We investigated the optical response of amorphous (a-) Ge thin films by a time-resolved reflectivity measurement and the observation of terahertz radiation from a-Ge photoconductive switch coupled with Au/Cr transmission lines. The results show that the lifetime of photocarrier in the a-Ge is about 0.9 ps, and the pulse width of terahertz beam radiated from the photoconductive switch is about 1 ps. These results indicate that a-Ge can be used as an optical-to-electrical converter in sub-terahertz frequency range. However, the peak width of a-Ge photoconductive switches with YBCO transmission lines is about 2 ps and broad compared with the result with Au/Cr transmission lines. © 2004 Published by Elsevier B.V.