Surface Damage in Cadmium Sulfide

Abstract

A new type of birefringent filter, called a dispersive birefringent filter (DBF), has been described by Yeh1. A DBF requires a material whose birefringence is dispersive in the wavelength region of interest. In the region from 5300 to 5500 Angstroms (Å), cadmium sulfide (CdS) has this property. Early calculations showed that a DBF made from CdS could be made to have a very narrow passband (approximately 2 Å) and very wide field-of-view (80 to 90 degrees half-angle). At a wavelength of 5320 Å (doubled Nd:YAG) this filter would require flat and parallel CdS plates as thin as 35 microns. When such plates were made, it was found that the absorption coefficient was several orders of magnitude larger than expected. This excessive absorption was traced to mechanically induced surface damage of the CdS. As a result, the ability to produce thin, parallel, and relatively damage-free CdS plates became crucial to the success of the DBF development effort.