Second harmonic generation in layered TunS2 in the vicinity of the low-temperature phase transitions

Abstract Second-harmonic generation in the ternary layered semiconductor TlInS2 excited with the wavelength λ = 1.06 μm of YAG: Nd3 + laser is investigated in the temperature range corresponding to the low-temperature phase transitions. It is shown that the intensity of the second-harmonic signal corresponding to the non-linear coefficients deff, d21, d22 reveals peculiarities close to the commensurate-incommensurate phase transitions. The temperature hysteresis of the second-harmonic signal in the low-temperature region (below and close to Ti2 = 206K) is explained assuming that an incomplete lock-in transition in TlInS2 takes place at Tc1 =204K within the temperature range between a ferroelectric (Tc2 = 201K) and an incommensurate (Ti2 = 206 K) phases. It is shown that the monoclinic point group symmetry C2 is preserved also at temperatures lower than the phase transition temperature Tc4 = 79K to a weak ferroelectric state.