The phase-matching condition is a key aspect in nonlinear wavelengthconversion processes, which requires the momenta of the photons involved in theprocesses to be conserved. Conventionally, nonlinear phase matching is achievedusing either birefringent or periodically poled nonlinear crystals, whichrequires careful dispersion engineering and are usually narrowband. In recentyears, metasurfaces consisting of densely packed arrays of optical antennashave been demonstrated to provide an effective optical momentum to bend lightin arbitrary ways. Here, we demonstrate that gradient metasurface structuresconsisting of phased array antennas are able to circumvent the phase-matchingrequirement in on-chip nonlinear wavelength conversion. We experimentallydemonstrate phase-matching-free second harmonic generation over many coherentlengths in thin film lithium niobate waveguides patterned with the gradientmetasurfaces. Efficient second-harmonic generation (1660% W-1cm-2) in themetasurface-based devices was observed over a wide range of pump wavelengths(1580-1650 nm).
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