A low drift high resolution cryogenic null ellipsometer

A cryogenic ellipsometer capable of measuring submonolayers of liquid helium has been developed. Methods for controlling drift and noise created by the cryostat windows and ellipsometer optics are discussed in detail. The cryostat vacuum windows were made from SF-57 glass due to its low stress-optic coefficient. Custom low-stress vacuum window mounts were designed and all windows were temperature controlled. Placing the compensator wave plate inside the cryostat was necessary to avoid room temperature fluctuation induced noise. These steps produced 12 h drifts in the measured polarization of less than 0.002°. A helium adsorption isotherm taken at 1.5 K on gold is presented to show the high resolution obtained once noise and drift associated with window birefringence is minimized.