A new type of beam profiler in the terahertz region using optical modulatio
n of transmission in a silicon wafer is proposed and demonstrated. The prof
ile of the beam emitted from a photoconductive antenna and focused by parab
oloidal mirrors is obtained as a function of frequency. The beam profile at
each frequency can be fitted well with a Gaussian function. The diameter o
f the beam at a focal plane decreases with increasing frequency. The spatia
l resolution of the beam profiler is approximately determined by the optica
l beam size on the silicon wafer.