Chalcogenide Glass
Chalcogenide glass (ChG) is a family of non-oxide glasses built from the chalcogen elements sulfur (S), selenium (Se), and/or tellurium (Te), usually combined with network formers such as As, Ge, Sb, Ga, or P (e.g. As₂S₃, As₂Se₃, Ge–As–Se, Ge–Sb–Se, GST). In lasers and photonics they are used as infrared-transmitting, high-index, highly nonlinear hosts rather than as ordinary window glass.
They sit between silica/fluoride glasses and crystalline IR materials: they can be melted, drawn into fiber, deposited as thin films, and patterned into waveguides, while transmitting far deeper into the mid-IR than silica.
Technical properties:
Composition and structure. Covalent networks of S/Se/Te with heavier cations. Bond strengths and phonon energies are much lower than Si–O, so the multiphonon absorption edge is pushed far into the infrared. Typical glass-transition temperatures are modest (~150–350 °C depending on composition).
Transmission window. Rough guide:
sulfides: ~0.6–11 µm (often some visible/NIR transmission)
selenides: ~1–12–15 µm
tellurides: ~2–20 µm and beyond
Fibers usually have a slightly narrower usable window than bulk glass because of impurity and scattering losses. Commercial IR optics often use Ge–Sb–Se or Ge–As–Se glasses that cover the 3–5 µm and 8–12 µm atmospheric windows.
Refractive index. Typically n≈2.0–3.5 (As₂S₃ ~2.4, As₂Se₃ ~2.8, many Ge–Sb–Se glasses ~2.5–2.7). High nnn gives strong mode confinement in waveguides and large Fresnel reflection unless AR-coated.
Phonon energy. ~200–350 cm⁻¹, much lower than silica (~1100 cm⁻¹) or ZBLAN (~550 cm⁻¹). That reduces non-radiative decay of rare-earth ions and enables mid-IR emission that oxide/fluoride hosts quench.
Nonlinear optics. Third-order Kerr index n2 is typically 100–1000× that of silica (As₂S₃ ~130×, As₂Se₃ several hundred×). Fast electronic response, low two-photon absorption in many bands of interest, and no free-carrier absorption like silicon. Some compositions also support Raman, Brillouin, and (in engineered films) χ(2) processes.
Other photonic traits. Photosensitivity (useful for writing gratings), rare-earth solubility (limited but usable), and—in GST-type alloys—reversible amorphous/crystalline phase change with large index contrast for nonvolatile photonic memory and switches.
Practical caveats: they are softer and more chemically/thermally fragile than silica, more lossy in fiber (tens to hundreds of dB/km typical vs silica’s 0.2 dB/km), and some As-containing glasses raise toxicity and handling issues.
Applications:
Passive IR optics. Molded or diamond-turned lenses, windows, and prisms for thermal imaging, night vision, and mid-IR sensors; cheaper and more formable than Ge or ZnSe for many systems.
IR fibers and laser delivery. Solid-core and (less commonly) hollow-core ChG fibers transmit 2–12+ µm light for spectroscopy, chemical sensing, and delivery of CO₂ (~10.6 µm) or other mid-IR lasers where silica is opaque. Losses are higher than telecom fiber, so lengths are meters rather than kilometers.
Mid-IR fiber lasers and amplifiers. Rare-earth doping (Dy, Pr, Er, Ce, etc.) in low-phonon ChG fiber enables emission beyond the ~2.5 µm silica and ~4 µm fluoride limits. Continuous-wave lasing past 5 µm has been demonstrated in Ce-doped selenide fiber; the target range is roughly 4–10 µm for sensing, countermeasures, and medical uses. Power is still low (mW class) pending better purity and loss.
Nonlinear photonics. High n2 plus tight confinement enables compact devices:
mid-IR supercontinuum sources (spectra from ~2 µm out to 10–16 µm in Te-rich fibers).
all-optical switching, regeneration, wavelength conversion, four-wave mixing.
Raman and Brillouin lasers / microwave photonics.
on-chip nonlinear circuits (waveguides, microresonators) for signal processing and frequency conversion.
Integrated and reconfigurable photonics. Thin-film ChG waveguides on Si or other platforms for mid-IR lab-on-chip sensing. GST and related phase-change chalcogenides provide electrically or optically switched, nonvolatile index change for programmable photonics, optical memory, and tunable metasurfaces.
Sensing and spectroscopy. Fiber evanescent-wave sensors and on-chip circuits exploit the mid-IR “fingerprint” region of molecules; broadband SC sources in ChG fiber are used as illumination for those measurements.
Chalcogenide glass is the workhorse soft-glass platform for mid-infrared light—high index, huge nonlinearity, and a transmission window silica cannot reach—used for IR optics, mid-IR fiber lasers, supercontinuum sources, and emerging integrated photonic circuits.