1. Introduction

With a fast, dramatic, and reversible change in refractive index, Phase Change Materials (PCMs)—whereas volatile alloys such as VO₂ or nonvolatile such as Ge₂Sb₂Te₅—are a versatile platform for free-space and on-chip light modulation. The significant technical progress achieved in the last decade has led to an increasing number of optical devices with unprecedented performance, new alloys with superior optical properties, and various techniques to control and characterize PCMs. Consequently, PCMs are attracting interest from scientists working in a broad spectrum of Optics and Photonics fields, along with industry and funding agencies. Because the driving force of such progress is the combined efforts of the Materials Science and the Photonics communities, Optical Materials Express, an ideal venue for this topic, dedicates this feature issue to contributions that expand our understanding of this family of materials and their applications.

2. Feature issue summary

From metasurfaces and optical filters to plasmonic and photonic integrated modulators, the topics covered in this feature issue reflect the versatility of PCMs in a broad spectrum of applications. Moreover, the various alloys (VO₂, Ge₂Sb₂Te₅, Sb₂Se₃, and Ge₂Sb₂Se₄Te) demonstrate an expanding family of application-tailored materials. Here, we summarize the different contributions divided into works exploring volatile and nonvolatile alloys.

2.1 Vanadium dioxide (VO₂)

The temperature-controlled modulation upon insulator to metal transition in VO₂ is ideal for tunable optics in free-space applications, which was the main focus of all the contributions using this material. Rahimi et al. [1] studied the temperature-dependent properties of VO₂/Si core-shell nanoparticles to achieve band-selective reflection control. Iman et al. [2] proposed a Graphene-VO₂-based defective photonic crystal platform to demonstrate a one-dimensional unidirectional terahertz absorber with thermal switching from broadband to narrowband absorption. D. Zhang et al. [3] numerically demonstrated a hybrid dielectric grating that uses a VO₂ thin film to switch between reflection and transmission responses upon insulator to metal phase transition. Y. Liu et al. [4] simulated four different structures for near-field multistage radiative thermal rectifiers combining two other phase-change materials: VO₂ and Ge₂Sb₂Te₅. Such devices can achieve multistage thermal rectification with varying ratios of rectification by exploiting the different transition temperatures of each PCM. Finally, Cunningham & Bradely [5] used various gold nanostructures coupled to a VO_{2 ­­}thin film to computationally demonstrate plasmon resonance tunability within the visible and near-infrared spectral regions.

2.2 Chalcogenide phase change materials

The nonvolatile family of PCMs had representation in works exploiting, studying, or applying the properties of mainly three alloys: Ge₂Sb₂Te₅, Sb₂Se₃, and Ge₂Sb₂Se₄Te. Developing on the characterization of PCM properties, Martin-Monier et al. [6] reviewed the different failure mechanisms that affect the endurance in state-of-the-art optical devices based on phase change materials. Such mechanisms represent a key challenge to overcome in moving toward PCM-based technologies. H. Zhang et al. [7] carried out ab initio calculations to understand how selenium substitution in Ge₂Sb₂Se_xTe_5-x, with $x = 1$ to 4, modifies the local structure and the optical response of the amorphous quaternary alloys. Sevison et al. [8] experimentally demonstrated an interferometric method to measure the phase accumulation through a switched layer of Ge₂Sb₂Te₅, which is relevant to applications in free-space dynamic reconfiguration. Finally, Prikryl et al. [9] characterized the differences in optical properties of as-deposited, annealed, laser-reamorphized, and laser-recrystallized Ge₂Sb₂Te₅.

Like volatile PCMs on metasurfaces, nonvolatile chalcogenides also drew significant attention in free-space optical applications. Barreda et al. [10] proposed to utilize phase change materials to tune the properties of light-emitting metasurfaces designed to support quasi-bound states in the continuum in the telecom wavelength range. They use both Sb₂Se₃ and Ge₂Sb₂Te₅ to provide strong refractive index contrast when switched between the amorphous and the crystalline states. Tsitas & Foteinopoulou [11] computationally demonstrated nonvolatile mid and long-wave infrared beam reconfigurability with PCM-based gratings featuring high-refractive index shift. The material of choice in this contribution was the broadband transparent and nonvolatile Ge₂Sb₂Se₄Te.

On integrated optics, Mohammadi-Pouyan et al. [12] proposed two ultra-compact Mach-Zehnder Interferometers (MZI) based on Ge₂Sb₂Se₄Te phase and amplitude modulators, including also a bend-less MZI design that exploits the modulator’s compact footprint. Lei et al. [13] carried out an exhaustive characterization and comparison of magnetron-sputtered and thermal-evaporated low-loss Sb₂Se₃ phase-change films in non-volatile integrated photonics. Gao et al. [14] used numerical methods to optimize the structural parameters of nonvolatile photonic memories based on Ge₂Sb₂Te₅ for high storage density and low energy consumption. T. Liu et al. [15] simulated a mode switch formed by three cascaded asymmetric directional couplers that selectively convert an input TE₁₁ mode into TE₂₁, TE₃₁, or TE₄₁ depending on Ge₂Sb₂Se₄Te state. C. Zhang et al. [16] proposed a class of photonic integrated circuits that use subwavelength metal gratings to couple and decouple light and explore the possibility of using phase change materials to tune the coupling efficiency via index matching. Lastly, Ghosh & Dhawan [17] performed numerical modeling to design two on-chip photonic devices: a reflector switch using a photonic crystal and a mode converter switch. Both devices were simulated using a silicon nitride platform with embedded Sb₂Se₃ phase change material as the nonvolatile switching platform.

It is our hope that this feature issue offers a timely overview of the field of optical phase change materials and devices, and will stimulate further research and development efforts in this area. We especially want to express our genuine gratitude to all the authors and reviewers for their contributions. We also thank Dr. Stavroula Foteinopoulou for her support of this feature issue, and the Optica staff for their exceptional work throughout the review and production processes.