Infrared second harmonic generation (IR-SHG) spectroscopy, an extension of spectroscopic SHG to the IR, is described and applied to the investigation of germanium-dielectric interfaces in the spectral region near the direct and indirect band gap of the bulk semiconductor. The spectrum of the Ge(111)-GeO2 interface, in the 1100-2000 nm fundamental wavelength range, is dominated by a resonance at 590 nm. This feature is assigned to the direct Gamma(25)>Gamma(2) transition between valence and conduction band states. Polarization and azimuth dependent IR-SHG spectroscopy revealed that the anisotropic contribution, containing bulk quadrupole, xi, and surface, partial derivative(11), nonlinear susceptibility terms, dominates the 590 nm resonance. S-termination of Ge(111) significantly modifies the interface nonlinear optical response. The IR-SHG spectrum of S-Ge(111) presents a new, possibly surface resonance at similar to565 nm, in addition to the resonance inherent to the bulk Ge at 590 nm, tentatively assigned to an interband transition of Ge atoms associated with the surface. (C) 2003 American Institute of Physics.