Photoelastic modulator polarization switching

Steiner et al. [14] have reported the measurement of maps of octadecanephos-phonic acid (OPA) molecules deposited on a microstructured aluminum oxide/gold surface using an PEL source that was modulated by a rapid-scanning interferometer. The beam was then passed through a wire-grid polarizer and a photoelastic modulator so that the polarization was switched at a rate of 75 kHz. 

In this method, a polarizer and a photoelastic modulator (PEM) are used to switch alternately between two linearly polarized infrared beams, the planes of polarization of which are orthogonal to each other. A PEM used in the mid-infrared region consists of an infrared-transparent isotropic crystal (usually ZnSe) and a quartz oscillator. The latter is used to make the ZnSe crystal anisotropic by applying pressure to the ZnSe crystal in one direction (called here the u axis), the anisotropic crystal shows different refractive indices in different planes of polarization. 

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