  We report on the observation of terahertz radiation in a non-centrosymmetric
insulating oxide, paratellurite ($\alpha$-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.
