All-Optical Polarization Encoding and Modulation by Nonlinear Interferometry at the Nanoscale

Metasurfaces have emerged as versatile, ultrathin platforms for light manipulation. Their rapidly evolving capabilities include precise control over light wavefronts and, in recent years, the ability to perform light-by-light manipulation. Nonlinear optical processes are among the key mechanisms enabling light modulation by light. We have recently showcased all-optical routing of telecom photons upconverted to the visible range through an all-dielectric metasurface [1]. We did so by leveraging the interference between two frequency-degenerate nonlinear optical processes - specifically third-harmonic and sum-frequency generation (THG and SFG) - excited by dual beam illumination at ω and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2\omega$</tex> (see a sketch of the experimental setup in Fig. 1a). Light routing with attosecond delay control is enabled by the unique interference mechanism caused by the different parity of the two processes, odd for THG and even for SFG, where nonlinear parity conversion occurs since it transforms two photons from the pump beam into one. The interference can be controlled with high accuracy at a frequency rate potentially close to the one corresponding to the optical cycle of the upconverted light, by the relative phase <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Delta \varphi=2 \omega \cdot \Delta \tau$</tex> (being <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Delta \tau$</tex> the temporal delay) between the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\omega$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2\omega$</tex> pump pulses.