Robust Plasmonic Structures Fabricated by Femtosecond Laser-Assisted Anisotropic Intaglio Engraving on Quartz for High-Sensitivity Surface Plasmon Resonance Sensing

Abstract

Fabrication of plasmonic structures on a gold film is a well-established approach to improve the performance of surface plasmon resonance (SPR) sensors. However, traditional fabrication techniques, such as photolithography, electron beam lithography (EBL), and focused ion beam (FIB) milling, often involve complex procedures and costly equipment. In this study, we present a mask-free, convenient method to fabricate robust plasmonic structures on the surface of z-cut α-quartz by combining femtosecond laser-induced modification with chemical etching for high-sensitivity Kretschmann configuration SPR sensing. We find that the etching process of laser-modified α-quartz in ammonium bifluoride (NH₄HF₂) solution proceeds in two distinct stages: isotropic etching and anisotropic etching. By comparing the etching morphology of the craters ablated by femtosecond laser with wavelengths of 400 and 800 nm, we observe that due to the lower threshold fluence and steeper crater profile, the 400 nm laser can induce more pronounced anisotropic etching, leading to sharper inverted pyramid structures. A 5-nm-thick Cr film and a 50-nm-thick Au film are then deposited on the patterned quartz surface, which is used to detect the refractive indices (RIs) of glycerol solutions as an SPR sensor. The inverted pyramid structures can enhance the localized electric fields, causing a red shift of the resonance peak and thereby improving the sensitivity of the SPR sensor. The sensor demonstrates a sensitivity of 4662.21 nm/RIU, achieving a 21.51% improvement compared with a traditional SPR sensor with a plain Au film under the same light incident angle. The refractive index (RI) resolution reaches 2.7 × 10^-5 RIU, and the figure of merit (FOM) is 85.36 RIU^-1. Femtosecond laser-assisted chemical etching offers an efficient and convenient method for fabricating plasmonic structures on α-quartz. The high-sensitivity SPR sensors developed through this approach demonstrate promising potential for applications in fields such as medical diagnostics, disease detection, and environmental monitoring.

Keywords: SPR sensor; anisotropic wet etching; femtosecond laser; plasmonic structures; α-quartz.