Third-Harmonic Generation in Crumpled Silicon Nanomembranes

Silicon nanomembranes provide mechanically compliant devices for strain engineering. Here, we investigate third-harmonic generation in flat and crumpled silicon nanomembranes of 15 and 35 nm using polarization-resolved third-harmonic generation microscopy with ab initio calculations. In flat silicon nanomembranes, the measured polarization dependence is reproduced by the calculated third-order susceptibility, identifying the intrinsic nonlinear response of silicon. In crumpled silicon nanomembranes, local strain variations between valleys and apexes generate distinct polarization-dependent responses. Uniaxial strain along the [110] direction breaks electronic structure symmetry and induces anisotropy between X- and Y-polarized components, while the real and imaginary parts of the third-order susceptibility exhibit quadratic strain dependence. Morphology-induced redistribution of the local optical field further modifies the response in crumpled geometry, and a finite-difference time-domain-based local-field correction substantially improves agreement with the experiment. Therefore, the third-harmonic response of crumpled silicon nanomembranes is controlled by both strain-induced electronic structure modification and morphology-induced local optical-field redistribution.