Methods for drift stabilization and photomultiplier linearization for photometric ellipsometers and polarimeters

We describe simple modifications to photometric polarimeter and ellipsometer systems that greatly reduce nonlinearity, component drift, and digital noise effects. A photomultiplier feedback circuit is described and analyzed that varies the effective system linearity by means of a single control. Linearity to 5 x 10(-4) over more than three orders of magnitude of incident light intensity is obtained. Effective double-beam stability to 5 x 10(-5) is achieved by blocking the source intensity over part of an extended measurement cycle. These refinements permit photometric instruments to approach accuracy capabilities compatible with their precision capabilities, and also allow measurements to be made in the presence of ambient scattered light, at high sample temperatures, or using solid-state detectors for which the zero reference is not well defined. As an example, we give the dielectric function of Ge measured from 1.5 to 5.8 eV at a sample temperature of 800 degrees C.