Abstract
Grüneisen parameter is a key temperature-dependent physical characteristic responsible for thermoelastic efficiency of materials. We propose a new methodology for accurate measurements of temperature dependence of Grüneisen parameter in optically absorbing solutions. We use two-dimensional optoacoustic (OA) imaging to improve accuracy of measurements. Our approach eliminates contribution of local optical fluence and absorbance. To validate the proposed methodology, we studied temperature dependence of aqueous cupric sulfate solutions in the range from 22 to 4 °C. Our results for the most diluted salt perfectly matched known temperature dependence for the Grüneisen parameter of water. We also found that Grüneisen-temperature relationship for cupric sulfate exhibits linear trend with respect to the concentration. In addition to accurate measurements of Grüneisen changes with temperature, the developed technique provides a basis for future high precision OA temperature monitoring in live tissues.
Highlights
Grüneisen parameter is one of the key material components contributing to optoacoustic phenomenon, which has a variety of applications in modern science, technology, biology, and medicine from extremely sensitive detection of molecular traces to sophisticated three dimensional imaging of biological tissues and even whole organisms [1,2,3,4,5,6]
Due to divergence of the laser illumination, the tubes experienced different fluence resulting in higher optoacoustic response of the top sample, which was situated closer to the optoacoustic probe
The difference in measured optoacoustic response was further amplified by increased acoustic diffraction and attenuation for the more distant sample
Summary
Grüneisen parameter is one of the key material components contributing to optoacoustic phenomenon, which has a variety of applications in modern science, technology, biology, and medicine from extremely sensitive detection of molecular traces to sophisticated three dimensional imaging of biological tissues and even whole organisms [1,2,3,4,5,6]. The optoacoustic effect is manifested when optical energy is rapidly deposited into an absorbing medium at rates faster than relaxation through thermal, acoustic, and other mechanisms [1,7]. It is achieved by illumination with a Q-switched laser with pulse duration on the order of 1100 ns. If the resultant stress (σ) has negligible shear component, than thermoelastic efficiency becomes a Grüneisen parameter of the medium (Γ) [1,7,8]. The amplitude of stress signals carried by travelling acoustic waves and measured by optoacoustic sensors is proportional to the Grüneisen parameter at the location of optoacoustic sources [9]:
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