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We calculate the second-harmonic-generation spectra of Si͑100͒ and ͑111͒ surfaces introducing the nonlinear surface local-field effect. Our model consists of four interpenetrated fcc lattices of polarizable bonds, each of which is centrosymmetric, but responds nonlinearly to the spatial inhomogeneities of the polarizing local field. The gradient of the field induced at a bond due to the dipole moment of a neighbor leads to a secondorder polarization that is canceled out in the bulk after summing over all other bonds, but it is not compensated at the surface, where it leads to a large nonlinear macroscopic response. Our model parameters are fitted to the nonlinear anisotropy measured at 1.17 and 2.34 eV. The linear anisotropy spectra calculated for the Si͑110͒ surface are in accordance with reflectance difference measurements. The same parameters yield a nonlinear spectrum that has peaks at 1.65 eV for a strained ͑100͒ surface and at 1.75 eV for a ͑111͒ surface, in agreement with recent experimental results.

We develop a hydrodynamic model for the calculation of sum and difference frequency generation ͑SFG/DFG͒ at the surface of nonlocal conductors with arbitrary equilibrium electronic density profiles n 0 . We apply our model to simple profiles and calculate the nonlinear surface susceptibility tensor zzz s ( 1 , 2 ) and the radiated efficiency R( 3 ϭ 1 Ϯ 2 ) as a function of the pump frequencies 1 and 2 . R is strongly enhanced due to the excitation of the dipolar surface plasmon characterized by a resonant frequency d ; it displays ridges whenever 1 , 2 , or 3 Ϸ d , an additional ridge at the bulk plasma frequency 3 Ϸ b , and very large double resonance peaks whenever two ridges cross each other. These results suggest that SFG/DFG spectroscopy might be a useful probe of surface collective modes.

We present a model for studying the adsorption of Sb atoms on the clean GaAs(110) surface and analyse the consequences of the adsorption on the reflectance anisotropy spectroscopy (RAS). A 1 × 8 unit cell is taken as a basis of the Sb-covered GaAs(110) structure. We allow the Sb-covered surface to be disordered by letting every surface atom move freely around its equilibrium position. In order to obtain a representative RAS spectrum of the surface we generate an ensemble with N different structural realizations of the surface and the ensemble RAS average is performed.