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Surface-normal electro-optic spatial light modulator using
graphene integrated on a high-contrast grating resonator TIANBO
SUN,1,6 JONGHWAN KIM,2,6 JONG MIN YUK,2,3,4,5 ALEX ZETTL,2,3,4 FENG
WANG,2,7 AND CONNIE CHANG-HASNAIN1,8 1EECS Department, University
of California at Berkeley, Berkeley, CA 94720, USA 2Physics
Department, University of California at Berkeley, Berkeley, CA
94720, USA 3Materials Sciences Division, Lawrence Berkeley National
Laboratory, Berkeley, CA 94720, USA 4Kavli Energy NanoScience
Institute at the Univeristy of California, Berkeley and the
Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
5Department of Materials Science and Engineering, Korea Advanced
Institute of Science and Technology (KAIST), Daejeon 305-701, South
Korea 6These authors contributed equally to this work
[email protected] [email protected]
Abstract: We demonstrate efficient optical modulation of
surface-normal reflection in a novel device structure integrating
graphene on a high contrast grating (HCG) resonator. As high as 11
dB extinction ratio is achieved by varying the voltage applied to a
single atomic layer of graphene on a HCG resonator. The device
topology facilitates easy fabrication of large 2D arrays, and
free-space operation. We also demonstrate a graphene-oxide-graphene
structure which can potentially operate at MHz operation speed. The
devices are fully fabricated by standard CMOS compatible processes
indicating that the integrated structure of graphene-on-HCG shows
great promise for display, imaging and interconnects applications
with low-cost and large scalability. © 2016 Optical Society of
America
OCIS codes: (140.4780) Optical resonators; (130.0130) Integrated
optics.
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#275113 http://dx.doi.org/10.1364/OE.24.026035 Journal © 2016
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published 1 Nov 2016
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1. Introduction Electro-optic (EO) modulation is an essential
operation in photonic and optoelectronic application ranging from
optical interconnect to bio/chemical sensing and imaging [1–3].
Recently, graphene has emerged as a novel optical medium for EO
modulators owing to its unique optoelectronic properties. Graphene,
as a zero band gap semiconductor, absorbs light in the extremely
broad spectral range from terahertz to ultraviolet frequency [4–7].
Remarkably, electrostatic gating of graphene can efficiently tune
this broadband light absorption [8, 9]. Such excellent
gate-tunability in light absorption is particularly attractive for
the EO applications at optical frequencies from visible to telecom
frequencies where conventional semiconductors exhibit limited
tunability. Furthermore, it has been shown that graphene can be
grown in the large scale and readily combined with CMOS process
[10, 11].
The most critical challenge for graphene toward practical
applications, however, lies in its small light absorption. A single
atomic thickness of graphene significantly limits the total light
absorption to ~2.3% upon surface normal incidence [6–8], although
its absorption coefficient is actually an order of magnitude larger
than the band edge absorption of a typical direct bandgap
semiconductor, e.g. GaAs. Integration of graphene with photonic
structures has demonstrated that light absorption can be greatly
enhanced via enhancing interaction cross-section from waveguide
mode or amplifying electric field from strong cavity resonance.
Various engineered structures have been explored including Si
waveguides, photonic crystal cavities or ring resonators [5,
12–18]. However, these structures do not facilitate the fabrication
of 2D arrays, and free-space operation for 3D free-space based
applications such as imaging, display, holographic and remote
sensing. Extending the dimension from a planar optical circuit into
a 3D free-space system can significantly expand the capability and
scalability of optical system in the aforementioned
applications.
Here, we demonstrate a new free-space graphene-based spatial
light modulator (SLM) with the use of a novel surface-normal high
contrast grating (HCG) resonator. High contrast
Vol. 24, No. 23 | 14 Nov 2016 | OPTICS EXPRESS 26036
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grating (HCG) is a single ultrathin layer of near-wavelength
gratings fully surrounded by media having a large contrast in
refractive indices. It exhibits many distinct features nonexistent
in conventional gratings [20]. In particular, it can be designed to
exhibit a very high quality factor resonance for optical beam
incident in the surface-normal direction with a high coupling
efficiency via free-space excitation [21, 22]. These attributes are
ideal for integrating with graphene for EO modulator in 3D
free-space applications. Integrated structure of graphene and HCG
is realized via a CMOS-compatible process, including standard
optical lithography and graphene transfer techniques, eliminating
the inefficiency and high-cost of e-beam lithography which is
commonly used for graphene modulators in the previous reports.
Reflection spectroscopy shows that optical resonances in HCG can be
efficiently tuned with a maximum modulation depth of 11 dB by
electrostatic gating of graphene, which is substantially superior
to other EO spatial light modulator scheme based on GaAs quantum
wells both in modulation depth and CMOS compatibility [19]. Free
space coupling scheme with large modulation depth and simple
fabrication process of our architecture facilitates the fabrication
of large 2D array which is extremely promising for applications in
optical imaging, telecommunication, bio/chemical sensing at optical
frequencies.
2. Optical Resonance in HCG for Efficient Free-space Coupling
with Graphene A high-Q HCG resonator for surface-normal incident
beam is formed on an ultra-thin layer (< 200 nm) of silicon on
insulator (SOI) substrate, schematically shown in Fig. 1(a).
Silicon gratings with near-wavelength dimensions, fully surrounded
by materials with low refractive indices, i.e. air and silicon
dioxide, form this unique resonator. The basic principle of HCG
works as follows. By choosing appropriate grating period and duty
cycle, a HCG design supporting only two grating modes can be easily
obtained. Leveraging the high contrast in refractive indices at
both the entrance and exit planes, a strong coupling is achieved
between the intrinsic grating modes at the boundaries. With the
right thickness, these two grating modes can be made to interfere
constructively at the entrance and exit boundaries, resulting in a
high-Q resonator with light propagating in the direction normal to
the grating plane. This unique phenomenon was analyzed and reported
previously [20–22]. A detailed quantitative analysis of the design
algorithm employed for the HGG resonator is described in Sections 1
& 4 of reference 20.
The cavity resonance of HCG has two outstanding advantages for
enhancing light modulation with graphene upon free space coupling.
First of all, the cavity mode exhibits excellent spatial overlap
with graphene on the grating surface, which is essential for the
light absorption in graphene. Figures 1(b) and 1(c) show intensity
profile of in-plane electric field for the cavity resonance under a
collimated Gaussian beam (10 um size) at normal incidence. In the
vertical direction (Fig. 1(b)), a large portion of the resonance
mode is exposed on the HCG surface where graphene can be readily
transferred (green dashed line). In comparison, a simple
Fabry-Perot cavity does not have strong electric field at the
surface, leading to weak light absorption in graphene. Figure 1(c)
shows the top view of the resonance sliced in the center of the
grating slab. The mode profile in the horizontal direction is only
limited by incident Gaussian beam (yellow dashed line in the
schematic), which can cover large area of graphene on the surface.
Secondly, total reflection from HCG in the free space is completely
dominated by the cavity resonance. Figure 1(d) shows a simulated
optical reflection spectrum where free space reflectivity exhibits
nearly 100% at the optical resonance. Therefore, remarkably large
modulation depth can be achieved in the free space reflection
efficiently via controlling the cavity resonance with light
absorption in graphene.
Vol. 24, No. 23 | 14 Nov 2016 | OPTICS EXPRESS 26037
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Fig. 1. (a) Schematic of HCG. (b) Side view of simulated optical
mode inside one period of HCG resonator. (c) Top view of HCG
resonator under excitation of 10um Gaussian beam. (d) Simulated
reflection spectrum for HCG resonator
3. Device Demonstration and Optical Characterization Figure 2(a)
schematically illustrates the device structure for the experimental
demonstration. HCG are patterned onto a 6 inch silicon-on-insulator
wafer using deep ultraviolet (DUV) lithography, followed by silicon
refractive ion etching, both standard processes in semiconductor
manufacturing. For HCG devices used in this study, the grating
period is 792 nm, the barwidth is 427 nm, and grating thickness is
500 nm. On top of the HCG resonator, we transferred a large-area
(~3cm x 3cm) graphene grown by chemical vapor deposition using the
standard growth and transfer processes [23, 24]. Figures 2(b) and
2(c) show representative optical microscope and scanning electron
microscope (SEM) images of the hybrid graphene-Si HCG structure,
respectively. Both microscope images show that monolayer graphene
is successfully transferred on the HCG surface. For electrostatic
gating, we use Au-graphene contact to form source, drain and a top
electrolyte gating with ion-gel. Optical cavity resonance is
characterized via reflection spectroscopy. A supercontinuum laser
as a broadband light source is illuminated in surface-normal
direction as shown in Fig. 2(a) where incident light is linearly
polarized, parallel to the HCG plane and the high contrast grating
is aligned at 45 degree with the input light polarization. We
collect cross polarized reflected light which is analyzed by a
spectrometer equipped with an InGaAs array detector.
Vol. 24, No. 23 | 14 Nov 2016 | OPTICS EXPRESS 26038
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Fig. 2on top
Figure 3(aBare HCG (reof ~1000. Remcurve). Cavityresonator.
Thelectromagnetinterband opti
Fig. 3of gateV).
4. ElectrostIn order to moelectrolyte. A
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At low drive vol
f HCG graphene sM of HCG structur
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spatial light modulre with mono-layer
f bare HCG ancavity resonan
yer graphene reby ~14 dB at onance is owinctromagnetic f
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Cavity Resone gate the grapvity reflection
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ng to light absfield in the c
hene transfer. (b) aresonance frequen
nance with Gphene by meanis rendered to
image of graphene.
structure with gm with the qualvity nearly opaqavelength of
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and (c) Illustrationncy Rw ( = 0.86
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e
graphene. lity factor que (Blue
bare HCG phene via pated via
n 6
el as a top te. As the
Vol. 24, No. 23 | 14 Nov 2016 | OPTICS EXPRESS 26039
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voltage is tuned to gate graphene hole-doping side, HCG cavity
reflection is rendering to be an ‘on’ state. We observe significant
narrowing in the cavity line width as well as dramatic increase in
the cavity reflectivity. This switching can be qualitatively
understood by simple illustration in Figs. 3(b) and 3(c). For
charge neutral case (Fig. 3(b)), Fermi level is located at Dirac
point and interband transition exhibits strong light absorption in
graphene at the cavity resonance energy, Rw . However, this strong
interband transition can be turned off by shifting Fermi level
below half of the cavity resonance energy 1/2 Rw via electrostatic
doping as shown in Fig. 3(c): the optical transition is forbidden
via Pauli blocking since initial electronic state is unoccupied.
Alternatively, interband transition can be also turned off via
Pauli blocking by shifting Fermi level above 1/2 Rw in which case
final electronic state is occupied.
Figure 4 shows the effect of the electric field on the
graphene-HCG device. We measure the cavity reflection spectrum
while varying the gate voltage from 0 V to −2 V where Fermi level
of graphene is tuned from charge neutral case to hole-doped case.
Black dots in Figs. 4(b)-4(d) summarize electrostatic gate
modulation of the cavity resonance in terms of resonance
wavelength, linewidth and reflectivity, respectively. Observed
cavity resonance energy shift and linewidth can be attributed to
the change in complex dielectric constant of graphene, which is
reported in the theory section of reference 9. To conclusively
prove that the change in resonance are due to graphene, we
theoretically fit our experimental observations following the model
in our previous work (Red curves). Excellent agreement between
experiment and theoretical predication indicates that cavity
modulation is dominated by graphene absorption. In terms of cavity
reflectivity, an 11dB modulation depth is achieved within 2V
voltage swing shown clearly in Fig. 4(d). Modulation saturates at
around −2V, at which interband transition at resonance wavelength
is blocked and graphene is transparent.
Fig. 4. (a) Cavity reflectivity for gating voltage from −2V to
0V. (b) Resonance wavelength as function of gate voltage. Red curve
shows theoretical fitting. (c) Resonance linewidth as function of
applied voltage. Red curve shows theoretical fitting. (d) Cavity
peak reflection as function of applied voltage. An 11dB modulation
is achieved within 2V voltage swing.
5. Towards Large Scale Fast Spatial Light Modulator In the
previous section, we have used ion-gel to electrostatically dope
graphene to demonstrate the capability to modulate the HCG
resonance by gating graphene. To show the high speed operation
potential of our proposed modulator, we employ the
graphene-oxide-graphene structure integrated with HCG, in which two
layers of graphene are separated by
Vol. 24, No. 23 | 14 Nov 2016 | OPTICS EXPRESS 26040
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Al2O3 grown by ALD (Cambridge) and integrated on top of HCG
cavities. Both electron and holes are injected into graphene
layers. When the driving voltage is close to zero, both graphene
layers are undoped. Therefore, Fermi levels are close to the Dirac
point, and both graphene sheets are light absorptive. Consequently,
cavity transmission is low and modulation state is ‘off’. When a
voltage is applied across graphene-oxide-graphene structure, two
graphene layer form a parallel capacitor with two graphene layer
doped by holes and electrons separately by the same carrier
concentration. When the Fermi level shift in both graphene layers
reach more than half of the cavity resonance energy, both graphene
layers become transparent simultaneously. In this case, cavity
transmission will be high and modulation state is ‘on’ [25]. An SEM
image of a fabricated graphene-oxide-graphene HCG modulator with
top and bottom contact is shown in Fig. 5(a). Device fabrication
process is similar to the method described in reference [25],
including HCG definition, bottom graphene layer transfer and
definition, bottom contact, ALD, top layer graphene transfer and
definition, and top layer contact. The whole fabrication is based
on photolithography and Si compatible process. Schematic from side
view of final structure is shown in Fig. 5(b).
To demonstrate the high speed operation, sinusoidal driving
voltage with 1 MHz frequency (shown as red curve in Fig. 5(c)) is
applied across top and bottom graphene. Reflection signal is
collected and shown in the blue curve in Fig. 5(c). Speed
measurement in the experiment is limited by the poor electrical
contact between Au and graphene and with process improvement, we
expect higher operation speed. In theory, a 20um x 20um
graphene-oxide-graphene structure has capacitance around 6.8 pf
(assuming 5nm ALD layer). Resistance for the device can be as low
as 30 Ω / sq with moderately doped graphene [13, 26]. These lead to
0.78GHz operation speed theoretically. Only 9% modulation depth is
shown in this measurement, due to graphene damage in the transfer
process (shown in the SEM) and oversized illumination of the
incident beam, which can be improved with optimized fabrication
process and measurement techniques.
Additionally, the fabrication process we demonstrated in this
study is based on optical lithography and fully CMOS compatible,
which allows a low-cost and large scale modulator array desired in
many optical applications. In Fig. 6(a), we show a fabricated 10 x
10 HCG resonator array (before graphene transfer) with as high as
95% filling factor. Single resonator is around 1mm x 1mm. Together
with recent progress of wafer scale graphene transfer techniques
[10, 11], we can foresee a low-cost large scale spatial modulator
array using graphene integration with HCG.
Resonance wavelength of HCG resonator is determined by grating
period and duty cycle, which are controlled accurately by optical
lithography [20]. With the same Si layer thickness, we can vary the
resonance wavelength of single device by adjusting its lateral
dimension within the same resonator array. In Fig. 6(b), we show
that by controlling the period and duty cycle inside same array we
can shift resonance wavelength by 200nm. This unique properties
make the modulator array possible for hyperspectral operation in
the aforementioned applications.
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Fig. 5. (a) SEM image of a fabricated graphene-oxide-graphene
HCG modulator with top and bottom contact. (b) Schematic of
modulator structure from a side view. (c) Cavity reflection
response to 1 MHz drivig voltage.
Fig. 6. (a) SEM image of a fabricated 10 x 10 HCG array with 95%
filling factor and 1mm x 1mm single device footprint. (b) HCG
resonance wavelength varies with different period and duty cycle
inside the same array in (a).
6. Conclusion In conclusion, we have designed and experimentally
demonstrated a spatial light modulator using mono-layer graphene
integrated with HCG resonator. Modulator can be effciently coupled
via free-space optics. An 11dB modulation depth is observed within
2V voltage swing with ion-gel gating. We also demonstrate a
graphene-oxide-graphene structure with MHz operation speed. We show
the great potential of such structure for low-cost and large-scale
integration and possible hyperspectral operation, which is strongly
desired in applications such as display, imaging and
interconnects.
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Funding This material is based upon work supported by the
National Science Foundation under Grant No. 1335609 and by the
National Natural Science Foundation of China (NSFC) under grant
61320106001, and State Key Laboratory of Advanced Optical
Communication Systems and Networks, China. Additional support was
provided by the Director, Office of Science, Office of Basic Energy
Sciences, Materials Science and Engineering Division, of the U.S.
Department of Energy under contract NO. DE-AC02- 05CH11231, within
the sp2-bonded Materials Program (KC2207) which provided for
graphene transfer methods and structural characterization, and
within the van der Walls Heterostructures Program (KCWF16) which
provided for graphene synthesis.
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