Nano-Optomechanical Systems (NOMS) for Gas Chromatography Sensing

Abstract

Nano-optomechanical systems (NOMS) achieve high-precision measurement of displacement which enables very high sensitivity through mechanical resonance-shift sensing. A recent breakthrough [1] has shown that NOMS devices can operate in high-damping environment without sacrificing their frequency stability and sensing resolution. This is because stability losses from lower quality factor (Q) are offset by stability gains from a larger intrinsic signal-to-noise ratio. We take advantage of this excellent stability to do atmospheric pressure resonant mechanical gas sensing with high sensitivity using NOMS [2]. In particular, we have set up a traditional gas chromatograph to output to a NOMS detector and shown parts-per-billion level detection [2]. By modifying the NOMS devices to make the silicon cantilevers porous, we have improved sensitivity by a further factor of 10 [3]. The next challenge, and opportunity, in using NOMS for mass sensing, is to separate the gas loading signals that affect both the mechanical and optical resonances in high optomechanical-coupling devices [4]. This is because, at high coupling, changes in optical resonance produce changes in mechanical resonance and vice versa. This effect can be used to amplify the sensing signal obtained when measuring mechanical frequency changes. In fact, it scales particularly well with high-finesse optical cavities and could lead to a situation where evanescent field gas loading on the optical cavity is transduced as a mechanical frequency shift with orders of magnitude better mass sensitivity than could be realized from the NEMS alone or the optical cavity alone. As an example, our present mass loading sensitivity is around 50 kDa [5]; improving the cavity linewidth merely by a factor of 10 should shrink that mass sensitivity by a factor of 1000, bringing it down to 50 Da. This level of sensitivity would allow studying chemical adsorption and desorption of individual gas chromatography molecules on the sensor surface and could provide a simple path for obtaining orthogonal information in gas chromatography detectors. [1] S. K. Roy, V. T. K. Sauer, J. N. Westwood-Bachman, A. Venkatasubramanian, and W. K. Hiebert, Improving mechanical sensor performance through larger damping. Science 360, eaar5220 (2018); doi: 10.1126/science.aar5220. [2] A. Venkatasubramanian, et al., Nano-optomechanical systems for gas chromatography. Nano Lett. 16, 6975-6981 (2016); doi: 10.1021/acs.nanolett.6b03066. [3] A. Venkatasubramanian, et al., Porous nanophotonic optomechanical beams for enhanced mass adsorption. ACS sensors 4, 1197-1202 (2018); doi: 10.1021/acssensors.8b01366. [4] M. P. Maksymowych, J. N. Westwood-Bachman, A. Venkatasubramanian, and W. K. Hiebert, Optomechanical spring enhanced mass sensing. Appl. Phys. Lett. 115, 101103 (2019); doi: 10.1063/1.5117159 [5] Da = Dalton = 1 amu = 1 atomic mass unit ~ 1.66 x 10-27 kg.