Broadband second-harmonic generation in AlGaAs-on-insulator using subwavelength-grating metamaterials

Second-harmonic generation in integrated photonic waveguides is crucial for applications ranging from all-optical signal processing to quantum-state generation. While high-index-contrast waveguides can offer high conversion efficiencies due to tight modal confinement, their strong geometric dispersion typically results in narrow phase-matching bandwidths at a single wavelength. Here, we show that subwavelength gratings in aluminum gallium arsenide on insulator waveguides flatten this dispersion, merging two phase-matching points only 23.6 n m apart into a single broadband response with a 64 n m 3-dB bandwidth, over 100 × wider than that of a conventional strip waveguide on the same platform ( 0.6 n m ) . This bandwidth enhancement comes at the cost of efficiency, with the flat-top SWG design reaching 8.6 W − 1 c m − 2 , an order of magnitude below the 176.8 W − 1 c m − 2 of the narrowband strip design, for a 3 m m device including propagation loss. Furthermore, our design is thermally tunable over an unprecedented bandwidth of 170 n m for a 100 °C temperature change, compared with only 11.5 n m for the strip waveguide over the same range.