Separation of different loss channels in DUV optical elements

Abstract

A photo-thermal measurement device for quantitative determination of absorptance in DUV optics was developed. It is based upon a Hartmann-Shack wavefront sensor with extreme sensitivity, accomplishing spatially resolved monitoring of thermally induced wavefront distortions. Caused by the temperature dependence of the refractive index as well as thermal expansion, the initially plane wavefront of a test laser is distorted into a convex or concave lens, depending on sign and magnitude of index change and expansion. Since the extent of deformation is directly proportional to the absorption loss, the parallelized photo-thermal technique can be employed for a rapid assessment of the material quality. Monitoring the fluence dependence of the thermal lens effect accomplishes evaluation of both single- and two-photon absorption coefficients. Moreover, a separation of surface and bulk absorptance can be achieved. Along with a description of the technique we present results from absorption measurements on fused silica and CaF₂ under 193nm irradiation. The data are compared with theoretical results obtained from a solution of the heat diffusion equation.