Spatiotemporal Imaging of Waveguide Nonlinear Optics

Non-linear optical conversion is an essential aspect of modern photonic applications, and major efforts target the realization of efficient nonlinear processes in compact waveguides for miniaturization. Van der Waals structures, renowned for their pronounced light-matter interactions and nonlinear susceptibilities, have emerged as promising platforms to realize such nonlinear waveguides, but optimization requires precise knowledge of their linear and nonlinear optical properties, which are notoriously difficult to extract in highly anisotropic microstructures. I will describe an approach we developed to extract the optical properties and phase-matching conditions of nonlinear materials by directly imaging light propagation and harmonic conversion within van der Waals waveguides with extreme spatiotemporal resolution. Although it is generally assumed that waveguided light remains out the reach of far-field microscopy, our approach, based on far-field ultrafast microscopy, leverages strong light-matter interactions to track light fields even beyond the total internal reflection barrier, an unprecedented feat. We focus on slab waveguides of 3<i>R</i>-MoS2, which were recently found to exhibit highly efficient second harmonic generation. We show that spatiotemporal imaging of both fundamental and second harmonic waves provides several self-consistent methods to determine the phase-matching angle, mode profiles, harmonic generation efficiency and losses in nonlinear waveguides without any a priori knowledge of material properties. Our approach thus enables rapid identification of promising materials and optimization of waveguide structures for efficient harmonic generation and optical modulation in nanophotonic architectures.