Nonlinear Absorption of Terahertz Radiation by Free Charge Carriers in Polar Semiconductors

An analytical formula is obtained for the absorption coefficient of intense terahertz radiation by free electrons in a semiconductor when the main mechanism of electron scattering is spontaneous emission of polar optical phonons. The absorption coefficient is shown to increase sharply when the average energy of electron oscillations in the terahertz field exceeds the optical phonon energy.