All-optical spin-selective control and shaping of frequency conversion pathways
The interplay between optical spin and crystal symmetry provides a powerful mechanism to control light-matter interaction, enabling advances in imaging, holography, sensing, signal processing, and more. Here we reveal that this interplay can also be exploited as a programmable mechanism for selecting and shaping nonlinear frequency-conversion pathways. Using a z-cut lithium niobate thin film, we experimentally demonstrate spin-selective switching between complementary three-wave mixing pathways, where co- and cross-circular-polarization excitations activate sum- and difference-frequency generation, respectively. We further generalize this concept to establish a quantitative spin-to-frequency mapping, that encodes the pump spin composition into the generated frequencies, and enables the reverse retrieval of the pump spin state. Finally, we leverage this concept also to demonstrate mapping of spin patterns to a combination of frequency-dependent real space images, and Fourier-space diffraction patterns. These results pave the way for all-optical programable frequency-conversion and spatiotemporal nonlinear signal manipulation in free-space and integrated photonic systems.