Waveplate

Waveplates (or retardation plates) are light polarisation devices that introduce different phase retardations for different polarisation states. They are made up of a birefringent material, an anisotropic, dual refractive index material (usually a crystal of some sort) with two axes: a slow ($n_s$) and fast ($n_f$) axis. The refractive index depends on the direction that light travels through the material.

The core idea of waveplates is to modify the polarisation state of light. We achieve this with two common types of waveplates:

• Quarter waveplates (QWP) introduce a phase shift of $\frac{\pi }{2}$ between the fast and slow axes. We use them to convert from linear polarisation to circular or elliptical polarisation and vice versa.
  • This only holds if the input isn’t already aligned with one axis already, i.e., the input isn’t an eigenstate.
• Half waveplates (HWP) introduce a phase shift of $\pi$. They rotate linear polarisation by $2\theta$, where $\theta$ is the angle between the input polarisation and the optical axis.