Differential Polarized Reflectance Spectroscopy at Air/Water and Air/Metal Interfaces

We report the direct measurement of differential polarized reflectance spectra for samples at air/water and air/metal interfaces. The central component for these measurements is a polarization-division interferometer (PDI). This interferometer uses an in-house-designed beamsplitter constructed in-house from a BaF₂ polarizer and a matching substrate. In conjunction with a linear polarizer in front of the source and two rooftop mirrors, one in each arm of the interferometer, the PDI divides the input beam into two orthogonal linear polarization components, recombines them for interference at the beamsplitter, and directs the output beam at 90° to the direction of the input beam. Collimated light rays exiting the interferometer are focused by an f/5 lens and bent from the horizontal propagating axis by a ZnSe wedge to give an angle of incidence of ∼75°, at the sample. The reflected rays are brought back to the horizontal propagation direction by another ZnSe wedge and focused onto the detector by an f/1 lens. The interferogram obtained in this manner represents the differential polarized reflectance interferogram whose cosine Fourier transform directly gives the differential polarized reflectance spectrum. Thus, these spectra were obtained without need for the commonly used photoelastic modulator. Representative spectra for monolayers on a water surface and for samples on a gold surface are presented.