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1. Compact atto-joule-per-bit bus-coupled photonic crystal nanobeam switches

   https://doi.org/10.1117/12.2649250  

   Shen, Jianhao; Donnelly, Daniel; Chakravarty, Swapnajit (March 2023, Proceedings of SPIE)

   García-Blanco, Sonia M.; Cheben, Pavel (Ed.)

   The benefits of photonics over electronics in the application of optical transceivers and both classical and quantum computing have been demonstrated over the past decades, especially in the ability to achieve high bandwidth, high interconnectivity, and low latency. Due to the high maturity of silicon photonics foundries, research on photonics devices such as silicon micro ring resonators (MRRs), Mach-Zehnder modulators (MZM), and photonic crystal (PC) resonators has attracted plenty of attention. Among these photonic devices, silicon MRRs using carrier depletion effects in p-n junctions represent optical switches manufacturable in the most compact magnitude at high volume with demonstrated switching energies ~5.2fJ/bit. In matrix multiplication demonstrated with integrated photonics, one approach is to couple one bus straight waveguide to several MRRs with different resonant wavelengths to transport signals in different channels, corresponding to a matrix row or column. However, such architectures are potentially limited to ~30 MRRs in series, by the limited free-spectral range (FSR) of an individual MRR. We show that PC switches with sub-micron optical mode confinements can have a FSR >300nm, which can potentially enable energy efficient computing with larger matrices of ~200 resonators by multiplexing. In this paper, we present designs for an oxide-clad bus-coupled PC switch with 1dB insertion loss, 5dB extinction, and ~260aJ/bit switching energy by careful control of the cavity geometry as well as p-n junction doping. We also demonstrate that air-clad bus-coupled PC switches can operate with 1dB insertion loss, 3dB extinction, and ~80aJ/bit switching energy. 

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2. Non-volatile electrically programmable integrated photonics with a 5-bit operation

   https://doi.org/10.1038/s41467-023-39180-3  

   Chen, Rui; Fang, Zhuoran; Perez, Christopher; Miller, Forrest; Kumari, Khushboo; Saxena, Abhi; Zheng, Jiajiu; Geiger, Sarah J.; Goodson, Kenneth E.; Majumdar, Arka (December 2023, Nature Communications)

   Abstract Scalable programmable photonic integrated circuits (PICs) can potentially transform the current state of classical and quantum optical information processing. However, traditional means of programming, including thermo-optic, free carrier dispersion, and Pockels effect result in either large device footprints or high static energy consumptions, significantly limiting their scalability. While chalcogenide-based non-volatile phase-change materials (PCMs) could mitigate these problems thanks to their strong index modulation and zero static power consumption, they often suffer from large absorptive loss, low cyclability, and lack of multilevel operation. Here, we report a wide-bandgap PCM antimony sulfide (Sb₂S₃)-clad silicon photonic platform simultaneously achieving low loss (<1.0 dB), high extinction ratio (>10 dB), high cyclability (>1600 switching events), and 5-bit operation. These Sb₂S₃-based devices are programmed via on-chip silicon PIN diode heaters within sub-ms timescale, with a programming energy density of$$\sim 10\,{fJ}/n{m}^{3}$$ $~10fJ/n{m}^3$. Remarkably, Sb₂S₃is programmed into fine intermediate states by applying multiple identical pulses, providing controllable multilevel operations. Through dynamic pulse control, we achieve 5-bit (32 levels) operations, rendering 0.50 ± 0.16 dB per step. Using this multilevel behavior, we further trim random phase error in a balanced Mach-Zehnder interferometer. 

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3. Low-power quasi-continuous hybrid volatile/nonvolatile tuning of ring resonators

   https://doi.org/10.1063/5.0236098  

   Dutta, Jayita; Chen, Rui; Tara, Virat; Majumdar, Arka (February 2025, APL Photonics)

   Programmable photonic integrated circuits are expected to play an increasingly important role in enabling high-bandwidth optical interconnects and large-scale in-memory computing as needed to support the rise of artificial intelligence and machine learning technology. To that end, chalcogenide-based non-volatile phase-change materials (PCMs) present a promising solution due to zero static power. However, high switching voltage and a small number of operating levels present serious roadblocks to the widespread adoption of PCM-programmable units. Here, we demonstrate an electrically programmable wide bandgap Sb2S3-clad silicon ring resonator using a silicon microheater at a complementary-metal–oxide–semiconductor compatible voltage of <3 V. Our device shows a low switching energy of 35.33 nJ (0.48 mJ) for amorphization (crystallization) and reversible phase transitions with high endurance (>2000 switching events) near 1550 nm. Combining a volatile thermo-optic effect with non-volatile PCMs, we demonstrate 7-bit (127 levels) operation with excellent repeatability and reduced power consumption. Our demonstration of low-voltage and low-energy operation, combined with the hybrid volatile–nonvolatile approach, marks a significant step toward integrating PCM-based programmable units in large-scale optical interconnects. 

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4. Non-volatile electrically programmable integrated photonics with 5-bit operation based on phase-change material Sb2S3

   https://doi.org/10.1364/CLEO_SI.2023.STu3J.1  

   Chen, Rui; Fang, Zhuoran; Perez, Christopher; Miller, Forrest; Kumari, Khushboo; Saxena, Abhi; Zheng, Jiajiu; Geiger, Sarah J.; Goodson, Kenneth E.; Majumdar, Arka (January 2023, Optica Publishing Group)

   We report a hybrid phase-change mateial Sb₂S₃-silicon photonic tunable directional coupler, which exhibits low insertion loss (< 1.0 dB), large extinction ratio (> 10 dB), high endurance (> 1,600 switching events), and 32 operation levels. 

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5. Nonvolatile Electrically Reconfigurable Integrated Photonic Switch Enabled by a Silicon PIN Diode Heater

   https://doi.org/10.1002/adma.202001218  

   Zheng, Jiajiu; Fang, Zhuoran; Wu, Changming; Zhu, Shifeng; Xu, Peipeng; Doylend, Jonathan_K; Deshmukh, Sanchit; Pop, Eric; Dunham, Scott; Li, Mo; et al (June 2020, Advanced Materials)

   Abstract Reconfigurability of photonic integrated circuits (PICs) has become increasingly important due to the growing demands for electronic–photonic systems on a chip driven by emerging applications, including neuromorphic computing, quantum information, and microwave photonics. Success in these fields usually requires highly scalable photonic switching units as essential building blocks. Current photonic switches, however, mainly rely on materials with weak, volatile thermo‐optic or electro‐optic modulation effects, resulting in large footprints and high energy consumption. As a promising alternative, chalcogenide phase‐change materials (PCMs) exhibit strong optical modulation in a static, self‐holding fashion, but the scalability of present PCM‐integrated photonic applications is still limited by the poor optical or electrical actuation approaches. Here, with phase transitions actuated by in situ silicon PIN diode heaters, scalable nonvolatile electrically reconfigurable photonic switches using PCM‐clad silicon waveguides and microring resonators are demonstrated. As a result, intrinsically compact and energy‐efficient switching units operated with low driving voltages, near‐zero additional loss, and reversible switching with high endurance are obtained in a complementary metal‐oxide‐semiconductor (CMOS)‐compatible process. This work can potentially enable very large‐scale CMOS‐integrated programmable electronic–photonic systems such as optical neural networks and general‐purpose integrated photonic processors. 

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