Chalcogenide glasses contain as a major constituent one or more of the chalcogen elements from group 6a of the periodic table (i.e. Sulphur, Selenium and Tellurium, but excluding Oxygen) covalently bonded to other elements such as As, Ge, Sb, Ga, Si, or P. Chalcogenide glasses have been studied since the 1950s due to their amazing optical properties. They have already found important applications in a number of areas, including the electronics industry and in imaging applications. In the last decade there has been renewed interest in these materials because of their unique optical nonlinear and midinfared properties. The high material nonlinearity of chalcogenides combined with the strong confinement and dispersion engineering achievable in fiber and waveguide devices makes them attractive as ultrafast nonlinear devices or in efficient frequency conversion schemes; the absence of free-carriers or two-photon absorption means that the nonlinearity is near-instantaneous and pure. These glasses are transmissive well into the mid-infrared region (e.g. sulphides transmit to ~11μm) and are photosensitive to visible light.

Recent progress in chalcogenide photonics has been driven by scientific and technological challenges faced in different areas, for example the electronic bottleneck in optical communications, the emergence of bio-health hazards associated with hazardous microorganisms that absorb at mid-infrared wavelengths and defence applications that require bright mid-infrared sources. The chalcogenide platform also provides a platform for fundamental investigations of light-matter interactions in nanophotonics structures, such as photonic crystal and even metamaterial structures. The combination of their unique optical properties with the flexibility in tailoring the composition and fabrication methodology makes the chalcogenides compelling for photonics research and has stimulated research groups around the world to actively pursue this vibrant area.

Chalcogenide photonics research can be categorized in to a number of areas: (i) chalcogenide material and device science, which aims to optimize chalcogenide compositions in order to tailor or enhanced device performance (e.g. optical nonlinearity, transparency or photosensitivity); (ii) device fabrication, including optical fiber technology, which aims to develop novel planar waveguide components, photonic crystals or optical fibers with optimized optical nonlinearity or midinfrared properties; (iii) applications in nonlinear optics, which aim to exploit the optical nonlinearity for ultrafast all-optical signal processing, such as optical switching, broadband frequency conversion or active sources; and (iv) or sensing applications, which exploit the mid-infrared transparency for biosensing applications.

This special issue reviews recent progress in this field with 13 invited articles from the leading groups around the globe. This issue is comprehensive with articles that span the spectrum of activities mentioned above.

Gai et al. [1] provide a detailed report on the fabrication of chalcogenide waveguides for nonlinear optical signal processing at telecommunication wavelengths. Troles et al. [2] and El-Amraoui et al [3] report on advances in the fabrication of chalcogenide microstructured optical fibers for tailored optical nonlinearity with particular emphasis on generating mid-infrared wavelengths. Weiblen et al. [4] computationally investigate supercontinuum generation in chalcogenide photonic crystal fibers at mid-infrared wavelengths. Suzuki and Baba [5] report massive optical nonlinearity in photonic crystal waveguides based on Ag-As₂-Se₃ chalcogenide and report high-efficiency self-phase modulation and four wave mixing. Pelusi et al. [6] report the first demonstration of optical phase conjugation using four-wave mixing in an As₂S₃ planar waveguide for dispersion-free transmission of multiple channels that are phase encoded signals. Lee et al. [7] investigate nonlinear effects in chalocogenide photonic crystal cavities. They characterize photosensitivity and photothermal nonlinear effects at telecommunication wavelengths and consider the impact of these effects on ultrafast all-optical switching.

Seddon et al. [8] report on rare-earth doped mid-infrared fiber lasers based on chalcogenide glasses. Elliot et al. [9] report on active devices based on chalcogenide glass microspheres. They report laser action at 1075 and 1086 nm in sub millimeter neodymium-doped gallium lanthanum sulphide glass spheres. Carlie et al. [10] report on novel sensing architectures for midinfrared wavelengths based on chalcogenide waveguide resonators. They exploit the chalcogenide photosensitivity to post-trim resonators and compensate for fabrication imperfections. Tsay et al. [11] report on a novel fabrication methodology for chalcogenide glass materials for patterning and integrating chalcogenide glass waveguides from solution, a significant step towards developing chalcogenide based midinfrared active photonic circuits. Application to sensing bio-molecules is considered by Yang et al. [12] in developing a telluride based opto-electrophoretic sensor for the detection and identification of hazardous microorganisms. Sanghera et al. [13] report on microstructuring of chalcogenide fiber end faces in order to incorporate antireflection properties that enhance optical transmission.

In conclusion, this issue was created with the intent to represent the current state-of-the-art in the field of chalcogenide photonics. I am very grateful to all invited authors for their effort in preparing high quality manuscripts. My special thanks to Martijn de Sterke who strongly supported the idea of this Focus Issue. I would like to thank Meghan Cook for the technical assistance with publishing this Focus Issue.