Quasi-Bragg plasmon modes for highly efficient plasmon-enhanced second-harmonic generation at near-ultraviolet frequencies.

Boosting nonlinear frequency conversions with plasmonic nanostructures at near-ultraviolet (UV) frequencies remains a great challenge in nano-optics. Here we experimentally design and fabricate a plasmon-enhanced second-harmonic generation (PESHG) platform suitable for near-UV frequencies by integrating aluminum materials with grating configurations involved in structural heterogeneity. The SHG emission on the proposed platform can be amplified by up to three orders of magnitude with respect to unpatterned systems. Furthermore, the mechanism governing this amplification is identified as the occurrence of quasi-Bragg plasmon modes near second-harmonic wavelengths, such that a well-defined coherent interplay can be attained within the hot spot region and facilitate the efficient out-coupling of local second-harmonic lights to the far-field. Our work sheds light into the understanding of the role of grating-coupled surface plasmon resonances played in PESHG processes, and should pave an avenue toward UV nanosource and nonlinear metasurface applications.