We present a physical model that explains several sequential stages of the conversion of optical to acoustical energy when irradiating diluted suspensions of metal nanoparticles with laser pulses. Optical absorption and scattering of a single particle driven by plasmon resonance interactions in an aqueous medium are considered. Thermal effects produced by laser-irradiated nanoparticles, dynamics of vapor bubble formation, and acoustic signals from expanding bubbles formed around heated nanoparticles are calculated. Stochastic features of the pressure magnitude emitted as a result of low-fluence irradiation of suspensions are also discussed. The probabilistic distribution of pressure magnitude from individual bubbles was found to obey Zipf's law for low concentrations of nanoparticles, while increasing their concentration brings the pressure magnitude distribution into conformance with the Gaussian law.
Pulsed photoacoustic spectroscopy (PAS) technique for immediate measuring absolute optical absorption coefficient is presented. It is based on determination of amplitude-time characteristics of photoacoustic (PA) signal generated in air surrounding the sample under investigation. The PA signal is excited by a short laser pulse. The sample is placed in the lateral chamber of a three-chamber PA cell. The difference of PA signals excited in two other chambers is recorded. Time dependence of PA signal for each PA cell chamber is derived. The theoretical predictions are experimentally verified. The pulsed PAS technique presented was tested experimentally by measuring water absorption coefficient at an infrared wavelength.
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