A complex of solid-state physics problems concerned with symmetry, morphology, electronic changes under strain and their modification in the presence of surface and interface, is an important basic field of studies [1-4]. The correlation between the phenomena and nonlinear optical response of solids, their surfaces and buried interfaces is advanced area of studies during recent years and the main subject of the proposed paper. Crucial changes are expected in the second-harmonic generation (SHG) under external deformation due to its high sensitivity to the crystal symmetry, charge density and defects. The question of the separation of the DC-electric field-induced SHG (EFISH) and strain-induced SHG at silicon surface is still open. In this work, the intensity of the second-harmonic generation reflected from a natively oxidized silicon plate in p-in, p-out polarization combination is measured as a function of the magnitude of the applied strain. The strain-induced and electricfield induced SHG components are separated by using cylinder deformation geometry. A femtosecond Ti:Sapp laser tunable from 710 to 800 nm is used as a source of the fundamental radiation. The strain at the surface is achieved by pressing the back side of the 0.5 mm-thick (001) silicon plate with a metallic sphere or cylinder on a micrometric screw. The plate is bended by a machine with a micrometric accuracy, and probed area position fixed. Figure 1 shows the set of second-harmonic intensity dependences on the deformation magnitude measured for several fundamental photon energies under spherical deformation.figureFig. 1. Second-harmonic intensity as a function of the depth of the spherical plate deformation for a set of the fundamentalphoton energies.Fig.2 Second-harmonic intensity as a function of time with point-by-point modulation of the cylindrical strain from zero to maximum. (a) s-in, p-out polarization combination, cylinder axis is in the incidence plane (H-geometry); (b) s-in, p-out, cylinder axis is perpendicular to the incidence plane (Vgeometry); (c) p-in, p-out, H-geometry; (d) p-in, p-out, V-geometryThe upper limit of the bending axis is close to the crack threshold. Strong, mostly linear variations of the SH intensity are observed. The results demonstrate strong influence of the strain at the silicon surface on the quadratic nonlinear-optical response and its' spectrum. The curve change its' behavior while the fundamental wavelength is tuned through the resonance at 3.4 eV of combined density of states in silicon. This can be caused by modification of the crystal cell geometry and conduction and valence band shifts. Then, by modification of the charge distribution both in the crystal itself and in the dioxide traps. To clarify the nature of the effect and to separate strain and charge (EFISH) components, we change the deformation geometry to cylinder one to have one-dimensional strain at the interface. The strain effect appeared to be smaller and was measured in the modulation mode as shown in Fig 2. The variations of the sign, modulation amplitude for the different geometries confirms the anisotropic nature of the contribution observed. The EFISH contribution is expected to be in-plane isotropic, so the intensity variations with the deformation modulation can be attributed to the influence of strain via strain-induced modification of the electronic band structure.