Ultra-Sensitive Surface Spectroscopy of Silica with a Miniature Optical Resonator
A C. Pipino
The surface properties of silica are of great importance in many areas of technology including microelectronics, chemical separations, catalysis, and biosensing. As key reactive sites, the surface hydroxyl or silanol groups (SiOH) largely control the surface chemistry of silica. Yet a monolayer of silanols produces an absorption signal at or below the detection limit of conventional absorption techniques, requiring use of silica gels, porous silica, or powders to enhanced surface area in spectroscopic studies. Further, the surface silanol absorption is often dwarfed by large bulk-OH absorption. Now, a new technology enables high sensitivity spectroscopy of an ultra-smooth silica surface in vacuum, gas flow streams, or liquids. The technology employs a miniature (<1 cm3), high-Q optical resonator fabricated from extremely low-OH fused silica with at least one ultra-smooth (RMS roughness<0.05 nm) total internal reflection (TIR) surface, where even second-order vibrational overtones and combination bands of the surface silanols and adsorbed species can be probed at sub-monolayer coverage.