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Terahertz radiation induced by coherent phonon generation via impulsive stimulated Raman scattering in paratellurite

We report on the observation of terahertz radiation in a non-centrosymmetric insulating oxide, paratellurite ($α$-TeO$_2$) by irradiation of a femtosecond laser pulse at room temperature. In the power spectrum of the terahertz radiation, an intensity fringe pattern with a period of $\sim$ 0.25 THz shows up below 3 THz. It can be reproduced by taking into account the effective generation length for the terahertz radiation with a poor phase-matching condition. In addition, a temporal oscillation component appears in the radiated terahertz wave with a frequency of $\sim$ 3.71 THz, which is in good agreement with the center frequency of the Raman active longitudinal optical (LO) $E$ mode. On the basis of comprehensive polarized optical and Raman spectroscopic studies, we explain the generation mechanism of the temporal oscillation component in terms of the coherent phonon generation via impulsive stimulated Raman scattering.