We investigated photoconductive gratings in resonant semiconductor layers acting as light-controlled polarizers of millimeter (MM) waves. We compared the effects of strip-like, wire-like, and fin-like gratings excited by red light and IR radiation in Silicon wafers, respectively. The fin-like gratings are shown to be the preferred structures that can operate at limited light intensity. The light-sensitive shift of maxima of transmitted power and polarizing efficiency towards the lower frequency band is observed. The effect makes photoconductive gratings and similar patterns potentially suitable for the design of light-controlled frequency-tuning and frequency-modulating components of resonant quasi-optical devices.