Alloys of sulfur, selenium and tellurium, often referred to as chalcogenide semiconductors, offer a highly versatile, compositionally-controllable material platform for a variety of passive and active photonic applications. They are optically nonlinear, photoconductive materials with wide transmission windows that present various high- and low-index dielectric, low-epsilon and plasmonic properties across ultra-violet, visible and infrared frequencies, in addition to an, non-volatile, electrically/optically induced switching capability between phase states with markedly different electromagnetic properties. This roadmap collection presents an in-depth account of the critical role that chalcogenide semiconductors play within various traditional and emerging photonic technology platforms. The potential of this field going forward is demonstrated by presenting context and outlook on selected socio-economically important research streams utilizing chalcogenide semiconductors. To this end, this roadmap encompasses selected topics that range from systematic design of material properties and switching kinetics to device-level nanostructuring and integration within various photonic system architectures.

We demonstrate the first single-mode optical fiber couplers made with ZBLAN optical fiber. Couplers are fabricated using a controlled tapering procedure enabling high reproducibility while limiting glass crystallization. A coupling ratio of up to 41%/59% in cross/through ports with an excess loss of 2.5 dB is obtained at a wavelength of 2.73 μm. In addition, the stability of a coupler with traces of surface crystallization is tested at ambient atmosphere over a period of more than 90 days.

We demonstrate chalcogenide optical fiber couplers with a power-dependent coupling coefficient. The couplers are designed and fabricated using an As₂Se₃ fiber and characterized at a wavelength of 1938 nm, leading to a critical power of 126 W, the lowest ever reported for any optical fiber coupler. These nonlinear couplers enable all-optical switching and will be useful for passive mode-locking over a wide wavelength range from the telecommunication band to the mid-infrared.

We report on self-injection locking in InGaN/GaN (blue/green) and InGaP/AlGaInP (red) visible-light laser diodes. The free-space optical feedback path was accomplished via an external mirror. The effect of injection current, optical power injection ratio, and external cavity length on the spectral linewidth and modulation bandwidth of the lasers is investigated. Our results show that the laser performance was substantially improved. In particular, we achieved a significant increase of ~57% (1.53-2.41 GHz) and ~31% (1.72-2.26 GHz) in the modulation bandwidth, and ~9 (1.0-0.11 nm) and ~9 (0.63-0.07 nm) times reduction in spectral linewidth of the green and blue lasers, respectively. Consequently, side-mode-suppression-ratio was considerably increased in all the cases, reaching as high as ~20 dB in self-injection locked blue laser diode, thus enabling a close to single-mode operation. This paper paves the way for attaining high-speed optical wireless communications by overcoming the challenges of limited modulation bandwidth and multimode operation of visible laser diodes with this simple scheme.

We report, to the best of our knowledge, the first employment of a self-injection locking scheme for the demonstration of a tunable InGaN/GaN semiconductor laser diode. We have achieved a 7.11 nm (521.10–528.21 nm) tunability in a green color with different injection currents and temperatures. The system exhibited mode spectral linewidth as narrow as $\sim 69\mathrm{pm}$ and a side mode suppression ratio as high as $\sim 28\mathrm{dB}$, with a maximum optical power of $\sim 16.7\mathrm{mW}$. In the entire tuning window, extending beyond 520 nm, a spectral linewidth of $\le 100\mathrm{pm}$, high power, and stable performance were consistently achieved, making this, to the best of our knowledge, the first-of-its-kind compact tunable laser system attractive for spectroscopy, imaging, sensing systems, and visible light communication.

We present a soliton order preservation mechanism leading to broad tunability with high energy conversion efficiency (ECE) from a fiber-based soliton self-frequency shift (SSFS) system. Here, a pulse compressing fiber, placed before the Raman shifting nonlinear fiber (NLF), acts as a soliton order preserver. Thanks to this passive mechanism, the soliton order at the input of the NLF is preserved within $2.0\lt N \lt 2.3$ despite a $\sim6.7$ -fold energy increase. The resulting pulse leads to an SSFS that is spectrally tunable over 1.96-2.14 μm while keeping an ECE of 80-85%. It is the first time that an ECE $\gt 80\%$ is maintained over a tunable range as large as 180 nm in a passive fiber-based SSFS system. Solitons are also tunable up to 320 nm in the wavelength range of 1.96-2.28 μm with ECE $\gt 50\%$ . Such a soliton order preservation mechanism that enables a high ECE while maintaining a broad soliton tunability is of utmost practical interest for wavelength conversion systems.