Strain-designed strategy to induce and enhance second-harmonic generation in centrosymmetric and noncentrosymmetric materials

Second-harmonic generation is described by the second-order nonlinear susceptibility χ⁽²⁾ which, in the electric-dipole approximation, requires a noncentrosymmetric medium. It is very challenging and of high technological interest to search whether it is possible to find a way to break inversion symmetry in centrosymmetric crystals in order to induce second-order nonlinearities. A new intriguing way to observe second-order nonlinear phenomena is strain. Here, we present a detailed analysis of the correlation between the strain and the χ⁽²⁾ in both centrosymmetric and noncentrosymmetric materials. We considered Si and SiC as test materials and we studied different types of strain (tensile/compressive), in different directions (uniaxial/biaxial) and for different light-polarization directions. We found which is the type of strain necessary in order to induce, tune, and enhance second-harmonic generation in different energy regions for centrosymmetric and noncentrosymmetric materials.