  Polarization control of terahertz wave is a challenging subject in terahertz
science and technology. Here, we report a simple method to control polarization
state of the terahertz wave in terahertz generation process. At room
temperature, terahertz radiation from a noncentrosymmetric and chiral oxide,
sillenite Bi$_{12}$GeO$_{20}$, is observed by the irradiation of linearly
polarized femtosecond laser pulses at 800 nm. The polarization state of the
emitted terahertz wave is found to be elliptic with an ellipticity of
$\sim$0.37$\pm$0.10. Furthermore, the ellipticity was altered to a nearly zero
($\sim$0.01$\pm$0.01) by changing the polarization of the incident linearly
polarized femtosecond laser pulses. Such a terahertz radiation characteristic
is attributable to variation of the polarization state of the emitted terahertz
waves, which is induced by retardation due to the velocity mismatch between the
incident femtosecond laser pulse and generated terahertz wave and by the
polarization tilting due to the optical activity at 800 nm.
