Mapping near-field of the guided second-harmonic in lithium niobate nanowaveguides with photosensitive polymer

Nanowaveguides (NWs) are valuable components for building miniaturized optical devices since they have both nanoscale and microscale dimensions. One can further extend the NW application range by exploiting nonlinear optical effects such as second-harmonic (SH) and sum-frequency (SF) generation. Lithium niobate (LiNbO
₃
) NWs and nanopillars are demonstrated to generate and guide S and SF signals. However, to develop nonlinear NW applications, one should know behaviour of the guided light inside the NWs. This information can be obtained by scanning near-field optical microscopy, which requires complex equipment. As an alternative, one can also use a photopolymerization technique. This technique is based on a negative photosensitive polymer, which crosslinks when irradiated by light and, thus, maps the distribution of the electric field. Previously, this technique was applied to image plasmonic standing-waves in metallic NWs through two-photon absorption (TPA). Here, we apply this technique to study the guided SH signal in LiNbO
₃
NWs and map its behaviour at the NW output and along the NW, which may help to maximize the output power of the nonlinear signal.