The resonance behavior of metal transmission gratings and its impact on the response of metal-semiconductor-metal (MSM) photodetectors have been studied experimentally and theoretically. The metal gratings, with finger spacings in the subwavelength region of the visible light, were fabricated usinge-beam lithography and lift-off. Strong resonances have been observed only in theSpolarization. As a result, the light transmitted through a grating is primarilySpolarized if the grating’s finger spacing is less than one-third of the wavelength of the incident light, butPpolarized otherwise. Similar phenomenon has been observed in the response of MSM photodetectors since the fingers (electrodes) of an MSM photodetector basically form a grating. Theoretical simulations employing the rigorous modal-expansion theory fairly predict the observed phenomenon. ©1997 American Institute of Physics.