Diffraction from metal strip gratings with high spatial frequency in the infrared spectral region

A comparative theoretical and experimental study of the diffraction characteristics of high-spatial-frequency aluminum strip gratings as a function of grating thickness is presented. The gratings were made with ZnSe and ZnS substrates by using standard photolithographic techniques and reactive ion etching. A grating period of 3 μm and a fill factor of approximately 0.33 are used. The grating thicknesses vary from 2400 to 9500 Å. Transmittance measurements were carried out in the 7–13-μm-wavelength band in which only the forward and backward zeroth-order waves propagate. The gratings exhibit a long-wavelength bandstop, i.e., a high-pass filtering characteristic in transmission with the transmittance decreasing with increasing grating thickness. For the thickest grating tested (9500 Å), the transmittance was less than 5% for 8.5 μm or greater and dropped to approximately 1.5% at 13 μm. The measured transmittances are compared with results calculated by using the rigorous coupled-wave theory and are found to be in good agreement.