Peculiarities of second-harmonic generation in TlInS2 crystals

The temperature dependence of optical second-harmonic generation in TlInS2 crystals in the vicinity of phase transitions has been investigated. It is established that in an incommensurate phase the generation is observed only for propagation of the fundamental wave along the superstructure axis and its existence for certain experimental configurations confirms C23 space group of the low-temperature polar phase. The dependence of the spectra on thermocycling process is explained by the formation and the annihilation of the incommensurate-like structures inside the commensurate phase.