Andrew C. Beveridge

The photoacoustic effect generated by an incompressible sphere

An incompressible sphere with a vanishing thermal expansivity suspended in a fluid can generate a photoacoustic effect when the heat deposited in the sphere by a light beam diffuses into the surrounding liquid causing it to expand and launch a sound wave. The properties of the photoacoustic effect for the sphere are found using a Greens function solution to the wave equation for pressure with Neumann boundary conditions. The results of the calculation show that the acoustic wave for fast heat liberation is an outgoing compressive pulse followed by a reflected pulse whose time profile is modified as a result of frequency dependent reflection from the sphere. For slow heat release by the sphere, the photoacoustic effect is shown to be proportional to the first time derivative of the heat flux at the particle-fluid interface.

Photoacoustic shock generation in carbon suspensions

This letter discusses photoacoustic shock wave generation and the origin of nonlinear sound wave generation in carbon suspensions. The Burgers equation for an inviscid fluid is solved for an exponential acoustic wave. The solution shows an increasingly steep wave form that gradually coalesces into a shock front. Large dynamic range measurements of photoacoustic waves generated by a pulsed-laser beam in carbon suspensions show the pressure in the wave to depart significantly from the predictions of linear response theory. Acoustic sound speed and amplitude measurements indicate that weak shocks are produced from the photoacoustic sound generation process rather than from nonlinear propagational effects.

Imaging based on the ultrasonic vibration potential

An ultrasonic wave traversing a colloidal suspension causes distortion of the charge distributions at the sites of individual colloidal particles producing a voltage known as the ultrasonic vibration potential. We show how imaging of colloidal regions within a body can be carried out using a beam of ultrasound to produce a radio frequency vibration potential. A theory for image formation shows that Fourier transformation of vibration potential signals processed by a mixer and low pass filter gives the spatial distribution of colloid. The salient feature of the method, insofar as medical imaging is concerned, is its contrast mechanism.

SONOLUMINESCENCE INITIATED BY LASER IRRADIATION OF CARBON SUSPENSIONS

We report observation of sonoluminescence following irradiation of suspensions of carbon in water with a high power, pulsed laser. The mechanism for generation of a gas bubble is chemical in origin: the heating of the carbon particles by absorption of laser radiation causes the production of high pressure H2 and CO through reaction of carbon with the water surrounding the particles. Expansion of the bubble past its equilibrium diameter results in oscillation of the bubble diameter which, several hundred μs after the initial formation of the bubble, results in emission of optical and acoustic radiation. The effects of laser power, the concentration of carbon in the suspension, dissolved Ar, and temperature on the sonoluminescent intensity are reported.We report observation of sonoluminescence following irradiation of suspensions of carbon in water with a high power, pulsed laser. The mechanism for generation of a gas bubble is chemical in origin: the heating of the carbon particles by absorption of laser radiation causes the production of high pressure H2 and CO through reaction of carbon with the water surrounding the particles. Expansion of the bubble past its equilibrium diameter results in oscillation of the bubble diameter which, several hundred μs after the initial formation of the bubble, results in emission of optical and acoustic radiation. The effects of laser power, the concentration of carbon in the suspension, dissolved Ar, and temperature on the sonoluminescent intensity are reported.

Frequency domain vibration potential imaging: Objects with symmetry in one dimension

Frequency domain, ultrasonic vibration potential imaging can be carried out by irradiating a colloidal object with a plane ultrasonic wave and recording the magnitude and phase of the current in a pair of electrodes as a function of the frequency. The method is applied to imaging of objects with symmetry in one dimension including a thin layer, a thick layer, pairs of layers, and layers with differing colloidal concentrations. The experimental results show agreement with the theory of vibration potential imaging where the recorded signal is proportional to the integral of the concentration of colloidal or ionic species over the pressure gradient in the ultrasonic wave.

Vibration potential imaging: theory and preliminary results

The ultrasonic vibration potential refers to the generation of a potential when ultrasound traverses a colloidal or ionic solution. The vibration potential can be used for imaging of tissue by sending a burst of ultrasound into a body and recording the vibration potential on the surface of the body with a pair of electrodes attached to a preamplifier and signal processing electronics. The theory of imaging in one-dimension is based on an integral of the ultrasound burst over the colloid distribution in space. A complete theory gives the current from the vibration potential as an integral of the product of the pressure with the component of the gradient of the colloid distribution in space in the direction of propagation of the ultrasound.

Internally excited acoustic resonator for photoacoustic trace detection.

The quantum-cascade laser can be used as an infrared source for a small portable photoacoustic trace gas detector. The device that we describe uses a quantum-cascade laser without collimating optics mounted inside an acoustic resonator. The laser is positioned in the center of a longitudinal resonator at a pressure antinode and emits radiation along the length of the resonator exciting an axially symmetric longitudinal acoustic mode of an open-ended cylindrical resonator. Experiments are reported with an 8-microm, quasi-cw-modulated, room-temperature laser used to detect N2O.

Chemical generation of sound waves: shock waves and a "giant" photoacoustic effect

When a suspension of carbon particles in water is irradiated by a high power, pulsed laser an anomalously large photoacoustic effect is generated which has an amplitude on the order of 2000 times larger than that produced by a dye solution with an equivalent absorption coefficient. Transient gratings generated in carbon suspensions with high power lasers show a doubling of the acoustic frequency corresponding to the optical fringe spacing of the grating. Both effects are attributed to high temperature chemical reactions initiated by the laser that consume energy and produce gaseous reaction products. A theory for generation of the photoacoustic effect is given that is applied to the reactions taking place in the carbon suspension.

Transient grating studies of laser induced 1ΔgO2 production by photodynamic therapy agents

The transient grating method acts as a monitor of the evolution of thermal energy following optical excitation of an absorbing molecule. The signals produced by a photodynamic therapy agent are shown to be strongly dependent on presence of oxygen in solution indicating transfer of energy from a triplet state of the dye to form excited 1Δg oxygen. Analysis of the data shows that the efficiency of excited oxygen production can be determined by a recording of the diffracted light intensity versus time.

Tissue imaging using the ultrasonic vibration potential

An ultrasonic vibration potential is generated when an acoustic wave propagates in an ionic or colloidal suspension. Measurement of the potential as an ultrasonic wave propagates in a body offers the possibility of a method of imaging. The resolution of the method ultimately is limited by the wavelength of the ultrasound; the contrast of the technique will depend on inertial quantities and the relative zeta potentials of the irradiated regions. The prospects for tissue imaging using the ultrasonic vibration potential are discussed.