Maximizing the guided second-harmonic in lithium niobate nanowires

Nanowires made of a non-centrosymmetric crystal can both guide light and perform nonlinear wavelength conversion like second-harmonic generation. Such nanowires would help to develop various nanowire-based applications such as subwavelength microscopy [1] and nonlinear optical sources [2]. Strong nonlinearity and transparency in a broad wavelength range of the lithium niobate (LiNbO3) crystal makes LiNbO3 nanowires promising for this purpose. Indeed, the LiNbO3 nanowires have been shown to both generate and propagate the second-harmonic (SH) [3,4]. Moreover, we have even demonstrated fluorescent dye excitation with the propagated SH in LiNbO3 nanowires [4]. Nevertheless, it has not been studied where the propagated SH signal originates from. On one hand, the SH could be scattered into the nanowire at its input. On the other hand, the SH could be excited by the guided laser light inside the nanowire. In addition, the mechanisms to maximize this guided SH must be identified for further development of nanowire applications.