Waveguide Evanescent-Field Absorption Spectroscopy

The strong evanescent fields inherent in air-clad or polymer-clad sub-micron waveguides make these systems intriguing candidates for chemical sensing based on infrared absorption, fluorescence, or Raman scattering from adsorbed analyte species. We have recently shown that by coating ultra-thin silicon nitride waveguides (150 nm – 175 nm) with functionalized sorbent polymers, we can spectroscopically detect trace gases (such as simulants for CW agents) at concentrations as low as 30 ppb. Our measurements are presently based on near-infrared absorption spectroscopy at an overtone of the OH molecular resonance in sorbent materials. These weak absorption features can be probed with high fidelity using resonant waveguide cavities such as Fabry-Perot and microring resonators. Spectral features in the infrared absorption can be used to identify the toxicity of the analyte molecule, and perhaps even identify the molecule itself.

A SiN waveguide for evanescent field absorption spectroscopy. (Left): An SEM of a 150-nm thick SiN-on-SiO2 waveguide coated with a sorbent polymer. (Inset): A finite-element model of the waveguide, showing that 60% of the TM modal power is in the sorbent polymer. (Right): A SiN microring resonator coated with a sorbent polymer used to detect chemical vapor analytes.
A SiN waveguide for evanescent field absorption spectroscopy. (Left): An SEM of a 150-nm thick SiN-on-SiO2 waveguide coated with a sorbent polymer. (Inset): A finite-element model of the waveguide, showing that 60% of the TM modal power is in the sorbent polymer. (Right): A SiN microring resonator coated with a sorbent polymer used to detect chemical vapor analytes.

Micromechanical Photothermal Spectroscopy

A BCB-gold microbridge coated with a sorbent polymer. These structures are used to perform micromechanical photothermal spectroscopy of vapor-phase analytes.
A BCB-gold microbridge coated with a sorbent polymer. These structures are used to perform micromechanical photothermal spectroscopy of vapor-phase analytes.

Leading technologies for identifying trace gases, such as Gas Chromatography-Mass Spectrometry (GCMS), Ion Mobility Spectrometry (IMS) and Fourier Transform Infrared (FTIR) spectroscopy are quite effective in laboratory settings, but are difficult to miniaturize and field for low-cost sensing. We have invented a new method to spectroscopically detect trace gases using optical micromechanical systems coated with functionalized sorbent materials. This method, Micromechanical Photothermal Spectroscopy combines a tunable mid-infrared source, a bimaterial microbridge, interferometric displacement readout, and functionalized sorbent polymers to detect vapor-phase analytes. At infrared wavelengths that corresponds to rotational or vibrational molecular resonances of the sorbent material or adsorbed analyte, the microstructure absorbs that radiation and heats up. This heating results in bending of the bimaterial microbridge, which is read out optically. The presence of trace analyte vapors sorbed into the functionalized material changes this photothermal spectrum in unique and predictable ways.

A functionalized sorbent material placed on the micromechanical structure allows orders of magnitude more analyte molecules to be sorbed, enables reversible and long-term operation, and targets the device towards a particular class of analytes. The use of microcavity interferometry for readout in place of an optical lever-arm enables a much more compact design and an enhanced displacement sensitivity. Finally, we introduce the use of benzocyclobutene (BCB) as a movable MEMS material, which enables outstanding thermal isolation and robust fabrication.