Plasmon-enhanced second harmonic generation from DAST nanocrystals locally grown on a nanoporous gold substrate

The miniaturization of organic nonlinear optical crystals is critical for integrated photonics but remains fundamentally challenged by the drastic decline of nonlinear signals at sub-wavelength scales. While the surface plasmon resonance (SPR) of metallic nanostructures can concentrate local fields to enhance nonlinear processes, harnessing this effect effectively for organic crystals is nontrivial. Here, we exploit a three-dimensional nanoporous gold (NPG) substrate to form a hybrid composite with 4-N, N-dimethylamino-4'-N'-Methyl-stilbazolium tosylate (DAST). This NPG-DAST hybrid achieves a remarkable enhancement of second harmonic generation, with an absolute conversion efficiency of 1.22 × 10^-⁹ two orders of magnitude higher than the 1.63 × 10^-11obtained on silicon under identical excitation. This corresponds to a 75-fold intensity enhancement at 515 nm compared to DAST on flat silicon. This dramatic enhancement primarily originates from the dual-resonant plasmonic effect of the NPG substrate, where the localized SPRs are efficiently excited at both the fundamental (1030 nm) and the second-harmonic (515 nm) wavelengths, synergistically amplifying the local field and the nonlinear conversion process. This work provides a compelling strategy to overcome the fundamental limitation of signal reduction in miniaturized organic nonlinear optical materials, demonstrating significant potential for their application in micro- and nano-optoelectronic devices.