Metasurface-assisted phase-matching-free second harmonic generation in lithium niobate waveguides

TL;DR

This paper demonstrates a novel metasurface approach that enables phase-matching-free second harmonic generation in lithium niobate waveguides, simplifying nonlinear wavelength conversion and broadening operational bandwidth.

Contribution

The study introduces gradient metasurfaces with phased array antennas to bypass traditional phase-matching constraints in on-chip nonlinear processes.

Findings

Achieved efficient second harmonic generation over many coherence lengths.

Demonstrated wideband operation from 1580 to 1650 nm.

Observed 1660% W^-1cm^-2 conversion efficiency.

Abstract

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