Nonlinear Properties of Chalcogenide Crystals

Chalcogenide crystals are compounds of group 16 elements (chalcogens: sulfur S, selenium Se, tellurium Te) with metals. Due to their unique nonlinear optical properties, they are widely used in laser technology, spectroscopy, and optoelectronics.


Key nonlinear optical effects

Chalcogenide crystals exhibit the following nonlinear effects:

  1. Second‑harmonic generation (SHG) — doubling the frequency of incident radiation (wavelength conversion λ→λ/2).
  2. Optical parametric amplification — conversion of one pump photon into two photons with lower energy.
  3. Four‑wave mixing — interaction of four waves with energy exchange between them.
  4. Self‑focusing and self‑defocusing — change in the refractive index under the action of intense radiation.
  5. Supercontinuum generation — broadening the radiation spectrum to a wide band.

Key nonlinear characteristics

  • Second‑order nonlinear susceptibility χ(2) — responsible for quadratic nonlinearity effects (SHG, parametric conversion).
  • Nonlinear refractive index n2​ — determines cubic nonlinearity effects (self‑focusing, four‑wave mixing).
  • Nonlinear absorption coefficient — critical for high‑power laser systems.
  • Laser damage threshold — limits the maximum radiation intensity.

Crystal

Transparency range (µm)

Nonlinear coefficient dij​ (pm/V)

Application

AgGaS2​ (AGS)

0.5–12

~13

Mid‑IR radiation generation

AgGaSe2​ (AGSe)

0.7–18

~30

OPO for 3–12 µm

ZnGeP2​ (ZGP)

2–12

~75

3–5 µm generation with CO₂ laser pumping

HgGa2​S4​ (HGS)

0.5–13

~27

Broad transparency range

GaSe

0.6–18

~50–75

Highly efficient SHG in the IR range

Advantages of chalcogenides

  • Broad transparency range in the IR region (up to 18 µm).
  • High nonlinear coefficients (10–100 times higher than quartz).
  • Possibility of phase matching for efficient nonlinear conversion.
  • Resistance to laser radiation (for several crystals).

Applications

  • Optical parametric oscillators (OPOs) — tunable sources of IR radiation.
  • Frequency conversion — generation of radiation in the mid‑ and far‑IR ranges.
  • Spectroscopy — analysis of gases and biological tissues.
  • Remote sensing — lidars for environmental and meteorological monitoring.
  • Medical diagnostics — optoacoustic imaging.
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