Enhancing Guided Second-Harmonic Light in Lithium Niobate Nanowires

We experimentally demonstrate practical approaches to enhance second-harmonic (SH) generation in individual lithium niobate nanowires (NWs) with a sub-micrometer cross-section and length up to tens of micrometers. We establish that parametric interactions of guided modes propagating along the NW determine the SH output power, which can be therefore controlled by the NW length. We show that the SH power is increased by about 84 times at wavelengths corresponding to modal phase-matching. Importantly, at non-phase-matched wavelengths the SH power can be improved by a factor of up to 9.3 by adjusting the NW length with a focused ion beam. We also characterize SH emission directionality, which can be further tailored for applications in integrated optical circuits and nonlinear microscopy.