Pockels effect

The Pockels effect or Pockels electro-optic effect, also known as the linear electro-optic effect, is named after Friedrich Carl Alwin Pockels who studied the effect in 1893. The Pockels effect is a directionally dependent linear variation in the refractive index of an optical medium that occurs in response to the application of an electric field. The non-linear counterpart, the Kerr effect, causes changes in the refractive index at a rate proportional to the square of the applied electric field. In optical media, the Pockels effect causes changes in birefringence that vary in proportion to the strength of the applied electric field. The Pockels effect occurs in crystals that lack inversion symmetry, such as KH2PO4 (KDP), KD2PO4 (KDP or DKDP), lithium niobate (LiNbO3), beta-barium borate (BBO), and in other non-centrosymmetric media such as electric-field poled polymers or glasses. The Pockels effect has been elucidated through extensive study of electro-optic properties in materials like KDP. The key component of a Pockels cell is a non-centrosymmetric single crystal with an optic axis whose refractive index is controlled by an external electric field. In other words, the Pockels effect is the basis of the operation of Pockels cells. By controlling the refractive index, the optical retardance of the crystal is altered so the polarization state of incident light beam is changed. Therefore, Pockels cells are utilized as voltage-controlled wave plates as well as other photonics applications. See applications below for uses. Pockels cells are divided into two configurations depending on the crystals' electro-optic properties: longitudinal and transverse. Longitudinal Pockels cells operate with electric field applied along the crystal optic axis or along incident beam propagation. Such crystals include KDP, KDP, and ADP. Electrodes are coated as transparent metal oxide films on crystal faces where the beam is propagating through or metal rings (usually made out of gold) coated around the crystal body.