Penn State Annual Report),
enough to cause significant changes in the optical properties of
the photonic crystal.
[0074] To exemplify the magnitude of change in periodicity required
to cause significant changes in the optical properties of a 3D
photonic crystal, changes in periodicity of 5% result in the
transmission spectrum peak intensity being displaced by 150 nm (C.
C. Cheng, A. Scherer, V. Arbet-Engels, and E. Yablonovitch,
"Lithographic Bandgap tuning in photonic Bandgap cystals", J. Vac.
Sci. Technol. B, Vol. 14, No. 6, 1996, 4110-4114), or in the case
of a 2D photonic crystal, a shift of 40 nm (Hideki Masuda, Masayuki
Ohya, Hidetaka Asoh, Masashi Nakao, Masaya Nohtomi, and Toshiaki
Tamamura, "Photonic Crystal using anodic porous alumina", Japan
Journal of Applied Physics, Part 2, Vol. 38, No. 12A, 1999,
L1403-1405).
[0075] In Dense Wavelength-Division Multiplexing/Demultiplexing
(DWDM) systems, common channel spacings for DWDM are 100 GHz
(corresponding to a channel spacing of .about.0.8 nm at 1550 nm) or
lower frequencies (for example, 50 or 25 GHz), providing
approximately 50 channels in the C-band (1525 to 1570 nm).
Piezoelectric materials are suitable for "scanning" a photonic
crystal response over the complete wavelength band, using small
induced changes in local periodicity for channel-to-channel
modulation, over the relatively large range of the DWDM wavelength
range.
[0076] The absolute degree of movement of a given photonic crystal
element depends on the piezoelectric coefficient, the size of the
element, the voltage applied, and the movement mechanism. Under
appropriate conditions, these large piezoelectric coefficients
permit significant changes in the periodicity of photonic crystals
to be realized.
[0077] In FIG. 3, another embodiment of the invention is shown in
which the support may be configured through elongation/compression
due its intrinsic (or material) properties. The photonic device
includes pillars of a first dielectric 31 separated by air 32 on a
support 33. A voltage applied across the base of the support 33
will change the separation between the pillars (from the dotted
line to the solid line). The resulting change in periodicity will
change the optical properties of the photonic crystal.
[0078] Traditional actuators typically take many different
forms--some common examples include discs, rings, washers,
cylinders, tubes, bars, plates and hemispheres. More elaborate
flex-tensional devices (e.g., bimorphs) and stacked piezoelectric
elements are used to achieve increased displacements as compared
with conventional piezoelectric elements. In addition,
piezoelectric motors have been developed to add to the range of
piezoelectric actuators (e.g., inchworm motors and standing wave
motors).
[0079] In FIG. 4, another embodiment of the invention is shown in
which the support may be configured through bimorph deflection due
to its intrinsic (or material) properties. The photonic crystal 30
includes pillars of a first dielectric 31 separated by air 32 on a
support 33. A voltage applied across the base of the support 33
will reconfigure the pillar such that the separation between the
pillars will be changed (from the dotted line to the solid line).
The resulting change in periodicity will change the optical
properties of the photonic crystal.
[0080] In addition to intrinsic (or material) properties of the
support, the movement of the support may be accomplished using
extrinsic methods (e.g., a micro-actuation device, including
so-called micro-electro-mechanical-systems (MEMS) technology). MEMS
technology is a mature technology, well suited for fabricating such
arrays and readily offering the possibility of producing actuators
capable of yielding displacements with sub-nanometer
resolution.
[0081] Using the substrate configuration method, it is possible to
create a wide range of different displacement configurations,
particularly using the microactuator embodiment. Micro-actuation
devices work on a large number of operating principles (e.g.,
photothermal, electrostrictive, electrostatic, electromagnetic,
thermal, magnetostrictive, electro-thermal, pneumatic, stress,
etc.) resulting in different magnitudes of displacement. However,
the micro-actuators all result in configuration of the substrate,
regardless of operating principle.
[0082] FIGS. 5 to 12 represent schematic elevation views of 2D
photonic devices showing alternate embodiments of the invention in
which the support may be configured through the microactuator
embodiment (where the dotted and solid lines represent the changes
between the original and configured state). The photonic devices
include pillars of a first dielectric 31 separated by air 32 on a
support/actuator 34. Translation of an element in the horizontal
direction allows the periodicity of the photonic crystal to be
altered as shown in FIG. 5, while translation of a textured
photonic crystal oaemont (for example, a notched element) in the
vertical direction creates a change in fill factor as shown in FIG.
6. Rotation of the photonic crystal element may occur relative to
unactuated elements as in FIG. 7, or about its centre for an
asymmetrical photonic crystal element as in FIG. 8.
[0083] This support configuration method may be applied to a
significant portion of the photonic crystal (as well as to a single
element as described above). It is possible to use the
microactuator configuration method to translate and/or rotate a
group of photonic crystal elements as shown in FIGS. 9 to 12. In
particular, it is possible to rotate a group of elements about its
centre to change its local fill factor (FIG. 12), allowing
pre-existing defect states to be rotated relative to defect states
on another group of photonic crystal elements.
[0084] FIGS. 13 to 15 are schematic plan views of 2D photonic
devices representing possible configuration mechanisms of the
present invention, where the original state of the dielectric
pillars 31 surrounded by air 32 is represented by the dotted line
and the configured state is represented by the solid line. The
photonic crystal elements may move relative to each other (i.e.,
local change in structure symmetry and/or periodicity) due to
configuration of the photonic crystal or its support (FIG. 13), the
photonic crystal elements may change in size (i.e., local change in
fill factor and/or periodicity) due to configuration of the
photonic crystal or its support (FIG. 14), or the photonic crystal
elements may change in shape due to configuration of the photonic
crystal or its support (FIG. 15).
[0085] The configuration mechanisms of the present invention may be
used to change the direction(s) or electromagnetic field pattern(s)
of a fixed frequency (or frequencies) of light passing through the
photonic crystal, change the frequency of light passing through the
photonic crystal, change both the direction and frequency of light
passing through the photonic crystal, or any combination thereof,
and additionally to compensate for imperfections in photonic
crystals.
[0086] Photonic crystal cavities represent high quality factor (Q)
cavities owing to the high effective reflectivity of the cavity in
the range of the photonic bandgap. A photonic cavity can support at
least one mode of light depending on the cavity dimensions, The
mode characteristics are defined by the cavity dimensions (i.e.,
shape and size) and the dielectric properties of the surrounding
photonic crystal medium. Those characteristics include the mode
energy distribution within the cavity as well as that penetrating
into the cladding, mode symmetry, and mode polarization, in both
the steady-state and in time.
[0087] The higher the Q value of the cavity, the narrower the
frequency dispersion for a given mode. For a cavity that supports a
plurality of modes, the inter-mode frequency spacing or free
spectral range, is functionally dependent on the cavity dimensions.
By changing the dimensions of the cavity, the allowed frequencies,
mode symmetries, polarization(s), intensity distribution(s),
full-width half-maximum spectral distribution, and free spectral
range, can all be tuned.
[0088] For example, DWDM systems are anticipated to operate with
channel spacing as narrow as 6.25 GHz, and such high Q cavities are
feasible for such systems. In such systems, photonic crystal
cavities may be made proximate to each other such that tunneling of
light mode(s) between said cavities occurs, providing resonantly
coupled optical cavities. Examples of such coupled photonic crystal
cavity systems include coupled resonant optical waveguides (Amnon
Yariv, Yong Xu, Reginald K. Lee and Axel Scherer,
"Coupled-resonator Optical Waveguide; a Proposal and Analysis",
Optics Letters, Vol. 24, No. 11, 1999, 711-713) and compound
systems including photonic crystal cavities coupled to photonic
crystal waveguides (Steven G. Johnson, Christina Manolatou, Shanhui
fan, R. Villeneuve, J. D. Joannopoulos, H. A. Haus, "Elimination of
cross talk in waveguide intersections", Optics Letters, Vol, 23,
1998, 1855-1857).
[0089] Furthermore, in addition to providing a means for tuning the
properties of the photonic crystal cavity, the mechanism also
allows the relative location of the cavity with respect to other
photonic crystal and conventional photonic components to be
adjusted. This permits a configurable means for controlling light
propagation, confinement, or both in an optical integrated circuit.
Surprisingly, this approach can be used to control the intensity of
light confined within, or propagating through the photonic device.
Moreover, this can be used to equalize the intensity of individual
signals within a band.
[0090] Two applications for the invention that can benefit from the
ability to change the direction of the light frequency passing
through the structure for a specific mode in the photonic bandgap
are configurable waveguides and configurable spatial light
modulators (i.e., for free-space optical interconnects).
[0091] Waveguides are useful embodiments of photonic crystals as it
is possible to guide light efficiently around tight corners
(90.degree. bends) with radii of curvature on the order of the
wavelength of the guided light in photonic crystals, (P. R.
Villeneuve, S. Fan, A Mekis, and J. D. Joannopoulos, "Photonic
Crystals and their potential applications", IEE Colloquium on
Semiconductor Optical Microcavity Devices and Photonic Bandgaps,
IEE, London, UK; 1996, 1-7). FIG. 16 is a plan view schematic
representation of a configurable waveguide as an example of an
embodiment of the present invention. Light at a frequency within
the photonic bandgap 41 is incident on the photonic crystal 40. It
is feasible to introduce a defect state 42 in the photonic crystal
40 that lies within the photonic crystal bandgap, and then
spatially reconfigure this defect with time to create an alternate
light path through the photonic crystal (42 or 43). Light may exit
the photonic crystal in a chosen location (45, 46, or 47)
corresponding to the position of the reconfigured defect
states.
[0092] This embodiment may be constructed, for example, using a 2D
photonic crystal made of dielectric rods on a square lattice with
unit cell size "a", refractive index contrast of 3.4, and
dielectric rods of radius of 0.18 a, which was found to have a TM
photonic bandgap at wavelengths from 2.257 a to 3.322 a (P. R.
Villeneuve, S. Fan, A Mekis, and J. D. Joannopoulos, "Photonic
Crystals and their potential applications", IEE Colloquium on
Semiconductor Optical Microcavity Devices and Photonic Bandgaps,
IEE, London, UK; 1996, 1-7). It is possible to use the MEMS
configuration shown in FIG. 6, for example, to remove a row of rods
to cause light to propagate within the photonic bandgap in a
selected defect state. In accordance with the invention, light may
be routed to a path anywhere in the photons crystal, by selectively
introducing the defect to route light in a selected spatial
direction,
[0093] This same principle may be adopted to form a configurable
spatial light modulator that can be used to relieve the existing
interconnect bottleneck in the backplane of microelectronic chips.
FIG. 17 is a three-dimensional schematic representation showing a
configurable spatial light modulator as an example of an embodiment
of the present invention. The light emitter array 50 composed of
light emitting diodes 51, (or laser diodes, or superluminescent
diodes, or vertical cavity surface emitting lasers (VCSEL)) emits
light on the configurable spatial light modulator 52, which directs
light to the detector array 55 (e.g. multiple quantum well (MQW)
detector arrays). Using "free-space optics", which can make use of
a whole surface of a chip 50 for connections (i.e.,
three-dimensional computing with free-space optical interconnects
instead of wires), light signals from an array of light emitting
diodes 51 may be used to transmit information in free space
perpendicular to the chip surface (instead of traditional wires
traveling on the chip surface), significantly increasing the speed
of multiprocessor computing systems.
[0094] The invention may be used to guide light beams from one chip
to another (i.e. from the laser array 50 to the detector array 55),
through the spatial control of light beam propagation using a
photonic device (configurable spatial light modulator) 52
(individually reconfigurable over the fixed regions 53). At the
present time, MQW devices and VCSEL lasers have dimensions on the
order of tens of microns in size, well within the range of both
practical photonic crystal fabrication and MEMS devices.
[0095] Applications for this invention that would benefit from the
ability to change the frequency of light passing through the
photonic crystal for a specific mode in the photonic band gap are
optical filters, that select only a particular frequency from an
initial broadband wavelength spectrum, and optical switches, that
select certain wavelengths of light that can pass through the
photonic crystal as a function of time.
[0096] FIG. 18 is a plan view schematic representation showing an
optical filter as an example of an embodiment of the present
invention, The defect state 60 may be configured such that only a
selected frequency of the broadband light 41 (with a frequency
range within the photonic bandgap) incident on the photonic device
40 will pass through the photonic device 61. FIG. 19 is a plan view
schematic representation showing an optical switch as inn example
of an embodiment of the present invention. The defect state 62 may
be configured such that a light mode from the incident broadband
light 41 on the photonic device 40 will or will not pass through
the photonic crystal 63. In both these modes, the defect state can
be altered to permit/disallow a mode passing through the photonic
crystal. The switching time is limited by the mechanism of tuning
and geometry--commercially available piezoelectric actuators have
resonant frequencies in the 1 to 3 kHz range (available from EDO
Piezoelectric Products and Ferroperm).
[0097] The invention may be used to change both the direction and
frequency of light passing through the photonic crystal. Examples
of this configuration include a configurable DWDM demultiplexer, a
phase modulator, and a configurable channel drop filter.
[0098] FIG. 20 shows a photonic device 40 as a configurable DWDM
multiplexer. A broadband light input 41, containing a series of
different wavelengths 68, 69, 70, and 71, can be "unpacked" by
selectively guiding the different wavelengths along specific paths
64, 65, 66, and 67. Any or each of these paths (corresponding to
the defect states for a selected wavelength) may be turned on or
off and are spatially reconfigurable.
[0099] In DWDM systems, the number of wavelength channels that can
be transmitted depends on channel spacing, total optical bandwidth
of the system, and the modulation bandwidth of the individual
optical signals. At present, the common channel spacing for DWDM
are 100 GHz (corresponding to a channel spacing of .about.0.8 nm at
1550 nm) or lower frequencies (for example, 50 or 25 GHz),
providing approximately 50 channels in the C-band (1525 to 1570
nm). These wavelength ranges are well within the range of the
feasible unit cell sizes amenable for photonic crystal
fabrication.
[0100] The invention may also provide a means for tuning of other
dispersive photonic crystal components, for example, superprisms
(Hideo Kosaka, Takayuki Kawahima, Akihisa Tomita, Masaya Notomi,
Toshiaki Tamamura, Takashi Sato and Shojiuo Kawakami, "Superprism
Phenomena in Photonic Crystals: Toward Microscale Lightwave
Circuits", Journal of Lightwave Technology, Vol. 17, No. 11, 1999,
2032-2038), and ultrarefractive optical elements (S. Enoch, G.
Tayeb, and D. Maystre, "Numerical evidence of ultrarefractive
optics in photonic crystals", Optics Communications, Vol. 161,
1999, 171-176).
[0101] FIG. 21 shows a photonic crystal 40 configured as a tunable
phase modulator. The phase of the outgoing light 77 may be modified
by causing the incoming beam 73 (selected from a broad band light
input 72 incident on the photonic device 40) to be divided into two
paths 74 and 75 with different path lengths, recombining in path 76
with the phase depending on the relative path lengths of 74 and 75.
By configuring the length of paths 74 and 75 relative to each
other, the phase of the output beam 77 will be changed.
[0102] Traditionally, electro-optic crystals have been used as
phase modulators, but their input and output locations are
determined by their fixed waveguides. With the present invention,
not only can the phase of the outgoing beam be configured, any
number of incoming and outgoing beams can be accommodated with the
same device, in a spatially discretionary fashion.
[0103] FIG. 22 shows a photonic device 40 configured as a tunable
channel drop filter. In a channel drop filter, a single wavelength
channel 85 (e.g., .lambda..sub.2) from a multiplexed signal 80
(e.g., .lambda..sub.1, . . . .lambda..sub.n) traveling through a
waveguide 81 can be transferred into a second waveguide 83 using an
optical resonator system 82 without disturbing the other channels
(i.e., channels .lambda..sub.1, .lambda..sub.3, . . .
.lambda..sub.n will continue along waveguide 81 undisturbed).
[0104] Fixed optical resonator systems in 2D photonic crystals have
been investigated using different sized cavities (or defect modes)
to transfer a single wavelength channel from waveguide 81 to 83
(Shanhui Fan, P. R. Villeneuve, J. D. Joannopoulos, and H. A. Haus,
"Channel drop filters in photonic crystals", Optics Express, Vol.
3, No. 1, Jul. 6, 1998, 4-11). With the present invention, there is
the added benefit of mechanically configuring the defect mode such
that a range of wavelengths (instead of a single wavelength) may be
accessed and a range of directions (and exit locations) of the
dropped channel may be reconfigured.
[0105] The invention can also provide a means for controlling the
magnitude of the non-linear response of a photonic crystal. For
example, a photonic crystal may be comprised of element(s)
exhibiting non-linear optical response(s). These response(s) may be
tuned by adjusting the relative location(s) of these elements with
respect to each other.
[0106] The configurable photonic device of the present invention
may also be used to optimize desired photonic crystal-based devices
through optical feedback. This capability may occur through tuning
of the individual configurable cells/elements based on feedback
from a detection system (for one or more channels) and analysis
based on the value of said output(s). In this way, the desired
output may be optimized continually with little user intervention,
correcting for aging or degradation in device performance through
such self-adaptive fine-tuning of the structure with time.
[0107] Another application of the invention is to compensate an
imperfect real structure, FIG. 23 shows an imperfect (i.e. "real")
photonic crystal 40 in which the incident broadband light 86 is not
completely reflected by the photonic crystal 87 due to
imperfections (or defect states) in the photonic crystal. FIG. 24
shows a compensated photonc device in which the incident broadband
light 86 (lying within the photonic bandgap) is substantially
totally reflected by the photonic crystal.
[0108] It will of course be appreciated by those skilled in the art
that many variations are possible within the broad scope of the
invention. The invention may be utilized as a basis for designing
other structures, devices and circuits for carrying out the objects
of the invention. The disclosure is intended to be read by way of
illustration only, and the scope of the invention is to be measured
by the claims that follow.
* * * * *