Benefiting from high parallelism and low latency, photonic integrated circuits (PICs) constructed from on-chip building blocks with diverse functions have emerged as a promising technology in the realm of optical neural networks (ONN). Tunable components, through the utilization of physical mechanisms such as thermo-optic effect and free-carrier plasma dispersion effect, structural motion like microelectromechanical systems (MEMS), or material properties including liquid crystal and two-dimensional materials, play a pivotal role in enabling reconfigurability within PICs. Among these reconfiguration schemes, chalcogenide phase change materials (PCMs) based photonic devices have attracted extensive attention owing to their high energy efficiency and integration density brought by huge refractive index contrasts and nonvolatility of PCMs. However, this nonvolatile modulation method meets difficulty in scalability since the process flow of integrating PCMs into silicon photonics is insupportable in the foundries. Here, we demonstrated a back-end-of-line (BEOL) integration platform for the monolithic integration of PCMs into silicon photonic devices without modification in standard process design kits (PDK). This is achieved by fabricating a low-loss oxide trench to expose the waveguide core at the functional area from the top dielectric layer, with assistance from a silicon nitride etch stop layer. On this basis, integrated photonic devices with stable switching performance and repeatable multi-bit storage capability have been developed, possessing the potential for crucial blocks of PICs in ONN applications that require infrequent reconfiguration, such as hardware error correction before training and data storage in pre-trained models.
Currently, integrated optoelectronic technology has made significant progress in commercial applications. However, existing technologies are approaching their theoretical limits. How to introduce new materials to achieve novel on-chip optical field control and generate disruptive breakthroughs will be crucial for meeting the future demands of optical computing, optical communication, optical sensing, and other applications. Chalcogenide materials, also known as chalcogenide glass materials, mainly refer to compounds containing sulfur, selenium, tellurium, and other chalcogen elements. They not only possess excellent nonlinear optical properties and excellent micro-nano processing characteristics but also some specific compositions of chalcogenide glass exhibit nonvolatile phase transition characteristics for exploring nonvolatile reconfigurable photon platforms. This paper will mainly introduce some progress in our research on scalable fabrication techniques for integrated photonic devices based on chalcogenide materials.
Intelligent photonics, driven by silicon photonics, is revolutionizing high-speed data processing, low-power computing, and precision sensing. Leveraging these advances, photonic chips are enabling the development of optical neural networks and nonlinear activation mapping, which are crucial for addressing the demands of large generative models. However, traditional on-chip control methods struggle with high power consumption and volatility. To overcome these challenges, phase-change materials (PCMs) offer high optical contrast and non-volatility, enhancing integration density and reducing power usage. This article discusses the performance and reversible control of PCMs and their integration with silicon photonics. By incorporating PCMs into in-memory optical computing chips, we achieved 4-bit storage and over 88% accuracy on the MNIST dataset, marking significant progress in next-generation high-performance computing.
Nonvolatile light-field manipulation via electrically-driven phase transition of chalcogenide phase change materials (PCMs) is regarded as one of the most powerful solutions to low-power-consumption and compact integrated reconfigurable photonics. However, before the breakthrough in large-scale integration approaches linked to wafer foundries, phase-change non-volatile reconfigurable photonics could hardly see their widespread practical applications. Here we demonstrate nonvolatile photonic devices fabricated by back-end-of-line (BOEL) integration of PCMs into the commercial silicon photonics platform. A narrow trench etched into the BOEL dielectric layer exposed the waveguide core and allowed for the direct deposition of various PCM films on the waveguide in the functional areas. Fine-tuning the nonvolatile phase transition of Sb2Se3 via a PIN microheater was verified by realizing the post-fabrication trimming of silicon photonic devices. Our work highlights a reliable platform for large-scale PCM-integrated photonics and validates its precise nonvolatile reconfigurability.
Optical neural networks (ONNs), enabling low latency and high parallel data processing without electromagnetic interference, have become a viable player for fast and energy-efficient processing and calculation to meet the increasing demand for hash rate. Photonic memories employing nonvolatile phase-change materials could achieve zero static power consumption, low thermal cross talk, large-scale, and high-energy-efficient photonic neural networks. Nevertheless, the switching speed and dynamic energy consumption of phase-change material-based photonic memories make them inapplicable for in situ training. Here, by integrating a patch of phase change thin film with a PIN-diode-embedded microring resonator, a bifunctional photonic memory enabling both 5-bit storage and nanoseconds volatile modulation was demonstrated. For the first time, a concept is presented for electrically programmable phase-change material-driven photonic memory integrated with nanosecond modulation to allow fast in situ training and zero static power consumption data processing in ONNs. ONNs with an optical convolution kernel constructed by our photonic memory theoretically achieved an accuracy of predictions higher than 95% when tested by the MNIST handwritten digit database. This provides a feasible solution to constructing large-scale nonvolatile ONNs with high-speed in situ training capability.
The mid-wave infrared (MWIR) is an important band for numerous applications ranging from night vision to biochemical sensing. However, unlike visible or near-infrared (NIR) optical parts, which are economically available off the shelf, MWIR optics are plagued by much higher costs and often inferior performance compared to their visible or NIR counterparts. Optical metasurfaces, artificial materials with subwavelength-scale thicknesses and on-demand electromagnetic responses, provide a promising solution for cost-effective, high-performance infrared optics. Using high-refractive-index (> 5) chalcogenide materials deposited on IR-transparent substrates, we have experimentally demonstrated a MWIR transmissive metasurface device with diffraction-limited focusing and imaging performance and optical efficiency up to 75%. We further show that the metasurface design can accommodate ultra-wide field-of-view and the fabrication method can be extended to conformal integration of metasurface optics on curved surfaces. The projected size, weight and power advantages, coupled with the manufacturing scalability leveraging standard microfabrication technologies, makes the meta-optical devices promising for next-generation MWIR system applications.
We present recent development on integrated flexible and stretchable photonic devices. Conventional photonic devices are fabricated on rigid semiconductor or dielectric substrates and are therefore inherently incompatible with soft biological tissues. Recently, we have developed a suite of active and passive photonic devices and systems integrated on plastic substrates which can be bent, twisted, and stretched without compromising their optical performance. Key innovations are monolithic multi-material integration and advanced micro-mechanical structures co-designed with photonic devices, which enables devices with extreme mechanical flexibility and excellent optical performance.
Two-dimensional (2-D) materials are of tremendous interest to silicon photonics given their singular optical characteristics spanning light emission, modulation, saturable absorption, and nonlinear optics. To harness their optical properties, these atomically thin materials are usually attached onto prefabricated devices via a transfer process. Here we present a new route for 2-D material integration with silicon photonics. Central to this approach is the use of chalcogenide glass, a multifunctional material which can be directly deposited and patterned on a wide variety of 2-D materials and can simultaneously function as the light guiding medium, a gate dielectric, and a passivation layer for 2-D materials. Besides achieving improved fabrication yield and throughput compared to the traditional transfer process, our technique also enables unconventional multilayer device geometries optimally designed for enhancing light-matter interactions in the 2-D layers. Capitalizing on this facile integration method, we demonstrate a series of high-performance glass-on-graphene devices including ultra-broadband on-chip polarizers, energy-efficient thermo-optic switches, as well as mid-infrared (mid-IR) waveguide-integrated photodetectors and modulators based on graphene and black phosphorus.
Conventional photonic integration technologies are inevitably substrate-dependent, as different substrate platforms stipulate vastly different device fabrication methods and processing compatibility requirements. Here we capitalize on the unique monolithic integration capacity of composition-engineered non-silicate glass materials (amorphous chalcogenides and transition metal oxides) to enable multifunctional, multi-layer photonic integration on virtually any technically important substrate platforms. We show that high-index glass film deposition and device fabrication can be performed at low temperatures (< 250 °C) without compromising their low loss characteristics, and is thus fully compatible with monolithic integration on a broad range of substrates including semiconductors, plastics, textiles, and metals. Application of the technology is highlighted through three examples: demonstration of high-performance mid-IR photonic sensors on fluoride crystals, direct fabrication of photonic structures on graphene, and 3-D photonic integration on flexible plastic substrates.
Mid-infrared (MIR, 2-6 μm wavelength) transparent metal oxides are attractive materials for planar integrated photonic devices for sensing applications. In this study, we present reactive sputtering deposited ZrO2-TiO2 (ZTO) thin films as a new material candidate for integrated MIR photonics. We demonstrate that amorphous ZTO thin films can be achieved with Ti concentration of 40 at.%. With increasing Ti concentration, the optical band gap decreases monotonically from 4.34 eV to 4.11 eV, while the index of refraction increases from 2.14 to 2.24 at 1 μm wavelength. MIR micro-disk resonators on MgO substrates are demonstrated using Ge23/Sb7S70/Zr0.6Ti0.4O2 strip-loaded waveguides with a loaded quality factor of ~11,000 at 5.2 μm wavelength. By comparing with a reference device of Ge23Sb7S70 resonator on MgO and simulating the optical confinement factors, the ZTO thin film loss is estimated to be below 10 dB/cm. Single mode shallow ridge waveguides with a ridge height of 400 nm and a slab height of 1.7 μm are also demonstrated using ZrO2 thin films on MgO substrates. The low loss, relatively high index of refraction, superior stability and proven CMOS compatibility of ZTO thin films make them highly attractive for MIR integrated photonics.
A high bandwidth density chip-to-chip optical interconnect architecture is analyzed. The interconnect design leverages
our recently developed flexible substrate integration technology to circumvent the optical alignment requirement during
packaging. Initial experimental results on fabrication and characterization of the flexible photonic platform are also
presented.
High-index-contrast optical devices form the backbone of densely integrated photonic circuits. While these devices are
traditionally fabricated using lithography and etching, their performance is often limited by defects and sidewall
roughness arising from fabrication imperfections. This paper reports a versatile, roll-to-roll and backend compatible
technique for the fabrication of high-performance, high-index-contrast photonic structures in composition-engineered
chalcogenide glass (ChG) thin films. Thin film ChG have emerged as important materials for photonic applications due
to their high refractive index, excellent transparency in the infrared and large Kerr non-linearity. Both thermally
evaporated and solution processed As-Se thin films are successfully employed to imprint waveguides and micro-ring
resonators with high replicability and low surface roughness (0.9 nm). The micro-ring resonators exhibit an ultra-high
quality-factor of 4 × 105 near 1550 nm wavelength, which represents the highest value reported in ChG micro-ring
resonators. Furthermore, sub-micron nanoimprint of ChG films on non-planar plastic substrates is demonstrated, which
establishes the method as a facile route for monolithic fabrication of high-index-contrast devices on a wide array of
unconventional substrates.
Chalcogenide glasses, namely the amorphous compounds containing sulfur, selenium, and/or tellurium, have emerged as a promising material candidate for mid-infrared integrated photonics given their wide optical transparency window, high linear and nonlinear indices, as well as their capacity for monolithic integration on a wide array of substrates. Exploiting these unique features of the material, we demonstrated high-index-contrast, waveguide-coupled As2Se3 chalcogenide glass resonators monolithically integrated on silicon with a high intrinsic quality factor of 2 × 105 at 5.2 micron wavelength, and what we believe to be the first waveguide photonic crystal cavity operating in the mid-infrared.
Chalcogenide glasses, namely the amorphous compounds containing sulfur, selenium, and/or tellurium, have emerged as a promising material candidate for integrated photonics given their wide infrared transparency window, low processing temperature, almost infinite capacity for composition alloying, as well as high linear and nonlinear indices. Here we present the fabrication and characterization of chalcogenide glass based photonic devices integrated on silicon as well as on flexible polymer substrates for mid-IR sensing, optical interconnect and nonlinear optics applications.
Chalcogenide glasses, namely the amorphous compounds containing sulfur, selenium, and/or tellurium, have emerged as
a promising material candidate for integrated photonics given their wide infrared transparency window, low processing
temperature, almost infinite capacity for composition alloying, as well as high linear and nonlinear indices. Here we
present the fabrication and characterization of chalcogenide glass based photonic devices integrated on silicon as well as
on flexible polymer substrates for sensing, optical interconnect and nonlinear optics applications.
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