Far IR excitation of longitudinal optical phonons in triglycine sulphate

Abstract

In the parts of the Far IR (FIR) spectrum where Ferroelectric Triglycine Sulphate (TGS) is transparent for a given direction of the electric vector , the forced transverse vibrations that can be excited in that direction are shown to couple slightly with the Longitudinal Optical (LO) vibrations propagating along the wave vector (wave normal) of the FIR wave. Very clear LO phonons FIR transmission spectra at quasi-normal incidence with zero transmission sharp lines can thus be obtained at low temperature with thick samples, while classical spectra of Transversal Optical (TO) phonons are obtained with thin crystal plates. This coupling seems a second order effect, quite new, applicable to all low frequencies LO phonons of TGS and probably other crystals of low symetry. On the opposite, no coupling has been detected with an other direction than the vector.