Ultra-low-energy programmable non-volatile silicon photonics based on phase-change materials with graphene heaters

The rise of electrically tunable metasurfaces

Metasurfaces, which offer a diverse range of functionalities in a remarkably compact size, have captured the interest of both scientific and industrial sectors. However, their inherent static nature limits their adaptability for their further applications. Reconfigurable metasurfaces have emerged as a solution to this challenge, expanding the potential for diverse applications. Among the series of tunable devices, electrically controllable devices have garnered particular attention owing to their seamless integration with existing electronic equipment. This review presents recent progress reported with respect to electrically tunable devices, providing an overview of their technological development trajectory and current state of the art. In particular, we analyze the major tuning strategies and discuss the applications in spatial light modulators, tunable optical waveguides, and adaptable emissivity regulators. Furthermore, the challenges and opportunities associated with their implementation are explored, thereby highlighting their potential to bridge the gap between electronics and photonics to enable the development of groundbreaking optical systems.

Piezoelectricity in wide bandgap semiconductor 2D crystal GaN nanosheets

Gallium nitride (GaN) exhibits various potential applications in optics and optoelectronics due to its outstanding physical characteristics, including a wide direct bandgap, strong deep-ultraviolet emission, and excellent electron transport properties. However, research on the piezoelectric and related properties of GaN nanosheets are scarce, as previous small-scale GaN investigations have mainly concentrated on nanowires and nanotubes. Here, we report a strategy for growing 2D GaN nanosheets using chemical vapor deposition on Ga/W liquid-phase substrates. Additionally, utilizing scanning probe techniques, it has been observed that 700 nm-thick GaN nanosheets demonstrate a piezoelectric constant of deff33 = 1.53 ± 0.21 pm V-1 and possess the capability to effectively modulate the Schottky barrier. The piezoelectric characteristics of 2D GaN are offering new options for innovative applications in various fields, including energy harvesting, electronics, sensing, and communications.

Solution-derived Ge-Sb-Se-Te phase-change chalcogenide films

Ge-Sb-Se-Te chalcogenides, namely Se-substituted Ge-Sb-Te, have been developed as an alternative optical phase change material (PCM) with a high figure-of-merit. A need for the integration of such new PCMs onto a variety of photonic platforms has necessitated the development of fabrication processes compatible with diverse material compositions as well as substrates of varying material types, shapes, and sizes. This study explores the application of chemical solution deposition as a method capable of creating conformally coated layers and delves into the resulting modifications in the structural and optical properties of Ge-Sb-Se-Te PCMs. Specifically, we detail the solution-based deposition of Ge-Sb-Se-Te layers and present a comparative analysis with those deposited via thermal evaporation. We also discuss our ongoing endeavor to improve available choice of processing-material combinations and how to realize solution-derived high figure-of-merit optical PCM layers, which will enable a new era for the development of reconfigurable photonic devices.

Rewritable Photonic Integrated Circuit Canvas Based on Low-Loss Phase Change Material and Nanosecond Pulsed Lasers

Programmable photonic integrated circuits (PICs) are an increasingly important platform in optical science and engineering. However, current programmable PICs are mostly formed through subtractive fabrication techniques, which limits the reconfigurability of the device and makes prototyping costly and time-consuming. A rewritable PIC architecture can circumvent these drawbacks, where PICs are repeatedly written and erased on a single PIC canvas. We demonstrate such a rewritable PIC platform by selective laser writing a layer of wide-band-gap phase change material (PCM) Sb2S3 with a low-cost benchtop setup. We show arbitrary patterning with resolution up to 300 nm and write dielectric assisted waveguides with a low optical loss of 0.0172 dB/μm. We envision that using this inexpensive benchtop platform thousands of PIC designs can be written, tested, and erased on the same chip without the need for lithography/etching tools or a nanofabrication facility, thus reducing manufacturing cost and increasing accessibility.

Tailorable ITO thin films for tunable microwave photonic applications

Tunability is a fundamental prerequisite for functional devices and forms the backbone of reconfigurable microwave photonic (MWP) signal processors. In this paper, we explore the use of indium tin oxide (ITO) thin films, notable for their combination of optical transparency and electrical conductivity, to provide tunability for integrated MWP devices. We study the impacts of post-thermal annealing on the structural, electrical, and optical properties of ITO films. The annealed ITO microheater maintains a low total insertion loss of just 0.1 dB while facilitating the tunability of the microring across the entire free spectral range (FSR) using less than half the voltage required by its non-annealed counterpart. Furthermore, the post-annealed ITO film exhibits a 30% improvement in response time, enhancing its performance as an active voltage-controlled microheater. Leveraging this advantage, we employed the post-annealed device to demonstrate continuous tunable radio frequency (RF) phase shifts from 0-330° across a frequency range spanning 15 GHz to 40 GHz with only 5.58 mW of power. The flexibility in modifying the ITO thin film properties effectively bridges the gap between achieving low-loss and high-speed thermo-optic based microheaters.

Reconfigurable Micro/Nano-Optical Devices Based on Phase Transitions: From Materials, Mechanisms to Applications

In recent years, numerous efforts have been devoted to exploring innovative micro/nano-optical devices (MNODs) with reconfigurable functionality, which is highly significant because of the progressively increasing requirements for next-generation photonic systems. Fortunately, phase change materials (PCMs) provide an extremely competitive pathway to achieve this goal. The phase transitions induce significant changes to materials in optical, electrical properties or shapes, triggering great research interests in applying PCMs to reconfigurable micro/nano-optical devices (RMNODs). More specifically, the PCMs-based RMNODs can interact with incident light in on-demand or adaptive manners and thus realize unique functions. In this review, RMNODs based on phase transitions are systematically summarized and comprehensively overviewed from materials, phase change mechanisms to applications. The reconfigurable optical devices consisting of three kinds of typical PCMs are emphatically introduced, including chalcogenides, transition metal oxides, and shape memory alloys, highlighting the reversible state switch and dramatic contrast of optical responses along with designated utilities generated by phase transition. Finally, a comprehensive summary of the whole content is given, discussing the challenge and outlooking the potential development of the PCMs-based RMNODs in the future.

Nonvolatile Phase-Only Transmissive Spatial Light Modulator with Electrical Addressability of Individual Pixels

Active metasurfaces with tunable subwavelength-scale nanoscatterers are promising platforms for high-performance spatial light modulators (SLMs). Among the tuning methods, phase-change materials (PCMs) are attractive because of their nonvolatile, threshold-driven, and drastic optical modulation, rendering zero-static power, crosstalk immunity, and compact pixels. However, current electrically controlled PCM-based metasurfaces are limited to global amplitude modulation, which is insufficient for SLMs. Here, an individual-pixel addressable, transmissive metasurface is experimentally demonstrated using the low-loss PCM Sb2Se3 and doped silicon nanowire heaters. The nanowires simultaneously form a diatomic metasurface, supporting a high-quality-factor (∼406) quasi-bound-state-in-the-continuum mode. A global phase-only modulation of ∼0.25π (∼0.2π) in simulation (experiment) is achieved, showing ten times enhancement. A 2π phase shift is further obtained using a guided-mode resonance with enhanced light-Sb2Se3 interaction. Finally, individual-pixel addressability and SLM functionality are demonstrated through deterministic multilevel switching (ten levels) and tunable far-field beam shaping. Our work presents zero-static power transmissive phase-only SLMs, enabled by electrically controlled low-loss PCMs and individual meta-molecule addressable metasurfaces.

Sub-1-Volt Electrically Programmable Optical Modulator Based on Active Tamm Plasmon

Reconfigurable optical devices hold great promise for advancing high-density optical interconnects, photonic switching, and memory applications. While many optical modulators based on active materials have been demonstrated, it is challenging to achieve a high modulation depth with a low operation voltage in the near-infrared (NIR) range, which is a highly sought-after wavelength window for free-space communication and imaging applications. Here, electrically switchable Tamm plasmon coupled with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is introduced. The device allows for a high modulation depth across the entire NIR range by fully absorbing incident light even under epsilon near zero conditions. Optical modulation exceeding 88% is achieved using a CMOS-compatible voltage of ±1 V. This modulation is facilitated by precise electrical control of the charge carrier density through an electrochemical doping/dedoping process. Additionally, the potential applications of the device are extended for a non-volatile multi-memory state optical device, capable of rewritable optical memory storage and exhibiting long-term potentiation/depression properties with neuromorphic behavior.

Monolithic back-end-of-line integration of phase change materials into foundry-manufactured silicon photonics

Monolithic integration of novel materials without modifying the existing photonic component library is crucial to advancing heterogeneous silicon photonic integrated circuits. Here we show the introduction of a silicon nitride etch stop layer at select areas, coupled with low-loss oxide trench, enabling incorporation of functional materials without compromising foundry-verified device reliability. As an illustration, two distinct chalcogenide phase change materials (PCMs) with remarkable nonvolatile modulation capabilities, namely Sb2Se3 and Ge2Sb2Se4Te1, were monolithic back-end-of-line integrated, offering compact phase and intensity tuning units with zero-static power consumption. By employing these building blocks, the phase error of a push-pull Mach-Zehnder interferometer optical switch could be reduced with a 48% peak power consumption reduction. Mirco-ring filters with >5-bit wavelength selective intensity modulation and waveguide-based >7-bit intensity-modulation broadband attenuators could also be achieved. This foundry-compatible platform could open up the possibility of integrating other excellent optoelectronic materials into future silicon photonic process design kits.

Multi-channel broadband nonvolatile programmable modal switch

Mode-division multiplexing (MDM) in chip-scale photonics is paramount to sustain data capacity growth and reduce power consumption. However, its scalability hinges on developing efficient and dynamic modal switches. Existing active modal switches suffer from substantial static power consumption, large footprints, and narrow bandwidth. Here, we present, for the first time, to the best of our knowledge, a novel multiport, broadband, non-volatile, and programmable modal switch designed for on-chip MDM systems. Our design leverages the unique properties of integrating nanoscale phase-change materials (PCM) within a silicon photonic architecture. This enables independent manipulation of spatial modes, allowing for dynamic, non-volatile, and selective routing to six distinct output ports. Crucially, our switch outperforms current dynamic modal switches by offering non-volatile, energy-efficient multiport functionality and excels in performance metrics. Our switch exhibits exceptional broadband operating bandwidth exceeding 70 nm, with low loss (< 1 dB), and a high extinction ratio (> 10 dB). Our framework provides a step forward in chip-scale MDM, paving the way for future green and scalable data centers and high-performance computers.

Programmable Photonic Time Circuits for Highly Scalable Universal Unitaries

Programmable photonic circuits (PPCs) have garnered substantial interest for their potential in facilitating deep learning accelerations and universal quantum computations. Although photonic computation using PPCs offers ultrafast operation, energy-efficient matrix calculations, and room-temperature quantum states, its poor scalability hinders integration. This challenge arises from the temporally one-shot operation of propagating light in conventional PPCs, resulting in a light-speed increase in device footprints. Here we propose the concept of programmable photonic time circuits, utilizing time-cycle-based computations analogous to gate cycling in the von Neumann architecture and quantum computation. Our building block is a reconfigurable SU(2) time gate, consisting of two resonators with tunable resonances, and coupled via time-coded dual-channel gauge fields. We demonstrate universal U(N) operations with high fidelity using an assembly of the SU(2) time gates, substantially improving scalability from O(N^{2}) to O(N) in terms of both the footprint and the number of gates. This result paves the way for PPC implementation in very large-scale integration.

Recent Advances in Laser-Induced Graphene-Based Materials for Energy Storage and Conversion

Laser-induced graphene (LIG) is a porous carbon nanomaterial that can be produced by irradiation of CO2 laser directly on the polymer substrate under ambient conditions. LIG has many merits over conventional graphene, such as simple and fast synthesis, tunable structure and composition, high surface area and porosity, excellent electrical and thermal conductivity, and good flexibility and stability. These properties make LIG a promising material for energy applications, such as supercapacitors, batteries, fuel cells, and solar cells. In this review, we highlight the recent advances of LIG in energy materials, covering the fabrication methods, performance enhancement strategies, and device integration of LIG-based electrodes and devices in the area of hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, zinc-air batteries, and supercapacitors. This comprehensive review examines the potential of LIG for future sustainable and efficient energy material development, highlighting its versatility and multifunctionality in energy conversion.

Dynamic light manipulation via silicon-organic slot metasurfaces

Active metasurfaces provide the opportunity for fast spatio-temporal control of light. Among various tuning methods, organic electro-optic materials provide some unique advantages due to their fast speed and large nonlinearity, along with the possibility of using fabrication techniques based on infiltration. In this letter, we report a silicon-organic platform where organic electro-optic material is infiltrated into the narrow gaps of slot-mode metasurfaces with high quality factors. The mode confinement into the slot enables the placement of metallic electrodes in close proximity, thus enabling tunability at lower voltages. We demonstrate the maximum tuning sensitivity of 0.16nm/V, the maximum extinction ratio of 38% within ± 17V voltage at telecommunication wavelength. The device has 3dB bandwidth of 3MHz. These results provide a path towards tunable silicon-organic hybrid metasurfaces at CMOS-level voltages.

Non-volatile 2 × 2 optical switch using multimode interference in an Sb2Se3-loaded waveguide

We propose a non-volatile 2 × 2 photonic switch based on multimode interference in an Sb2Se3-loaded waveguide. The different modal symmetries of the TE0 and TE1 modes supported in the multimode region change their propagation constants distinctly upon the Sb2Se3 phase transition. Through careful optical design and FDTD optimization of the multimode waveguide dimensions, efficient switching is achieved despite the modest index contrast of Sb2Se3 relative to Ge2Sb2Te5. The fabricated optical switch demonstrates favorable characteristics, including low insertion loss of ∼1 dB, a compact length of ∼27 µm, and small cross talk below -15 dB across a 35 nm bandwidth. Such non-volatile and broadband components will be critical for future high-density programmable photonic-integrated circuits for optical communications and signal processing.

High-speed and energy-efficient non-volatile silicon photonic memory based on heterogeneously integrated memresonator

Recently, interest in programmable photonics integrated circuits has grown as a potential hardware framework for deep neural networks, quantum computing, and field programmable arrays (FPGAs). However, these circuits are constrained by the limited tuning speed and large power consumption of the phase shifters used. In this paper, we introduce the memresonator, a metal-oxide memristor heterogeneously integrated with a microring resonator, as a non-volatile silicon photonic phase shifter. These devices are capable of retention times of 12 hours, switching voltages lower than 5 V, and an endurance of 1000 switching cycles. Also, these memresonators have been switched using 300 ps long voltage pulses with a record low switching energy of 0.15 pJ. Furthermore, these memresonators are fabricated on a heterogeneous III-V-on-Si platform capable of integrating a rich family of active and passive optoelectronic devices directly on-chip to enable in-memory photonic computing and further advance the scalability of integrated photonic processors.

Arbitrary Programming of Racetrack Resonators Using Low-Loss Phase-Change Material Sb2Se3

The programmable photonic integrated circuit (PIC) is an enabling technology behind optical interconnects and quantum information processing. Conventionally, the programmability of PICs is driven by the thermo-optic effect, free carrier dispersion, or mechanical tuning. These effects afford either high speed or a large extinction ratio, but all require constant power or bias to maintain the states, which is undesirable for programmability with infrequent switching. Recent progress in programmable PICs based on nonvolatile phase-change materials (PCMs) offers an attractive solution to a truly "set-and-forget" switch that requires zero static energy. Here, we report an essential building block of large-scale programmable PICs─a racetrack resonator with independent control of coupling and phase. We changed the resonance extinction ratio (ER) without perturbing the resonance wavelength, leveraging a programmable unit based on a directional coupler and a low-loss PCM Sb2Se3. The unit is only 33-μm-long and has an operating bandwidth over 50 nm, a low insertion loss (∼0.36 dB), high ER (∼15 dB), and excellent fabrication yield of over 1000 cycles endurance across nine switches. The work is a crucial step toward future large-scale energy-efficient programmable PICs.

A system-on-chip microwave photonic processor solves dynamic RF interference in real time with picosecond latency

Radio-frequency interference is a growing concern as wireless technology advances, with potentially life-threatening consequences like interference between radar altimeters and 5 G cellular networks. Mobile transceivers mix signals with varying ratios over time, posing challenges for conventional digital signal processing (DSP) due to its high latency. These challenges will worsen as future wireless technologies adopt higher carrier frequencies and data rates. However, conventional DSPs, already on the brink of their clock frequency limit, are expected to offer only marginal speed advancements. This paper introduces a photonic processor to address dynamic interference through blind source separation (BSS). Our system-on-chip processor employs a fully integrated photonic signal pathway in the analogue domain, enabling rapid demixing of received mixtures and recovering the signal-of-interest in under 15 picoseconds. This reduction in latency surpasses electronic counterparts by more than three orders of magnitude. To complement the photonic processor, electronic peripherals based on field-programmable gate array (FPGA) assess the effectiveness of demixing and continuously update demixing weights at a rate of up to 305 Hz. This compact setup features precise dithering weight control, impedance-controlled circuit board and optical fibre packaging, suitable for handheld and mobile scenarios. We experimentally demonstrate the processor's ability to suppress transmission errors and maintain signal-to-noise ratios in two scenarios, radar altimeters and mobile communications. This work pioneers the real-time adaptability of integrated silicon photonics, enabling online learning and weight adjustments, and showcasing practical operational applications for photonic processing.

Freeform direct-write and rewritable photonic integrated circuits in phase-change thin films

Photonic integrated circuits (PICs) with rapid prototyping and reprogramming capabilities promise revolutionary impacts on a plethora of photonic technologies. We report direct-write and rewritable photonic circuits on a low-loss phase-change material (PCM) thin film. Complete end-to-end PICs are directly laser-written in one step without additional fabrication processes, and any part of the circuit can be erased and rewritten, facilitating rapid design modification. We demonstrate the versatility of this technique for diverse applications, including an optical interconnect fabric for reconfigurable networking, a photonic crossbar array for optical computing, and a tunable optical filter for optical signal processing. By combining the programmability of the direct laser writing technique with PCM, our technique unlocks opportunities for programmable photonic networking, computing, and signal processing. Moreover, the rewritable photonic circuits enable rapid prototyping and testing in a convenient and cost-efficient manner, eliminate the need for nanofabrication facilities, and thus promote the proliferation of photonics research and education to a broader community.

Fully reconfigurable MEMS-based second-order coupled-resonator optical waveguide (CROW) with ultra-low tuning energy

Integrated microring resonators are well suited for wavelength-filtering applications in optical signal processing, and cascaded microring resonators allow flexible filter design in coupled-resonator optical waveguide (CROW) configurations. However, the implementation of high-order cascaded microring resonators with high extinction ratios (ERs) remains challenging owing to stringent fabrication requirements and the need for precise resonator tunability. We present a fully integrated on-chip second-order CROW filter using silicon photonic microelectromechanical systems (MEMS) to adjust tunable directional couplers and a phase shifter using nanoscale mechanical out-of-plane waveguide displacement. The filter can be fully reconfigured with regard to both the ER and center wavelength. We experimentally demonstrated an ER exceeding 25 dB and continuous wavelength tuning across the full free spectral range of 0.123 nm for single microring resonator, and showed reconfigurability in second-order CROW by tuning the ER and resonant wavelength. The tuning energy for an individual silicon photonic MEMS phase shifter or tunable coupler is less than 22 pJ with sub-microwatt static power consumption, which is far better than conventional integrated phase shifters based on other physical modulation mechanisms.

Tunable Tamm plasmon cavity as a scalable biosensing platform for surface enhanced resonance Raman spectroscopy

Surface enhanced Resonance Raman spectroscopy (SERRS) is a powerful technique for enhancing Raman spectra by matching the laser excitation wavelength with the plasmonic resonance and the absorption peak of biomolecules. Here, we propose a tunable Tamm plasmon polariton (TPP) cavity based on a metal on distributed Bragg reflector (DBR) as a scalable sensing platform for SERRS. We develop a gold film-coated ultralow-loss phase change material (Sb2S3) based DBR, which exhibits continuously tunable TPP resonances in the optical wavelengths. We demonstrate SERRS by matching the TPP resonance with the absorption peak of the chromophore molecule at 785 nm wavelength. We use this platform to detect cardiac Troponin I protein (cTnI), a biomarker for early diagnosis of cardiovascular disease, achieving a detection limit of 380 fM. This scalable substrate shows great promise as a next-generation tunable biosensing platform for detecting disease biomarkers in body fluids for routine real-time clinical diagnosis.

Roadmap for phase change materials in photonics and beyond

Phase Change Materials (PCMs) have demonstrated tremendous potential as a platform for achieving diverse functionalities in active and reconfigurable micro-nanophotonic devices across the electromagnetic spectrum, ranging from terahertz to visible frequencies. This comprehensive roadmap reviews the material and device aspects of PCMs, and their diverse applications in active and reconfigurable micro-nanophotonic devices across the electromagnetic spectrum. It discusses various device configurations and optimization techniques, including deep learning-based metasurface design. The integration of PCMs with Photonic Integrated Circuits and advanced electric-driven PCMs are explored. PCMs hold great promise for multifunctional device development, including applications in non-volatile memory, optical data storage, photonics, energy harvesting, biomedical technology, neuromorphic computing, thermal management, and flexible electronics.

Time-Resolved Temperature Mapping Leveraging the Strong Thermo-Optic Effect in Phase-Change Materials

Optical phase-change materials are highly promising for emerging applications such as tunable metasurfaces, reconfigurable photonic circuits, and non-von Neumann computing. However, these materials typically require both high melting temperatures and fast quenching rates to reversibly switch between their crystalline and amorphous phases: a significant challenge for large-scale integration. In this work, we use temperature-dependent ellipsometry to study the thermo-optic effect in GST and use these results to demonstrate an experimental technique that leverages the thermo-optic effect in GST to enable both spatial and temporal thermal measurements of two common electro-thermal microheater designs currently used by the phase-change community. Our approach shows excellent agreement between experimental results and numerical simulations and provides a noninvasive method for rapid characterization of electrically programmable phase-change devices.

Recent developments in Chalcogenide phase change material-based nanophotonics

There is now a deep interest in actively reconfigurable nanophotonics as they will enable the next generation of optical devices. Of the various alternatives being explored for reconfigurable nanophotonics, Chalcogenide phase change materials (PCMs) are considered highly promising owing to the nonvolatile nature of their phase change. Chalcogenide PCM nanophotonics can be broadly classified into integrated photonics (with guided wave light propagation) and Meta-optics (with free space light propagation). Despite some early comprehensive reviews, the pace of development in the last few years has shown the need for a topical review. Our comprehensive review covers recent progress on nanophotonic architectures, tuning mechanisms, and functionalities in tunable PCM Chalcogenides. In terms of integrated photonics, we identify novel PCM nanoantenna geometries, novel material utilization, the use of nanostructured waveguides, and sophisticated excitation pulsing schemes. On the meta-optics front, the breadth of functionalities has expanded, enabled by exploring design aspects for better performance. The review identifies immediate, and intermediate-term challenges and opportunities in (1) the development of novel chalcogenide PCM, (2) advance in tuning mechanism, and (3) formal inverse design methods, including machine learning augmented inverse design, and provides perspectives on these aspects. The topical review will interest researchers in further advancing this rapidly growing subfield of nanophotonics.

Reconfigurable TE-pass polarizer based on lithium niobate waveguide assisted by Ge2Sb2Te5 and silicon nitride

On-chip polarization management components play a critical role in tackling polarization dependence in the lithium-niobate-on-insulator (LNOI) platform. In this work, we proposed a reconfigurable TE-pass polarizer based on optical phase change material (GST) and the LNOI wafer. The key region is formed by a hybrid GST-S i 3 N 4 layer symmetrically deposited atop the centerline of the LNOI waveguide along the propagation direction where the GST is sandwiched in the middle of the S i 3 N 4 layer. Whether the polarizer will take effect depends on the phase states of the GST layer and the graphene and aluminum oxide layers are coated atop the G S T-S i 3 N 4 layer as the microheater to control the conversion of phase states. The proposed device length is 7.5 µm with an insertion loss (IL)=0.22 dB and extinction ratio (ER)=32.8 dB at the wavelength of 1550 nm. Moreover, it also has a high ER (>25d B) and a low IL (<0.5d B) in the operating bandwidth of 200 nm. Such a high-performance TE-pass polarizer paves a new way for applications of photonics integrated circuits.

Integrated Bragg grating filters based on silicon-Sb2Se3 with non-volatile bandgap engineering capability

Integrated optical filters show outstanding capability in integrated reconfigurable photonic applications, including wavelength division multiplexing (WDM), programmable photonic processors, and on-chip quantum photonic networks. Present schemes for reconfigurable filters either have a large footprint or suffer from high static power consumption, hindering the development of reconfigurable photonic integrated systems. Here, a reconfigurable hybrid Bragg grating filter is elaborately designed through a precise, modified coupling mode theory. It is also experimentally presented by integrating non-volatile phase change material (PCM) Sb2Se3 on silicon to realize compact, low-loss, and broadband engineering operations. The fabricated filter holds a compact footprint of 0.5 µm × 43.5 µm and maintains a low insertion loss of < 0.5 dB after multiple levels of engineering to achieve crystallization. The filter is able to switch from a low-loss transmission state to the Bragg reflection state, making it a favorable solution for large-scale reconfigurable photonic circuits. With a switching extinction ratio over 30 dB at 1504.85 nm, this hybrid filter breaks the tradeoff between insertion loss and tuning range. These results reveal its potential as a new candidate for a basic element in large-scale non-volatile reconfigurable systems.

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

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 [Formula: see text]. Remarkably, Sb₂S₃ is 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.

Neuromorphic Photonics Based on Phase Change Materials

Neuromorphic photonics devices based on phase change materials (PCMs) and silicon photonics technology have emerged as promising solutions for addressing the limitations of traditional spiking neural networks in terms of scalability, response delay, and energy consumption. In this review, we provide a comprehensive analysis of various PCMs used in neuromorphic devices, comparing their optical properties and discussing their applications. We explore materials such as GST (Ge₂Sb₂Te₅), GeTe-Sb₂Te₃, GSST (Ge₂Sb₂Se₄Te₁), Sb₂S₃/Sb₂Se₃, Sc_0.2Sb₂Te₃ (SST), and In₂Se₃, highlighting their advantages and challenges in terms of erasure power consumption, response rate, material lifetime, and on-chip insertion loss. By investigating the integration of different PCMs with silicon-based optoelectronics, this review aims to identify potential breakthroughs in computational performance and scalability of photonic spiking neural networks. Further research and development are essential to optimize these materials and overcome their limitations, paving the way for more efficient and high-performance photonic neuromorphic devices in artificial intelligence and high-performance computing applications.

In-memory photonic dot-product engine with electrically programmable weight banks

Electronically reprogrammable photonic circuits based on phase-change chalcogenides present an avenue to resolve the von-Neumann bottleneck; however, implementation of such hybrid photonic-electronic processing has not achieved computational success. Here, we achieve this milestone by demonstrating an in-memory photonic-electronic dot-product engine, one that decouples electronic programming of phase-change materials (PCMs) and photonic computation. Specifically, we develop non-volatile electronically reprogrammable PCM memory cells with a record-high 4-bit weight encoding, the lowest energy consumption per unit modulation depth (1.7 nJ/dB) for Erase operation (crystallization), and a high switching contrast (158.5%) using non-resonant silicon-on-insulator waveguide microheater devices. This enables us to perform parallel multiplications for image processing with a superior contrast-to-noise ratio (≥87.36) that leads to an enhanced computing accuracy (standard deviation σ ≤ 0.007). An in-memory hybrid computing system is developed in hardware for convolutional processing for recognizing images from the MNIST database with inferencing accuracies of 86% and 87%.

Rewritable photonic integrated circuits using dielectric-assisted phase-change material waveguides

Photonic integrated circuits (PICs) can drastically expand the capabilities of quantum and classical optical information science and engineering. PICs are commonly fabricated using selective material etching, a subtractive process. Thus, the chip's functionality cannot be substantially altered once fabricated. Here, we propose to exploit wide-bandgap non-volatile phase-change materials (PCMs) to create rewritable PICs. A PCM-based PIC can be written using a nanosecond pulsed laser without removing any material, akin to rewritable compact disks. The whole circuit can then be erased by heating, and a new circuit can be rewritten. We designed a dielectric-assisted PCM waveguide consisting of a thick dielectric layer on top of a thin layer of wide-bandgap PCMs Sb₂S₃ and Sb₂Se₃. The low-loss PCMs and our designed waveguides lead to negligible optical loss. Furthermore, we analyzed the spatiotemporal laser pulse shape to write the PICs. Our proposed platform will enable low-cost manufacturing and have a far-reaching impact on the rapid prototyping of PICs, validation of new designs, and photonic education.

Compact nonvolatile polarization switch using an asymmetric Sb2Se3-loaded silicon waveguide

We propose and simulate a compact (∼29.5 µm-long) nonvolatile polarization switch based on an asymmetric Sb₂Se₃-clad silicon photonic waveguide. The polarization state is switched between TM₀ and TE₀ mode by modifying the phase of nonvolatile Sb₂Se₃ between amorphous and crystalline. When the Sb₂Se₃ is amorphous, two-mode interference happens in the polarization-rotation section resulting in efficient TE₀-TM₀ conversion. On the other hand, when the material is in the crystalline state, there is little polarization conversion because the interference between the two hybridized modes is significantly suppressed, and both TE₀ and TM₀ modes go through the device without any change. The designed polarization switch has a high polarization extinction ratio of > 20 dB and an ultra-low excess loss of < 0.22 dB in the wavelength range of 1520-1585 nm for both TE₀ and TM₀ modes.

On-Chip Reconfigurable and Ultracompact Silicon Waveguide Mode Converters Based on Nonvolatile Optical Phase Change Materials

Reconfigurable mode converters are essential components in efficient higher-order mode sources for on-chip multimode applications. We propose an on-chip reconfigurable silicon waveguide mode conversion scheme based on the nonvolatile and low-loss optical phase change material antimony triselenide (Sb₂Se₃). The key mode conversion region is formed by embedding a tapered Sb₂Se₃ layer into the silicon waveguide along the propagation direction and further cladding with graphene and aluminum oxide layers as the microheater. The proposed device can achieve the TE₀-to-TE₁ mode conversion and reconfigurable conversion (no mode conversion) depending on the phase state of embedded Sb₂Se₃ layer, whereas such function could not be realized according to previous reports. The proposed device length is only 2.3 μm with conversion efficiency (CE) = 97.5%, insertion loss (IL) = 0.2 dB, and mode crosstalk (CT) = -20.5 dB. Furthermore, the proposed device scheme can be extended to achieve other reconfigurable higher-order mode conversions. We believe the proposed reconfigurable mode conversion scheme and related devices could serve as the fundamental building blocks to provide higher-order mode sources for on-chip multimode photonics.

Phase change metamaterial for tunable infrared stealth and camouflage

In the paper, a type of phase change metamaterial for tunable infrared stealth and camouflage is proposed and numerically studied. The metamaterial combines high temperature resistant metal Mo with phase-changing material GST and can be switched between the infrared "stealthy" and "non-stealthy" states through the phase change process of the GST. At the amorphous state of GST, there is a high absorption peak at the atmospheric absorption spectral range, which can achieve infrared stealth in the atmospheric window together with good radiative heat dissipation in the non-atmospheric window. While at the crystalline state of GST, the absorption peak becomes broader and exhibits high absorption in the long-wave infrared atmospheric window, leading to a "non-stealthy" state. The relationship between the infrared stealth performance of the structure with the polarization and incident angle of the incident light is also studied in detail. The proposed infrared stealth metamaterial employs a simple multilayer structure and could be fabricated in large scale. Our work will promote the research of dynamically tunable, large scale phase change metamaterials for infrared stealth as well as energy and other applications.