GaN surface micro-nanostructures induced by femtosecond vector beams for high-sensitivity SERS applications

Abstract

Fabricating rich surface micro/nanostructures on gallium nitride (GaN)-based surface-enhanced Raman scattering (SERS) substrates represents a critical approach to boosting their detection performance. In this study, we propose a method of fabricating hierarchical micro-nanostructures on GaN surfaces using femtosecond vector light fields, while systematically assessing its potential applications in SERS. By modulating the polarization state of vector beams, diverse microstructures were induced on the GaN thin film surfaces. Additionally, by varying the overlap ratio of adjacent laser spots, we successfully fabricated three types of periodic surface structures, namely micropillar arrays, hybrid arrays, and micropit arrays. Experimental results reveal that depositing a 50 nm-thick silver film on these structures leads to the formation of abundant electromagnetic field hotspots, thereby significantly enhancing SERS performance. Among these structures, the hybrid structure prepared with a 15% spot overlap rate exhibits the optimal SERS enhancement effect, attributed to its richer nanoscale features. This structure achieves a detection limit of 10⁻¹¹ M for methylene blue (MB) solution, and an enhancement factor (EF) of 1.43 × 10⁹. Furthermore, the Ag/GaN SERS substrate demonstrated excellent signal uniformity and temporal stability. This work provides a new insight into the functionalizing GaN surfaces and holds substantial potential applications in biochemical sensing and optoelectronic devices.