Nonlocal nonlinear optical response of solids

2026-07-22

Understanding the light-matter interactions taking place inside a crystalline solid is a challenging task, but of great significance both for fundamental science and technological applications. When laser light shines on a crystal, light waves and electric currents interact and give rise to a multitude of interesting and relevant nonlinear optical (NLO) effects: harmonic generation, current injection, the Kerr effect, and many more.1 Their study forms a rich and broad subject that sits at the intersection of nonlinear optics and condensed matter physics. Most of these effects are captured by a perturbative expansion of the current density on the optical fields. The coefficients of the expansion are the linear, second, third and higher order conductivities. They are in general functions of time and space (or frequency and momentum) and, for light intensities that are not too extreme, encapsulate the entire optical response of the material. Indeed, the nonlinear conductivities provide a unifying framework for understanding the aforementioned nonlinear phenomena, with each nonlinear effect being ascribed to a particular frequency component of a conductivity. Material-specific calculations of the nonlinear conductivity remain difficult, however, mostly relying on the electric-dipole (infinite wavelength) approximation, whereby the spatial dependence of the conductivity is neglected (local response) on account of the approximate uniformity of the optical fields at the scale of the unit cell. This approximation leaves out important magneto-optical and magnetoelectric effects, as well as the possibility of second order nonlinearities in centrosymmetric materials. The proposed project (NL2Optic) aims to overcome this limitation by developing and applying a theory of the nonlocal nonlinear optical response of bulk crystalline solids. The proposal merges perturbative methods introduced by the applicant and a well established ab initio approach developed by the host.

Spain

2026-01-01