QWP

QWP stands for Quarter-Wave Plate, an optical device that introduces a phase delay of one quarter of a wavelength between the two orthogonal polarization components of light. It is a type of birefringent retarder. When linearly polarized light enters a QWP with its polarization at 45 degrees to the plate's fast and slow optical axes, the emerging light becomes circularly polarized at the design wavelength; otherwise, the output polarization is modified according to the input orientation and the retardance.

Construction and design rely on birefringent materials. QWPs are commonly made from crystals such as quartz, calcite, or other birefringent substrates, and sometimes from polymer films. The plate thickness is chosen so that the retardance between the fast and slow axes is exactly λ/4 at the target wavelength. For broader spectral use, achromatic quarter-wave plates employ multiple layers or combinations of materials to maintain approximately 90-degree retardation over a range of wavelengths, though performance remains best near the design wavelength.

Applications include preparing or analyzing polarization states in optical experiments, polarimetry, and ellipsometry. In quantum optics, QWPs are used to implement polarization-qubit operations. They are commonly paired with linear polarizers or other waveplates in setups that require controlled rotation of polarization, conversion between linear and circular states, or measurement of Stokes parameters.

Limitations and considerations include wavelength dependence, as retardance departs from 90 degrees away from the design wavelength. Temperature fluctuations, mechanical stress, and imperfect alignment of the fast axis can degrade performance. Accurate orientation and calibration are essential for precise polarization control.