A2 – Control of tailored nonlinear optical excitations in functionalized 2D heterostructures

This project aims at controlling tailored nonlinear optical properties (e.g. excited-state dynamics, sum frequency generation, two-photon absorption) of functionalized 2D heterostructures of transition metal dichalcogenides (TMDCs), as MoX₂ and WX₂ (X=S, Se, Te) with their alloys, combined among themselves or with graphene and BN. This goal will be pursued by developing and applying theoretical approaches for the simulation of nonlinear excitations induced by moderately strong lasers in quasi-2D systems.

Spatial confinement is common in both optics and quantum mechanics, but it usually happens on very different scales for photons and electrons. In particular, quantum mechanical confinement of electrons is only observed at the nanometer scale, where it is usually difficult to achieve confinement of light. Atomically thin films can establish significant exceptions of that rule, as enhanced light-matter interaction allows for the simultaneous confinement of both electrons and light. These systems have enormous potential for ultra-small and low-power optoelectronic applications. In the first funding period, project A2|Botti/Peschel focused on the real-time description of nonlinear processes in crystals. We developed tools to extract nonlinear optical coefficients from real-time simulations using time-dependent density functional theory, designed exchange-correlation functionals for accurate band diagrams at interfaces and surfaces, modeled electromagnetic fields in TMDCs with surrounding nanostructures. Inspired by the gained insight, in the second period we will focus on nonlinear optics in heterostructures decorated with molecules. We will play with "twist" angles between layers, doping, defects, disorder and reversible external control, such as strain and electric fields, to investigate how atomic-scale modification and functionalization manifest themselves in the nonlinear optical properties. We will tackle this problem from two complementary sides, combining accurate methods of quantum chemistry for finite systems with advanced approaches for periodic materials, producing in this way a large amount of data on nonlinear optical properties. To cope with the increasing complexity of the system, we will accelerate calculations and identify patterns in the data with reliable machine learning predictions.