Construction of an extreme ultraviolet polarimeter based on high- order harmonic generation

We report on the development of a polarimeter for characterizing reflective surfaces throughout the extreme ultraviolet (EUV). The instrument relies on laser high-order harmonics generated in helium, neon, or argon gas. The 800 nm laser generates a discrete comb of odd harmonics up to order 100 (wavelengths from 8-62 nm). The flux of EUV light is a couple orders of magnitude less than a synchrotron source but 30,000 times greater than a plasma source currently in operation at BYU. The polarimeter determines the reflectance from surfaces as a function of incident angle, linear light polarization orientation, and wavelength. The instrument uses a wave plate in the laser beam to control the orientation of the harmonic polarization (linear, same as laser). After reflecting from the sample, the harmonic beams are dispersed by a grating and focused onto a micro-channel plate coupled to a phosphor screen. We have demonstrated the feasibility of this project with a simple prototype instrument, which measured the reflectance of samples from 30 nm to 62 nm. The prototype demonstrated that sensitivity is sufficient for measuring reflectances as low as 0.5% for both sand ppolarized light. The full instrument employs extensive scanning mobility as opposed to the fixed angle and fixed wavelength range of our earlier prototype. An advantage of employing harmonics as a source for EUV polarimetry is that a wide range of wavelengths can be measured simultaneously. This project represents an authentic ‘work-horse’ application for high-order harmonics, as opposed to merely demonstrating proof of concept.