Fig. 4: Second harmonic generation enhancement.

a SHG spectra of the Au–LiNbO₃ hybrid particles as the FW is tuned from 800 nm (violet) to 960 nm (red). b Power dependence of SHG at 800 nm. The power scattered at the SH (blue dots) is proportional to the input laser power at the FW to the power b = 1.99 (95% confidence interval of ±0.02, see fit [black line]) times a normalization constant C. c Enhancement of the scattered SHG power as a function of the fundamental wavelength for an individual hybrid Au–LiNbO₃ nanostructure recorded by sweeping the excitation laser wavelength from 850 to 1070 nm with a step size of 5 nm. The experimental results (blue, red) are overlaid by the modeled near-field coupling enhancements (black). The plotted curves are shifted phenomenologically by 10–25 nm from the solutions of Eq. (1) to align with the experiment, as is highlighted in the “Discussion”. Inset: response functions (SI, Section S.2.4.2) of the narrow Mie resonances of a bare LiNbO₃ sphere as a function of the sphere radius and photon energy. The response function maxima (gold/white) lie at the same resonance energies ω_β as the SHG enhancement peaks, implying a red (blue) shift in the narrow SHG enhancement features with increasing (decreasing) a₂. The white dots indicate the shifted peak positions shown in the main panel. Further details are available in the SI. d A diagram of the pathways for energy transfer of light within the system. Clockwise from left: incident light (dark red), SHG (blue arrows) through energy transfer from nonlinear polarization (blue vector field) to Mie resonances (halo), superradiant Purcell-enhanced SHG, and near-field enhanced SHG