Photo elastic modulator (PEM) is one the critical components in a phase modulated ellipsometer, which modulates phase and amplitude changes in detected signal, based on that Psi and Delta will be measured and n, k, d, and others accordingly in principle.
A PEM comprises of a photo elastic crystal and piezoelectric actuator, as shown in above figure. When a sinusoidal driving voltage is applied, actuator excites and maintains the mechanical vibration in the photo elastic crystal (stress standing wave), forming a periodic variation of birefringence. Therefore a PEM can be considered as a dynamic retarder with controllable retardation.
There are different types of polarization modulators, including Faraday rotator, electro-optic modulator, liquid crystal variable retarder, and PEM. Among them, PEMs have a high modulation frequency, a high modulation purity and efficiency, good retardation stability and a large acceptance angle and aperture, which provides accurate and flexible modulation of the polarization state of the incident light over a wide range of wavelengths, from vacuum ultraviolet (UV) (~l20 nm) to mid-infrared (mid-IR) (~16 μm). X-cut quartz crystal plate is chosen as the piezoelectric actuator. Photo elastic crystals are fused silica, CaF2, ZnSe, and mono crystalline silicon etc.
In the ellipsometer the PEM works at the frequency of 50KHz. The following is an image of a typical PEM.
Reference 1: www.hindsinstruments.com
Reference 2: Appl. Sci. 2019, 9, 341