Birefringent materials, characterized by their direction-dependent indices of refraction, are powerful tools for manipulating light’s polarization. Held one way, a birefringent crystal serves as a polarization analyzer that separates a light beam into orthogonally polarized components—or puts those components back together again. Held another way, the crystal becomes a wave plate that retards one polarization component with respect to the other. As figure 1 shows, the sum of two suitably phased linearly polarized waves is circularly polarized. So with the right arrangement of birefringent components, one can engineer light with linear or circular polarization or any of the continuum of elliptical polarization states in between.
Circularly polarized light is important because of its capacity to distinguish between the mirror-image forms of molecules that are asymmetric, or chiral—a category that includes almost all biomolecules and many of the substances that interact with them (see Physics Today, July 2018, page 14...
