Pan Species Chemical Agent Detector Through Optomechanics of Surface Acoustic Waves

Andrew Oswald

Abstract


Present commercial and military methods of chemical detection are highly sensitized, some yield many false positives, and most require specialized training – all of which makes them impractical as deployable chemical agent detectors. Our analytical and empirical models show the possibility of detecting various pollutants in the environment by observing frequency shifts of the Brillouin acoustic mode excited on spherical or toroidal whispering gallery resonators (WGR) pumped by a telecom-wavelength evanescent field surrounding a length of tapered fiber optic cable. Due to photon-phonon interplay, the WGR experiences electrostriction, which yields both a Stokes line and Brillouin surface acoustic wave (SAW).  The SAW propagates at the air-WGR interface at the speed of sound, which is determined as a weighted average of the speed of sound in the “air” material and the WGR material.  Via conservation of momentum, in order to maintain resonance, the wavelength of the SAW cannot change, so a change in the speed of sound of the SAW is manifested as a change in frequency of the SAW.  Therefore, a changed SAW frequency indicates that the materials through which the SAW propagates has changed.  In our case, the WGR is composed of silica glass with a constant speed of sound, so a rapid shift in SAW frequency is most likely a function of a change in the “air” surrounding the WGR.  Our models show it likely that we can determine the chemical agent based upon this shift in SAW frequency, and we have developed formulae based upon material properties to determine the species of chemical agent present.  Here we will present our analytical and empirical methods, measurements, and results in the detection of a variety of chemical agent species.


Keywords


optomechanics, surface acoustic waves, whispering gallery, brillouin scattering

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