Lens-free laser nanopatterning of large-scale metal film areas with structured light for biosensing applications.

Pulsed laser nanotexturing of metal films represents an ultra-fast, high-performance and cost-effective processing technology for fabrication of various functional surfaces widely used in plasmonics, biosensing, and photovoltaics. However, this approach usually requires high-NA lenses to focus a laser beam onto a few-micron spot as well as a micropositioning platform to move this spot along the sample surface, which increases the cost of the produced functional surfaces and limits the performance of laser-assisted nanotexturing techniques. In this paper we report on a laser-assisted technology for the fabrication of large-scale nanotextured metal substrates. In our approach, speckle-modulated patterns obtained by passing nanosecond laser pulses through the simplest diffusive object were utilized to cover a thin gold film with closely packed micron-sized structures - nanojets, nanobumps and through holes - previously reported only for single-shot nanoablation with tightly focused laser beams. The presented easy-to-implement technology, being one of the simplest of ever reported, since it requires neither focusing lenses nor micropositioning platforms, was shown to provide a way to pattern millimeter-size areas with the nano-sized jets at an average recording density of 35∙10³ nanostructures per square millimeter and an average recording speed of 4.5·10³ nanostructures per pulse. The fabricated nanotextured Au substrates were shown to yield spatially uniform surface-enhanced fluorescence signals from the Rhodamine 6G organic dye with an averaged 5.3-fold enhancement factor as compared with non-treated Au surface.