Puoliaaltolevyjä

Puoliaaltolevyjä, often translated as half-wave plates, are optical components used to manipulate the polarization state of light. They are a type of wave plate, specifically designed to introduce a phase difference between two orthogonal polarization components of light as it passes through the material. This phase difference is approximately half a wavelength, hence the name "half-wave".

The operation of a puoliaaltolevy relies on birefringence, a property of certain materials where the refractive index depends on the polarization and direction of light. In a puoliaaltolevy, there are two principal axes, often referred to as the extraordinary and ordinary axes. Light polarized parallel to one axis travels at a different speed (and thus experiences a different refractive index) than light polarized parallel to the other axis.

When linearly polarized light strikes a puoliaaltolevy at an angle to its principal axes, it can be resolved into two components, one along each axis. As these components pass through the plate, they experience a slight delay relative to each other due to the birefringence. The thickness of the plate and the material's birefringence are precisely engineered so that this delay corresponds to half a wavelength.

Upon exiting the puoliaaltolevy, the two components recombine. Because of the half-wavelength phase shift, their relative phase is shifted by 180 degrees. This recombination effectively rotates the plane of polarization of the incident linearly polarized light. The angle of rotation is twice the angle between the incident polarization direction and the optic axis of the puoliaaltolevy. This makes them useful for controlling the orientation of polarized light.