We have measured the reflectance and transmittance of thorium dioxide thin films from 50-280 eV. We have developed several methods for fitting this data that gives the most reliable values for the complex index of refraction, n = 1 - δ + iβ. These fitting methods included fitting film thickness using interference fringes in highly transmissive areas of the spectrum and fitting reflectance and transmittance data simultaneously. These techniques give more consistent optical constants than solitary unconstrained fitting of reflectance as a function of angle. Using these techniques, we have found approximate optical constants for thorium dioxide in this energy range. We found that the absorption edges of thoria were shifted 4 eV and 2 eV to lower energies from those of thorium. We also found that the peak in δ was shifted by 3 eV to lower energy from that of thorium.
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 s- and p-polarized 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.
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