Second harmonic generation (SHG) is a nonlinear optical process in which photons can be combined to produce new photons with double the frequency, and hence half the wavelength, of the original photons. SHG materials can be used, for example, in the laser industry to convert infrared lasers (1064 nm) into green lasers (532 nm).

Li-Ming Wu, Chinese Academy of Sciences, Fuzhou, and colleagues have investigated SHG crystals in the mid-infrared range, where there is currently a lack of good materials. They are searching for materials with a number of useful properties, the most important of which are good SHG response and a high laser-induced damage threshold. The team has made a series of chalcogenides with a diamond-like framework (AZn₄Ga₅S₁₂ with A = K, Rb, Cs; structure pictured) and compared their properties to AgGaS₂ (AGS), a commercially used material.

All three materials have a good SHG response (1.2–1.4 times better than AGS) and are particularly resistant to laser damage (36 times more resistant than AGS). All materials are also very stable at high temperatures up to 1400K. An additional computational study allowed the researchers to identify the structural features responsible for the SHG properties, which they hope will allow them to design even better materials in the future.

• Coexistence of Strong Second Harmonic Generation Response and Wide Band Gap in AZn₄Ga₅S₁₂ (A=K, Rb, Cs) with 3D Diamond-like Frameworks,
  Hua Lin, Hong Chen, Yu-Jun Zheng, Ju-Song Yu, Xin-Tao Wu, Li-Ming Wu,
  Chem. Eur. J. 2017.
  DOI: 10.1002/chem.201701679