Guest editors Nathan Youngblood, Qian Wang, Robert E. Simpson, and Juejun Hu introduce the Special Section on Phase-Change Reconfigurable Photonics.

Chalcogenide phase-change materials (PCMs) offer paradigms for programmable photonic integrated circuits thanks to their zero static energy and significant refractive index contrast. However, prototypical PCMs, such as Ge2Sb2Te5 (GST), are lossy in their crystalline phase, albeit transparent in the amorphous state. Moreover, electrically switching PCMs to intermediate states is a stochastic process, limiting programming accuracy. As a result, achieving both low-loss and deterministic multilevel operations with GST remains challenging. Although low-loss PCMs, such as Sb2S3 and Sb2Se3, have been discovered in recent years, they are much less technologically mature. We propose a design with multiple GST segments to overcome the challenge of deterministic multilevel operation. GST segments are individually controlled by interleaved silicon p++-doped-intrinsic-n++-doped diode heaters in a binary but reliable fashion, and multiple levels are encoded in their phase sequence. A 1×1 programmable unit with two unequal GST segments is experimentally demonstrated, showcasing four distinct operation levels and negligible thermal crosstalk with only one pair of metal contacts. We then extend the design to 1×2 and 2×2 programmable units. For the 2×2 programmable unit design, we propose a phase-detuned three-waveguide directional coupler structure to mitigate the absorption and radiation loss, showing <1.2 dB loss and three splitting ratios. We provide a new path toward low-loss and multilevel optical switches using lossy PCMs.

The development of functional chalcogenide optical phase change materials holds significant promise for advancing optics and photonics applications. Our comprehensive investigation into the solution processing of Sb2Se3 thin films presents a systematic approach from solvent exploration to substrate coating through drop-casting methods and heat treatments. By employing characterization techniques such as scanning electron microscopy, dynamic light scattering, energy-dispersive X-ray spectroscopy, Raman spectroscopy, and X-ray diffraction, we reveal crucial insights into the structural, compositional, and morphological properties of the films as well as demonstrated techniques for control over these features to ensure requisite optical quality. Our findings, compared with currently reported deposition techniques, highlight the potential of solution deposition as a route for scalable Sb2Se3 film processing.

The on-chip photonic switch is a critical building block for photonic integrated circuits and the integration of phase change materials (PCMs) enables non-volatile switch designs that are compact, low-loss, and energy-efficient. Existing switch designs based on these materials typically rely on weak evanescent field interactions, resulting in devices with a large footprint and high energy consumption. Here, we present a compact non-volatile 2×2 switch design leveraging optical concentration in slot waveguide modes to significantly enhance interactions of light with PCM, thereby realizing a compact, efficient photonic switch. To further improve the device’s energy efficiency, we introduce an integrated single-layer graphene heater for ultrafast electrothermal switching of the PCM. Computational simulations demonstrate a 2×2 switch crosstalk (CT) down to −24 dB at 1550 nm wavelength and more than 55 nm 0.3 dB insertion loss (IL) bandwidth. The proposed photonic switch architecture can constitute the cornerstone for next-generation high-performance reconfigurable photonic circuits.

The dynamic color control of solid-state electrochromic (EC) devices has sparked broad interest in both academia and industry. In this work, we report a combination of an optical stack of a Bragg reflector and Fabry–Pérot cavity into a solid-state EC device that achieves high brightness and saturation. The Bragg reflector and Fabry–Pérot optical stacks are designed as multi-color selectors through dynamically varying the optical constants (n and k) of the EC layer, allowing selective reflection of different wavelengths. By employing EC-inactive mixed-tungsten-vanadium oxide as the counter electrode, high reflectivity and contrast can be ensured. The demonstrated combination of the Bragg reflector and Fabry–Pérot cavity into a solid-state EC device offers a practical approach to designing and fabricating high-performance reflective EC devices.

Metasurfaces have attracted immense interest across various scientific disciplines due to their ability to manipulate light wave parameters with numerous functionalities. However, these functionalities have historically been static, lacking the capability for dynamic, real-time control. In this study, we introduce a highly efficient, tunable waveplate by incorporating a thin layer of the phase change material Sb2Se3 into a silicon all-dielectric metasurface. This structure demonstrates the ability to transition from a half-waveplate to a quarter-waveplate as Sb2Se3 shifts from an amorphous to a crystalline state at the telecom wavelength of 1.55 μm. Remarkably, it maintains consistent performance across a range of rotation angles. In addition, we have performed comprehensive electro-thermal simulations to validate the phase change process, confirming the practical feasibility of this technology. This tunable metasurface represents a significant advancement in adaptive photonics, offering customizable and sophisticated functionalities.

We demonstrate a high-efficiency silicon optical phase shifter based on a silicon-Sb2Se3 hybrid integrated waveguide. The optical field has large confinement in the Sb2Se3 material, leading to high optical wave modulation efficiency upon phase change of Sb2Se3. The phase change is initiated by electro-thermal heating generated by a highly durable graphene microheater positioned between the Sb2Se3 strip and the silicon slab of the hybrid waveguide. To effectively couple the phase shifter with single-mode silicon waveguides, we design a two-layer taper structure as a mode spot size converter. Utilizing this phase shifter, we implemented a Mach–Zehnder interferometer structure to function as an optical switch. The number of effective switching events exceeds 30,000, and 66 non-volatile switching levels are obtained. Our work provides an effective solution for introducing highly durable graphene microheaters on silicon-based phase-change platforms.

The integration of phase change materials (PCMs) with photonic devices creates a unique opportunity for realizing application-specific, reconfigurable, and energy-efficient photonic components with zero static power consumption and low thermal crosstalk. In particular, photonic waveguides based on silicon or silicon nitride can be integrated with PCMs to realize nonvolatile photonic memory cells, which are able to store data in the phase state of the PCMs. We delve into the performance comparison of PCM-based programmable photonic memory cells based on silicon photonic and silicon nitride platforms using known PCMs (GST and GSST) for photonic memory applications while showcasing the fundamental limitations related to each design in terms of the maximum number of bits that they can store as well as their optical insertion loss. Moreover, we present comprehensive design-space exploration for analyzing the energy efficiency and cooling time of the photonic memory cells depending on the structure of the heat source. The results show that the silicon-based strip waveguide integrated with GST is the best option to realize a photonic memory cell with the highest bit density (up to 4-bits per cell given 6% spacing between the optical transmission levels). In addition, considering a microheater integration on top of a waveguide on which PCM is deposited, multi-physics simulation results show that as the heat source is placed above the PCM with a gap of 200 nm, the photonic memory cell tends to become more energy-efficient, and the cooling time of the PCM (for set and reset) becomes significantly shorter than the case where the heat source is placed further from the PCM.

Raising the mechanical frequency of atomic force microscopy (AFM) probes to increase the measurement bandwidth has been a long-standing expectation in the field and a technically difficult challenge. Recent advances in cavity optomechanics and in-plane probe designs have yielded significant progress. In situations in which an AFM tip extends a few micrometers from a planar optomechanical resonator, we present an approach to make it overhang from the probe die with precise control of the edge-line position. This fabrication step, which exposes the tip apex to the sample surface, is a prerequisite for any AFM experiment with optomechanical probes. We utilize a combination of saw dicing and time-controlled isotropic plasma etching to undercut the 725-μm-thick silicon substrate beneath the tip. The technique is easy to implement without any lithography steps. The overhang length of the tip is controlled to less than 5 μm with very good smoothness of the edge, reproducibility, and yield.

To improve data security and authentication, we designed and fabricated a cryptographic key generator based on a photonic integrated circuit that converts a digital input key into a digital output key by means of a physical unclonable function (PUF). The PUF is realized by an imperfect multi-mode interferometer controlled by low-power micro-electromechanical system (MEMS) phase shifters. An analytical model was derived and used to prove the randomness of the generated digital keys, and the model was validated by measurements of a first proof-of-concept demonstrator. The demonstrator was fabricated using a silicon nitride photonic integrated circuit (PIC) platform and our recently developed MEMS-on-PIC technology.

We have developed a compact scattering-based light sheet microscopy (sLSM) probe capable of imaging unstained tissues with cellular resolution. In the compact sLSM probe, a custom miniature objective lens was developed to achieve a high lateral resolution, large field of view (FOV), and small field curvature. The measured resolution of the custom objective was 1.65 to 1.97 μm across the FOV of ±1.0 mm. The compact probe had dimensions of 4 cm in width and height and 10 cm in length. The compact sLSM probe achieved an axial resolution better than 5.6 μm over a depth range of 206.2 μm and a lateral resolution of 1.9 μm. Preliminary results showed that the compact sLSM probe could visualize cellular details of fixed human anal epithelial tissues in a similar manner to a bench light sheet microscopy device using off-the-shelf objective lenses.

Ultrasound imaging is typically based on the use of arrays of piezoelectric transducers that can both emit and receive ultrasound. It has recently been shown that on-chip optical microresonator transducers can achieve massive improvements in minimizing footprint and increasing both ultrasound sensitivity and bandwidth; however, the construction of practical arrays remains an open problem. We study the feasibility of making an array of optical microresonators for ultrafast imaging. As a proof of concept, we propose the design of a linear array of polymer microring resonators with equally spaced resonance frequencies. Optical dual-comb setup simultaneously interrogates the whole array’s ultrasound perturbation by assigning each microring to a single comb tooth. Using an optical frequency comb for detection provides an efficient way of sampling a large array of transducers while using only a single balanced heterodyne detection scheme per branch.