All-optical polarization encoding and modulation by nonlinear interferometry at the nanoscale.

Optical metasurfaces allow complex light manipulation within subwavelength thicknesses and are thus rapidly emerging as a key enabling technology for nanophotonics applications. The control over light polarization already provided a route towards ultracompact metasurface-based polarimetry devices. If translated to the nonlinear optical regime it may become a transformative tool for nonlinear imaging, optical holography, and sensing. Here, we report ultrafast all-optical polarization modulation of upconverted light by all-dielectric metasurfaces via nonlinear interferometry. By controlling the relative phase between a pump beam at ω and its frequency-doubled replica at 2ω, we can set the phase relation between two frequency-degenerate upconversion processes at 3ω - sum-frequency generation and third-harmonic generation - stemming from an AlGaAs metasurface. By leveraging the opposite parity of the two nonlinear processes and adjusting their relative intensities, we achieve a modulation of the polarization state of the upconverted light between linear and circular states with a circular polarization degree of up to 83%. Remarkably, circularly polarized light of opposite handedness is symmetrically mapped in the Fourier space, at coincidence with the first diffraction orders of the metasurface. Furthermore, the handedness can be completely reversed within the same diffraction order by applying a phase delay of π. Our work adds an additional modulation layer beyond intensity to all-optical routing with precise phase control: polarization. The capability to encode and modulate simultaneously different polarization states in the k-space holds potential for chiral sensing and advanced imaging techniques.