In this paper, we report on sequential decreases in the amplitude of photoacoustic (PA) signals from nanosecond laser
pulse irradiation of various samples. These samples include biological tissues, such as dental-enamel and chicken/turkey
breast as well as some non-biological samples. Laser energy densities in the range of 80mJ/cm2 to 300mJ/cm2 were used
in our experiments, typical of those used in PA imaging regimes. Induced temperature rises are modelled to show that
the average temperature rise for each pulse in those biological tissues is less than one degree centigrade. Measurements
reveal a rapid decay of photoacoustic signals within the first few laser pulses absorbed by the sample and this decay is
irreversible in the short term. The phenomenon indicates that laser irradiation interacts with biological samples, causing
long-term physical changes that can be attributed to a reduction of optical absorption within the samples.
We describe the phenomenon of a sudden decrease in the amplitude of photoacoustic signals arising from nanosecond laser pulse irradiation of biological samples, measured in vitro. Several dental enamel and chicken/turkey breast samples are examined. Moderate optical energy densities (i.e., about 300 mJ/cm2) are used, typical of those exploited in photoacoustic investigations. Measurements show a rapid decay of photoacoustic signals within the first few laser pulses absorbed by the sample. This phenomenon indicates that laser irradiation interacts with biological samples, causing long-term physical changes that can be attributed to a reduction of optical absorption within the samples.
A theoretical model is presented for the study of the near field of light diffracted by ultrasound in an isotropic medium which becomes birefringent in the presence of an acoustic wave. Due to the interference with the sound wave, the diffracted light wave in its near field is seen to act as a quasi-monochromatic light wave which is partially polarized. The degree of coherence ins sen to coincide with the degree of polarization and can be controlled acoustically. Hence coherent laser light can be transformed into incoherent light by sending it through an isotropic material with acoustically induced birefringence. In the experimental part of the work, the non-diagonal elements of the diffracted light beam coherency matrix were measured and a good agreement with the theoretical model was obtained.
Light diffraction by ultrasonic waves has been studied for many years, however we still can not say that the phenomenon has been examined completely. During the last decade the near field investigations received a particular attention. The new results obtained, both in the near field and in the far field of ultrasonically difracted light, forced us not only to change our point of view concerning this phenomenon but also to modify the formulas describing it. A brief review is given of some of the results which have been obtained up until now, both theoretical and experimental ones. Some new theoretical suggestions, based on the N-th order approximation (NOA) method are presented.
Polarization effects that arise during diffraction of an arbitrarily polarized light by an acoustic wave in an anisotropic medium are studies both theoretically and experimentally. Peculiarities of these effects in the cases of isotropic and anisotropic scattering of the light are considered in detail. Results of polarization state measurements performed with a paratellurite acousto-optic cell in the zeroth order of diffraction are presented as well.
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