On-Fly Femtosecond-Laser Fabrication of Self-Organized Plasmonic Nanotextures for Chemo- and Biosensing Applications

Surface-enhanced Raman scattering (SERS) and surface-enhanced photoluminescence (SEPL) are emerging as versatile widespread methods for biological, chemical, and physical characterization in close proximity of nano structured surfaces of plasmonic materials. Meanwhile, single-step, facile, cheap, and green technologies for large-scale fabrication of efficient SERS or SEPL substrates, routinely demonstrating both broad plasmonic response and high enhancement characteristics, are still missing. In this research, single-pulse spallative micron-size craters in a thick Ag film with their internal nanotexture in the form of nanosized tips are for the first time shown to demonstrate strong polarization dependent enhancement of SEPL and SERS responses from a nanometer-thick covering Rhodamine 6G layer with average enhancement factors of 40 and 2 x 10(6), respectively. Additionally, the first detailed experimental study is reported for physical processes, underlying the formation mechanisms of ablative nanotextures on such thick metal films. Such mechanisms demonstrate a complex hybrid fluence-dependent ablation character-appearance of spallative craters, typical for bulk material, at low fluences and formation of upright standing nanotips (frozen nanojets), usually associated with thin-film ablation, in the crater centers at higher fluences. Moreover, special emphasis was made on the possibility to reshape the nanotopography of such spallative craters through multipulse laser-induced merging of their small nanotips into larger ones. The presented approach holds promise to be one of the cheapest and easiest-to-implement ways to mass-fabricate various efficient spallation-nanotextured single-element plasmonic substrates for routine chemo- and biosensing, using MHz-repetition-rate femtosecond fiber laser sources with multiplexed laser-beams.