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Laser-photoacoustic and photothermal spectroscopy in thermophysical sensing applications


M. Sigrist1 and J. Rey1

1ETH Zürich, Switzerland

Keywords: laser photoacoustic spectroscopy, diffuse reflection
property: optical, thermal
material: gases, turbid media

Photoacoustic and photothermal spectroscopy are versatile techniques for the analysis of optical and thermal properties of media. Both methods have been widely used for gas sensing, nondestructive evaluation, medical diagnostics, etc., in various kinds of samples including biological tissue. The potential of these methods will be illustrated with several examples. In particular the photoacoustic identification and quantification of trace gases in multi-component gas mixtures in the ppm to sub-ppb concentration range [1] has attracted great interest as it represents a key issue for applications in ambient air monitoring, in industrial process monitoring, in homeland security or in breath monitoring for medical diagnosis. Also mobile, fully automated systems have been successfully applied for continuous sensing. Furthermore, a low-cost LED-based photoacoustic gas sensor has recently been introduced [2]. It is based on the simultaneous excitation of two acoustic resonances in a specially designed photoacoustic gas cell.

A further approach is related to scattering of radiation in samples which is of particular interest for studying turbid media with applications in biology, medicine, physics, and industry. The spatial distribution of diffuse reflected or transmitted light in an irradiated scattering medium represents a new tool for material investigations like temperature-driven structural changes, i.e. phase transitions, or for investigations in biological tissue in view of non-invasive medical diagnosis. Recently we performed studies on temperature-dependent scattering in different solids in the near-infrared. The samples were irradiated with light from a diode laser at 785 nm and two closely placed fibers in contact with the irradiated sample surface were used to detect the scattered light. The temperature dependence of the scattering was studied by heating the samples either with resistive heaters or, spatially very confined, with a focused beam of a CO2 laser. Temperature-dependent scattering was observed in samples like polymers, powders and natural fibers. The rather different results from the various samples will be discussed. The minimum detectable temperature variation obtained with the proposed setup is currently 0.1 K. The perspectives of this new tool for future studies will be outlined.

References
  1. M.W. Sigrist, R. Bartlome, D. Marinov, J.M. Rey, D.E. Vogler, H. Wächter, Appl. Phys. B 90, 289 (2008)

  2. J.M. Rey, C. Romer, M. Gianella, M.W. Sigrist, Appl. Phys. B 100, 189 (2010)

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