Graded plasmonic metagratings for directional nonlinear optical sensing

Plasmon-enhanced sensing based on localized resonances conventionally relies on the variation of the mode frequency of a nano-resonator, owing to its dependence on the local dielectric environment. In this work, a novel approach to optical refractometric sensing is introduced, using second-harmonic emission from metagratings of plasmonic nanoantennas. Leveraging the angular shift of the nonlinear emission patterns induced by changes in the local refractive index, a directional sensing strategy complementary to plasmonic resonance-based detection is demonstrated. This approach is first assessed on periodic metagratings, where the effects of the resonance spectral shift and the variation of the emission pattern are disentangled through Fourier-space imaging. A graded aperiodic metagrating layout is then introduced to enable the direct measurement of the angular shift in the emission. In this configuration, a comparable performance between angular and spectroscopic sensing is obtained, highlighting the potential of emission directionality as an independent and synergistic mechanism for enhanced nonlinear optical sensing.