Polarization modulation can be used to obtain infrared (IR) spectra of any sample which absorbs light of two orthogonal polarizations to different extents. Examples of applications include vibrational circular dichroism (VCD), infrared reflection-absorption spectroscopy (IRRAS), infrared linear dichroism of statically-oriented samples, and time-dependent dynamic infrared linear dichroism (DIRLD) of samples under small-amplitude oscillatory strain. Modulation spectroscopy can be performed on either Fourier transform or dispersive IR spectrometers. A photoelastic modulator (PEM) placed in a linearly-polarized beam of light can alternate the state of polarization at frequencies on the order of 50 kHz. This enables a small difference signal to be obtained in a single measurement without having to subtract or ratio two separately-measured large signals. The principles of polarization modulation are developed and examples of VCD, IRRAS, and DIRLD presented.
A. E. Dowrey,
"Modulation Spectroscopy", Proc. SPIE 0553, Fourier and Computerized Infrared Spectroscopy, (20 December 1985); doi: 10.1117/12.970717; https://doi.org/10.1117/12.970717