A novel fabrication method for surface-enhanced Raman scattering (SERS) sensors that used a fast femtosecond (fs) laser scanning process to etch uniform patterns and structures on the endface of a fused silica optical fiber, which is then coated with a thin layer of silver through thermal evaporation is presented. A high quality SERS signal was detected on the patterned surface using a Rhodamine 6G (Rh6G) solution. The uniform SERS sensor built on the tip of the optical fiber tip was small, light weight, and could be especially useful in remote sensing applications.

A novel low-temperature SnO₂ gas sensor was prepared and studied on silicon nanostructures formed by femtosecond laser irradiation. By applying a bias voltage on the silicon substrate to alter the charge distribution on the surface of the SnO₂, carbon monoxide (CO), and ammonia (NH₃) gas can be distinguished by the same sensor at room temperature. The experimental results are explained with a mechanism that the sensor works at low temperature because of adsorption of gas molecules that trap electrons to the surface of the SnO₂.

Tin oxide (SnO₂) thin film gas sensors that function at room temperature have been fabricated on nanostructured substrates. After femtosecond laser irradiation of the surface of the SnO₂, the sensitivity to gases, for example, carbon monoxide, increased noticeably. The dependence of the sensitivity on the number of laser pulses has been investigated. It is believed that the femtosecond laser pulses generate defects in a thin layer on the SnO₂ sensor surface. These defects may result in a potential energy well creating surface bound states for electrons to move on the surface, which increases the sensitivity to gases.

SnO₂ thin film room-temperature gas sensors have been fabricated on silicon nanospike surfaces prepared by femtosecond pulsed laser irradiation. The surface morphologies of the as-fabricated silicon nanospikes and SnO₂ thin film gas sensors indicate that the surface roughness increased significantly after the SnO₂ layer was deposited. The surface morphology and electric field distribution of the silicon nanospikes were studied with atomic force microscopy (AFM) and the electric force microscopy (EFM), respectively. The comparison between AFM morphology and EFM images shows that the aspect ratio of the nanostructures in the EFM image was larger than that in the AFM image, which indicates that the nanospikes on the silicon surface can induce an enhanced electric field around their sharp features. The electric field around the tips is further enhanced when there is electric current flowing through the SnO2 layer. The enhanced electric field and increased surface area on the nanospike structures are the main contributors to the high sensitivity of these room temperature gas sensors.

SnO₂ gas sensors were fabricated on polyurethane (PU) polymer surfaces with nanospike structures. These nanospikes are replicated with a low-cost soft nanolithography method from silicon nanospike surfaces formed by femtosecond pulsed laser irradiation. The hydrophobicity of the sensing surface was enhanced by a monolayer coating of silane (1H,1H,2H,2H-perfluorooctyltrichlorosilane, PFOTS). The resulting self-cleaning behavior enabled sensing in environments with high moisture and heavy particulate content, while performing cleaning-in-place operations to prolong the lifetime of the sensors. Failure studies were performed to quantify the effects on the sensitivity of water washing. Contact angle measurements showed that the hydrophobicity was weakened after many cycles of droplet washing due to wear of the PFOTS film and/or damage of the nanoscale spike structure. It was also found that the baseline signal increased with droplet washing, while the sensitivity changed randomly within about 7.5%, so that the sensitivity of the gas sensor remained at a constant level after several thousand cycles of water washing.