Grover C. Wetsel

Generalized theory of the photoacoustic effect

The theory of the photoacoustic effect is extended to include the contribution of mechanical vibration of the sample. Coupled equations for thermal and acoustic waves are solved in both sample and gas. It is shown that the pressure signal in the gas may be significantly affected by acoustic coupling in the sample, and experimental confirmation of this extended theory is given. The results of the fully coupled treatment are shown to be accurately reproduced by an extension of the Rosencwaig piston model: the pistonlike motion of the gas boundary layer adjoining the sample is superimposed on the mechanical vibration of the sample surface to give a composite piston displacement which then produces the pressure signal in the gas. The composite‐piston model provides relatively simple algebraic results applicable to many cases of physical interest.

Photoacoustic determination of absolute optical absorption coefficient

We propose and demonstrate that the photoacoustic effect can be used for absolute determination of the optical absorption coefficient. The photoacoustic signal is measured as a function of chopping frequency and compared to the theory of the photoacoustic effect. The essential agreement of theory and experiment over a restricted frequency makes possible the determination of the optical absorption coefficient (to within 10% in a test case). Observation of a characteristic leveling off of the photoacoustic signal at low frequencies for several materials is also reported.

Subsurface‐structure determination using photothermal laser‐beam deflection

Photothermal imaging using laser‐beam deflection is shown to be a successful means of detecting subsurface structure in solids. Experimental data for known and unknown subsurface structures are reported. The existing theory agrees well with data on broad subsurface structures, but small subsurface structures produce signal variations which are better represented by a subsurface thermal contact resistance. The first photothermal‐image characterization of a microscopic, unknown subsurface defect is presented.

Absolute measurement of optical attenuation

We have discovered that laser beam deflection spectroscopy can be used for the absolute measurement of wave or particle beam attenuation in condensed matter. The concept has been experimentally evaluated by successfully measuring the absolute optical attenuation in a crystal of U3+:CaF2 at 514 nm. A theoretical model that explains the experiment and characterizes the range of applicability of the method has been developed.

Effects of Magnetic Field on Attenuation of Ultrasonic Waves in a Nematic Liquid Crystal

The effects of the magnitude and direction of a steady magnetic field on the propagation of longitudinal ultrasonic waves in N‐(p‐methoxybenzylidene)—p‐butylaniline have been investigated as a function of frequency and path length. The anisotropy of the measured ultrasonic attenuation is in good agreement with a theory of ultrasonic‐wave propagation in nematic liquid crystals based on micropolar continuum mechanics. The variation of the attenuation with frequency from 5–85 MHz is, however, inconsistent with present theories. Measurements of velocity and dispersion are also reported.

Resolution and definition in photothermal imaging

The influences of pump and probe beam sizes, as well as chopping frequency, on resolution and definition in photothermal laser‐beam‐deflection imaging of subsurface structure are investigated experimentally and theoretically. It is shown that the resolution of nearby subsurface structures is improved by decreasing pump and probe beam sizes to dimensions less than, but not necessarily much less than, the characteristic dimension of the subsurface structures. It is also shown that the photothermal image width (full width at half maximum) may be different from the structure size, and that the width may vary with frequency, for certain structure geometry. A theoretical model of thermal‐wave scattering gives results consistent with the present experimental data and with previous, apparently contradictory, results in the appropriate geometric limits.

Ultrasonic‐wave generation by harmonic heating in composite structures

A one‐dimensional theory of the generation of ultrasonic waves by photoacoustic absorption in a composite structure is described. The model consists of a backing material, an absorbing bulk or surface film, and a sample. Variations of ultrasonic amplitude and intensity with structure dimensions, optical absorption coefficient, frequency, and material parameters are discussed and compared with reported experimental data. It was found that the important material group—for both backing and film or sample—is α(D)1/2, where α is effectively a thermal‐expansion coefficient and D is the thermal diffusivity. The relevance of the parameters in the heated volume to imaging is discussed.

Capillary oscillations on liquid jets

Capillary oscillations on modulated liquid jets have been investigated using laser illumination and electronic detection of the magnified jet shadow. The amplitudes of several wave harmonics of a growing spatial instability were measured as a function of distance from the orifice for a range of jet velocities and initial‐disturbance amplitudes. The experimentally determined growth rates at the fundamental frequency are compared with theories of capillary‐wave propagation. An empirically derived explanation of the suppression of satellite formation is given. Experimental evidence for infinite‐wavelength capillary oscillations is reported; a description of these oscillations in terms of the Rayleigh theory is presented.

Ultrasonic-Wave Generation by Surface and Bulk Heating in Multimaterial Structures

In many light-absorbing materials a substantial fraction of the annihilated photon’s energy is transformed into heat. If the light is pulsed, then pulses of heat are produced which, in turn, generate pulsed elastic disturbances; this phenomenon is commonly known as the photoacoustic effect. Whereas the heat pulses are exponentially damped as they move away from the source, the elastic waves propagate with relatively little attenuation. The elastic waves so generated contain information about the thermal, elastic, and optical properties of the material in which they are produced as well as information about materials in contact with the generating material. The information carried by the elastic wave is more localized--that is, the resolution is greater-- the higher is the frequency of the light pulses; and if the frequency is increased to the microwave range, the resolution can approach or even surpass that of the optical microscope. The generation of high-frequency elastic waves by photothermal means is of current interest because of its relevance to several areas of physics, including: the development of the photoacoustic microscope1 , thermal-wave imaging2 , determination of thermoelastic material parameters, nondestructive evaluation of devices3 , and laser annealing and melting phenomena in semiconductors.

Multiple‐Phonon Interactions in Ultrasonic Paramagnetic Resonance of Uranium Ions in Calcium Fluoride

The method of ultrasonic paramagnetic resonance (UPR) has been used to study direct spin‐lattice interactions between tetravalent uranium ions in sites of trigonal symmetry and the host lattice, CaF2. Intense absorption and dispersion of 9.0‐GHz longitudinal elastic waves propagated along the [111] direction of calcium fluoride were observed when an external magnetic field was adjusted to the spin‐resonance value B1, characteristic of the absorption of one phonon by each paramagnetic ion. Resonant phonon absorption of progressively lower intensity was observed at multiples of two, three, four, and five of the one‐phonon resonant field, B1. The absorption was measured as a function of magnetic field, angle between B and the symmetry axis of the paramagnetic ion, ultrasonic intensity, ultrasonic pulsewidth, and temperature in the range 1.5° to 4.2°K. Analysis of the results indicates that two processes are extant: (1) the generation of phonons at harmonics of the applied frequency due to the anharmonic latt...