24 January 2011 Recent advances in resonant optothermalacoustic detection
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Abstract
Optothermal detection is a spectroscopic technique where the energy input into a gas or other media caused by absorption of optical radiation is measured directly by means of a thermal detector.1-3 A fraction of the absorbed energy is transported to the thermal detector by heat conduction or molecular diffusion. In this work a conventional thermal sensor was replaced by a quartz tuning fork (QTF), and the optical power input into the gas was modulated at the QTF resonant frequency. We call this approach "resonant optothermoacoustic detection", or ROTADE. The same experimental setup can be used to conduct a closely related technique, quartz enhanced photoacoustic spectroscopy (QEPAS).4 QEPAS relies on energy transfer from the initially excited molecular vibrational state to the translational degrees of freedom. In some cases this process is too slow to follow the modulation required for QEPAS. In other cases, the resonant energy transfer can result in vibrational excitation of nitrogen, which relaxes very slowly. ROTADE, on the other hand, detects the energy delivered by molecules even if this energy is still in the form of vibrational excitation. The molecules will then release their energy to the QTF upon collision with its surface. Experimental investigations of ROTADE and its comparison with QEPAS were performed in pure CO2 and 0.5% acetylene in N2 using near-infrared diode lasers. A fiber collimator and a refocusing lens were used to focus the laser to a ≈15 μm diameter waist. Its position was scanned in the QTF plane using a 3D translation stage with computer-controlled actuators. Different QTFs were used to compare the effect of modulation frequency on the ROTADE signal.
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James H. Doty, James H. Doty, Anatoliy A. Kosterev, Anatoliy A. Kosterev, Frank K. Tittel, Frank K. Tittel, } "Recent advances in resonant optothermalacoustic detection", Proc. SPIE 7945, Quantum Sensing and Nanophotonic Devices VIII, 79450Q (24 January 2011); doi: 10.1117/12.875105; https://doi.org/10.1117/12.875105
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