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Abstract
We propose a microring resonator (MRR) optical switch based on III-V/Si hybrid metal-oxide-semiconductor (MOS) optical phase shifter with an ultrathin InP membrane. By reducing the thickness of the InP membrane, we can reduce the insertion loss of the phase shifter, resulting in a high-quality-factor (Q-factor) MRR switch. By optimizing the device structure using numerical analysis, we successfully demonstrated a proof-of-concept MRR optical switch. The optical switch exhibits 0.3 pW power consumption for switching, applicable to power-efficient, thermal-crosstalk-free, Si programmable photonic integrated circuits (PICs) based on wavelength division multiplexing (WDM).
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Facing the scaling limitation of current electrical computing schemes due to the limits of Moore’s law, optical neural networks (ONNs) using programmable photonic integrated circuits (PICs) based on a Si photonics platform has a lot of attention as a novel computing architecture to accelerate calculations in deep learning, which is one of the most growing technologies widely used in various applications, with the speed of optical signals [1–8]. To perform multiply–accumulate (MAC) operation, meshed Mach–Zehnder interferometers (MZIs) have been used, while the size of an MZI imposes the scalability of the circuit size. For better scalability in ONNs, a Si microring resonator (MRR) is favorable since a Si MRR integrated with an optical phase shifter works as a small-dimensional optical switch that selectively transmits optical signals of specified wavelengths [4–6,9–12]. In particular, we have proposed the use of a Si MRR crossbar array, which can increase the speed of MAC operation in deep learning with wavelength division multiplexing (WDM) optical signals [13]. To program an ONN based on the MRR crossbar array, a thermo-optic (TO) phase shifter can be used [14], while thermal crosstalk is a major obstacle in the controllability of an MRR [15]. For the replacement of a TO phase shifter, we have proposed the use of a III-V/Si hybrid metal-oxide-semiconductor (MOS) optical phase shifter that consists of a MOS capacitor of an n-InGaAsP/Al2O3/p-Si stack [16–22]. Owing to the plasma dispersion and band-filling effects induced by electron accumulation at an MOS interface, an efficient and loss-loss phase modulation is achievable [16]. Moreover, the power consumption of the hybrid MOS phase shifter, which is dominated by a gate leakage current, is extremely low, which is 107 times lower than that of the TO-effect-based optical phase shifter [17,18]. Thus, the MRR crossbar array consisting of the hybrid MOS optical phase shifter can solve the thermal crosstalk issue. In the conventional design of the hybrid MOS optical phase shifter, however, a 50-µm-long III-V taper structure required to eliminate insertion loss between the Si waveguide and the phase shifter [23] makes it difficult to integrate the phase shifter on the MRR with a radius of approximately 10 µm. Previously, a racetrack resonator modulator with the hybrid MOS optical phase shifter was reported to keep its III-V taper structure [19], which requires a large circuit. Also, an MRR optical switch based on the hybrid MOS optical phase shifter consisting of a 150-nm-thick III-V structure bonded onto the entire ring waveguide has been fabricated [20–22] but showed poor phase-matching condition at the gap of MRR and high attenuation due to the side roughness of a III-V layer, resulting in a low quality factor (Q-factor). In this study, we propose an MRR optical switch based on a III-V/Si hybrid MOS optical phase shifter with an ultrathin InP membrane as shown in Fig. 1. To simplify the structure for the feasibility study, we adopted the all-pass structure instead of the add-drop one. By using a 25-nm-thick III-V membrane, we can eliminate the insertion loss between the Si waveguide and the phase shifter even without tapers [24]. We discuss the details of the MRR design and show the proof-of-concept device integrated into a III-V/Si hybrid MOS optical phase shifter.
2. Device design using numerical analysis
To obtain a high-Q-factor, efficient MRR optical switch that meets the critical coupling condition, we numerically analyzed an MRR optical switch based on the hybrid MOS phase shifter of various structures. Figure 2 shows the plan-view and cross-sectional schematics of the simulated structure. We designed the device operating at the O-band because the design of a grating coupler operating at the O-band was only available in the foundry service. A 300-nm-high, 550-nm-wide Si rib waveguide was assumed for a single-mode operation. The radius of the Si MRR was assumed to be 10 µm. The InP membrane was assumed to be bonded onto the Si waveguide with a 5-nm-thick SiO2 bonding interface. The doping concentrations of p-Si and n-InP layers were both 5×1017 cm−3. The free-carrier absorption in the p-Si and n-InP layers was taken into account in the simulation. We investigated the optimal thickness of the InP membrane and the gap between the ring waveguide and the bus waveguide through simulations.
Fig. 2. (a) Plan-view and (b) cross-sectional schematics of MRR switch based on the hybrid MOS phase shifter.
First, we analyzed the attenuation of the ring waveguide with varied thicknesses of the InP membrane. To obtain the propagation loss of the ring waveguide and the insertion loss of the phase shifter, the fundamental transverse electric (TE) mode in the MRR was calculated using the finite-difference eigenmode solver of Lumerical MODE. The free-carrier absorption was considered by using the models described in [16] for InP and [25] for Si. The insertion loss at the connection between the Si and hybrid waveguides was evaluated from the mismatch of the fundamental TE mode between the Si ring waveguide and the phase shifter, which can be calculated with Lumerical MODE. Figure 3(a) shows the obtained propagation loss, insertion loss, and the total loss per round-trip of the ring waveguide. The total loss was defined as the sum of the propagation loss and doubled insertion loss. The inset of Fig. 3(a) shows the optical mode distribution. When there is no InP membrane, the propagation loss matches the loss of p-Si. As the thickness of the InP membrane increases, the optical confinement in the InP membrane increases. Since the free-carrier absorption of n-InP is smaller than that of p-Si, the propagation loss of the ring waveguide gradually decreases. When there is no InP membrane, the propagation loss is 0.15 dB, which corresponds to approximately 24 dB/cm. In this simulation, we ignored the propagation loss due to the sidewall roughness of the waveguide, which is much smaller than that due to the free-carrier absorption. On the other hand, the insertion loss between the Si waveguide and the InP/Si hybrid structure increases. In total, the loss of the ring waveguide increases as the thickness of the InP membrane increases. Figure 3(b) shows the Q-factor of MRR as a function of the thickness of the InP membrane calculated from the results in Fig. 3(a) when a critical coupling condition was assumed. When the thickness of the InP membrane was smaller than 40 nm, the Q-factor of the MRR can be greater than 104.
Fig. 3. (a) Simulated optical loss of the ring waveguide and (b) Q-factor of the MRR with various thicknesses of InP.
Next, we evaluated the Q-factor and the coupling condition of the MRR switch by sweeping both the thickness of the InP membrane and the gap of the MRR. When the MRR meets a critical coupling condition, the power attenuation coefficient of the ring waveguide equals to the self-coupling coefficient of the MRR. The power attenuation coefficient was calculated from the results in Fig. 3(a). The coupling region of the MRR was divided into short sections, each of which was considered a short directional coupler. Thus, we calculated each self-coupling coefficient based on the odd and even modes. Finally, the self-coupling coefficient of the whole structure can be obtained by multiplication of the short directional couplers [26,27]. Figure 4(a) shows the calculated Q-factor (color map) and the critical coupling condition (black curve) of the MRR with various thicknesses of the InP membrane and gaps of the MRR. When the thickness of the InP membrane is 30 nm and the gap is 150 nm, the Q-factor is more than 104. Also, this condition meets the critical coupling condition, which can exhibit the highest extinction ratio at the resonance wavelength. Figure 4(b) shows the calculated optical transmissions of the MRR with varied applied voltages as a function of an operating wavelength when the thickness of the InP membrane and the gap were 30 and 150 nm, respectively. Theoretical equation of an MRR was used to calculate the output spectra [9]. The optical phase shift of the hybrid MOS optical phase shifter was simulated in conjunction with Sentaurus technology computer aided design (TCAD) and Lumerical MODE described in [16]. We assumed that it is the equivalent oxide thickness (EOT) was 5 nm and there were no interface traps at the Si and InP MOS interfaces. As shown in Fig. 4(b), a high Q-factor and high extinction resonance are expected to be obtained with the optimal design. Moreover, by applying voltage to the optical phase shifter, we can expect the resonance peak to be clearly shifted with carrier-induced phase modulation, which is applicable to an optical switch.
Fig. 4. (a) Simulated Q-factor and the critical coupling condition of MRR and (b) transmission of optimized MRR optical switch based on the hybrid MOS optical phase shifter.
3. Fabrication procedure
We fabricated the MRR optical switch with a taperless III-V/Si hybrid MOS optical phase shifter based on the optimized design discussed in the previous section. Figure 5 shows the fabrication procedure. We fabricated rib waveguides and grating couplers operating at a 1300 nm wavelength on a 300-nm-thick Si-on-insulator (SOI) wafer. According to the optimized design, the radius and gap of the MRR were determined to be 10 µm and 150 nm, respectively. Boron was implanted to form p-Si and p+-Si regions. The doping concentrations of the p-Si and p+-Si regions were 5×1017 and 1×1019 cm−3, respectively. Then, to form SiO2-embedded Si waveguides, SiO2 was formed by chemical vapor deposition (CVD), followed by planarization by chemical mechanical polishing (CMP). The flat surface of SiO2 after CMP enabled uniform wafer bonding, contributing to the high-yield device fabrication. Then, an InP epitaxial wafer including a 25-nm-thick InP membrane and etch-stop layers was bonded onto the Si waveguide via a 4-nm-thick Al2O3 layer formed by atomic layer deposition (ALD). After removing the InP substrate and etch-stop layers, the InP mesa was defined using electron-beam (EB) lithography and wet chemical etching. After the forming of the SiO2 cladding layer by CVD, contact via holes were opened by inductively coupled plasma (ICP) etching. Finally, electrode pads were formed with a Ni/Ti/Pt/Al metal stack by sputtering and lift-off. Finally, post-metallization annealing (PMA) at 400°C for 1 min was performed to improve the electrical contact.
Fig. 5. Fabrication procedure for MRR optical switch based on III-V/Si hybrid MOS optical phase shifter.
Figure 6(a) shows a plan-view microscopy image of the fabricated MRR optical switch based on the III-V/Si hybrid MOS optical phase shifter. A 25-nm-thick InP membrane was successfully bonded onto the Si ring waveguide. Figure 6(b) shows a scanning electron microscopy (SEM) image of the device focusing on the gap between the ring waveguide and the bus waveguide. The width and gap of the waveguides were measured to be approximately 150 nm and 550 nm, respectively, as designed.
Fig. 6. (a) Plan-view microscopy image and (b) expanded SEM image of fabricated MRR optical switch based on III-V/Si hybrid MOS optical phase shifter.
Figure 7 shows a transmission electron microscopy (TEM) image of the hybrid MOS optical phase shifter in the MRR optical switch. As shown in Fig. 7(a), the heights of the ridge and slab regions of the Si waveguide were determined to be 300 nm and 150 nm, respectively, which agree with the design. From Fig. 7(b), the thicknesses of SiO2 and InP was revealed to be approximately 6 nm and 25 nm, respectively. The thickness of Al2O3 formed by ALD was measured to be approximately 1 nm. Some defects were found at the top surface of the InP. By measuring a capacitance–voltage (C-V) curve of the InP/Al2O3/SiO2/Si MOS capacitor fabricated at the same time, the EOT of the hybrid MOS phase shifter was determined to be approximately 8 nm.
Fig. 7. Cross-sectional TEM images of (a) III-V/Si hybrid MOS phase shifter in the MRR optical switch and (b) InP/Al2O3/SiO2/Si stack.
4. Measurement
We evaluated the MRR switch by measuring its output spectra at various gate voltages. A TE-mode, O-band continuous-wave light was injected from a tunable laser to the MRR via a grating coupler. The output was coupled into a cleaved fiber via a grating coupler, and the output power was measured using an InGaAs optical power meter. Figures 8(a) and (b) show the obtained output spectra. From Fig. 8(a), the free spectrum range (FSR) of the MRR was obtained to be 6.5 nm. Figure 8(b) shows the output spectra at various gate voltages. A clear blue-shift of the resonance peak was successfully obtained. Since the attenuation of the ring waveguide was changed by the free-carrier absorption at the MOS interface, the extinction ratio of the MRR changed when applying the gate voltage. When the gate voltage was 2 V, the extinction ratio of the MRR switch was approximately 35 dB. Thus, owing to the optimized design, the MRR met the critical coupling condition. The Q-factor of the MRR switch was obtained to be approximately 8500, which is greater than those of resonance-based optical switches with hybrid MOS optical phase shifters reported in [20–22]. By removing the surface roughness of the InP shown in Fig. 7(b) and reducing optical scattering loss, we can obtain further improvement of the Q-factor.
Fig. 8. (a) Output spectrum of the MRR optical switch based on III-V/Si hybrid MOS optical phase shifter and (b) output spectra at various gate voltages.
From the results in Fig. 8, we evaluated the phase shift of the hybrid MOS phase shifter as shown in Fig. 9(a). From the slope of the phase shift, the modulation efficiency VπL of the phase shifter was evaluated to be 1.08 Vcm. Although the measured EOT of the hybrid MOS phase shifter was 8 nm, the obtained modulation efficiency corresponded to EOT of 20 nm. This discrepancy might be attributed to the large interface trap density attributable to the too thin Al2O3 bonding interface. When the EOT is reduced to be 5 nm, the modulation efficiency will be improved to be approximately 0.3 Vcm. Also, InGaAsP (λg = 0.92 µm) can be used to further improve of the modulation efficiency [24]. On the other hand, since the clear peak shift was observed as shown in Fig. 8(b), this MRR switch is still applicable for Si programmable PICs despite the small modulation efficiency. Figure 9(b) shows the Q-factor of the MRR switch at various applied voltages obtained from Fig. 8(b). Owing to the small carrier-induced attenuation of the hybrid MOS phase shifter [28], the Q-factor does not change, which is an advantage for tuning a resonance wavelength. In this device fabrication, we used the n-InP membrane with no heavily doped contact layer, resulting in the large contact resistance and small modulation bandwidth. We expect that the modulation bandwidth exceeds 10 GHz when an n+-InGaAs contact layer is introduced as reported in [29].
Fig. 9. (a) Phase shift and (b) Q-factor of the MRR switch at various gate voltages on the hybrid MOS phase shifter.
We also evaluated the power consumption of the MRR switch by leakage current measurement of the hybrid MOS phase shifter at various applied voltages. Red points in Fig. 10 show the evaluated power consumption of the phase shifter. Blue points in the figure show the transmission when the wavelength of the optical signal was about 1276.97 nm, which corresponds to the resonance wavelength of the MRR when the applied gate voltage was 0 V. The power consumption for 0.022π phase shift, corresponds to a 25 dB change in the output power, was evaluated to be approximately 0.3 pW which is more than 109 times smaller than that of the MRR switch based on the TO effect [10]. We can increase an available phase shift by introducing InGaAsP and thinner EOT as mentioned. Thus, the MRR switch based on the hybrid MOS phase shifter can eliminate thermal crosstalk when integrated on a Si programmable PIC, which improves its controllability.
Fig. 10. Power consumption of the hybrid MOS optical phase shifter and the change in transmission of the MRR switch.
Finally, we compared the fabricated device with several MRR switches consisting of Si-based MOS optical phase shifters with III-V/insulator/Si [20], poly-Si/insulator/Si [30], and transparent conductive oxide (TCO)/insulator/Si stacks [31,32], which can perform power-efficient, low-thermal-crosstalk switching. Figure 11 shows a benchmark of the Q-factor as a function of the thickness of the top layer in MOS structures. Owing to the MOS structure with an ultrathin InP membrane, our device exhibits the highest Q-factor among MOS-based optical phase shifters. Since the higher Q-factor of MRR provides higher finesse, the available number of channels in a WDM-based programmable PIC can be increased by a high-Q MRR [6]. Also, as shown in Fig. 8(b), a high-Q MRR can show clear switching at a resonance wavelength of optical signal with a relatively small optical phase shift. Considering that the resonance wavelength can be roughly tuned by changing the radius of the ring waveguide [11], we can use our device for power-efficient, high-dense WDM-based Si programmable PICs such as MRR crossbar arrays.
5. Conclusion
We proposed an MRR optical switch based on the III-V/Si hybrid optical phase shifter with an ultrathin InP membrane. Through design optimization by numerical analysis, we successfully demonstrated a high-Q, high-extinction-ratio MRR optical switch with 0.3 pW power consumption, ensuring the elimination of thermal crosstalk. Since the hybrid MOS phase shifter with an ultrathin membrane exhibits small insertion loss, our device is applicable for use in WDM-based Si programmable PICs such as MRR crossbar arrays for ONNs.
Funding
New Energy and Industrial Technology Development Organization; Core Research for Evolutional Science and Technology (JPMJCR2004); Ministry of Education, Culture, Sports, Science and Technology (JPMXP09F20UT0021).
Acknowledgements
This work was partly commissioned by the New Energy and Industrial Technology Development Organization (NEDO) and supported by JST, CREST Grant Number JPMJCR2004. Part of this work was conducted at Takeda Sentanchi super cleanroom, The University of Tokyo, supported by the “Nanotechnology Platform Program” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (Grant Number JPMXP09F20UT0021).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. K. Kitayama, M. Notomi, M. Naruse, K. Inoue, S. Kawakami, and A. Uchida, “Novel frontier of photonics for data processing–Photonic accelerator,” APL Photonics 4(9), 090901 (2019). [CrossRef]
2. L. D. Marinis, M. Cococcioni, P. Castoldi, and N. Andriolli, “Photonic Neural Networks: A Survey,” IEEE Access 7(1), 175827–175841 (2019). [CrossRef]
3. Y. Shen, N. C. Harris, S. Skirlo, M. Prabhu, T. Baehr-Jones, M. Hochberg, X. Sun, S. Zhao, H. Larochelle, and D. Englund, “Deep learning with coherent nanophotonic circuits,” Nat. Photonics 11(7), 441–446 (2017). [CrossRef]
4. L. Yang, R. Ji, L. Zhang, J. Ding, and Q. Xu, “On-chip CMOS-compatible optical signal processor,” Opt. Express 20(12), 13560–13565 (2012). [CrossRef]
5. A. N. Tait, T. F. De Lima, E. Zhou, A. X. Wu, M. A. Nahmias, B. J. Shastri, and P. R. Prucnal, “Neuromorphic photonic networks using silicon photonic weight banks,” Sci. Rep. 7(1), 1–10 (2017). [CrossRef]
6. A. Tait, A. Wu, T. F. De Lima, E. Zhou, B. J. Shastri, M. A. Nahmias, and P. R. Prucnal, “Microring Weight Banks,” IEEE J. Sel. Top. Quantum Electron. 22(6), 312–325 (2016). [CrossRef]
7. Y. Shen and Y. Bai, “Statistical computing with integrated photonics system,” in 24th OptoElectron. Commun. Conf. (OECC) Int. Conf. Photon. Switching Comput. (PSC), paper J1-2, Fukuoka, Japan (2019).
8. C. Ramey, “Silicon Photonics for Artificial Intelligence Acceleration,” in Hot Chips 2020, paper 32 (2020).
9. W. Bogaerts, P. D. Heyn, T. V. Vaerenbergh, K. D. Vos, S. K. Selvaraja, T. Claes, P. Dumon, P. Bienstman, D. V. Thourhout, and R. Beats, “Silicon microring resonators,” Laser Photonics Rev. 6(1), 47–73 (2012). [CrossRef]
10. N. S. Droz, H. Wang, L. Chen, B. G. Lee, A. Biberman, K. Bergman, and M. Lipson, “Optical 4 ( 4 hitless silicon router for optical Networks-on Chip (NoC),” Opt. Express 16(20), 15915–15922 (2008). [CrossRef]
11. M. S. Dahlem, C. W. Holzwarth, A. Khilo, F. X. Kärtner, H. I. Smith, and E. P. Ippen, “Reconfigurable multi- channel second-order silicon microring-resonator filterbanks for on-chip WDM systems,” Opt. Express 19(1), 306–316 (2011). [CrossRef]
12. F. Testa, C. J. Oton, C. Kopp, M. Lee, R. Ortuño, R. Enne, S. Tondini, G. Chiaretti, A. Bianchi, P. Pintus, S. Kim, D. Fowler, J. A. Ayúcar, M. Hofbauer, M. Mancinelli, M. Fournier, G. B. Preve, N. Zecevic, C. L. Manganelli, C. Castellan, G. Parès, O. Lemonnier, F. Gambini, P. Labeye, M. Romagnoli, L. Pavesi, H. Zimmermann, F. D. Pasquale, and S. Stracca, “Design and Implementation of an Integrated Reconfigurable Silicon Photonics Switch Matrix in IRIS Project,” IEEE J. Sel. Top. Quantum Electron. 22(6), 155–168 (2016). [CrossRef]
13. S. Ohno, K. Toprasertpong, S. Takagi, and M. Takenaka, “Si microring resonator crossbar arrays for deep learning accelerator,” Jpn. J. Appl. Phys. 59(SG), SGGE04 (2020). [CrossRef]
14. G. Cocorullo and I. Rendina, “Thermo-optical modulation at 1.5 µm in silicon etalon,” Electron. Lett. 28(1), 83–85 (1992). [CrossRef]
15. A. N. Tait, T. F. De Lima, M. A. Nahmias, B. J. Shastri, and P. R. Prucnal, “Multi-channel control for microring weight banks,” Opt. Express 24(8), 8895–8906 (2016). [CrossRef]
16. J.-H. Han, F. Boeuf, J. Fujikata, S. Takahashi, S. Takagi, and M. Takenaka, “Efficient low-loss InGaAsP/Si hybrid MOS optical modulator,” Nat. Photonics 11(8), 486–490 (2017). [CrossRef]
17. Q. Li, J.-H. Han, C. P. Ho, S. Takagi, and M. Takenaka, “Ultra-power-efficient 2×2 Si Mach-Zehnder Interferometer Optical Switch based on III-V/Si Hybrid MOS Phase Shifter,” Opt. Express 26(26), 35003–35012 (2018). [CrossRef]
18. M. Takenaka, J.-H. Han, F. Boeuf, J.-K. Park, Q. Li, C. P. Ho, D. Lyu, S. Ohno, J. Fujikata, S. Takahashi, and S. Takagi, “III-V/Si Hybrid MOS Optical Phase Shifter for Si Photonic Integrated Circuits,” J. Lightwave Technol. 37(5), 1474–1483 (2019). [CrossRef]
19. Q. Li, C. P. Ho, H. Tang, M. Okano, S. Takagi, and M. Takenaka, “Si racetrack optical modulator based on the III–V/Si hybrid MOS capacitor,” Opt. Express 29(5), 6824–6833 (2021). [CrossRef]
20. D. Liang, G. Kurezvell, M. Fiorentino, S. Srinivasan, J. E. Bowers, and R. G. Beausoleil, “A Tunable Hybrid III-V-on-Si MOS Microring Resonator with Negligible Tuning Power Consumption,” in Optical Fiber Communication Conference (OFC), paper Th1 K.4, Anaheim, USA (2016).
21. X. Huang, D. Liang, C. Zhang, G. Kurczveil, X. Li, J. Zhang, M. Fiorentino, and R. Beausoleil, “Heterogeneous MOS microring resonators,” in IEEE Photonics Conference (IPC), paper MD 3.4, Orlando, USA (2017).
22. D. Liang, G. Kurczveil, Z. Huang, B. Wang, A. Descos, S. Srinivasan, Y. Hu, X. Zeng, W. V. Sorin, S. Cheung, S. Liu, P. Sun, T. V. Vaerenbergh, M. Fiorentino, J. E. Bowers, and R. G. Beausoleil, “Integrated Green DWDM Photonics for Next-Gen High-Performance Computing,” in Optical Fiber Communication Conference (OFC), paper M2B.6, San Diego, USA (2020).
23. Q. Huang, J. Cheng, L. Liu, Y. Tang, and S. He, “Ultracompact tapered coupler for the Si/III-V heterogeneous integration,” Appl. Opt. 54(14), 4327–4332 (2015). [CrossRef]
24. S. Ohno, Q. Li, N. Sekine, J. Fujikata, M. Noguchi, S. Takahashi, K. Toprasertpong, S. Takagi, and M. Takenaka, “Taperless Si hybrid optical phase shifter based on a metal-oxide-semiconductor capacitor using an ultrathin InP membrane,” Opt. Express 28(24), 35663–35673 (2020). [CrossRef]
25. R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. Quantum Electron. 23(1), 123–129 (1987). [CrossRef]
26. M. Bahadori, M. Nikdast, S. Rumley, L. Y. Dai, N. Janosik, T. V. Vaerenbergh, A. Gazman, Q. Cheng, R. Polster, and K. Bergman, “Design Space Exploration of Microring Resonators in Silicon Photonic Interconnects: Impact of the Ring Curvature,” J. Lightwave Technol. 36(13), 2767–2782 (2018). [CrossRef]
27. G. Calo, A. D’Orazio, and V. Petruzzelli, “Broadband Mach-Zehnder Switch for Photonic Networks on Chip,” J. Lightwave Technol. 30(7), 944–952 (2012). [CrossRef]
28. I. Datta, S. H. Chae, G. R. Bhatt, M. A. Tadayon, B. Li, Y. Yu, C. Park, J. Park, L. Cao, D. N. Basov, J. Hone, and M. Lipson, “Low-loss composite photonic platform based on 2D semiconductor monolayers,” Nat. Photonics 14(4), 256–262 (2020). [CrossRef]
29. Q. Li, C. P. Ho, J. Fujikata, M. Noguchi, S. Takahashi, K. Toprasertpong, S. Takagi, and M. Takenaka, “Low Parasitic Capacitance III-V/Si Hybrid MOS Optical Modulator toward High-speed Modulation,” in Optical Fiber Communication Conference (OFC), paper Th2A. 16, San Diego, USA (2020).
30. J. V. Campenhout, M. Pantouvaki, P. Verheyen, S. Selvaraja, G. Lepage, H. Yu, W. Lee, J. Wouters, D. Goossens, M. Moelants, W. Bogaerts, and P. Absil, “Low-voltage, low-loss, multi-Gb/s silicon micro-ring modulator based on a MOS capacitor,” in Optical Fiber Communication Conference (OFC), paper OM2E.4, Los Angeles, USA (2012).
31. E. Li, B. A. Nia, B. Zhou, and A. X. Wang, “Transparent conductive oxide-gated silicon microring with extreme resonance wavelength tunability,” Photonics Res. 7(4), 473–477 (2019). [CrossRef]
32. W.-C. Hsu, B. Zhou, Z. Cheng, and A. X. Wang, “Silicon Microring Resonator Driven by High-Mobility conductive Oxide Capacitor,” in IEEE Photonics Conference (IPC), paper MF3.2 (2020).
