Polarization-ratio reflectance measurements in the extreme ultraviolet

We demonstrate a technique for determining optical constants of materials in the extreme UV from the ratio of p-polarized to s-polarized reflectance. The measurements are based on laser-generated high-order harmonics, which have easily rotatable linear polarization but that are prone to brightness fluctuations and systematic drifts during measurement. Rather than measure the absolute reflectance, we extract the optical constants of a material from the ratio of p-polarized to s-polarized reflectance at multiple incident angles. This has the advantage of dividing out long-term fluctuations and possible systematic errors. We show that the reflectance ratio is as sensitive as the absolute reflectance to material optical properties.