Influence of nanoscale temperature rises on photoacoustic generation: discrimination between optical absorbers based on nonlinear photoacoustics at high frequency

Abstract

In the thermoelastic regime, photoacoustic sensing of optical absorption relies on conversion from light to acoustic energy via the coefficient of thermal expansion β. In this work, we confront confront experimental measurements to theoretical predictions of nonlinear photoacoustic generation based on the dynamic variation of β(T) during the optical excitation of absorbers in aqueous solution. The photoacoustic generation from solutions of organic dye and gold nanospheres (with same optical densities), illuminated with 532 nm nanosecond pulses, was detected using a high frequency ultrasound transducer (center frequency 20 MHz). Photoacoustic emission was observed with gold nanospheres at low fluence (a few mJ/cm2) for an equilibrium temperature around 4°C, where the linear photoacoustic effect in water vanishes, highlighting the nonlinear emission from the solution of nanospheres. Under the same condition, no emission was observed with the absorbing organic dye. At a fixed fluence of 5 mJ/cm2, the photoacoustic amplitude was studied as a function of the equilibrium temperature from 2°C to 20°C. While the photoacoustic amplitude from the organic dye followed β(T) in water and vanished around 4°C, the photoacoustic amplitude from the gold nanospheres remained significant over the whole temperature range. These experimental results are shown to be in qualitative agreement with theoretical calculations based on the nanoscale and nanosecond transient changes of the coefficient of thermal expansion of water around the gold nanospheres. Our results suggest that in the context of high frequency photoacoustic imaging, nanoparticles may be discriminated from molecular absorbers based on either the nonlinear fluence-dependence or the equilibrium temperature-dependence of the photoacoustic amplitude.