a change in the static, or low-frequency, dielectric constant of a medium that occurs when the medium is exposed to electromagnetic radiation.
An electromagnetic field may change both the real part ε1 and the imaginary part ε2 of the complex dielectric constant. The change in ε2 is associated with photoconductivity. When a medium is exposed to electromagnetic radiation, ε1 is changed as a result of the transition of some of the atoms or molecules to excited states in which the polarizability of the atoms or molecules differs from their polarizability in the ground state.
In semiconductors, the photodielectric effect is observed even at relatively low radiation intensities, owing to optical excitation or optical charge exchange of impurity atoms. When impurity atoms are optically excited or undergo charge exchange, the effective radius of the atoms increases. As a result, the polarizability of the impurity atoms and the polarizability of the crystal as a whole increase.