ISOTROPIC AND ANISOTROPIC BULK CONTRIBUTIONS TO 2ND-HARMONIC GENERATION FROM SIMPLE METALS

Quantum-mechanical calculations are carried out for the non-linear cur rent in the bulk of simple metals. The energy bands and matrix element s for optical transitions are calculated using a two-band model which is known to give accurate results for the description of the linear di electric response of these systems. The frequency dependence of the is otropic and anisotropic components of the non-linear polarizability te nsor shows characteristic resonances due to various interband transiti ons in the optical region. In the adiabatic limit, excellent agreement is obtained with results derived within the Boltzmann equation approa ch. The emitted harmonic radiation is calculated to illustrate the inf luence of the non-linear volume current on the variation of the intens ity with polar and azimuthal angles of incidence. Using recent experim ental data for optical second-harmonic generation from Al(111) and the calculated bulk polarizability, predictions are made concerning the m agnitude and relative phase of the surface anisotropy.