Pockels

The Pockels effect is the linear electro-optic effect observed in certain non-centrosymmetric crystals, in which the refractive index changes in direct proportion to an applied electric field. This allows fast, voltage-controlled modulation of light without moving parts, making it central to many optical modulators and switching devices.

The effect arises from the electric-field–induced distortion of the crystal’s optical indicatrix. Mathematically, it is described by the relation Δ(1/n^2)_i = sum_j r_ij E_j, where E is the electric field and r_ij are the electro-optic coefficients of the material. The resulting birefringence Δn is typically small and depends on wavelength. The Pockels effect occurs only in crystals lacking a center of symmetry, such as lithium niobate (LiNbO3), potassium dihydrogen phosphate (KDP), and related materials.

Applications of the Pockels effect are widespread in photonics. The Pockels cell, a crystal-based electro-optic modulator, is used to control the polarization or intensity of light in lasers and optical communication systems. These devices support high-speed modulation, enabling gigahertz-range operation, and are used in pulse shaping, Q-switching, and fast optical switching. Material choice, crystal orientation, and electrode geometry determine the required drive voltage and modulation bandwidth for a given application.

Historically, the effect is named after Friedrich Carl Rudolph Pockels, who reported the phenomenon in the late 19th to early 20th century. It is also referred to as the linear electro-optic effect and remains a foundational principle in modern electro-optic technologies.