U.S. patent application number 10/910216 was filed with the patent office on 2006-02-09 for photonic crystal resonator apparatus with improved out of plane coupling.
Invention is credited to Annette Grot, Laura Mirkarimi, Mihail Sigalas.
Application Number | 20060029347 10/910216 |
Document ID | / |
Family ID | 35694939 |
Filed Date | 2006-02-09 |
United States Patent
Application |
20060029347 |
Kind Code |
A1 |
Sigalas; Mihail ; et
al. |
February 9, 2006 |
PHOTONIC CRYSTAL RESONATOR APPARATUS WITH IMPROVED OUT OF PLANE
COUPLING
Abstract
A two-dimensional photonic crystal resonator apparatus in which
the power of light coupled out of the apparatus in one direction is
greater than the power of light coupled out of the apparatus in the
opposite direction The apparatus has a photonic crystal slab
waveguide structure having a waveguide and a resonator in the
vicinity of the waveguide such that light propagated through the
waveguide is extracted from the waveguide through the resonator and
is coupled out of the plane of the apparatus. The apparatus has
upper and lower cladding layers on the photonic crystal slab
waveguide structure having different indices of refraction, and the
power of light coupled out of the apparatus in the direction of the
cladding layer having the higher index of refraction is greater
than the power of the light coupled out of the apparatus in the
direction of the cladding layer having the lower index of
refraction.
Inventors: |
Sigalas; Mihail; (Santa
Clara, CA) ; Grot; Annette; (Cupertino, CA) ;
Mirkarimi; Laura; (Sunol, CA) |
Correspondence
Address: |
AGILENT TECHNOLOGIES, INC;Legal Department, DL 429
Intellectual Property Administration
P.O. Box 7599
Loveland
CO
80537-0599
US
|
Family ID: |
35694939 |
Appl. No.: |
10/910216 |
Filed: |
August 3, 2004 |
Current U.S.
Class: |
385/129 ;
385/122; 385/130; 385/131; 385/14 |
Current CPC
Class: |
G02B 6/1225 20130101;
B82Y 20/00 20130101; G02B 2006/12109 20130101 |
Class at
Publication: |
385/129 ;
385/014; 385/122; 385/130; 385/131 |
International
Class: |
G02B 6/00 20060101
G02B006/00; G02B 6/10 20060101 G02B006/10; G02B 6/12 20060101
G02B006/12 |
Claims
1. A two-dimensional photonic crystal resonator apparatus,
comprising: a photonic crystal slab waveguide structure, the
photonic crystal slab waveguide structure having a waveguide and a
resonator in the vicinity of the waveguide; a first cladding layer
having a first index of refraction on at least a portion of a first
surface of the photonic crystal slab waveguide structure; and a
second cladding layer having a second index of refraction higher
than the first index of refraction on at least a portion of a
second surface of the photonic crystal slab waveguide structure
such that a second power of light coupled out of the apparatus
through the second cladding is greater than a first power of light
coupled out of the apparatus through the first cladding layer.
2. The apparatus according to claim 1, wherein the waveguide
comprises a line of non-circular shaped holes.
3. The apparatus according to claim 2, wherein the line of
non-circular shaped holes comprises a line of elliptically-shaped
holes.
4. The apparatus according to claim 3, wherein the first cladding
layer comprises SiO.sub.2 and the second cladding layer comprises
Si.sub.3N.sub.4.
5. The apparatus according to claim 4, wherein the photonic crystal
slab waveguide structure comprises Si.
6. The apparatus according to claim 5, wherein the first cladding
layer comprises a lower cladding layer on at least a portion of a
lower surface of the photonic crystal slab waveguide structure in
the vicinity of the resonator, wherein the second cladding layer
comprises an upper cladding layer on at least a portion of an upper
surface of the photonic crystal slab waveguide structure in the
vicinity of the resonator, and wherein the second power of light
coupled out of the apparatus through the upper cladding layer is
greater than the first power of light coupled out of the apparatus
through the lower cladding layer.
7. The apparatus according to claim 1, wherein the photonic crystal
slab waveguide structure has an index of refraction that is higher
than the second index of refraction.
8. The apparatus according to claim 1, wherein the photonic crystal
slab waveguide structure further includes an array of
circular-shaped holes defining a periodic lattice of the photonic
crystal slab waveguide structure.
9. The apparatus according to claim 1, wherein the waveguide
comprises a line of circular-shaped holes having a radius less than
a radius of the circular-shaped holes defining a periodic lattice
of the photonic crystal slab waveguide structure.
10. The apparatus according to claim 9, wherein the first cladding
layer is air and the second cladding layer is SiO.sub.2.
11. The apparatus according to claim 1, wherein the two-dimensional
photonic crystal resonator apparatus comprises an add-drop
device.
12. The apparatus according to claim 1, wherein the first and
second cladding layers cover at least portions of the first and
second surfaces respectively, of the photonic crystal slab
waveguide structure in the vicinity of the resonator.
13. The apparatus according to claim 12, wherein the first and
second cladding layers fully cover the first and second surfaces,
respectively, of the photonic crystal slab waveguide structure.
14. A two-dimensional photonic crystal resonator apparatus,
comprising: a photonic crystal slab waveguide structure, the
photonic crystal slab waveguide structure having a first index of
refraction and including a waveguide comprising a line of air holes
and a resonator in the vicinity of the waveguide; a first
cladding-layer having a second index of refraction on at least a
portion of a first surface of the photonic crystal slab waveguide
structure in the vicinity of the resonator; and a second cladding
layer having a third index of refraction that is lower than the
first index of refraction and higher than the second index of
refraction on at least a portion of a second surface of the
photonic crystal slab waveguide structure in the vicinity of the
resonator such that a second power of light coupled out of the
apparatus through the second cladding layer is greater than a first
power of light coupled out of the apparatus through the first
cladding layer.
15. The apparatus according to claim 14, wherein the photonic
crystal slab wave guide structure further includes an array of
circular-shaped holes defining a periodic lattice of the photonic
crystal slab waveguide structure, and wherein the waveguide
comprises a line of elliptically-shaped holes.
16. The apparatus according to claim 14, wherein the photonic
crystal slab waveguide structure further includes an array of
circular-shaped holes defining a periodic lattice of the photonic
crystal slab waveguide structure, and wherein the waveguide
comprises a line of circular-shaped holes having a radius that is
less than the radius of the circular-shaped holes defining the
periodic lattice.
17. A method for fabricating a two-dimensional photonic crystal
resonator apparatus, comprising: providing a photonic crystal slab
waveguide structure having a waveguide and a resonator in the
vicinity of the waveguide; providing a first cladding layer having
a first index of refraction on at least a portion of a first
surface of the photonic crystal slab waveguide structure; and
providing a second cladding layer having a second index of
refraction higher than the first index of refraction on at least a
portion of a second surface of the photonic crystal slab waveguide
structure such that a second power of light coupled out of the
apparatus through the second cladding layer is greater than a first
power of light coupled out of the apparatus through the first
cladding layer.
18. The method according to claim 17, wherein the waveguide
comprises a line of elliptically-shaped holes.
19. The method according to claim 17, wherein the photonic crystal
slab waveguide structure further includes an array of
circular-shaped holes defining a periodic lattice of the photonic
crystal slab waveguide structure, and wherein the waveguide
comprises a line of circular-shaped holes having a radius that is
less than the radius of the circular-shaped holes defining the
periodic lattice.
20. The method according to claim 17, wherein the photonic crystal
slab waveguide structure has an index of refraction that is higher
than the second index of refraction.
Description
DESCRIPTION OF RELATED ART
[0001] Photonic crystals are periodic dielectric structures that
can prohibit the propagation of light in certain frequency ranges.
In particular, photonic crystals are structures that have spatially
periodic variations in refractive index; and with a sufficiently
high refractive index contrast, photonic bandgaps can be opened in
the structures' optical transmission characteristics. A "photonic
bandgap" is a frequency range within which propagation of light
through a photonic crystal is prevented.
[0002] A photonic crystal that has spatial periodicity in three
dimensions can prevent the propagation of light having a frequency
within the crystal's photonic bandgap in all directions, however,
the fabrication of a three-dimensional structure is often
technically challenging. An alternative is to utilize a
two-dimensional photonic crystal slab that has a two-dimensional
periodic lattice incorporated within it. In a two-dimensional
photonic crystal slab, light propagating in the slab is confined in
the direction perpendicular to a major surface of the slab via
total internal reflection, and light propagating in the slab in
directions other than perpendicular to a major surface is
controlled by properties of the photonic crystal slab. In a
two-dimensional photonic crystal slab, light propagating in the
slab in directions other than perpendicular to a slab face and
having a frequency within a photonic bandgap of the slab will not
propagate through the slab, while light having a frequency outside
the photonic bandgap is transmitted through the slab unhindered. In
addition to being easier to fabricate, two-dimensional photonic
crystal slabs provide the advantage that they are compatible with
the planar technologies of standard semiconductor processing.
[0003] It is known that the introduction of defects in the periodic
lattice of a photonic crystal allows the existence of localized
electromagnetic states that are trapped at the defect site, and
that have resonant frequencies within the bandgap of the
surrounding photonic crystal material. By providing a line of such
defects in a two-dimensional photonic crystal slab, a
two-dimensional photonic crystal slab waveguide is created that can
be used in the control and guiding of light. In a two-dimensional
photonic crystal slab waveguide, light of a given frequency that
would otherwise be prevented from propagating in the photonic
crystal slab may propagate in the defect region of the slab.
[0004] A two-dimensional photonic crystal resonator apparatus can
be formed by providing a resonator in the form of a resonant
chamber in a two-dimensional photonic crystal slab waveguide. One
implementation of a two-dimensional photonic crystal resonator
apparatus is as an add-drop device wherein light of a particular
frequency propagating along the waveguide couples to the resonator,
and from the resonator, couples out of the plane of the apparatus
(see "Investigation of a channel-add/drop filtering device using
acceptor-type point defects in a two-dimensional photonic crystal
slab", Takashi Asano et al., Applied Physics Letters, Vol. 83, No.
3, pages 407-409, 2003). The light coupled out of the plane of the
apparatus is light that is extracted from the light propagating
along the waveguide and the extracted light can, for example, be
redirected to a second waveguide for utilization in various
applications.
[0005] One example of a known two-dimensional photonic crystal
resonator apparatus implemented as an add-drop device is fabricated
from a bulk material having a periodic lattice of circular
air-filled columns, referred to as "holes", extending through the
bulk material in a height direction and periodic in the planar
direction. One line of the holes is omitted to provide a waveguide
having a narrow frequency width. For two-dimensional photonic
crystal membrane structures comprised of a high refractive index
photonic crystal slab having air cladding layers both above and
below the slab, the output power splits equally well from above and
below such that 50 percent of the output power goes to the top of
the slab, and 50 percent of the power goes to the bottom of the
slab.
[0006] In a photonic crystal resonator apparatus implemented as an
add-drop device, however, it is desirable to maximize the output
power of the light extracted from the apparatus in one direction at
the expense of the light extracted from the apparatus in the
opposite direction so that the extracted light can be more
efficiently used. Although it has been suggested that the light
extracted from the waveguide can be increased in one direction by
providing an asymmetry in the photonic crystal slab, such a
structure is very difficult to fabricate (see "Design of a channel
drop filter by using a donor-type cavity with high-quality factor
in a two-dimensional photonic crystal slab", Yoshihiro Akahane et
al., Applied Physics Letters, Vol. 82, No. 9, pages 1341-1343,
2003).
SUMMARY OF THE INVENTION
[0007] In accordance with the invention, a two-dimensional photonic
crystal resonator apparatus is provided in which the power of light
coupled out of the apparatus in one direction is greater than the
power of light coupled out of the apparatus in the opposite
direction. The apparatus has a photonic crystal slab waveguide
structure having a waveguide and a resonator in the vicinity of the
waveguide such that light propagated through the waveguide is
extracted from the waveguide through the resonator and is coupled
out of the plane of the apparatus. The apparatus has upper and
lower cladding layers on the photonic crystal slab waveguide
structure having different indices of refraction, and the power of
light coupled out of the apparatus in the direction of the cladding
layer having the higher index of refraction is greater than the
power of the light coupled out of the apparatus in the direction of
the cladding layer having the lower index of refraction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] Furthermore, the invention provides embodiments and other
features and advantages in addition to or in lieu of those
discussed above. Many of these features and advantages are apparent
from the description below with reference to the following
drawings.
[0009] FIG. 1a is a schematic cross-sectional top view that
illustrates a two-dimensional photonic crystal resonator apparatus
according to an exemplary embodiment of the invention;
[0010] FIG. 1b is a graph that shows light transmission
characteristics of the photonic crystal resonator apparatus of FIG.
1a;
[0011] FIG. 2 is a graph that shows light transmission
characteristics of the two-dimensional photonic crystal resonator
apparatus of FIG. 1a modified to include a Si.sub.3N.sub.4 upper
cladding layer;
[0012] FIG. 3a is a schematic cross-sectional top view that
illustrates a two-dimensional photonic crystal resonator apparatus
according to a further exemplary embodiment of the invention;
[0013] FIG. 3b is a graph that shows light transmission
characteristics of the photonic crystal resonator apparatus of FIG.
3a;
[0014] FIG. 4 is a schematic cross-sectional side view that
illustrates a two-dimensional photonic crystal resonator apparatus
according to a further exemplary embodiment of the invention;
and
[0015] FIG. 5 is a flowchart that illustrates a method for
fabricating a two-dimensional photonic crystal resonator apparatus
according to a further exemplary embodiment of the invention.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS OF THE
INVENTION
[0016] Embodiments in accordance with the invention provide a
two-dimensional photonic crystal resonator apparatus and a method
for fabricating a two-dimensional photonic crystal resonator
apparatus.
[0017] FIG. 1a is a schematic cross-sectional top view that
illustrates a two-dimensional photonic crystal resonator apparatus
according to an exemplary embodiment in accordance with the
invention. The apparatus is generally designated by reference
number 100 and comprises a Si photonic crystal slab 102 having a
periodic lattice formed by an array of circular air holes 106
extending through slab 102 from a top surface to a bottom surface
thereof. One line of elliptical air holes 108 is provided in slab
102 to define waveguide 110 that extends through slab 102 from
input end 112 to output end 114 thereof. In addition, an enlarged
circular air hole is provided in slab 102 in the vicinity of
waveguide 110 to provide a resonant chamber defining resonator
116.
[0018] In the exemplary embodiment in accordance with the invention
illustrated in FIG. 1a, circular air holes 106 each have a radius
of r=0.29a (a is the lattice constant). The thickness of Si slab
102 is t=0.6a. Elliptical air holes 108 have a short axis of 0.66a
and an ellipticity of 2.236. Resonator 116 has a radius of r=0.41a.
The Si slab is placed on a SiO.sub.2 substrate (not shown in FIG.
1a) as is typical in most two-dimensional photonic crystal devices
because photonic crystal membrane structures are difficult to
fabricate and are usually quite fragile. The SiO.sub.2 substrate
(refractive index 1.414) functions as a lower cladding layer of the
apparatus, and the upper cladding layer of the apparatus is air
(refractive index 1.0).
[0019] FIG. 1b is a graph that shows light transmission
characteristics of photonic crystal resonator apparatus 1100 of
FIG. 1a. A three-dimensional finite difference time domain method
was used to calculate the light transmission along waveguide 110,
and the power leaking out of plane of apparatus 100 through
resonator 116. As shown in FIG. 1b, at the resonance frequency of
resonator 116 (0.254c/a), there is an 8 dB drop of the light
transmission along waveguide 110, and a peak on the transmitted
power leaking out of the plane of photonic crystal slab 102. Eight
percent of the power goes to the SiO.sub.2 substrate functioning as
a lower cladding layer of the apparatus and seven percent of the
power leaks to air functioning as an upper cladding layer of the
apparatus. Thus, in two-dimensional photonic crystal resonator
apparatus 100, there is a slight increase in the light transmission
through the higher index of refraction SiO.sub.2 substrate relative
to the light transmission to the lower index of refraction air.
[0020] Light transmission along one direction can be further
increased in the two-dimensional photonic crystal resonator
apparatus of FIG. 1a by providing a dielectric slab such as
Si.sub.3N.sub.4 (refractive index 2) on top of photonic crystal
slab 102 as an upper cladding layer for apparatus 100 instead of an
air cladding layer. The SiO.sub.2 substrate again functions as the
lower cladding layer. FIG. 2 is a graph that shows light
transmission characteristics of the two-dimensional photonic
crystal resonator apparatus of FIG. 1a modified to include a
Si.sub.3N.sub.4 upper cladding layer. The resonant frequency moves
to lower frequencies (0.248c/a) due to the increase in the
dielectric constant of the upper cladding layer. As shown in FIG.
2, there is a 7 dB drop of the transmission along the waveguide.
Twenty-three percent of the power leaks to the higher index of
refraction Si.sub.3N.sub.4 upper cladding layer and thirteen
percent leaks to the lower index of refraction SiO.sub.2 lower
cladding layer. Thus, the transmitted power from the
Si.sub.3N.sub.4 upper cladding layer is almost three times higher
than in the case of an air upper cladding layer.
[0021] There is an optimum ratio of the index of refraction between
the upper and lower cladding layers of two-dimensional photonic
crystal resonator apparatus 100 to effectively increase the
transmission power in one direction out of the plane of photonic
crystal slab 102. The optimum ratio to effectively increase the
transmission power from the upper cladding layer depends on the
particular defect configuration, the waveguide geometry, the radius
of the circular air holes, and the thickness and the refractive
index of photonic crystal slab 102. The refractive index of the
upper cladding layer should be higher than the refractive index of
the lower cladding layer and lower than the refractive index of the
photonic crystal slab.
[0022] FIG. 3a is a schematic top cross-sectional view that
illustrates a photonic crystal resonator apparatus according to a
further exemplary embodiment in accordance with the invention. The
apparatus is generally designated by reference number 300 and
comprises a Si photonic crystal slab 302 having a periodic lattice
formed by an array of circular air holes 306 extending through slab
302 from a top surface to a bottom surface thereof, and having a
radius of 0.29a. A waveguide 310 extends through slab 302 from
input end 312 to output end 314 and is formed by a line of air
holes 308 having a radius of 0.1a. A circular air hole having a
radius of 0.59a is also provided in slab 302 to define resonator
316. Photonic crystal slab 302 was placed on top of a relatively
low index of refraction material such as SiO.sub.2 (refractive
index 1.414) to function as a lower cladding layer (not shown). The
upper cladding layer was air (refractive index 1.0).
[0023] FIG. 3b is a graph that shows the light transmission
characteristics of photonic crystal resonator apparatus 300 of FIG.
3a. In particular, FIG. 3a shows the transmission along the
waveguide and the transmitted power leaking to the substrate
through resonator 316, and the transmitted power leaking to the air
through resonator 316. The resonance frequency is at
a/.lamda.=0.274 and most of the power leaks to the lower cladding
layer that has a higher index of refraction than the upper air
cladding layer.
[0024] FIG. 4 is a schematic cross-sectional side view that
illustrates a two-dimensional photonic crystal resonator apparatus
according to a further exemplary embodiment in accordance with the
invention. The apparatus is generally designated by reference
number 400, and includes photonic crystal slab waveguide 402, for
example, photonic crystal slab waveguide 102 illustrated in FIG.
1a, upper cladding layer 404, lower cladding layer 406 and
substrate 408. Photonic crystal slab waveguide 402 comprises a
single crystalline material having a high index of refraction (n=3
to 4) such as Si, Ge or a compound semiconductor such as GaAs and
InP. For a Si photonic crystal slab waveguide, lower cladding layer
406 comprises a material having an index of refraction of
approximately 1.5 such as SiO.sub.2 or spin on glass. The
refractive index of upper cladding layer 404 is higher than the
index of refraction of lower cladding layer 406 and lower than the
index of refraction of slab waveguide 402.
[0025] The upper cladding layer can, for example, be
Si.sub.3N.sub.4 which has a refractive index of 1.9. Other
materials that can also be used for upper cladding layer 406
include MgO (n=1.8), Al.sub.2O.sub.3 (n=1.76), ZrSiO.sub.4
(n=1.95), SrO (n.about.2.0), Ta.sub.2O.sub.5 (n=2.2),
Sr.sub.xBa.sub.(1-x)TiO.sub.3 (n=2.2) and TiO.sub.2
(n=2.4-2.7).
[0026] For compound semiconductor devices, lower cladding layer 406
can be Al.sub.2O.sub.3 due to the ease of formation of epitaxial
layers with aluminum containing compounds which are later
controllably oxidized to form Al.sub.2O.sub.3. In this exemplary
embodiment in accordance with the invention, upper cladding layer
404 should have a refractive index higher than Al.sub.2O.sub.3
(n=1.76), and in particular, should be a material having a
refractive index n>2 such as SrO, Ta.sub.2O.sub.5 and TiO.sub.2.
Substrate 408 is preferably formed of materials such as Si, Ge,
GaAs and InP.
[0027] Although, in FIG. 4, upper and lower cladding layers are
shown as fully covering the upper and lower surfaces of slab
waveguide 402, in alternative embodiments, the cladding layers can
cover only the portions of the slab waveguide surfaces in the
vicinity of the resonator.
[0028] Photonic crystal resonator apparatus 400 illustrated in FIG.
4 can be fabricated by being patterned into a resist by using
electron beam lithography or another nanolithography technique. The
pattern is then transferred into the upper cladding layer by a
selective etch technique. The reverse pattern can also be
fabricated so that a metal lift-off technique may be used to
prepare a hard mask of metal. This may be useful in obtaining good
etch selectivity when good etch selectivity does not exist between
the resist and the upper cladding layer. After transferring the
pattern into the upper cladding layer, a selective etch is used to
etch away the Si without removing the upper cladding layer.
[0029] FIG. 5 is a flow chart that illustrates a method for
fabricating a two-dimensional photonic crystal resonator apparatus
according to a further exemplary embodiment in accordance with the
invention. The method is generally designated by reference number
500 and includes providing a photonic crystal slab waveguide
structure having a waveguide and a resonator in the vicinity of the
waveguide (step 502). An upper cladding layer having a first index
of refraction and that covers at least a portion of an upper
surface of the photonic crystal slab waveguide in the vicinity of
the resonator is provided (step 504). A lower cladding layer having
a second index of refraction different from the first index of
refraction and that covers at least a portion of a lower surface of
the photonic crystal slab waveguide in the vicinity of the
resonator is also provided (step 506) to complete the
apparatus.
[0030] While what has been described constitute exemplary
embodiments in accordance with the invention, it should be
recognized that the invention can be varied in numerous ways
without departing from the scope thereof. Because embodiments in
accordance with the invention can be varied in numerous ways, it
should be understood that the invention should be limited only in
so far as is required by the scope of the following claims.
* * * * *