Reflection polarizers for the vacuum ultraviolet using Al + MgF_2 mirrors and an MgF_2 plate

The performance of a three mirror polarizer wherein one of the reflecting surfaces is a MgF(2) plate illuminated at, or close to, the Brewster angle and the other two surfaces are Al + MgF(2) coatings is calculated. The polarization, R(s)/R(p), and throughput R(s). were obtained by taking the product of the individual values calculated at the appropriate angles of incidence. This polarizer is most efficient in the 1200-2000-A wavelength region. By choosing the proper angle of incidence on the MgF(2) plate, polarization values of 100 and more are obtained from about 900 A to 3000 A. The polarizer is also useful to wavelengths as short as 500 A although the polarization value decreases to about 10 at this wavelength. Comparing this polarizer to a four mirror polarizer wherein one reflecting surface is MgF(2) at, or close to, the Brewster angle and the other three surfaces are Al + MgF(2) coatings showed that the four mirror polarizer had a somewhat larger polarization but a lower throughput. A three mirror polarizer utilizing one MgF(2) surface and two Al + MgF(2) surfaces was constructed and used to measure the R(p) component of a MgF(2) test plate. The measurements showed a deep minimum at the Brewster angle. Similar measurements were made to obtain R(s) and R(p) for an evaporated film of ZnS. At 45 degrees , R(s)(2) = R(p), within the limits of accuracy of the measurement, and the residual polarization due to the monochromator grating was calculated to be less than 1.13 or in terms of fractional polarization, (I(s) - I(p))/(I(s) + I(p)), less than 6%.