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    Gas sensing through evanescent coupling of spoof surface acoustic waves
    (Elsevier Science Sa, 2019) Cicek, Ahmet; Arslan, Yasin; Trak, Digdem; Okay, Fatih Can; Kaya, Olgun Adem; Korozlu, Nurettin; Ulug, Bulent
    An ultrasonic gas sensor based on evanescent coupling of spoof surface acoustic waves between two surface phononic crystals containing trapezoidal grooves on rigid slabs is theoretically and experimentally demonstrated. Sensing properties for carbon dioxide in dry air at 25 degrees C and 760 Torr are investigated as an example. Band structure analyses reveal two spoof surface acoustic wave bands with opposite parities when the separation of surface phononic crystals is 1.5 times the periodicity of grooves. The beat length varies with frequency and carbon dioxide volume fraction, where the increase of the latter results in red shift of a sharp intense output peak at 59.69 kHz at a rate of 17.70 mHz/ppm and 16.20 mHz/ppm for carbon dioxide volume fractions up to 10,000 ppm, as measured through Finite-Element Method simulations and experiments, respectively. Gas sensing can also be achieved by measuring the output acoustic intensity at constant frequency, which exhibits a steep decrease with carbon dioxide volume fraction up to 2000 ppm.
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    One-dimensional surface phononic crystal ring resonator and its application in gas sensing
    (Amer Inst Physics, 2019) Kaya, Olgun Adem; Korozlu, Nurettin; Trak, Digdem; Arslan, Yasin; Cicek, Ahmet
    We introduce a ring resonator, which employs a one-dimensional phononic crystal on its inner surface, and investigate its performance as a gas sensor both numerically and experimentally. Having periodic equilateral trapezoidal protrusions, the ring resonator with 207 periods is optimized through band structure calculations via the finite-element method. A surface band linear around 58kHz is observed. The resonator exhibits sharp transmission peaks with a broad free-spectral range of 0.54kHz. Accordingly, a peak at 58.49kHz with a high-quality factor of 8196 appears. Application in detection of the carbon dioxide level in air with high sensitivity is demonstrated. The 58.49kHz peak red shifts linearly at 17.3mHz/ppm and 17.8mHz/ppm rates, as obtained from numerical calculations and experiments, respectively. Besides, the peak shape and maximum intensity are preserved. Due to the linear shift of the resonance peak with respect to the carbon dioxide concentration, acoustic intensity at initial peak frequency can be utilized as an auxiliary means for concentrations up to 1000ppm. The proposed ring resonator can be adapted to a variety of acoustic devices such as liquid concentration sensors based on phononic crystals, surface acoustic wave sensors, and micromechanical resonators.
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    Ultrasonic Gas Sensing by Two-Dimensional Surface Phononic Crystal Ring Resonators
    (Amer Chemical Soc, 2019) Cicek, Ahmet; Trak, Digdem; Arslan, Yasin; Korozlu, Nurettin; Kaya, Olgun A.; Ulug, Bulent
    An acoustic ring resonator employing a two-dimensional surface phononic crystal is proposed for high-sensitivity detection in binary gas mixtures. Band analyses and frequency-domain simulations via the finite-element method reveal that a single band for spoof surface acoustic waves appears at ultrasonic frequencies around 58 kHz where modification of its dispersion due to varying gas composition results in a linear shift of the resonance frequency. The shift rate is -17.3 and 8.8 mHz/ppm for CO2 and CH4, respectively. The linear shift of resonance frequency is experimentally validated. In addition, the ring resonator can also be employed to track acoustic intensity variation with gas concentration, where exponentially decaying intensity for low concentrations leverages high-sensitivity operation.