Nanogratings fabricated by wet etching assisted femtosecond laser modification of silicon for surface plasmon resonance sensing

Surface plasmon resonance (SPR) sensors based on nanostructures have received much attention in recent years for constructing portable devices. A main barrier for their widespread use is that traditional nanofabrication methods are expensive and time-consuming. Here, we demonstrate a mask-free, low-cost and high-production method to fabricate large-scale uniform plasmonic gratings with variable depths. Regular amorphous silicon strips were firstly triggered on the silicon surface via scanning by a cylindrically focused femtosecond laser. Subsequently, the laser processed silicon was etched by alkaline to form highly regular gratings, whose depth is flexible to be adjusted by controlling the etching time. After depositing a 53 nm gold film, the fabricated plasmonic gratings are able to efficiently generate SPR absorption dips as observed in the reflection spectra. Moreover, a SPR sensor was built based on the optimal grating, which has a sensitivity of 707.4 nm/RIU, a figure of merit of 13.37 RIU−1 and a refractive index resolution of 5.3×10-4 RIU. The combination process of bovine serum albumin and the gold grating was also studied. The fabrication method introduced here paves the way for mass-production of high-quality plasmonic gratings. The SPR sensor can be potentially used in applications like medical diagnoses, biotechnology, security and food safety.