Third-harmonic generation of spatially structured light in a quasi-periodically poled crystal

Nonlinear optical processes of spatially structured light such as optical vortices and vector optical fields have exhibited many interesting physical effects and found a variety of important applications from classical to quantum regimes. However, high-harmonic generation of a vector optical field with a space-varying polarization state is still a challenge. Here we present a solution to realize third-harmonic generation of spatially structured light including vector optical fields in a nonlinear Sagnac interferometer containing only one carefully designed quasi-periodically poled potassium titanyl phosphate, for the first time, to our knowledge. The experimental results well confirm our theoretical predictions. Our results will enable to manipulate spatially structured light or photons carrying higher orbital angular momentum at new wavelengths. Our scheme has potential applications such as in optical skyrmions and may open up new opportunities to produce spatially structured entangled photons for quantum communication and computation.