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

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.