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.

An optoacoustic (OA) sensor was designed, fabricated, and used to detect spherical gold nanoparticles (NPs) in diluted suspensions. The sensor, operating in the backward mode, was designed to measure signals from microscopic volumes of nanoparticulate suspensions in water. Thermal nonlinearity was observed in the course of OA signal generation. The irradiation of a microvolume of gold nanoparticles at the wavelength matching the peak of their plasmon resonance absorption gives rise to a multitude of thermomechanical processes, including heating of NPs below the critical temperature of water (374 K). The thermal diffusion from nanoparticles to water takes place; however, formation of vapor nanobubbles is avoided. As a result, a specific acoustic signal is produced exhibiting nonlinear behavior with respect to the incident laser pulse energy. The optoacoustic profile of the laser-induced signal generated in a thin layer of highly diluted suspensions of gold nanospheres was examined, thereby providing a basis for a method for detection of metal nanoparticles with high sensitivity.

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An optoacoustic device consisting of a XeC1 excimer laser and a measurement cell with an attached piezotransducer was used for detecting microparticles suspended in liquid probes. The potential of optoacoustic diagnostics of microinhomogeneous liquids was enhanced by applying informative parameters of optoacoustic response. Probes of distilled water and Dow Chemical latex suspension were used in experiments. It was shown that cavitation mechanism of optoacoustic conversion taking place at laser fluences below the optical breakdown threshold was well-suited for individual microparticle detection. The approach proposed is different from the well-known breakdown counting optoacoustic technique.KEY WORDS: Microinhomogeneous liquids, laser-induced cavitation. 63 64 S.S. ALIMPIEV et al.

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