Evidence for a Considerable Nonlinear Dipole Absorption in Gallium Arsenide

A comparison is made of the infrared lattice absorption spectra of Ge and GaAs. For this purpose an approximate damping function is derived for GaAs. The comparison of the damping functions shows that both the nonlinear dipole moment and the anharmonicity should be important for the absorption of GaAs in the two-phonon and three-phonon summation bands. The matrix elements for these two mechanisms are found to have nearly the same order of magnitude. In the absorption coefficient and in the imaginary part of the dielectric constant, the anharmonicity in connection with the infrared-active lattice mode dominates at frequencies near the resonance frequency ${\omega }_0$ while the nonlinear dipole moment dominates in the three-phonon summation-band region where $(\frac{{\omega }^2}{\omega _0^{}{}_{}{}^2}-1)\gg 1$.