U.S. patent application number 17/137806 was filed with the patent office on 2022-06-30 for photonic device structure and method of manufacturing the same, and electro-optic waveguide.
The applicant listed for this patent is City University of Hong Kong. Invention is credited to Kin Seng Chiang, Jingdong Luo.
Application Number | 20220206324 17/137806 |
Document ID | / |
Family ID | |
Filed Date | 2022-06-30 |
United States Patent
Application |
20220206324 |
Kind Code |
A1 |
Chiang; Kin Seng ; et
al. |
June 30, 2022 |
PHOTONIC DEVICE STRUCTURE AND METHOD OF MANUFACTURING THE SAME, AND
ELECTRO-OPTIC WAVEGUIDE
Abstract
A photonic device structure includes: an electro-optical
structure including a layer of optical material sandwiched by a
pair of electrodes, wherein the layer of optical material is
arranged to undergo an electro-optic activity when subjected to a
voltage bias across the pair of electrodes; and a cladding layer
adjacent to the electro-optical structure.
Inventors: |
Chiang; Kin Seng; (Kowloon,
HK) ; Luo; Jingdong; (Kowloon, HK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
City University of Hong Kong |
Kowloon |
|
HK |
|
|
Appl. No.: |
17/137806 |
Filed: |
December 30, 2020 |
International
Class: |
G02F 1/065 20060101
G02F001/065; G02F 1/01 20060101 G02F001/01 |
Claims
1. An electro-optic waveguide comprising: an electro-optical
structure including a layer of optical material sandwiched by a
pair of electrodes, each of the pair of electrodes comprising a
non-metallic portion having graphene, wherein the layer of optical
material is arranged to undergo an electro-optic activity when
subjected to a voltage bias across the pair of electrodes; and a
cladding layer adjacent to the electro-optical structure.
2. The electro-optic waveguide in accordance with claim 1, wherein
the electro-optic activity includes a change in the electro-optic
coefficient of the layer of optical material.
3. The electro-optic waveguide in accordance with claim 2, wherein
the electro-optic coefficient is 80-90 pm/V at a wavelength of 1541
nm or 100-120 pm/V at a wavelength of 1300 nm.
4. The electro-optic waveguide in accordance with claim 1, wherein
the electro-optic activity includes a change in the refractive
index of the layer of optical material.
5. The electro-optic waveguide in accordance with claim 4, wherein
the refractive index is changed from an isotropic refractive index
to an anisotropic refractive index.
6. (canceled)
7. (canceled)
8. The electro-optic waveguide in accordance with claim 1, wherein
the non-metallic portion comprises one or more layers of graphene
films.
9. The electro-optic waveguide in accordance with claim 1, wherein
each of the pair of electrodes further comprises a metallic
portion.
10. The electro-optic waveguide in accordance with claim 9, wherein
the metallic portion comprises gold.
11. The electro-optic waveguide in accordance with claim 9, wherein
the metallic portion is positioned on an edge of each of the pair
of electrodes.
12. The electro-optic waveguide in accordance with claim 1, wherein
the layer of optical material comprises an organic non-linear
optical material.
13. The electro-optic waveguide in accordance with claim 12,
wherein the layer of optical material comprises a host polymer
containing a guest non-linear optical material.
14. The electro-optic waveguide in accordance with claim 13,
wherein the host polymer is selected from a group consisting of: a
side-chain polymer and a dendrimer.
15. The electro-optic waveguide in accordance with claim 13,
wherein the guest non-linear optical material is a plurality of
non-linear optical chromophores.
16. The electro-optic waveguide in accordance with claim 15,
wherein the electro-optic activity includes a change in alignment
of the guest non-linear optical material.
17. The electro-optic waveguide in accordance with claim 16,
wherein the guest non-linear optical material is aligned with a
non-centrosymmetric order when subjected to the voltage bias across
the pair of electrodes.
18. The electro-optic waveguide in accordance with claim 1, wherein
the layer of optical material comprises a pyroelectric
material.
19. The electro-optic waveguide in accordance with claim 18,
wherein the layer of optical material comprises a ferroelectric
material.
20. The electro-optic waveguide in accordance with claim 19,
wherein the ferroelectric material is arranged to generate an
electrostatic field when subjected to the voltage bias.
21. (canceled)
22. The electro-optic waveguide in accordance with claim 1, wherein
the layer of optical material is further arranged to undergo the
electro-optic activity when heated at the glass transition
temperature of the optical material.
23. The electro-optic waveguide in accordance with claim 1, wherein
the layer of optical material is further arranged to undergo the
electro-optic activity when cooled from the glass transition
temperature of the optical material.
24. (canceled)
25. The electro-optic waveguide in accordance with claim 1, wherein
the pair of electrodes comprises an upper electrode above the layer
of optical material, and a lower electrode between the layer of
optical material and the cladding layer, and wherein the upper
electrode comprises a patterned structure.
26. A method of manufacturing a photonic device structure,
comprising the steps of: wet-transferring a first layer of
conductive material onto a cladding layer to form a first
electrode; coating a layer of optical material on the first
electrode; and transferring a second layer of conductive material
onto the layer of optical material to form a second electrode,
wherein the layer of optical material is arranged to undergo an
electro-optic activity when subjected to a voltage bias across the
first and second electrodes.
27. The method in accordance with claim 26, further comprising,
after forming the second electrode, heating to a temperature close
to the glass transition temperature of the optical material to
obtain a uniform contact between the first and second electrodes
and the layer of optical material.
28. The method in accordance with claim 26, wherein each of the
first and second layers of conductive material comprises
graphene.
29. The method in accordance with claim 28, wherein the first layer
of conductive material further comprises a polymer, and wherein the
method further comprises, before coating the layer of optical
material, the step of removing the polymer from the first layer of
conductive material.
30. The method in accordance with claim 26, wherein the step of
transferring a second layer of conductive material comprises
transferring a plurality of the second layer of conductive material
to form the second electrode.
Description
TECHNICAL FIELD
[0001] The invention relates to a photonic device structure and a
method of manufacturing the same. The invention also relates to an
electro-optic waveguide including the photonic device
structure.
BACKGROUND
[0002] Photonic devices, such as lasers, LEDs, modulators and
photodetectors, are exploited in various applications due to their
capability in light generation, detection, or manipulation. For
example, photonic devices may be used as a waveguide for guiding
electromagnetic waves in the optical spectrum.
[0003] Recently, photonic devices including electro-optic (EO)
polymer materials, e.g., high-performance EO modulators containing
EO polymers that possess strong, ultrafast EO effects, silicon
modulators based on EO polymer that show low driving voltages and
large bandwidths, etc., are gaining attentions in the field.
SUMMARY
[0004] In a first aspect, there is provided a photonic device
structure comprising: an electro-optical structure including a
layer of optical material sandwiched by a pair of electrodes,
wherein the layer of optical material is arranged to undergo an
electro-optic activity when subjected to a voltage bias across the
pair of electrodes; and a cladding layer adjacent to the
electro-optical structure.
[0005] In one embodiment of the first aspect, the electro-optic
activity includes a change in the electric-optic coefficient of the
layer of optical material.
[0006] In one embodiment of the first aspect, the electric-optic
coefficient is 80-90 pm/V at a wavelength of 1541 nm or 100-120
pm/V at a wavelength of 1300 nm.
[0007] In one embodiment of the first aspect, the electro-optic
activity includes a change in the refractive index of the layer of
optical material.
[0008] In one embodiment of the first aspect, the refractive index
is changed from an isotropic refractive index to an anisotropic
refractive index.
[0009] In one embodiment of the first aspect, each of the pair of
electrodes comprises a non-metallic portion.
[0010] In one embodiment of the first aspect, the non-metallic
portion comprises graphene.
[0011] In one embodiment of the first aspect, the non-metallic
portion comprises one or more layers of graphene films.
[0012] In one embodiment of the first aspect, each of the pair of
electrodes further comprises a metallic portion.
[0013] In one embodiment of the first aspect, the metallic portion
comprises gold.
[0014] In one embodiment of the first aspect, the metallic portion
is positioned on an edge of each of the pair of electrodes.
[0015] In one embodiment of the first aspect, the layer of optical
material comprises an organic non-linear optical material.
[0016] In one embodiment of the first aspect, the layer of optical
material comprises a host polymer containing a guest non-linear
optical material.
[0017] In one embodiment of the first aspect, the host polymer is
selected from a group consisting of: a side-chain polymer and a
dendrimer.
[0018] In one embodiment of the first aspect, the guest non-linear
optical material is selected from a group consisting of: a
plurality of non-linear optical chromophores, a plurality of binary
non-linear optical chromophores, and a plurality of non-linear
optical chromophores.
[0019] In one embodiment of the first aspect, the electro-optic
activity includes a change in alignment of the guest non-linear
optical material.
[0020] In one embodiment of the first aspect, the guest non-linear
optical material is aligned with a non-centrosymmetric order when
subjected to the voltage bias across the pair of electrodes.
[0021] In one embodiment of the first aspect, the layer of optical
material comprises a pyroelectric material.
[0022] In one embodiment of the first aspect, the layer of optical
material comprises a ferroelectric material.
[0023] In one embodiment of the first aspect, the ferroelectric
material is arranged to generate an electrostatic field when
subjected to the voltage bias.
[0024] In one embodiment of the first aspect, the electrodes are
arranged to generate an electric field to the layer of optical
material upon subjected to a voltage bias.
[0025] In one embodiment of the first aspect, the layer of optical
material is further arranged to undergo the electro-optic activity
when heated at a temperature close to the glass transition
temperature of the optical material.
[0026] In one embodiment of the first aspect, the layer of optical
material is further arranged to undergo the electro-optic activity
when cooled from a temperature close to the glass transition
temperature of the optical material.
[0027] In a second aspect, there is provided an electro-optic
waveguide comprising the photonic device structure in the first
aspect.
[0028] In one embodiment of the second aspect, the pair of
electrodes comprises an upper electrode above the layer of optical
material, and a lower electrode between the layer of optical
material and the cladding layer, the upper electrode comprises a
patterned structure.
[0029] In a third aspect, there is provided a method of
manufacturing a photonic device structure. The method comprises the
steps of: wet-transferring a first layer of conductive material
onto a cladding layer to form a first electrode; coating a layer of
optical material on the first electrode; and transferring a second
layer of conductive material onto the layer of optical material to
form a second electrode. The layer of optical material is arranged
to undergo an electro-optic activity when subjected to a voltage
bias across the first and second electrodes.
[0030] In one embodiment of the third aspect, the method further
comprises, after forming the second electrode, heating to a
temperature close to the glass transition temperature of the
optical material to obtain a uniform contact between the first and
second electrodes and the layer of optical material.
[0031] In one embodiment of the third aspect, each of the first and
second layers of conductive material comprises graphene.
[0032] In one embodiment of the third aspect, the first layer of
conductive material further comprises a polymer. The method further
comprises, before coating the layer of optical material, the step
of removing the polymer from the first layer of conductive
material.
[0033] In one embodiment of the third aspect, the step of
transferring a second layer of conductive material comprises
transferring a plurality of the second layer of conductive material
to form the second electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] Embodiments of the invention will now be described, by way
of example, with reference to the accompanying drawings in
which:
[0035] FIG. 1 is a schematic diagram of an example photonic device
structure, where light propagates in the y direction;
[0036] FIG. 2 is a schematic diagram of a photonic device structure
in accordance with one embodiment of the invention, where light
propagates in the y direction;
[0037] FIG. 3 is a flow chart of a method of manufacturing a
photonic device structure in accordance with one embodiment of the
invention;
[0038] FIG. 4A is a graph of the dependence of graphene-induced
loss of the TM.sub.0 mode at 1550 nm on the thickness of the
electro-optic (EO) polymer film calculated at different refractive
indices (in the z direction shown in FIG. 2) of the EO polymer of a
photonic device structure with only a bottom graphene
electrode;
[0039] FIG. 4B is a graph of the dependence of graphene-induced
loss of the TM.sub.0 mode at 1550 nm on the thickness of the EO
polymer film calculated at different refractive indices (in the z
direction shown in FIG. 2) of the EO polymer of a photonic device
structure with both a top graphene electrode and a bottom graphene
electrode;
[0040] FIG. 5 is a graph showing variations of the leakage current
with the temperature during poling of four sample photonic device
structures;
[0041] FIG. 6A is an image of a poled sample 2 in FIG. 5, showing
the poled and unpoled areas on the sample; and
[0042] FIG. 6B is an image of a poled sample 4 in FIG. 5, showing
the poled and unpoled areas on the sample, as well as the locations
of the prism together with the optical paths for the measurement of
the transmission of the TM.sub.0 mode through the poled and unpoled
areas.
DETAILED DESCRIPTION
[0043] The inventors have devised that, for electro-optic (EO)
devices using an EO polymer film, it is desired to obtain a large
EO coefficient, which would favorably affect the
transverse-magnetic (TM) polarization. To obtain a large EO
coefficient from an EO polymer film, it is necessary to align the
chromophores in the EO polymer. This can be done by an electric
poling process, where a sufficiently strong electric field is
applied across the EO polymer film, while the film is heated up to
its glass transition temperature. Traditionally, metal and indium
tin oxide (ITO) electrodes are used in electric poling.
[0044] FIG. 1 shows an example photonic device structure 100
configured as an EO polymer thin-film waveguide. The structure 100
includes a bottom ITO electrode 102 deposited on a substrate 104, a
cladding 106 between the bottom electrode 102 and an EO polymer
film 108, which in turn is in direct contact with a top (Au)
electrode 110. The top electrode 110 must be removed after poling;
otherwise it would adversely affect the operation. Because the
bottom electrode 102 cannot be removed after poling, the cladding
106 must be thick enough to serve as a buffer layer between the
electrode 102 and the EO polymer film 108 to keep the metal-induced
absorption loss low.
[0045] The inventors have found that the waveguide 100 may exhibit
limitations such as requiring a high poling voltage and complicated
manufacturing processes. Due to high loss of metal electrode in the
conventional waveguides, there has to be a buffer layer between
metal electrode and EO polymer layer to attenuate the optical loss.
Such device configuration leads to complication of waveguide
fabrication and reduces the poling efficiency of EO material. The
choice of a buffer material that has both high conductivity and
good optical transparency is a challenge. Sol-gel conductive
cladding, whose resistivity is smaller than that of EO polymer by
orders of magnitude, is usually used as the buffer material, but
its synthesis involves complicated chemical processes.
[0046] The inventors have devised, through their own research,
trials and experiments, that an improved photonic device structure
may be provided with the use of graphene electrodes for electric
poling of EO polymer to exempt the need of using buffer layers and
minimize the poling voltage, based on a finding that a graphene
film embedded in or placed on a low-index-contrast waveguide does
not induce a significant loss to the TM waves. This property of
graphene has been explored for the realization of a lithium niobate
EO grating, a polymer thermo-optic switch, and an all-optical
switch to improve their performances. Graphene holds the advantages
of excellent conductivity and low-loss at communication band. As
such, using graphene poling electrodes, buffer layers between EO
layer and electrodes are no longer necessary. This will effectively
improve the poling efficiency and simplify the device
fabrication.
[0047] FIG. 2 shows a photonic device structure 200 in accordance
with a preferred embodiment of the invention. The illustrated
photonic device structure 200 is configured as a slab waveguide
capable of exhibiting an electro-optic effect that alters the
optical properties of the photonic device structure 200 in response
to an electric field. The illustrated photonic device structure 200
may be used in applications other than as an EO waveguide. For
example, the structure 200 can be widely applied to the processing
of organic and polymeric non-linear optical (NLO) materials in a
variety of optoelectronic and photonic devices, including
electro-optic modulators, silicon-organic hybrid waveguides,
optical electric field sensors, new frequency generation, and
all-optical Switches. In one example embodiment, the photonic
device structure 200 is used in an EO waveguide, with a patterned
structure on the upper graphene electrode, forming a modulation
electrode.
[0048] The photonic device structure 200 includes an
electro-optical structure including a layer of optical material 208
sandwiched by a pair of electrodes 202, 210, and a cladding layer
206 adjacent to the electro-optical structure. Specifically, the
photonic device structure 200 includes a layered structure formed
by different components in the following order: a substrate 204,
the cladding 206, and the EO structure including the lower
electrode 202, the layer of optical material 208, and the upper
electrode 210. In other words, both electrodes are in direct
contact with the EO material. In operation, the poling electric
field is loaded by both the upper and lower electrodes 202, 210,
which in turn is applied to the layer of optical material 208
sandwiched therebetween.
[0049] The substrate 204 may be any substrate for fabrication of
photonic device, which can be made of glass, silicon, borosilicate,
or ceramic. Preferably, the substrate 204 is opaque to minimize any
adverse impact on the electro-optic effect during operation. The
cladding 206 may be any conductive material which has a refractive
index smaller than that of the optic material, e.g., EpoClad.
[0050] The optic material may be any materials capable of
undergoing an electro-optic activity when subjected to a voltage
bias across the pair of electrodes 202, 210. Preferably, the optic
material is an organic non-linear optical material, e.g. a host
polymer containing a guest non-linear optical material, such as a
side-chain polymer containing non-linear optical chromophores, a
side-chain polymer containing binary non-linear optical
chromophores, a dendrimer containing non-linear optical
chromophores, a cross-linkable polymer containing non-linear
optical chromophores, and a cross-linkable dendrimer containing
non-linear optical chromophores. In addition, the optic material
may include a pyroelectric material, such as a ferroelectric
material that is arranged to generate an electrostatic field when
subjected to the voltage bias. In one embodiment, the optical
material is in the form of an EO polymer film.
[0051] Each electrode 202, 210 is made of a conductive material and
includes a non-metallic portion 212, 214, e.g. made of graphene,
and a metallic portion 216, 218, e.g. made of gold, and connected
to the non-metallic portion 212, 214, as well as to the respective
positive and negative terminals of a voltage source which generates
the poling electric field. The non-metallic portions 212, 214 may
include one or more layers of graphene films. Preferably, the
non-metallic portions 212, 214 are the main component of the
electrodes 202, 210 arranged to generate and apply an electric
field to the optic material upon subjected to a voltage bias to
activate the electro-optic effect. As such, the metallic portions
216, 218 are only required to occupy a small area of the electrodes
202, 210 that is sufficient for serving as an electrical contact
for connection to the voltage bias. In the illustrated embodiment,
both metallic portions 216, 218 are arranged on an edge of the
electrodes 202, 210.
[0052] Such arrangement of EO structure allows the electric field
in the EO polymer generated by the applied voltage to depend only
on the resistance of the EO polymer film, thus minimizing the
poling voltage required. More importantly, such arrangement
provides numerous options of the material and the thickness of the
cladding 206.
[0053] In addition to being activated when subjected to a voltage
bias as discussed above, the EO activity may be activated when the
layer of optic material 208 is heated from, e.g. an ambient
temperature, to a temperature close to the glass transition
temperature or a cross-linking temperature of the optic material,
or when the layer of optic material 208 is cooled from a
temperature close to the glass transition temperature of the optic
material to, e.g. an ambient temperature. The EO activity of the
optic material may, alternatively or additionally, include a change
in the EO coefficient of the optic material after poling, e.g. to
80-90 pm/V at a wavelength of 1541 nm or 100-120 pm/V at a
wavelength of 1300 nm, a change in the refractive index of the
layer of optical material 208 after poling, e.g. from an isotropic
refractive index to an anisotropic refractive index, a change in
alignment of the guest non-linear optical material after poling,
e.g. to being aligned with a non-centrosymmetric order.
[0054] FIG. 3 shows a method 300 of manufacturing the photonic
device structure 200. The method 300 begins in step 302, where a
first layer of conductive material, e.g. graphene, is
wet-transferred onto the cladding layer 206 to form the lower
electrode 202. In some embodiments, the first layer of conductive
material may include a polymer in addition to graphene, the method
300 further includes a step of removing the polymer from the first
layer of conductive material, to ensure for an effective
conductivity of the final electrode 202. Then, in step 304, a layer
of optical material 208, e.g. EO polymer, is coated on the lower
electrode 202. For example, the layer of optical material 208 may
be spin-coated onto the lower electrode 202. Next, in step 306, a
second layer of conductive material, e.g. graphene, is transferred
onto the layer of EO polymer 208, forming the upper electrode 210.
This step 306 may be repeated to obtain an upper electrode 210 with
different layers of the conductive material. Optionally, to obtain
a uniform contact between the layer of EO polymer 208 and both
electrodes 202, 210, the three layers 208, 202, 210 may be heated
to a temperature close to the glass transition temperature of the
EO polymer.
[0055] For example, the structure 200 of FIG. 2 may be fabricated
in an exemplary embodiment, in which a thick (>10 .mu.m) EpoClad
film was first spin-coated on a silicon substrate and the film was
thermally cured. Then, a 5-mm wide Au patch (.about.50 nm) was
sputtered near the edge of the EpoClad film to facilitate bottom
electrode contact. Next, a 1.8.times.1 cm2 graphene/PMMA bilayer
(SixCarbon technology, Shenzhen) was wet-transferred onto the
EpoClad film and the PMMA was removed by acetone. The graphene film
covered part of the Au patch. Next, an EO polymer film was
spin-coated on the graphene film. Due to the large viscosity of the
EO polymer, the quantity of the EO polymer in the spinning process
can be controlled as desired, so that the Au-connected part of the
bottom graphene electrode was left uncoated. Next, a 5-mm wide Au
patch (.about.50 nm) was sputtered on one side of the EO polymer
film to facilitate top electrode contact. Then, two separate
graphene/PMMA bilayers (with an area of .about.1.0.times.1.0 cm2
each) were transferred onto the EO polymer film side-by-side, one
of which covered part of the Au patch. To ensure uniform contact
between the graphene films and the EO polymer film, the sample was
heated to a temperature near the glass transition temperature of
the EO polymer. The thin PMMA layer (.about.300 nm) was kept in the
transfer of the graphene films, as removal of the PMMA layer by
acetone would damage the EO polymer film. This thin PMMA layer does
not significantly affect the loss of the sample, as confirmed by
the simulation.
[0056] The inventors also tested the devices fabricated in
accordance with embodiments of the present inventinon to determine
the dependence of the graphene-induced loss on the parameters of
poled EO polymers. In one experimentation, EpoClad (Micro Resist
Technology GmbH), which has a refractive index of 1.5595 measured
at the wavelength 1537 nm, was used as the cladding material.
EpoClad is a durable polymer material with a high glass transition
temperature that has been employed to form waveguides embedded in
printed circuit boards. The graphene-induced losses of the TM.sub.0
mode was calculated with a commercial mode solver (COMSOL) for
different parameters of the EO polymer film, where the monolayer
graphene electrodes are modeled as conductive boundaries with a
complex conductivity of
6.084.times.10.sup.-5-j7.519.times.10.sup.-6 at 1550 nm and the
material losses of the substrate, the cladding, and the EO polymer
are ignored. The refractive index of the EO polymer film after
poling depends on the strength of the poling electric field and the
EO polymer. The poling process results in an EO coefficient
r.sub.33, which mainly affects the TM polarization. Before poling,
the EO polymer film has an isotropic refractive index. After
poling, the EO polymer becomes anisotropic; the refractive index
perpendicular to the surface of the waveguide (i.e., along the z
direction shown in FIG. 2) increases significantly, while the
refractive indices parallel to the waveguide surface (i.e., along
the x and y directions shown in FIG. 2) decrease slightly. In the
calculation, the refractive index of the EO polymer film along the
z direction was varied from 1.58 to 1.68, while the refractive
indices along the x and y directions were fixed at 1.58.
[0057] FIG. 4A shows the results for the case where only the bottom
graphene electrode is present and FIG. 4B shows the results for the
case that both the bottom and the top graphene electrode are
present. A comparison of these two sets of results shows that the
top electrode induces a slightly larger loss than the bottom
electrode. As shown in FIGS. 4A and 4B, the graphene-induced loss
decreases with an increase in the thickness of the EO polymer film,
which can be explained by the fact that a stronger light
confinement in the EO polymer film with a thicker film leads to a
weaker mode field at the graphene electrodes and hence a smaller
graphene-induced loss. In general, a large refractive index of the
EO polymer leads to a larger graphene-induced loss, which can be
explained by the increase in the evanescent field caused by the
larger refractive-index contrast. To keep the graphene-induced loss
low, we should choose a sufficiently large thickness for the EO
polymer film. With a thick enough EO polymer film, the
graphene-induced loss becomes insensitive to the refractive index
of the EO polymer.
[0058] The EO polymer used is a simple guest-host system by doping
15 wt % of dipolar polyene chromophore AJLZ53 into the random
copolymer poly(styrene-co-methyl methacrylate) (hereafter
AJLZ53/P(S-co-MMA)). A 6.3 wt % of the solid polymer was dissolved
in dibromomethane and filtered with a 0.22-.mu.m
polytetrafluoroethylene filter to form an EO polymer solution ready
for spin-coating.
[0059] A reference waveguides as shown FIG. 1 was fabricated for
comparison. To fabricate this device, an EpoClad film was first
spin-coated on an ITO-coated glass substrate and the film was
thermally cured. Next, an EO polymer film was spin-coated onto the
cured EpoClad film. To expose the ITO electrode, some of the
polymer film was scraped off the substrate. After baking the sample
in a vacuum oven at 70.degree. C. overnight, a thin layer of Au
(.about.50 nm) was sputtered onto the EO polymer film as the top
electrode. Two reference samples with Au/ITO electrodes were
prepared: Sample 1, where the thicknesses of the EpoClad and the EO
polymer film were 5.1 .mu.m and 4.0 .mu.m, respectively, and Sample
2, where the thicknesses of the EpoClad and the EO polymer film
were 4.5 .mu.m and 3.8 .mu.m, respectively.
[0060] In subsequent experiments, to evaluate the effect of poling
process of the photonic device structure of FIG. 2 being
fabricated, only the part of the EO polymer film covered by the
graphene film connected to the Au patch was poled. The other part
covered by the graphene film that was not connected to the Au patch
was not poled and served as a reference. Two samples with graphene
electrodes were prepared: Sample 3 and Sample 4, where the
thicknesses of the EO polymer film were 3.5 .mu.m and 3.8 .mu.m,
respectively.
[0061] Each sample was poled by applying a voltage ramping up
gradually from 0 V to a final voltage with a voltage source
(Keithley 2657A), while heating up the sample from 40.degree. C. to
115.degree. C. (the glass transition temperature of the EO polymer)
at a rate of 10.degree. C./min with a hot stage. The resistivity of
EO polymer was measured to be in the range of
6.times.10.sup.8-2.times.10.sup.9 .OMEGA.m and that of EpoClad was
comparable (.about.10.sup.9 .OMEGA.m). For Samples 1 and 2, the
final voltages used were 910 V and 800 V, respectively, and the
corresponding electric fields in the EO polymer films were
estimated to be .about.100 V/.mu.m (assuming the same resistivity
for EpoClad and the EO polymer). For Samples 3 and 4, the final
voltages used were 350 V and 420 V, respectively, and the
corresponding electric fields in the EO polymer films were 100
V/.mu.m and 110 V/.mu.m. The poled areas of Samples 1, 2, 3, and 4
were .about.1.2 cm.sup.2, .about.0.66 cm.sup.2, .about.1.0
cm.sup.2, and .about.1.0 cm.sup.2, respectively.
[0062] During the poling process, the leakage current was
monitored. The rotation of chromophores and polymer chains at the
poling voltage and the high temperature gave rise to a rapid
increase in the leakage current by more than two orders of
magnitude, which is a signature of the completion of the poling
process. The variations of the leakage current with the temperature
during the poling of the four samples are shown in FIG. 5. The
rising of the leakage current reflects the poling characteristics.
The leakage currents for Samples 1 and 2 are smaller than those for
Samples 3 and 4, which is due to the presence of buffer layers in
the first two samples. As the buffer of Sample 1 is thicker than
that of Sample 2 (5.1 .mu.m versus 4.5 .mu.m), the poling voltage
required for Sample 1 is higher than that of Sample 2. On the other
hand, the poling voltages for Samples 3 and 4, which do not contain
any buffer layers, are much lower than those for Samples 1 and 2.
Furthermore, the leakage current for Sample 4 has the largest
value, which indicates that this sample should experience the
highest electric-field strength and hence show the best poling
result.
[0063] After poling, the samples were cooled down to room
temperature for further analysis and measurement. For Samples land
2, the Au top electrodes was removed with an Au etchant solution.
Because the poling process changed the optical absorbance and the
refractive index of the EO polymer, the poled and unpoled areas on
Samples 1 and 2, which were formed on transparent glass substrates,
showed slightly different colors and could be easily differentiated
by naked eyes, as shown in FIG. 6A. On the other hand, visible
differentiation of the poled and unpoled areas on Samples 3 and 4
was difficult, as shown in FIG. 6B, as these samples were formed on
opaque silicon substrates.
[0064] The refractive indices, the EO coefficients r.sub.33, and
the propagation losses of the samples were measured with a
commercial prism-coupler system (Metricon 2010). Before poling, the
refractive indices of the EO polymer films were .about.1.58 and
.about.1.59 for both polarizations at the wavelengths 1541 nm and
1300 nm, respectively. After poling, the refractive indices for the
TM polarization were increased significantly, which means that the
poled EO polymer films became highly birefringent. The EO
coefficients r.sub.33 of Samples 3 and 4 were measured by directly
applying a voltage to each sample and measuring the
refractive-index change .DELTA.n. The EO coefficient can be
estimated from the expression r.sub.33=2 .DELTA.n/n.sup.3E, where n
is the refractive index of the EO polymer after poling and E is the
applied electric field. Because the resistance ratio between
EpoClad and the EO polymer was not accurately known, an accurate
estimate of the r.sub.33 values for Samples 1 and 2 could not be
provided with this direct measurement method. Nevertheless, by
comparing the birefringence values of the EO polymer films of the
samples and assuming that the r.sub.33 value is linearly
proportional to the birefringence, the r.sub.33 values of Samples 1
and 2 could be estimated.
[0065] Table 1 summarizes the refractive indices for the TE and TM
polarizations (denoted as n.sub.TE and n.sub.TM, respectively) and
the r.sub.33 values for the four poled samples at 1300 nm and 1541
nm. In consistency with the poling characteristics shown in FIG. 5,
Sample 4 shows the largest r.sub.33 value, as well as the largest
birefringence, while Sample 1 shows the smallest r.sub.33 value,
regardless of the highest poling voltage used. Poled with a lower
voltage, Sample 2 exhibits a larger EO effect than Sample 1, which
is due to the use of a thinner buffer layer. Sample 3 shows a
smaller r.sub.33 value than Sample 4, because of the use of a lower
poling electric field. The highest birefringence values achieved in
our work (from Sample 4) are 0.044 at 1541 nm and 0.06 at 1300 nm,
and the corresponding r.sub.33 values are 82 pm/V and 110 pm/V,
which are about three times of those of lithium niobate. These
values compare well with those reported for similar EO polymer
poled with conventional electrodes. These experiments confirm the
feasibility of poling EO polymer at low voltages with graphene
electrodes.
TABLE-US-00001 TABLE 1 Summary of poling results for Samples 1, 2,
3, and 4 (denoted as S1, S2, S3, and S4, respectively). Poling
r.sub.33 at r.sub.33 at Voltage n.sub.TE/n.sub.TM at
n.sub.TE/n.sub.TM at 1300 nm 1541 nm (V) 1300 nm 1541 nm pm/V pm/V
S1 910 1.59207/ 1.57676/ 70 33 1.62082 1.59824 S2 800 1.58802/
1.57362/ 100 67 1.63826 1.61040 S3 350 1.58635/ 1.57256/ 91 54
1.62818 1.60368 S4 420 1.58532/ 1.57276/ 110 82 1.64593 1.61682
[0066] No visible damages on the EO polymer films were observed
after poling. To check whether poling with graphene electrodes
might affect the optical loss of the EO polymer, the transmission
losses of the TM.sub.0 modes for Samples 3 and 4 were measured
using the prism-coupler system. The TM.sub.0 mode guided by the EO
polymer film was excited with a collimated laser beam (at 1537 nm)
through a high-index prism placed on the area without graphene
electrodes. The guided TM.sub.0 mode passed through the area with
graphene electrodes and was detected at the end of the waveguide.
The transmission was measured for the poled area and the unpoled
area, respectively. The locations of the prism and the optical
paths for the poled and unpoled areas for Sample 4 are shown in
FIG. 6B (the layout for Sample 3 is similar). As the optical paths
for the probing of the poled and unpoled areas were the same,
which, as shown in FIG. 6B, consist of a 6-mm path without graphene
electrodes and a 10-mm path with graphene electrodes, a comparison
of the output powers from the two measurements gave the
poling-induced loss directly. The poling-induced losses measured in
this way for Samples 3 and 4 were .about.0.3 dB and .about.1.0 dB,
respectively. The larger poling-induced loss in Sample 4 could be
attributed to the larger birefringence in Sample 4, which, as shown
in FIG. 4B, should lead to a larger graphene-induced loss.
[0067] Several reference samples were prepared, including one that
contained only a 3.7-.mu.m thick EO polymer film on a thick EpoClad
cladding (i.e., no graphene electrodes), one that contained a
waveguide with the bottom graphene electrode, and one that
contained a waveguide with both the bottom and the upper graphene
electrode. The sliding prism method was applied to these samples
and a propagation loss of 2.2 dB/cm obtained for the sample without
graphene, 6.2 dB/cm for the sample with the bottom graphene
electrode, and 10.6 dB/cm for the sample with both graphene
electrodes were obtained. The experimental results agree reasonably
well with the calculated results, considering the uncertainties in
the measurements and the presence of non-uniformity for such large
area waveguides. In most practical applications, only a short
length of EO polymer film is needed and an upper cladding with a
matching refractive index is usually applied, which can reduce the
graphene-induced losses. The use of a thicker EO polymer film, as
shown in FIGS. 4A and 4B, can significantly reduce the
graphene-induced loss. The reason of using relatively thin EO
polymer films in herein is to limit the poling voltage required for
ITO/Au electrodes and ensure single-mode operation.
[0068] The above embodiments of the invention have provided a
photonic device using low-loss graphene electrodes for producing
electro-optic activity in EO materials with a poling method.
Advantageously, the application of graphene as poling electrodes
not only simplifies the fabrication of an EO device, but also helps
reduce the poling voltage, without introducing significant
additional optical loss to the EO material.
[0069] In addition, the use of graphene electrodes can do away with
buffer layers required for traditional metal electrodes thus
minimize the poling voltage. High EO coefficients of 82 pm/V at
1541 nm and 110 pm/V at 1300 nm were achieved with a poling voltage
of only 420 V, which is much lower than that required by using
traditional metal electrodes. The use of graphene electrodes for
poling allows the use of inexpensive cladding material and more
flexible waveguide designs. The upper graphene electrode can be
further patterned to form low-loss modulation electrodes for the EO
device to be made. Such a graphene/EO polymer hybrid platform can
be explored as a key solution to overcome the traditional tradeoff
between poling efficiency and optical loss and significantly
simplify the design and fabrication of waveguide devices based on
EO polymer.
[0070] It will be appreciated by persons skilled in the art that
numerous variations and/or modifications may be made to the
invention as shown in the specific embodiments without departing
from the spirit or scope of the invention as broadly described. The
described embodiments of the invention should therefore be
considered in all respects as illustrative, not restrictive. For
example, the photonic device structure may have a different
structure as that illustrated, provided that the cladding is
adjacent to the electro-optic structure. The illustrated photonic
device structure may be made by a method different from that
illustrated. The illustrated method may be used to manufacture a
photonic device structure other than that illustrated.
[0071] It will be appreciated by persons skilled in the art that
numerous variations and/or modifications may be made to the
invention as shown in the specific embodiments without departing
from the spirit or scope of the invention as broadly described. The
present embodiments are, therefore, to be considered in all
respects as illustrative and not restrictive.
[0072] Any reference to prior art contained herein is not to be
taken as an admission that the information is common general
knowledge, unless otherwise indicated.
* * * * *