Second-harmonic generation in lithium niobate nanowires for local fluorescence excitation

Abstract

We study the nonlinear optical properties of lithium niobate (LiNbO₃) nanowires (NWs) fabricated by a top-down ion beam enhanced etching method. First, we demonstrate generation and propagation of the second-harmonic (SH) light in LiNbO₃ NWs of typical rectangular cross-sections of 400 x 600 nm² and length from 10 to 50 μm. Then, we show local fluorescent excitation of 4',6-diamidino-2-phenylindole (DAPI) dye with the propagated SH signal in standard concentrations as for biological applications. By measuring the detected average power of the propagated fundamental harmonic (FH) and the SH signal at the output of the NWs, we directly prove the dominating role of the SH signal over possible two-photon excitation processes with the FH in the DAPI dye. We estimate that 63 ± 6 pW of the propagated SH average power is required for detectable dye excitation. Finally, we model the waveguiding of the SH light to determine the smallest NW cross-section (around 40x60 nm²) which is potentially able to excite fluorescence with a FH intensity below the cell damage threshold.