Highly polarized optical harmonic generation into zero-order diffraction from gradient semiconductor metasurfaces

All-dielectric optical metasurfaces consist of two-dimensional arrangements of nanoresonators and are of great importance for shaping polarization, phase and amplitude of both linear and harmonic light waves. Up to date, their reported nonlinear optical properties have been dominated by the local features of the individual nanoresonators [1-3]. However, collective responses typical of either Mie-resonant metamaterials or photonic crystals can potentially boost the control over such optical properties. In this work, we demonstrate the generation of second harmonic with zero-order diffraction from quadratically nonlinear metasurfaces, made out of AlGaAs-on-AlOx nanocylinders with spatial period comparable with a pump wavelength in the optical telecom range. Upon normal incidence of the pump beam, we demonstrate that the modulation of Mie-resonances via Bragg scattering at both fundamental and second harmonic frequencies enables paraxial second harmonic light diffraction back into the zero order, based on effective optical properties. This zero-order second harmonic (SH) diffraction results in a 50-fold increase of the detected harmonic power within a solid angle of 5° (see Figure 1). Moreover, 90% of the incident polarization is converted in the SH cross-polarized component, in contrast to around 40% measured previously for isolated nano-antennas [4-7]. This is achieved by a stronger coupling of the incident light with the metasurface resonances at both fundamental and harmonic frequencies with respect to the original Mie-resonances of the isolated constituents. In addition, upon pumping a polarization-gradient χ(2) metasurface with varying incident polarizations, highly horizontal and diagonal linear polarized zero-order SH states can be achieved. Our results demonstrate that higher control of polarization, phase, and amplitude of the harmonic wavefront can be achieved in diffractive all-dielectric nonlinear metasurfaces, with applications for on-axis optical imaging.