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The objective of this Opinion is to stimulate new research into materials that can meet the needs of tomorrow’s programmable photonics components. Herein, we argue that the inherent property portfolios of the common telluride phase change materials, which have been successfully applied in data storage technologies, are unsuitable for most emerging programmable photonics applications. We believe that newer PCMs with wider bandgaps, such as Sb₂S₃, Sb₂Se₃, and Ge₂Sb₂Se₄Te (GSST), can be optimized to meet the demands of holographic displays, optical neural network memories, and beam steering devices.

The development of compact and fieldable mid-infrared (mid-IR) spectroscopy devices represents a critical challenge for distributed sensing with applications from gas leak detection to environmental monitoring. Recent work has focused on mid-IR photonic integrated circuit (PIC) sensing platforms and waveguide-integrated mid-IR light sources and detectors based on semiconductors such as PbTe, black phosphorus and tellurene. However, material bandgaps and reliance on SiO₂substrates limit operation to wavelengthsλ ≲ 4 μm. Here we overcome these challenges with a chalcogenide glass-on-CaF₂PIC architecture incorporating split-gate photothermoelectric graphene photodetectors. Our design extends operation toλ = 5.2 μm with a Johnson noise-limited noise-equivalent power of 1.1 nW/Hz^1/2, no fall-off in photoresponse up tof = 1 MHz, and a predicted 3-dB bandwidth off_3dB > 1 GHz. This mid-IR PIC platform readily extends to longer wavelengths and opens the door to applications from distributed gas sensing and portable dual comb spectroscopy to weather-resilient free space optical communications.

Integrated nanophotonics is an emerging research direction that has attracted great interests for technologies ranging from classical to quantum computing. One of the key-components in the development of nanophotonic circuits is the phase-change unit that undergoes a solid-state phase transformation upon thermal excitation. The quaternary alloy, Ge₂Sb₂Se₄Te, is one of the most promising material candidates for application in photonic circuits due to its broadband transparency and large optical contrast in the infrared spectrum. Here, we investigate the thermal properties of Ge₂Sb₂Se₄Te and show that upon substituting tellurium with selenium, the thermal transport transitions from an electron dominated to a phonon dominated regime. By implementing an ultrafast mid-infrared pump-probe spectroscopy technique that allows for direct monitoring of electronic and vibrational energy carrier lifetimes in these materials, we find that this reduction in thermal conductivity is a result of a drastic change in electronic lifetimes of Ge₂Sb₂Se₄Te, leading to a transition from an electron-dominated to a phonon-dominated thermal transport mechanism upon selenium substitution. In addition to thermal conductivity measurements, we provide an extensive study on the thermophysical properties of Ge₂Sb₂Se₄Te thin films such as thermal boundary conductance, specific heat, and sound speed from room temperature to 400 °C across varying thicknesses.

Mid-infrared photonic integrated circuits (PICs) that combine on-chip light sources with other optical components constitute a key enabler for applications such as chemical sensing, light detection, ranging, and free-space communications. In this paper, we report the monolithic integration of interband cascade lasers emitting at 3.24 µm with passive, high-index-contrast waveguides made of chalcogenide glasses. Output from the chalcogenide waveguides exhibits pulsed peak power up to 150 mW (without roll-over), threshold current density 280 A/cm², and slope efficiency 100 mW/A at 300 K, with a lower bound of 38% efficiency for coupling between the two waveguides. These results represent an important step toward the realization of fully integrated mid-infrared PICs.

Since its advent in the 1970s, optical tweezers have been widely deployed as a preferred non-contact technique for manipulating microscale objects. On-chip integrated optical tweezers, which afford significant size, weight, and cost benefits, have been implemented, relying upon near-field evanescent waves. As a result, these tweezers are only capable of manipulation in near-surface regions and often demand high power since the evanescent interactions are relatively weak. We introduce on-chip optical tweezers based on freeform micro-optics, which comprise optical reflectors or refractive lenses integrated on waveguide end facets via two-photon polymerization. The freeform optical design offers unprecedented degrees of freedom to design optical fields with strong three-dimensional intensity gradients, useful for trapping and manipulating suspended particles in an integrated chip-scale platform. We demonstrate the design, fabrication, and measurement of both reflective and refractive micro-optical tweezers. The reflective tweezers feature a remarkably low trapping threshold power, and the refractive tweezers are particularly useful for multiparticle trapping and interparticle interaction analysis. Our integrated micro-optical tweezers uniquely combine a compact footprint, broadband operation, high trapping efficiency, and scalable integration with planar photonic circuits. This class of tweezers is promising for on-chip sensing, cell assembly, particle dynamics analysis, and ion trapping.

3D photonics promises to expand the reach of photonics by enabling the extension of traditional applications to nonplanar geometries and adding novel functionalities that cannot be attained with planar devices. Available material options and device geometries are, however, limited by current fabrication methods. In this work, we pioneer a method that allows for placement of integrated photonic device arrays at arbitrary predefined locations in 3D using a fabrication process that capitalizes on the buckling of a 2D pattern. We present theoretical and experimental validation of the deterministic buckling process, thus demonstrating implementation of the technique to realize what we believe to be the first fully packaged 3D integrated photonics platform. Application of the platform for mechanical strain sensing is further demonstrated.

The extraordinary optical properties of single-layer graphene have spurred the development of a variety of photonic components. We have previously demonstrated a scalable and versatile platform to facilitate the integration of graphene and other 2-D materials with chalcogenide glass-based planar photonics. In this paper, we detail the design criteria and optimization guidelines towards high-performance graphene-integrated thermo-optic (TO) switches based on the chalcogenide glass-on-graphene platform. Notably, absorption loss of graphene can be reduced to < 20 dB/cm when it is sandwiched inside photonic structures capitalizing on the anisotropic absorption property of graphene. We quantify energy efficiency of the TO switch, showing that the choice of cladding materials plays a critical role in improving device efficiency. Furthermore, we report a record TO switching efficiency of 10 nm/mW via judicious engineering of the overlap between optical mode and thermal profile.