Acoustic Microresonator Research Articles

Multi-quartz-enhanced photoacoustic spectroscopy

A multi-quartz-enhanced photoacoustic spectroscopy (M-QEPAS) sensor system for trace gas detection is reported. Instead of a single quartz tuning fork (QTF) as used in QEPAS technique, a dual QTF sensor platform was adopted in M-QEPAS to increase the signal strength by the addition of the detected QEPAS signals. Water vapor was selected as the target analyte. M-QEPAS realized a 1.7 times signal enhancement as compared to the QEPAS method for the same operating conditions. A minimum detection limit of 23.9 ppmv was achieved for the M-QEPAS sensor, with a calculated normalized noise equivalent absorption coefficient of 5.95 × 10−8 cm−1W/√Hz. The M-QEPAS sensor performance can be further improved when more QTFs are employed or an acoustic micro-resonator architecture is used.

Multi-Quartz Enhanced Photoacoustic Spectroscopy with Different Acoustic Microresonator Configurations

Acoustic microresonators were added to the recently developed multi-QTF based QEPAS spectrophone to enhance the signal amplitude. Two kinds of “on-beam” configurations were experimentally investigated in detail. The developed multi-QTF based “on-beam” spectrophone had a signal enhancement of 1.6 times compared with the traditional single QTF based “on-beam” spectrophone, with the approximate noise level. A normalized noise equivalent absorption coefficient (1σ) of 1.24 × 10−9 W·cm−1·Hz−1/2was obtained for water vapor detection at atmospheric pressure.

Position effects of acoustic micro-resonator in quartz enhanced photoacoustic spectroscopy

The impact of acoustic micro-resonator (AmR) positions with respect to quartz tuning fork on signal amplitude, Q-factor and signal-to-noise ratio (SNR) of the quartz enhanced photoacoustic spectroscopy spectrophone was investigated. The replacement of the result plots’ abscissas makes the highest signal amplitude and the lowest Q-factor for different AmRs appear at the two absolute positions, respectively. These positions are independent on the AmR geometrical parameters, which facilitates the assembly of the spectrophone. The noncoincidence between the positions of the two extreme values results in a flat peak of the SNR curve, which is different from previously reported results. The spectrophone designs for three different applications are discussed in detail.

Fiber-Coupled Ozone Sensor Based on Tuning Fork-Enhanced Interferometric Photoacoustic Spectroscopy

In this paper, a completely fiber-coupled ozone sensor based on tuning fork (TF)-enhanced photoacoustic spectroscopy is presented. Crystalline silicon micro-TFs are processed and characterized. Their photoacoustically induced motion is monitored by a compact intensity interferometer. An acoustic microresonator for this new type of TFs is optimized. A frequency-doubled Nd:YAG laser is used as an excitation source. The sensor performance is assessed by calibration measurements. It reaches a detection limit of S = 2.13 ppm ± 0.02 ppm and a noise-equivalent absorption sensitivity of D = (1.27 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> ± 1.2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-11</sup> ) cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> W(Hz) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1/2</sup> .

T-shape microresonator-based high sensitivity quartz-enhanced photoacoustic spectroscopy sensor

A novel spectrophone sensor prototype consisting of a T-shaped acoustic microresonator (T-mR) in off-beam quartz-enhanced photoacoustic spectroscopy (T-mR QEPAS) is introduced for the first time. Its performance was evaluated and optimized through an acoustic model and experimental investigation via detection of water vapor in the atmosphere. The present work shows that the use of T-mR in QEPAS based sensor can improve the detection sensitivity by a factor of up to ~30, compared with that using only a bare QTF. This value is as high as that obtained in a conventional "on-beam" QEPAS, while keeping the advantages of "off-beam" QEPAS configuration: it is no longer necessary to couple excitation light beam through the narrow gap between the QTF prongs. In addition, the T-mR is really suitable for mass production with high precision.

Trace gas detection based on off-beam quartz enhanced photoacoustic spectroscopy: Optimization and performance evaluation

A gas sensor based on off-beam quartz enhanced photoacoustic spectroscopy was developed and optimized. Specifically, the length and diameter of the microresonator tube were optimized, and the outer tube shape is modified for enhancing the trace gas detection sensitivity. The impact of the distance between the quartz tuning fork and an acoustic microresonator on the sensor performance was experimentally investigated. The sensor performance was evaluated by determining the detection sensitivity to H(2)O vapor in ambient air at normal atmospheric pressure. A normalized noise equivalent absorption coefficient (1σ) of 6.2×10(-9) cm(-1) W/Hz(1/2) was achieved.

AlN films sputtered on iridium electrodes for bulk acoustic wave resonators

We report on the growth of highly c-axis-textured aluminum nitride (AlN) films sputtered on iridium layers for thin film bulk acoustic microresonator applications. Iridium films were evaporated on oxidized silicon covered with Ti or Mo/Ti seed layers to improve crystal quality and adhesion. The crystal quality of the AlN films as a function of the crystal structure of the iridium electrode and its pre-conditioning by Ar + bombardment was analyzed by X-ray diffraction, atomic force microscopy and wet chemical etching. Solidly mounted resonators using a single low-impedance layer of silicon dioxide for acoustic isolation were fabricated for a preliminary assessment of the piezoelectric activity of AlN films and the influence of the AlN/Ir stack on the performance of the devices. The electromechanical coupling factor of the AlN films was derived by fitting the electrical response of the resonators to Mason's physical model. AlN films exhibited very high coupling factors (7.5%) barely dependent on the width of the rocking-curve around the AlN 00·2 reflection. The high acoustic impedance of iridium electrodes provided resonators with quality factors higher than those of similar resonators built on lighter electrodes, such as molybdenum.

Distributed feedback guided surface acoustic wave microresonator

Surface acoustic wave resonators have been used in a number of applications: high-Q frequency filtering, very accurate frequency sources, etc. A major disadvantage of conventional resonators is their large dimensions, which makes them inadequate for integrated acoustics applications. In order to overcome these size limitations a new type of microresonator was designed, developed, and tested. In this paper, theoretical calculations and measurements on two kinds of such devices (a corrugated waveguide filter and a microresonator structure) are presented and their possible applications are discussed.