Micro OptoFluidic Ring Resonator for Micro Gas-Chromatographic Detectors

Kee Scholten, Xudong Fan, Edward T. Zellers

(a) SEM microimage of 100 um diameter mOFRR in etched placement channel (b) Raw data (blue) with Lorentzian fit (red) for mode centered at 984.83 nm for a 200 mm diameter mOFRR

This project explores a new generation of vapor sensitive micro-transducers based on optical whispering-gallery-mode (WGM) resonators. Optical resonators, as a class, operate by probing sorptive interfacial films for changes in the effective refractive index caused by reversible vapor interaction. This project concerns the fabrication and testing of the first microfabricated optofluidic ring resonator (µOFRR), which integrates the microfluidic and sensing functions in a single structure. The device is a cylindrical tube embedded in a Si frame, with a spherical expansion section to provide lateral confinement of optical modes (see SEM image above). Deep reactive ion etching is used to define a Si mold from which the SiOx µOFRR is grown by thermal oxidation. Devices with 50-200 µm diameters were fabricated with 2 µm thick walls on 9 mm2 chips. In experiments, light from a coherent source was coupled into the cavity evanescently; monitoring output intensity while varying wavelength produced a series of charactristic “peaks” with quality factors exceeding 104 (see plotted data above), and measurements of the free spectral range confirmed that the resonant behavior mimics that of planar ring resonators. Future work will focus on integrating microfluidic capillary connections and on-chip waveguides, as well as demonstrating repeatable vapor sensing. Toward that end, we have demonstrated the ability to deposit 100-nm thick interface layers of thiolate-monolayer protected gold nanoparticles on the inner surface of the mOFRRs, and plan to explore the ‘complementarity’ of the optical and electrical (resistive) response characteristic of these materials, with a goal of creating multi-transducer arrays with greater diversity of responses to organic vapor target analytes. This work is supported by the Department of Homeland Security, Science and Technology Directorate and by the National Institute of Health, Microfluidics in Biomedical Sciences Training Program.