Subwavelength amorphous silicon transmission gratings with a period from 100 to 800 nm and a thickness of 180 nm on silica substrates have been fabricated and studied. For a normal incident light (&lgr;=633 nm), in addition to strong birefringence (as large as 170° in phase difference), it was observed that the transmission of TE (the electric field parallel to the grating finger) wave is significantly different from that of TM (the electric field perpendicular to the grating fingers) wave (as large as 12 dB), and that both polarization and birefringence effects strongly depend on the ratio of grating period to wavelength, having the largest variations at the grating periods around a half of the wavelength. This observation deviates significantly from simple form birefringence theory and effective medium theory. Applications such as polarization selection mirrors for vertical cavity surface emitting lasers and waveplates are discussed. ©1995 American Institute of Physics.