Abstract

Experimental and theoretical diffraction characteristics of aluminum strip gratings on ZnSe and ZnS substrates in the visible and infrared spectral regions are presented. The grating thicknesses vary from 2400 to 9500 Å with a grating period of 3 μm. In the visible and near-infrared regions the gratings exhibit multiple diffracted orders. Diffraction efficiency measurements were made for both transmitted and reflected orders. The measurements are compared with results calculated with coupled-wave theory. The grating thickness and width are varied around the nominal measured values to obtain the best agreement between theory and experiment. These altered thicknesses and widths are also used in the calculation of transmittance as function of wavelength (7-13 μm) in the far IR region where only zeroth-order waves propagate. Reasonably good agreement is found between theory and experiment in these two wavelength regions using a single set of grating parameters and accounting for the wavelength dependence of the refractive indices. Furthermore, the diffraction efficiency variation as a function of grating thickness for different grating duty cycles and for different lossy materials was studied. For low-loss strips, the diffraction efficiency of the forward and backward diffracted orders oscillates rapidly with increasing grating thickness (0-5 μm). As the loss in the strips increases, the backward diffracted orders continue to oscillate whereas the forward diffracted waves stabilize.

© 1991 Optical Society of America