Transient gratings in particle suspensions: The effect of thermal nonlinearity and bubble formation

This thesis describes a theoretical and experimental investigation of the photoacoustic effect generated by the thermal nonlinearity of water, and the subsequent bubble production in laser irradiated aqueous particle suspensions. Experiments were carried out using the transient grating method creates an optical interference pattern in an absorbing sample by placing the sample at the intersection of two crossed laser beams from a pulsed Q-switched Neodynium-Yttrium Aluminum Garnet (Nd-YAG) laser. The dynamics of the transient grating were probed with a low-power continuous wave (CW) laser that was aligned at the Bragg angle for first order diffraction. Transient grating experiments with platinum (Pt) particle suspensions in water resulted in an imbalance between the acoustic and thermal modes of wave motion in the grating, which resulted in the appearance of overtones in the acoustic frequency of the diffracted light signal. Overtone production in the transient grating signal was explained by the temperature dependence in the thermal expansion coefficient of water, referred herein as thermal nonlinearity. It is well known that laser irradiation of metal particle suspensions in water results in a high particle surface temperature that leads to a vapor bubble. The density decrease in changing water from liquid to vapor corresponds to a large volume expansion that provides an additional mechanism for launching photoacoustic waves.;Aside from transient gratings, a theory is discussed in this thesis that considers the non-adiabatic launching of photoacoustic waves from a laser irradiated half-space, a layer, and sphere in acoustically and thermally identical media. The solutions show that heat conduction from the optical absorber results in a non-causal compressive spike that accompanies the adiabatic pressure wave for all three cases. The absence of causality is a consequence of using the heat diffusion equation which does not give causal solutions for temperature. Although no experiments are presented to verify this theory, previously reported experimental evidence supports the possibility of the theoretically predicted features.