Micro-optofluidic ring resonator structures for selective detection of organic vapors

A microfabricated optofluidic ring resonator (μOFRR) sensor for volatile organic compounds (VOC) is described. SiO_x optical resonators integrated into a microfluidic pathway were defined lithographically and fabricated via Si micromachining. Whispering gallery mode resonances with quality factors exceeding 10⁴ were excited in devices with diameters of 50-250 μm and wall thicknesses of 1.5-2 μm. Fluidic interconnections and structures for alignment of optical fiber waveguides were integrated into the larger μOFRRs. Devices coated internally with a thin PDMS layer exhibit shifts in resonant wavelengths during VOC exposure with sub-ng detection limits. In related work, selective VOC measurements obtained with plasmonic films of functionalized nanoparticles are described, with a goal of using them in place of polymer coatings in the μOFRR. A thin film of octanethiolate monolayer protected gold nanoparticles (MPN) was coated on a planar substrate and capped with a microfluidic cell. Reflected light from one of two single wavelength sources (783 nm and 488 nm) was measured during separate dynamic exposures to two VOCs. The decrease in reflected light intensity, due to film selling and changes in the local surface plasmon resonance band, was proportional to the VOC concentration. The ratio of responses at each wavelength provided a pattern unique to each VOC.