IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 27, NO. 3, MAY/JUNE 2021 3400108

Design of Hybrid Plasmonic Multi-Quantum-WellElectro-Reflective Modulators Towards <100 fJ/bit

Photonic LinksXiaoxin Wang, Shaoliang Yu , Haijie Zuo, Xiaochen Sun, Member, IEEE, Juejun Hu, Member, IEEE,

Tian Gu , Member, IEEE, and Jifeng Liu , Senior Member, IEEE

Abstract—Realization of on-board and inter-chip optical inter-connects requires a photonic data link with power consumptionwell below their electrical counterparts (i.e., <<1 pJ/bit). Cur-rently, directly modulating 850 nm vertical cavity surface emittinglasers at >50 Gb/s requires 2–4 pJ/bit/channel. External reverse-biased modulators could drastically reduce this power consump-tion. Here we design ultralow power GaAs/AlGaAs multi quantumwell electro-reflective modulators operating at 1 V for facile inte-gration with polymer “optical bridges”, utilizing coupled quantumconfined Stark effect between adjacent quantum wells and opticalcoupling to hybrid surface plasmon-slab modes for significantlyenhanced extinction ratio and spectral bandwidth. Distinctive fromconventional electro-optical or electro-absorption modulators, thisnew design synergistically leverages ultra-large changes in bothrefractive index (|Δn|∼0.05) and absorption coefficient (Δα∼104

cm−1), achieving 35-50 dB extinction ratio at 1 V reverse biaswith a low insertion loss of 1–3 dB, an incident angle toleranceof ∼5°, and a spectral bandwidth of 7–10 nm. The modulatorpower consumption is ∼1.9 fJ/bit without the need of thermaltuning, and the RC-limited bandwidth well exceeds 100 GHz.This new modulator enables high bandwidth and ultralow poweroptical interconnect networks at >100 Gb/s/channel and <100fJ/bit/channel compatible with ever-scaling CMOS technologies.

Index Terms—Electromagnetic propagation in plasma media,optical interconnections, optical modulation, optical surface waves,quantum confined stark effect, quantum well devices.

I. INTRODUCTION

O PTICAL interconnects (OI) have been playing anincreasingly important role in large-scale data centers

and high-performance computing systems for rack-to-rack andboard-to-board communication to address the rapidly growing

Manuscript received December 19, 2019; revised March 18, 2020; acceptedMarch 31, 2020. Date of publication April 20, 2020; date of current versionJune 22, 2020. This work was supported by the U.S. Department of Energythrough ARPA-E ENLITENED Program under the award #DE-AR0000847.(Corresponding author: Jifeng Liu.)

Xiaoxin Wang and Jifeng Liu are with Dartmouth College, Thayer Schoolof Engineering, Hanover, NH 03755-4401 USA (e-mail: Xiaoxin.Wang@dartmouth.edu; Jifeng.Liu@dartmouth.edu).

Shaoliang Yu, Haijie Zuo, Juejun Hu, and Tian Gu are with the MassachusettsInstitute of Technology, Department of Materials Science and Engineering,Cambridge, MA 02139-4307 USA (e-mail: yusl@mit.edu; hzuo@mit.edu;hujuejun@mit.edu; gutian@mit.edu).

Xiaochen Sun is with LaXense, Inc., West Covina, CA 91790 USA (e-mail:sun@laxense.com).

Color versions of one or more of the figures in this article are available onlineat https://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2020.2987174

Fig. 1. (a) Schematics of a chip-to-chip SHINE layout co-packaged withintegrated circuit chips showing the integration of VCSEL, MQW modulator andphotodetector (PD) with polymer waveguide optical bridges. (b) The modulationprinciple of the hybrid plasmonic MQW (HP-MQW) modulator is schematicallyillustrated for “ON” state (0 V) and “OFF” state with a low applied voltage ≤1 V. In the “ON” state the incident light from the input waveguide is mostlyreflected by a 20 nm Au thin film to the output waveguide. In the “OFF”state, the incident light is coupled to a hybrid plasmonic-MQW slab mode toachieve a high extinction ratio. (c) A more detailed schematic cross-section of theHP-MQW modulator structure. (d) FDTD simulation of the coupling betweenthe polymer waveguides (TM polarization) and the HP-MQW modulator region,demonstrating a low coupling loss of 0.5 dB. The inset on the left shows theprofile of the focal spot projected on the surface of the HP-MQW modulator.The inset on the right shows the mode profile of the output waveguide.

demand for higher energy-efficiency and bandwidth. The imple-mentation of on-board OI at chip-to-chip and chip-to-memorylevel will ultimately fulfill an all-optical interconnect withlink power consumption well below state-of-the-art electricalinterconnects (< 1 pJ/bit) [1]. However, no mature technicalpathways have yet been available for inter-chip OI. Onecandidate technology is Si photonics (SiP), which are fabricatedusing CMOS processes to interconnect electronic chips throughmulti-level integration [1], [2]. However, high density opticalI/O packaging and chip-to-chip coupling remains a significantchallenge. Recently, we proposed a Seamless Hybrid-integratedInterconnect Network (SHINE) architecture (Fig. 1(a)) [3], inwhich an “optical bridge” platform comprising flexible polymerribbon waveguide arrays integrated with III-V active devicesis employed for OIs between Si-CMOS chips co-packaged

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3400108 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 27, NO. 3, MAY/JUNE 2021

with the optical components. Instead of utilizing planarwaveguide integration of photonic components in SiP, theoptical bridge integrates compact and efficient surface-incidentactive photonic components with single mode flexible polymerwaveguides via freeform optical couplers [3]. This uniquecombination enables high bandwidth density, low optical lossesand low-cost packaging compatible with standard flip-chip andhigh-throughput pick-and-place technologies.

The active photonic components in the optical bridge includecontinuous-wave single-mode vertical-cavity-surface-emittinglasers (VCSELs), modulators and detectors (PDs). Nowadays,directly modulated VCSELs emitting at 850 nm dominate short-reach communications (<300 m) in data centers. However, thebandwidth of direct modulation is approaching the limit set bythe internal carrier dynamics (∼30 GHz or 56 Gb/s) [4]–[6].The laser driver for >50 Gb/s direct modulation also consumesa large amount of power on the order of pJ/bit due to thenecessity of ultrafast, high-level carrier injection [7], leadingto an energy consumption on the order of several pJ/bit/channel[6]. Therefore, we adopt reverse biased external modulators inthe SHINE architecture to significantly reduce the power con-sumption. In contrast to high speed laser drivers, state-of-the-artdrivers for reserve biased modulators is∼1.2×C Vpp

2, where Cis the capacitance of the modulator and Vpp is the peak-to-peakvoltage swing [8]. With compact micro/nanophotonic modulatorstructures, the modulator driver only consumes 10–100 fJ/bit,i.e., one to two orders of magnitude more energy efficient thanthe high-speed lasers drivers. The external modulator is requiredto simultaneously meet the challenges of high bandwidth, largeextinction ratio (ER), low insertion loss (IL), low driving voltage,and reasonably broad operation spectral range to circumvent theneed for active thermal stabilization.

For facile coupling with VCSELs, surface-incident modula-tors are preferred for the SHINE architecture. Conventionally,electro-optical modulators (EOMs) and electro-absorption mod-ulators (EAMs) are the two types of devices used for externalmodulation. They utilize the change in the real part (Δn) andimaginary part (Δk) of the refractive index under an appliedvoltage, respectively. EOMs usually require a waveguide and/orresonator structure for a sufficiently long light-matter interactionlength to implement π phase shift since Δn is usually limited to10−4–10−3. They are seldom used in surface incident configura-tion. Recent advances in integrated photonics have demonstratedthat EAMs are very promising to achieve ultralow power mod-ulation at high bandwidth. These devices utilize Franz-Keldysheffect (FKE) in bulk semiconductors and quantum-confinedStark effect (QCSE) in multiple quantum well (MQWs) [2], [9].Waveguide-integrated Ge/GeSi [10], [11] and hybrid III-V-on-SiEAMs [12] have presented a modulation bandwidth of 50-74GHz and an extinction ratio (ER) of ∼ 5-10 dB at 2-4 V drivingvoltage in the wavelength range of λ= 1.5∼1.6 μm and 1.3 μm,respectively, at a power consumption of 10-100 fJ/bit. Previousresearch has also demonstrated high-density, high-yield hybridintegration of surface incident GaAs/AlxGa1-xAs MQW EAMson Si CMOS via flip-chip solder-bonding [13]. However, thesedevices usually require a high driving voltage of 7–10 V toachieve an adequate ER due to the limited MQW thickness, evenwhen embedded in a resonant Fabry-Pérot (FP) cavity to acquire

multiple-pass absorption based on QCSE [13]–[20]. These highdriving voltages are not compatible with the current CMOStechnology nodes. By comparison, the recent demonstrationof polymer waveguide-coupled surface-normal reflective EAMswith ER = 4 dB at a bias voltage of 4 V proves their capabilityof full integration within a single photonic layer for chip-levelOIs, although the driving voltage should be further reduced andER further enhanced [21].

In this paper, we design ultralow power hybrid plasmonicMQW (HP-MQW) GaAs/AlGaAs electro-reflective modulatorsoperating at 1 V for facile integration with waveguide inter-connects exemplified by SHINE. Distinctive from conventionalEOM and EAMs, this new design synergistically leveragesultra-large changes in both the refractive index (|Δn| ∼ 0.05)and absorption coefficient (Δα∼ 104 cm−1), thereby efficientlycoupling the incident light into hybrid plasmonic slab modesunder 1 V reverse bias for an ER > 30 dB at a low IL of1–3 dB. The coupled QCSE (Co-QCSE) between adjacent QWsincreases the spectral bandwidth to 7–9 nm, significantly broaderthan micro-resonator-based modulators (< 0.1 nm) [22]–[23] ata similar footprint (3–5 μm in diameter) and capacitance (∼1fF) to allow even lower power consumption by avoiding activethermal tuning. This high-performance modulation and high ef-ficiency coupling with polymer waveguides enables an ultralowtotal power consumption of <100 fJ/bit/link in the SHINE net-work. The same device structure and coupling scheme are alsoapplicable to surface-incident PDs in the SHINE architecture,thereby further facilitating photonic integration.

II. PRINCIPLES OF HIGH-PERFORMANCE HP-MQWELECTRO-REFLECTIVE MODULATORS

It has been well known that the absorption change Δαdue to QCSE in GaAs/AlxGa1-xAs MQW can be as large as5000-10000 cm−1 under an electric field of ∼70 kV/cm [24].Accordingly, the refractive index change |Δn| reaches as much as∼0.05 near 850 nm wavelength, as deduced by Kramers-Kronigrelation [25] and first experimentally verified by Glick et al[26]. Note that this huge |Δn| (∼0.05) is two to three orders ofmagnitude higher than those in LiNbO3 and Si EOMs. However,conventionally such a large Δn near the direct bandgap ofGaAs QWs cannot be directly applied to EOMs due to thelarge absorption near the direct gap, which leads to a hugeIL. Considering that surface plasmon resonance (SPR) is verysensitive to the refractive index of the dielectric material nearby,we adopt a new approach by coupling the incident light intoa hybrid plasmonic-MQW slab mode in the OFF-state at areverse bias, thereby drastically reducing the driving voltage andincreasing the ER (see Fig. 1(b)). Such coupling synergisticallyutilizes the largeΔn andΔα from Co-QCSE. On the other hand,at ON-state (i.e., 0 V) there is almost no coupling to the hybridplasmonic mode, and the modulator structure exhibits a highreflection with minimal IL.

A more detailed schematic cross-section of the proposedHP-MQW electro-reflective modulator structure is shown inFig. 1(c). The device comprises a p-i-n diode with an intrinsicGaAs (10 nm)/Al0.3Ga0.7As (3.5 nm) MQW region. The thin Al-GaAs barrier enables quantum state coupling between adjacent

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WANG et al.: DESIGN OF HYBRID PLASMONIC MULTI-QUANTUM-WELL ELECTRO-REFLECTIVE MODULATORS TOWARDS <100 FJ/BIT 3400108

QWs to benefit from Co-QCSE for broader spectral bandwidthand higher ER. A low index dielectric layer, such as CYTOP(n = 1.34) [27] or SiO2 (n = 1.46), is sandwiched between athin Au layer (20 nm thick) and the p-i-n diode structure. Infact, a hybrid mode between the Au/dielectric surface plasmonand the MQW slab can be supported as long as the refractiveindex of the dielectric layer is lower than the polymer claddinglayer (n = 1.51) sitting on top of the Au film. The free-formmicro-optical coupler [3], [28], [29], which can be fabricatedby the two-photon polymerization process with high accuracy,is a critical element to couple the light from the single-modeflexible polymer waveguide to surface-incident modulators andPDs with well-controlled beam properties, such as angle ofincidence, divergence angle, focal spot size, mode profile, etc. Asshown in Fig. 1(d), 3D finite-difference-time-domain (FDTD)simulation indicates that the free-form facets based on totalinternal reflection provide a high coupling efficiency of ∼90%(or coupling loss <0.5 dB) for TM polarization from the inputwaveguide to the modulator then to the output waveguide overhalf an octave around the targeted wavelength [29]. The incidentlight on the surface of the MQW is p-polarized (i.e., electric fieldparallel to the plane of incidence), enabling surface plasmonexcitation and facilitating optimal coupling conditions requiredby the HP-MQW modulators. This coupling loss is added tothe total IL in the device design and optimization. The focalspot size incident on the modulator is ∼2 μm in diameter (seethe left inset in Fig. 1(d)), enabling a very compact HP-MQWmodulator on the order of 5 μm in diameter with metal ringcontacts in the peripheral area (see Fig. 1(c)). Experimentally,commercially available two-photon lithography technology hasbeen used to fabricate these free-form facets for micro-scalecouplers for high performance 3-D optical interfacing betweenwaveguides, fibers, and active photonic devices [3], [28], [29].In the “ON” state, the incident light from the input waveguide ismostly reflected by the thin Au layer to the output waveguide,thereby achieving a low IL. In the “OFF” state, a small reversebias ≤1 V is applied on the p-i-n diode to induce a large Δn andΔα in the MQW stack, and the light is coupled to the hybridplasmonic-MQW slab mode for a high ER. The thicknessesof the CYTOP layer (t) and MQW thickness (d) are importantdesign parameters to optimize the HP-MQW modulator, as willbe detailed next.

III. HP-MQW ELECTRO-REFLECTIVE MODULATOR DESIGN

AND OPTIMIZATION

The HP-MQW modulator and its surrounding are treatedas a 7-layer structure: polymer cladding (semi-infinite)/Au(20 nm)/ CYTOP (t)/p-Al0.37Ga0.63As (100 nm)/MQW (d)/n-Al0.37Ga0.63As (100 nm)/air (semi-infinite), as shown inFig. 1(c). In practical device fabrication, the air region canbe implemented by etching a recess in the substrate beforebonding the MQW stack [30]. Reflectance of this structureas a function of the incident angle is calculated by using themultilayer transfer-matrix method incorporating both the realand imaginary parts of the refractive indices. Finite elementapproach based on the Wave Optics/RF Modules of COMSOL

TABLE ILIST OF EPITAXIAL LAYERS IN THE MQW P-I-N DIODE TEST STRUCTURE

Multiphysics is also employed to confirm the analytical reflec-tion spectra, and to compute the electric field distribution andmodes. The thicknesses of the CYTOP layer (t) and the MQW(d) are optimized for several figure of merits (FOMs), includingthe reflectance change (ΔR), angular integrated ER/IL, and theangular peak shift of the hybrid plasmonic mode resonance.

A. Experimentally Verification of Large Δn From Co-QCSE inCoupled MQWs

The electric field-dependent refractive index spectrum of theMQWs were first obtained experimentally as an input for furtherdesign optimization. For this purpose, p-i-n diode GaAs/AlGaAsMQW test structures were grown by molecular beam epitaxy(MBE) on a Si-doped n+ GaAs, with 30 pairs of 10.5 nmGaAs/3.5 nm Al0.3Ga0.7As MQWs in the intrinsic region. De-tails of the growth stack is provided in Table I. The refractiveindex and the refractive index change of the MQW stack underan applied electric field for TE polarized light (ê parallel to theMQW plane) are determined using a combination of UV-Vis-IRspectroscopy as well as normal-incidence, spectrally-resolvedelectro-reflectance measurements [31] and photo-responsivity[32], [33] at different reverse biases. Although the optical prop-erties of the MQWs for TM polarized light (ê vertical to MQWplane) [34] cannot be directly derived from the normal incidencemeasurements, using theΔn measured for TE polarization givesus a conservative evaluation of the device performance, as willbe discussed shortly.

Fig. 2 shows the experimentally derived refractive indexchange Δn. The maximum |Δn| in the wavelength range of 845-865 nm is 0.055. It is interesting to notice that theΔn spectrum ismuch broader than the theoretical prediction of QCSE in isolatedQWs (dashed line in Fig. 2). Such spectral broadening is due tothe coupling of the wavefunctions between adjacent QWs thatcauses splitting of energy levels. More transitions between thesplit levels in the conduction band and valence bands broadentheΔα andΔn spectra under an applied electric field, leading toCo-QCSE in MQWs with thin barrier layers (i.e., 3.5 nm thickAlGaAs barrier layers in our case). To model the Co-QCSE incoupled MQWs, we first calculated the eigenvalues of energy

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3400108 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 27, NO. 3, MAY/JUNE 2021

Fig. 2. Comparison of experimentally derived vs. theoretically modeled re-fractive index (real part) change Δn at 80 kV/cm electric field. The dotsrepresent the experimental Δn for TE polarization. The dashed line representsthe theoretical Δn from QCSE in isolated QWs, and the orange line shows thetheoretical Δn from the coupled QW model. Theoretical Δn from coupled QWmodel for TM polarization is also shown here by the green curve. Here “TEpolarization” means that the electric field of the incident light is parallel to theMQW plane, while “TM polarization” means it is perpendicular to the MQWplane.

levels and wavefunctions of the MQW stack. We then computedthe corresponding absorption spectra of the MQWs, taking intoaccount both the continuum and the exciton transitions for bothTE (electric field parallel to the MQWs) and TM modes (electricfield perpendicular to the MQWs) [35], [36] and derived the Δnspectra using Kramer-Kronig relation for comparison with theexperimental data. Clearly, the measuredΔn for TE polarizationlargely agrees with the theoretical modeling of the coupled QWsrather than that of the isolated QWs. We also noted that thetheoretical |Δn| of coupled QWs for TM polarization is evenlarger than that of TE polarization in the wavelength range of845∼855 nm and reaches a maximum |Δn| of 0.11 at ∼850 nm.For other wavelength ranges of 835∼845 nm and 855∼865 nm,Δn under TM polarization is very close to that of TE. For the de-vice configuration shown in Fig. 1, the incident p-polarized lightwould be mixed between TE and TM in the MQWs. Consideringthat the Δn from TE polarization is generally slightly smallerthan that of TM, we will use the experimentally measured Δnunder TE polarization for a conservative design. This is alsoa good approximation for most of the device structures to bediscussed in the later text (e.g., Fig. 4) with an incident angle of∼50° in the cladding. The corresponding incident angle insidethe MQW is ∼20°, rendering a TE-dominated scenario.

B. Device Performance Modeling and HP-MQW SlabMode Analysis

Based on the experimentally measured/derived Δn spectrashown in Fig. 2 for Co-QCSE in coupled QWs, we modeledthe performance of the proposed HP-MQW electro-reflectiveMQW modulators. We found that this device structure exhibitsa large reflection change ΔR > 60% and ER > 30 dB at 1V reverse bias, far superior to the performance of previouslyreported asymmetric FP cavity, surface-incident reflection mod-ulators operating at 7-10 V driving voltages [14]–[20]. Fig. 3(a)

Fig. 3. (a) Calculated reflectance as a function of incident angle under 0 V bias(R0) vs. 1 V reserve bias (Rh) for p-polarized light. The CYTOP thickness ist= 200 nm, and the MQW thickness is d= 300 nm [see Fig. 1(c)]. The numericalcalculation using COMSOL agrees very well with the analytical approach.(b) Reflectance change (ΔR) vs. the incident angle in the polymer claddingdeduced from (a).

Fig. 4. (a) Distribution of the z-component of the magnetic field (Hz) under 1V reverse bias (80 kV/cm) at an incident angle of 54° for p-polarized light. Herethe CYTOP thickness is t = 200 nm, and the MQW thickness is d = 300 nm.The effective refractive index of the hybrid mode is 1.223–0.0059i correspondsto the excitation angle of 54o in the Kretschmann configuration. The red arrowsin (a) and (b) indicate the excitation port and the direction of input optical power.The operating wavelength is 858 nm.

compares the calculated reflectance as a function of incidentangle under 0 V bias (R0) vs. 1 V reserve bias (Rh) appliedon a 300 nm thick intrinsic MQW region (d = 300 nm). TheCYTOP layer thickness is t = 200 nm and the electric fieldat 1 V reverse bias is 80 kV/cm. The numerical modelingwith COMSOL agrees very well with the analytical approach,both indicating a significant decrease in reflectance under 1 Vreserve bias at 47°−55° incident angles. The resonance anglefor coupling into the hybrid plasmonic-MQW slab mode, atwhich the reflectance reaches a minimum, shifts from 56° (R0)to 54° (Rh) when a reserve bias of 1 V is applied due to thelarge |Δn| of ∼ 0.05. Based on COMSOL simulation of themode coupling, the nearly 100% absorption (i.e., Rh ∼ 0) at 54°incidence angle in the “OFF” state is mostly contributed by theelectro-absorption in the MQWs (85%), while the Au film alsoabsorbs 15% of the incident light. Fig. 3(b) further reveals thatthe maximum reflectance change (ΔR = R0-Rh) is 62.3% at52.3° incidence angle, which is predominantly contributed bythe electro-absorption in MQWs (57.1%) and slightly boosted

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WANG et al.: DESIGN OF HYBRID PLASMONIC MULTI-QUANTUM-WELL ELECTRO-REFLECTIVE MODULATORS TOWARDS <100 FJ/BIT 3400108

by the absorption in the Au film (5.2%). Therefore, the largeΔn from Co-QCSE helps to couple the incident light into thehybrid plasmonic-MQW slab mode at OFF state, while the largeΔα in the MQWs leads to high absorption and low reflectance.As such, this new device design synergistically utilizes both Δnand Δα to achieve a high ER at a low driving voltage of 1 V.

To further verify the coupling of the incident light intothe hybrid plasmonic-MQW slab modes, Fig. 4(a) plots the zcomponent of the magnetic field (Hz) under 1 V reverse bias(∼ 80 kV/cm) at the resonant incident angle of 54°. We notedthe evanescent field decay in CYTOP and air (also see Fig. 6(a)),respectively, the former being typical of surface plasmon whilethe latter typical of dielectric waveguides. Therefore, this fielddistribution analysis confirms that the surface-incident light iscoupled into a hybrid mode between the surface plasmon at theAu/CYTOP interface and the dielectric slab mode supported bythe high index MQWs. There is no interference pattern in thecladding layer in Fig. 4(a), indicating negligible reflectance andnearly complete coupling into the hybrid plasmonic-MQW stackmode. This result agrees very well with Fig. 3(a) showing

Rh ≈ 0 at 54° incident angle. To further verify the couplingbetween the hybrid plasmonic mode and surface-incident wave,we also launched the optical excitation from the side (i.e., par-allel to the Au film and the MQW) to calculate the mode profileand evaluate its coupling to the free space. As shown in Fig. 4(b),in this case the magnetic field distribution perfectly replicatesthat in Fig. 4(a), thereby confirming the coupling between thesurface-incident wave and the hybrid plasmonic mode in bothdirections. Based on COMSOL modeling, the complex effectiverefractive indexes neff of the hybrid plasmonic mode for t = 200nm and d = 300 nm are 1.253-0.0013i at 0 V and 1.223-0.0059iat 1 V reserve bias, respectively. These also agree very well withthe resonant incident angles of 56° and 54° at 0 V and 1 V reversebias in Fig. 3(a), respectively, when using asin(neff/1.51) in theKretschmann configuration to calculate the incident angles forcoupling.

To map the design space for optimal modulator performance,we also investigate the mode behavior vs. CYTOP thickness t ata fixed MQW thickness of d = 300 nm. Fig. 5 shows the colormappings of reflectance at 1 V reverse bias (Rh), reflectancechange ΔR = R0-Rh, ER (dB), and the ratio of ER/IL vs. tand incident angle. Fig. 6 further reveals the normalized elec-tric field intensity (|E|2/|E0|2) distribution along the modulatorstack for t = 200 nm, 425 nm, 905 nm and 1200 nm at thecorresponding resonant angles indicated in Fig. 5(b). This fielddistribution plot helps us to identify the nature of the opticalmodes under resonant coupling. In Fig. 5(a)-(d), the feature at∼ 41.5° incident angle corresponds very well to the criticalangle for total internal reflection in the modulator stack, i.e.,θc = asin(1/1.51) = 41.47°. Above this critical angle, light cancoupled into the slab mode if the in-plane vector is matched,forming an evanescent tail in the air region (see Fig. 6(a)-(d)).On the other hand, we can also see a local minimum of Rh atan incident angle of ∼ 68.5° in Fig. 5(a). This corresponds to apartial coupling into the SPR mode at the Au/CYTOP interface,as clearly shown in Fig. 6(c)-(d) by the exponential decay tailwell confined in the CYTOP layer at incident angles close to

Fig. 5. Mappings of (a) reflectance Rh at an electric field of 80 kV/cm,(b) reflectance change ΔR, (c) ER (dB), and (d) the ratio of ER to IL vs.CYTOP thickness t and incident angle in the cladding. The coupling loss withwaveguides is included in the IL. The MQW thickness is set as 300 nm. Theoperating wavelength is 858 nm, and the incident light is p-polarized.

Fig. 6. Normalized electric field (|E|2/|E0 |2) distribution for different CYTOPthickness t at the resonant incident angles under 1 V reserve bias (i.e., 80 kV/cmelectric field applied on the MQW region), revealing coupling into differentkinds of optical modes. (a) t = 200 nm, incident angle = 54° (hybrid plasmonicmode); (b) t = 425 nm, incident angles of 41.46° (dielectric supermode) and63.92° (hybrid plasmonic mode); (c) t = 905 nm, incident angles of 41.46°(dielectric supermode 2), 52.75° (dielectric supermode 1) and 68.40° (SPR);(d) t = 1200 nm, incident angles of 46.63° (dielectric supermode 2), 56.74°(dielectric supermode 1), and 68.78° (SPR). The MQW thickness is set as 300nm. The operating wavelength is 858 nm and the incident light is p-polarized.

68.5°. The corresponding ΔR at SPR coupling is low, though(see Fig. 5(b)), because the evanescent tail of the SPR has littleoverlap with the MQWs and does not sample its refractive indexchange under reverse bias.

Depending on the thickness of the CYTOP layer (t), weidentify two different mode coupling mechanism that lead toa significant ER:

1) For CYTOP thickness < 420 nm, the incident light canbe coupled into a hybrid plasmonic mode between the

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surface plasmon at Au/CYTOP interface and the dielectricslab mode of the MQWs, as discussed earlier. This hybridmode is clearly manifested in Fig. 4, Fig. 5(a) and Fig. 6(a),where the evanescent tail of the surface plasmon in theCYTOP layer overlaps and couples with the MQW di-electric slab mode. In this case, the MQW refractive indexchange has a notable impact on the effective index of theoverall hybrid plasmonic mode. Therefore, this couplingmechanism can offer a reflectance change of ΔR = 0.63,an ER up to 60 dB and an ER/IL ratio up to 20 at 1 Vreverse bias, as shown in Fig. 3 and Fig. 5.

2) For CYTOP thickness> 420 nm, the coupling between thesurface plasmon and MQW slab mode becomes very weakor negligible since the CYTOP is too thick. Instead, theCYTOP dielectric layer itself is thick enough to supportat least one dielectric slab mode that couples with thehigh-index MQW slab mode, thereby forming a dielectricsupermode. The thicker the CYTOP layer, the greaternumber of supermodes are supported (see Fig. 5(a)). Theincident light can also couple into these dielectric super-modes at resonant incident angles to induce a large ER. Forexample, Fig. 6(b) shows that the incident light couplesinto the 1st dielectric supermode at 41.46° and partiallycouples into the hybrid plasmonic mode at 63.92° for aCYTOP thickness of t = 425 nm. When the CYTOP layerthickness further increases beyond 900 nm, the incidentlight can be coupled into ≥ 2 dielectric supermodes atdifferent resonant incident angles, as shown in Figs. 6(c)and 6(d). Coupling into either supermode results in a lowreflectance and high ER in these two cases, as indicated by∼ 0 magnitude of the electric field in the polymer claddinglayer at the corresponding resonant incident angles.

Comparing these two coupling regimes, we found that cou-pling into hybrid plasmonic mode is more advantageous becauseit allows high ER in a broader range of incident angles andCYTOP thicknesses, thereby offering a more robust design.This is due to a stronger modal confinement in the MQW forthe hybrid plasmonic mode, comparing Figs. 6(a)-(d). Indeed,Fig. 5(b) shows that the red and dark red regions (indicatinga large ΔR) is broader for the hybrid plasmonic mode branchthan any of the dielectric supermode branches. Coupling into thehybrid plasmonic mode allows a high ΔR in a relatively broadincident angle range of 50°-55° for t = 200 nm, i.e., an angularbandwidth of 5°. By comparison, coupling into the dielectricsupermode only allows an angular bandwidth of ∼ 2° (50°-52°)for t = 905 nm. Similarly, having a low index CYTOP layerwith optimal thickness offers a larger incident angle bandwidthcompared to the case of no CYTOP layer. Therefore, in thedevice optimization we will focus on coupling into the hybridplasmonic mode induced by Co-QCSE in the MQW modulators.

C. Design Optimization of HP-MQW Modulators

Several FOMs are used to optimize the HP-MQW electro-reflective modulators, including reflectance change ΔR (be-tween 0 V and 1 V reverse bias), angularly integrated ER/ILratio, and the resonance peak-shift (between 0 V and 1 V reverse

Fig. 7. Mapping of the maximum reflectance change ΔR and the corre-sponding incident angle as a function of CYTOP and MQW thicknesses atwavelengths of 858 nm (a), (b) and 862 nm (c), (d). The white arrows indicatethe corresponding coupled modes at “OFF” states, including hybrid plasmonicmode (HP), dielectric supermode (DS) and dielectric MQW slab mode (D). Theincident light is p-polarized.

bias). Fig. 7 maps the maximum reflectance change ΔR andthe corresponding incident angle as a function of CYTOP andMQW thicknesses at the wavelengths of 858 nm (Fig. 7(a)-(b))and 862 nm (Fig. 7(c)-(d)). Several broad optimal regimes (inred and dark red) are found in Fig. 7(a) withΔR> 0.6 at 858 nm,which mostly overlap those at λ = 862 nm shown in Fig. 7(c)for MQW thickness d > 250 nm. The corresponding range ofoptimal incident angles (∼5° angular bandwidth) also overlapquite well for these two wavelengths (comparing the blue regionsin Fig. 7(b) with those in Fig. 7(d)). These results indicate thatthe HP-MQW design is not only robust to the variations inCYTOP thickness, MQW thickness and incident angles, butalso offers a spectral bandwidth ≥ 4 nm. Referring to Fig. 5,the broad optimal regimes located along a MQW thickness ofd∼300 nm in Figs. 7(a) and 7(c) are ascribed to the couplinginto hybrid plasmonic modes (labelled “HP”) and dielectricsupermodes (labelled “DS”). The corresponding incident anglesfor hybrid plasmonic mode and dielectric supermode couplingare ∼ 55° and ∼ 45°, respectively, as shown in Figs. 7(b) and7(d). Both optimal regimes accommodate a MQW thicknesstolerance of ∼25 nm and a CYTOP thickness tolerance of ∼100nm. Similarly, two broad secondary optimal regimes are alsofound for MQW thickness d∼150 nm. The maximum reflectancechange ΔR for the regions of d∼150 nm is ∼ 0.1 less than thatof d ∼300 nm due to less optical confinement (therefore lesselectro-absorption) in a thinner MQW region.

It is interesting to note that narrow optimal regimes also existat incident angles >70°, as indicated by the arrows in Figs. 7(a)and 7(c) and labelled “D”. These regimes are not observed inFig. 5 for d = 300 nm. They are identified as pure dielectricMQW slab modes with the electric field mostly confined in theMQW stack. In the case of d = 163 nm and t = 460 nm, thecalculated neff of the dielectric MQW slab mode is 1.487-0.005i

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WANG et al.: DESIGN OF HYBRID PLASMONIC MULTI-QUANTUM-WELL ELECTRO-REFLECTIVE MODULATORS TOWARDS <100 FJ/BIT 3400108

Fig. 8. Mappings of (a) angularly integrated ER/IL ratio over the range ofincidence angles satisfying ER >5 dB and IL <3 dB and (b) peak shift vs.MQW thickness and CYTOP thickness. The coupling loss with waveguides isincluded in the IL. The operating wavelength is 858 nm. The incident light isp-polarized.

for 0V and 1.467-0.0183i for 1V reverse bias at λ = 862 nm,respectively. It leads to a resonant peak shift from 79.8° to 76°and a large ΔR of 0.74. The region centered around d = 285nm, t = 400 nm revealed in Fig. 7(b) for λ = 858 nm is notidentified in Fig. 7(a) due to its overlap with other HP andDS modes. Compared to hybrid plasmon modes and dielectricsupermodes, coupling to pure dielectric MQW slab modes ismuch less tolerant to the MQW thickness (∼5 nm) and it requiresmuch larger incident angle (>70°). These factors will bringdifficulty to practical device fabrication, so we will not furtherconsider this coupling scheme.

Fig. 8(a) further maps the angularly integrated ER/IL ratio,evaluated by integrating ER/IL over the range of incidenceangles satisfying ER >5 dB and IL <3 dB. This FOM syn-ergistically characterizes the potential tradeoff between angularbandwidth and ER/IL ratio. We found two bands of optimalMQW thicknesses, one around d = 300 nm and the other aroundd = 150 nm. These thicknesses offer optimal coupling betweenthe MQW slab mode and the surface plasmon mode at the OFFstate to establish the hybrid plasmonic mode. On the other hand,the angularly integrated ER/IL ratio generally decreases with theincrease of CYTOP thickness due to less modal overlap betweenthe MQW slab and the evanescent tail of the surface plasmon. Wealso noted that the optimized thickness ranges of CYTOP andMQW for high angularly integrated ER/IL ratio largely overlapswith that of the largest resonant peak shift (in degrees between0 V and 1 V) in Fig. 8(b). It confirms that the resonant peak shiftfor coupling into the hybrid plasmonic mode caused by the largeΔn change in MQWs leads to large ER/IL ratios. The optimizedthicknesses for CYTOP and MQW layers are in the range of200 nm <t < 400 nm and 275 nm <d < 325 nm. The relativelysmall thickness of the MQW layer enables a low operatingvoltage ≤1 V for the HP-MQW modulator.

As an example, Fig. 9 exhibits the IL, ER, and ER/IL ratioas a function of incident angle and wavelength for an optimizedmodulator structure. With MQW thickness d = 300 nm andCYTOP thickness t = 200 nm, the maximum extinction ratioER is 35 dB at 1 V reverse bias, and the angular bandwidth is5° for ER > 5 dB. The spectral bandwidth for ER > 5 dB is9 nm. Assuming a device diameter of 5 μm to fully cover theincident light spot (∼ 2 μm in diameter, as shown in Fig. 1(d))and provide sufficient peripheral area for lateral contact, thedevice has an ultralow capacitance of 7.5 fF. Considering that

Fig. 9. IL, ER, and ER/IL ratio as a function of (a) incident angle and(b) wavelength for an optimized structure with MQW thickness d = 300 nm andCYTOP thickness t = 200 nm. The incident light is p-polarized. The couplingloss with waveguides is included in the IL.

the modulator itself consumes a power of (1/4)CVpp2 in on-off

keying, the power consumption of the modulator itself is as lowas 1.9 fJ/bit under 1V operation. Further considering a modulatordriver power consumption of ∼1.25 CVpp

2, as mentioned in theintroduction, the total power consumption of the modulator anddriver combined is as low as 11 fJ/bit. This is nearly 3 orderslower than the laser driver for direct modulation of VCSELs[7]. The bandwidth is only limited by RC delay (f = 1/(2πRC)),which exceeds 400 GHz using C = 7.5 fF (as discussed earlier)and 50 Ω load resistance. These analyses shows that, evenconsidering the parasitic capacitance of metal contacts [37],the bandwidth of the device can still exceeds 100 GHz at apower consumption on the order of 10-100 fJ/bit for ultralowpower chip-scale photonic links. Furthermore, the same devicestructure can also be applied to the PDs in Fig. 1(a), which greatlyfacilitates photonic integration.

IV. CONCLUSION

We design ultralow power, high bandwidth density HP-MQWGaAs/AlGaAs electro-reflective modulators operating at 1 Vfor facile integration with polymer waveguide interconnect net-works for co-packaged chip-scale photonic links. Distinctivefrom conventional electro-optical or electro-absorption modula-tors, this new design synergistically employs ultra-large changesin both refractive index (|Δn| ∼0.05) and absorption coefficient(Δα∼ 104 cm−1). This strategy efficiently couples the incidentlight into hybrid modes between the surface plasma and theMQW slab under 1 V reverse bias, achieving >30 dB extinc-tion ratio at a low insertion loss of 1–3 dB with an incidentangle tolerance of 5° and spectral bandwidth of 7-10 nm. Theinherent power consumption of the modulator and its driveris as low as ∼11 fJ/bit, and the RC limited bandwidth wellexceeds 100 GHz. The footprint (∼5 μm in diameter) and powerconsumption of these HP-MQW modulators are both notablysmaller than conventional waveguide-integrated GaAs/AlGaAsEAMs and comparable to micro-resonator-based modulators,without sacrificing the spectral bandwidth. This new modulatorenables high bandwidth and ultralow power inter-chip OIs at>100 Gb/s/channel and <100 fJ/bit/channel. The same devicestructure also applies to the PDs, thereby further facilitatingphotonic integration.

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3400108 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 27, NO. 3, MAY/JUNE 2021

REFERENCES

[1] Y. Shen, et al., “Silicon photonics for extreme scale systems,” J. Lightw.Technol., vol. 37, no. 2, pp. 245–259, Jan. 2019.

[2] X. Wang, and J. Liu, “Emerging technologies in Si active photonics,” J.Semicond., vol. 39, no. 6, pp. 061001-1–061001-29, Jun. 2018.

[3] S. Yu, et al., “Seamless hybrid-integrated interconnect network (SHINE),”in Proc. Opt. Fiber Commun. Conf., 2019, Paper M4D.5.

[4] P. Westbergh, et al., “High-speed oxide confined 850-nm VCSELs oper-ating error-free at 40 Gb/s up to 85°C,” IEEE Photonics Technol. Lett.,vol. 25, pp. 768–771, 2013.

[5] E. Haglund, et al., “30 GHz bandwidth 850 nm VCSEL with sub-100 fJ/bitenergy dissipation at 25-50 Gbit/s,” Electron. Lett., vol. 51, pp. 1096–1098,2015.

[6] D. Kuchta, “Multi-wavelength optical transceivers integrated on node(MOTION),” 2018 DOE ARPA-E ENLITENED Program ReviewMeeting, [Online]. Available: https://arpa-e.energy.gov/sites/default/files/Kuchta_ENLITENED2018.pdf

[7] G. Belfiore, M. Khafaji, R. Henker, and F. Ellinger, “50 Gb/s 190 mWasymmetric 3-Tap FFE VCSEL driver,” IEEE J. Solid-State Circuits,vol. 52, no. 9, pp. 2422–2429, Sep. 2017.

[8] J. L. González, R. Polster, G. Waltener, Y. Thonnart, and E. Cassan, “10Gbps, 560 fJ/b TIA and modulator driver for optical networks-on-chip inCMOS 65 nm,” in Proc. 2016 14th IEEE Int. New Circuits Syst. Conf.(NEWCAS), 26–29 Jun. 2016, Vancouver, Canada.

[9] Q. Cheng, M. Bahadori, M. Glick, S. Rumley, and K. Bergman, “Recentadvances in optical technologies for data centers: A review,” Optica, vol. 5,no. 11, pp. 1354–1370, Nov. 2018.

[10] S. A. Srinivasan, et al., “56 Gb/s germanium waveguide electron-absorption modulator,” J. Lightw. Technol., vol. 34, no. 2, pp. 419–424,2016.

[11] L. Mastronardi, et al., “High-speed Si/GeSi hetero-structure electron-absorption modulator,” Opt. Express, vol. 26, no. 6, pp. 6663–6673,Mar. 2018.

[12] Y. Tang, J. D. Peters, and J. E. Bowers, “Over 67 Ghz bandwidth hybridsilicon electroabsorption modulator with asymmetric segmented electrodefor 1.3 µm transmission,” Opt. Express, vol. 20, no. 10, pp. 11529–11535,May 2012.

[13] K. W. Goossen, et al., “GaAs MQW modulators integrated with siliconCMOS,” IEEE Photon. Technol. Lett., vol. 7, no. 4, pp. 360–362, Apr. 1995.

[14] G. D. Boyd, et al., “Multiple quantum well reflection modulator,” Appl.Phys. Lett., vol. 50, no. 17, pp. 1119–1121, Apr. 1987.

[15] P. Zouganeli, P. J. Stevens, D. Atkinson, and G. Parry, “Design trade-offs and evaluation of the performance attainable by GaAs–Al0.3Ga0.7asasymmetric Fabry-Perot modulators,” IEEE J. Quantum. Electron., vol. 31,no. 5, pp. 927–943, May 1995.

[16] R. H. Yan, R. J. Simes, and L. A. Coldren, “Surface-normal electroabsop-tion reflection modulators using asymmetric fabry-perot structures,” IEEEJ. Quantum Electron., vol. 27, no. 7, pp. 1922–1931, Jul. 1991.

[17] D. S. Gerber, R. Droopad, and G. N. Maracas, “GaAs/AlGaAs asymmetricFabry-Perot reflection modulator with very high contrast ratio,” IEEEPhoton. Technol. Lett., vol. 5, no. 1, pp. 55–58, Jan. 1993.

[18] C. C. Barron, C. J. Mahon, B. J. Thibeault, G. Wang, W. Jiang, and L. A.Coldren, “Millimeter-wave asymmetric fabry-perot modulators,” IEEE J.Quantum Electron., vol. 31, no. 8, pp. 1484–1493, Aug. 1995.

[19] R. M. Audet, E. H. Edwards, P. Wahl, and D. A. B. Miller, “Investigation oflimits to the optical performance of asymmetric fabry-perot electroabsorp-tion modulators,” IEEE J. Quantum Electron., vol. 48, no. 2, pp. 198–209,Feb. 2012.

[20] W. Sievater, W. S. Rabinovich, P. G. Goetz, R. Mahon, and S. C. Binari,“A surface-normal coupled-quantum-well modulator at 1.55 µm,” IEEEPhoton. Technol. Lett., vol. 16, no. 9, pp. 2036–2038, Sep. 2004.

[21] T. Gu, R. Nair, and M. W. Haney, “Chip-level multiple quantum wellmodulator-based optical interconnects,” J. Lightw. Technol., vol. 31, no.24, pp. 4166–4174, 2013.

[22] E. Timurdogan, C. Sorace-Agaskar, J. Sun, E. S. Hosseini, A. Biberman,and M. R. Watts, “An ultralow power athermal silicon modulator, ” Nat.Commun., vol.5, pp. 4008-1–4008-11, Jun. 2014.

[23] Z. Ying, Z. Wang, Z. Zhao, S. Dhar, D. Z. Pan, R. Soref, and R. T.Chen, “Comparison of microrings and microdisks for high-speed Opticalmodulation in silicon Photonic,” Appl. Phys. Lett., vol. 112, Mar. 2018,Art. no. 111108.

[24] G. Lengyel, K. W. Jelley, R. W. H. Engelmann, “A semi-empirical modelfor electroabsorption in GaAs/AlGaAs multiple quantum well modulatorstructures,” IEEE J. Quantum Electron., vol. 26, no. 2, pp. 296–304,Feb. 1990.

[25] J. S. Weiner, D. A. B. Miller, and D. S. Chemla, “Quadratic electro-opticaleffect due to quantum confined stark effect in quantum wells,” Appl. Phys.Lett., vol. 50, no. 13, pp. 842–847, Mar. 1987.

[26] M. Glick, F. K. Reinhart, G. Weimann, and W. Schlapp, “Quadraticelectron-optic light modulation in a GaAs/AlGaAs multiquantum het-erostructure near the excitonic gap,” Appl. Phys. Lett., vol. 48, no. 15,pp. 989–991, Apr. 1986.

[27] Asahi Glass Company, Technical Information CYTOP, distributedby Bellex International Corporation, [Online]. Available: http://www.bellexinternational.com/

[28] H. Zuo, S. Yu, T. Gu, and J. Hu, “Low loss, Flexible single-modepolymer photonics,” Opt. Express, vol. 27, no. 8, pp. 1152–1159, Apr.2019.

[29] S. Yu, H. Zuo, X. Sun, J. Liu, T. Gu, and J. Hu, “Optical free-form couplersfor high-density integrated photonics: A universal optical interface,” J.Lightw. Technol., doi: 10.1109/JLT.2020.2971724.

[30] A. W. Fang, H. Park, O. Cohen, R. Jones, M. J. Paniccia, and J. E. Bowers,“Electrically pumped hybrid AlGaInAs-silicon evanescent laser,” Opt.Express, vol. 14, no. 20, pp. 9203–9210, Oct. 2006.

[31] H. Nagai, M. Yamanishi, Y. Kan, and I. Suemune, “Field-induced modu-lations of refractive index and absorption coefficient in a GaAs/AlGaAsquantum well structure,” Electron. Lett., vol. 22, no. 17, pp. 888–889,Aug. 1986.

[32] P. J. Stevens, M. Whitehead, G. Parry, and K. Woodbridge, “Computermodeling of the electric field dependent absorption spectrum of multiplequantum well material,” IEEE J. Quantum. Electron., vol. 24, no. 10,pp. 2007–2016, Oct. 1988.

[33] M. Xu, J. M. Dell, J. F. Siliquini, and P. Chavarkar, “Near band-edgefield-dependent absorption coefficient and refractive index determinedby photocurrent and transmittance measurements,” Appl. Opt., vol. 38,no. 24, pp. 5127–5132, Aug. 1999.

[34] J. S. Weiner et al., “Strong polarization-sensitive electroabsorption inGaAs/AlGaAs quantum well waveguides, ” Appl. Phys. Lett., vol. 47,no. 24, pp. 1148–1150, 1985.

[35] L. Chuang, Physics of Optoelectronic Devices. New York, NY, USA:Wiley, 1995, ch. 13.

[36] P. J. Mares and S. L. Chuang, “Modeling of self-electro-optic-effectdevices,” J. Appl. Phys., vol. 74, no. 2, pp. 1388–1397, Jul. 1993.

[37] J. Liu, Z. Yu, and L. Sun, “A broadband model over 1–220 GHz for GSGpad structures in RF CMOS,” IEEE. J. Electron. Device Lett., vol. 35,no. 7, pp. 696–698, Jul. 2014.

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