Photoacoustic Generation Research Articles

Effect of photoacoustic generation mechanisms on transient grating measurements of thin films on silicon

Laser induced transient gratings technique is widely used for non-contact measurements of surface acoustic waves (SAWs). One major application of such measurements is characterization of thin films used in microelectronics. A peculiar phenomenon has been observed in transient gratings measurements of thin films on a silicon substrate: if the film is transparent, there is a systematic discrepancy between the measured and expected SAW velocity values that cannot be accounted for by instrumental errors. However, as soon as the sample is coated by an opaque metal film, the discrepancy disappears. In this presentation, we will show that the origin of the effect is in the competition of thermal and electronic mechanisms of the optical generation of sound in silicon. We will also describe another effect of the same origin observed on very thin semi-transparent metal films: at a certain film thickness, the SAW signal vanishes as the thermal expansion of the film and the contraction of silicon caused by photoexcited carriers cancel each other.

Photoacoustic study of explosive boiling of a 2-propanol layer of variable thickness on a KrF excimer laser-heated Si substrate

The dynamics of explosive boiling of a 2-propanol layer of variable thickness on a Si substrate heated by a nanosecond KrF excimer laser was studied using a contact photoacoustic technique. The transition from acoustic generation at a free Si boundary to that at a rigid alcohol/Si boundary accompanied by a sharp increase of acoustic generation efficiency was found above a laser fluence threshold of 0.17 J/cm2 and a liquid layer thickness greater than 0.25 μm due to subnanosecond near-critical explosive boiling of the superheated liquid layer near the hot absorbing Si substrate. The gradual increase of the photoacoustic response of the superheated alcohol with increasing thickness of the liquid film at fluences above the explosive boiling threshold was attributed to a diffraction effect due to the fluence- and time-dependent increase of the area undergoing explosive boiling. A model describing photoacoustic generation and subsequent lift-off of the entire liquid layer in this experimental “thin transparent liquid layer/solid absorbing substrate” geometry under near-critical explosive boiling conditions has been proposed.

Gas-phase generation of photoacoustic sound in an open environment.

The photoacoustic effect is commonly exploited for molecular spectroscopy, nondestructive evaluation, and trace gas detection. Photoacoustic sound is produced when a photoactive material absorbs electromagnetic radiation and converts it to acoustic waves. This article focuses on the generation of photoacoustic sound from thermal expansion of photoactive gases due to unsteady heating from a laser light source, and extends the work of prior studies on photoacoustic sound generation in an open environment. Starting with the forced free-space wave equation, a simple model is constructed for photoacoustic sounds produced by both acoustically distributed and compact gas clouds. The model accounts for laser absorption through the Lambert-Beer law and includes the effects of photoactive gas cloud characteristics (shape, size, and concentration distribution), but does not include molecular diffusion, thermal conduction, convection, or the effects of acoustic propagation through sound-absorbing inhomogeneous media. This model is compared to experimentally measured photoacoustic sounds generated by scanning a 10.6-micron carbon dioxide (CO2) laser beam through small clouds of a photoactive gas, sulfur hexafluoride (SF6). For the current investigation, the photoactive gas clouds are formed either by low flow-rate calibrated leak sources or by a laminar jet emerging from a 1.6-mm-diam tube. Model-measurement comparisons are presented over a 3- to 160-kHz bandwidth. Signal pulse shapes from simple gas cloud geometries are found to match calculated results when unmeasured gas cloud characteristics within the model are adjusted.

Photoacoustic method for the simultaneous acquisition of optical and ultrasonic spectra

A novel method for the simultaneous acquisition of optical absorption and ultrasonic attenuation spectra is described. The technique is based on the photoacoustic generation of bi-directional ultrasonic pulses by a thin optical absorber that is remote from the sample. A single piezoelectric transducer ultimately detects the two pulses of ultrasound, and the captured signals are used to determine both the optical absorption and ultrasonic attenuation properties of the sample. Measurements on a polycarbonate resin sample using the simultaneous photoacoustic method and conventional techniques show good agreement, indicating the validity of the method.

Photoacoustic characterization of the mechanical properties of thin films

Narrow band photoacoustics (laser ultrasonics) are used to characterize the properties of free-standing nanometer-sized thin films. Photoacoustic generation is achieved by use of a microchip laser which deposits pulsed laser energy in the form of a spatially periodic source on the structure. The resulting narrow band ultrasonic modes are monitored using a Michelson interferometer. By varying the geometry of the spatially periodic source, a wide range of acoustic wave numbers is probed. Results are presented for two-layer thin film aluminum/silicon-nitride (Al/Si3N4) membranes. For such thin films, only the two lowest order guided modes are generated and these in turn can be related to sheet and flexural modes in plates. The mechanical properties and residual stress in the thin films are evaluated from measured acoustic dispersion curves for these two lowest order modes.

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.

Application of pulsed photoacoustics in water at high pressure

The application of pulsed photoacoustics to the study of liquids at pressures of up to 350 bars is discussed. The design and development of an in-line sensor for the subsea monitoring of crude oil concentrations in water is reported. Crude oil detection sensitivities at parts per million concentrations were achieved with prototype instrumentation. A comparison of experimental results and a theoretical prediction of the pressure dependence of the pulsed photoacoustic response from water is outlined. The results demonstrate that existing models that describe pulsed photoacoustic generation in liquids are applicable to high-pressure conditions.

Photoacoustic cavitation in spherical and cylindrical absorbers

Photomechanical damage in absorbing regions or particles surrounded by a non-absorbing medium is investigated experimentally and theoretically. The damage mechanism is based on the generation of thermoelastic pressure by absorption of pulsed laser radiation under conditions of stress confinement. Principles of photoacoustic sound generation predict that the acoustic wave generated in a finite-size absorbing region must contain both compressive and tensile stresses. Time-resolved imaging experiments were performed to examine whether the tensile stress causes cavitation in absorbers of spherical or cylindrical shape. The samples were absorbing water droplets and gelatin cylinders suspended in oil. They were irradiated with 6-ns-long pulses from an optical parametric oscillator. Photoacoustic cavitation was observed near the center of the absorbers, even if the estimated temperature caused by absorption of the laser pulse did not exceed the boiling point. The experimental findings are supported by theoretical simulations that reveal strong tensile stress in the interior of the absorbers, near the center of symmetry. Tensile stress amplitudes depend on the shape of the absorber, the laser pulse duration, and the ratio of absorber size to optical absorption length. The photoacoustic damage mechanism has implications for the interaction of ns and sub-nslaser pulses with pigmented structures in biological tissue.

Photoacoustic generation for a spherical absorber with impedance mismatch with the surrounding media.

Pressure generation in a spherical absorber due to energy deposition from pulsed lasers is studied. For a variety of conditions, analytical solutions are derived that allow quick computation of exact results. For the special case of identical acoustic impedance, the pressure transient spreads to the surrounding medium by a single compressive pulse followed by a tensile pulse at the end of illumination. For the general case of impedance mismatch, the pressure transient is in the form of a series of dampened compressive and tensile pressure pulses. In this paper both the amplitude ratio and the sign of consecutive pressure pulses are determined analytically, and are shown to be dependent upon the impedance mismatch. For laser pulses of duration much less than the absorber's characteristic oscillation time, a stress confinement limit is reached for most of the absorber, but a sharp tensile stress in the core region of the sphere is predicted. This region of high stress is defined by r < or =r(c), and we show that r(c) is proportional to the laser pulse duration tau(0). Upon further shortening of the laser pulse duration, the strength of this tensile stress continues to increase while its spatial distribution is sharpened. This observation has relevance to a number of experiments where laser-induced pressure transients cause the absorber to fracture.

Photoacoustic Generation: Optical Fiber Ultrasonic Sources for Non-Destructive Evaluation and Clinical Diagnosis

An innovative photoacoustic source for ultrasonic non destructive evaluation (NDE) and clinical diagnosis is proposed. The working principle is based on thermal conversion of laser pulses into a metallic film evaporated directly onto the tip of an optical fiber. Attractive advantages of the proposed transducer are high frequency, large bandwidth and good efficiency.

The application of the photoacoustic transmittance oscillations for determining elastic constants in gallium and indium selenides

Transmittance periodic oscillations are observed in GaSe and InSe on excitation with optical pulses. Such oscillations are explained in terms of photoacoustic generation of dilatational waves, which become resonant within the crystal. Spectral analysis of those oscillations in samples of different thickness has led to an accurate determination of the longitudinal acoustic-wave velocity along the crystallographic axis c.

Photoacoustic investigation of multilayer semiconductor materials

A one-dimensional model to investigate the photoacoustic (PA) generation and depth profiling in multilayer semiconductor materials has been developed by considering the contributions of thermal and plasma waves. The theoretical results show that the contribution of the plasma wave induced by photo-generated carriers (PGC) to the photoacoustic signal through the electro-elastic effect is predominant at high modulation frequencies and the contribution of the thermal wave, which is caused by the recombination of PGC, to PA signal through the thermo-elastic effect is more important at low modulation frequencies. We also investigated the depth profiling ability of PA detection by changing either the modulation frequency or the phase shift of the reference signal. The theoretical analyses are consistent with the experimental results.

Photoacoustic generation in liquids with low optical absorption

Short laser pulses (250 ns) from a 1064 nm Nd: YAG laser have been used to generate acoustic waves in distilled water and methanol. The mechanism of this process has been studied both experimentally and theoretically with good overall agreement. The measurement of the acoustic signal utilized a small wideband (0.1–20 MHz) piezoelectric hydrophone. The advantages of this detector in the measurement of photoacoustically induced ultrasonic waves in liquids with low optical absorption are discussed.

Distributed source photoacoustic generation of ultrasound

By juxtaposing a series of optically thin and transparent layers, each having the same sonic transit time, it is possible to construct a distributed feedback generator of ultrasonic waves based on the photoacoustic effect. The response of the device to sinusoidally modulated, continuous radiation is determined by summing the plane wave amplitudes from each absorbing layer, taking into account the effects of reflection and transmission at the interfaces between layers. The frequency domain solution shows the effects of constructive and destructive interference with maxima in the photoacoustic amplitude at points where the modulation frequency of the optical radiation is such that the acoustic wavelength equals twice the layer thickness divided by an odd integer. Solutions for short pulse excitation of the layers show the emitted photoacoustic wave to be a series of square waves in time. Experimental waveforms generated by a series of layers excited with the 532 nm output of a frequency doubled Nd:YAG laser are presented. The relationship between the layered photoacoustic source and similar devices such as the acoustic filter, multilayer dielectric interference filters, superlattices and materials with photonic band gaps is discussed.

A laser photoacoustic sensor for analyte detection in aqueous systems

Pulsed laser photoacoustic spectroscopy has been used as the basis for sensing analytes in aqueous systems. The spectra obtained with the photoacoustic system are significantly different from conventional transmission spectra and, in particular, the spectra of some organic solvents and alcohols show a marked enhancement in feature strength due to the dependence of physical and thermal factors in the photoacoustic generation process. The photoacoustic process has been shown to be almost immune to the effects of optical scattering, which gives this sensing technique advantages over conventional spectroscopic techniques in the potential application to water-pollution monitoring and to measuring blood-glucose concentrations.

Explosion of a liquid film in contact with a pulse-heated solid surface detected by the probe-beam deflection method

The threshold for explosive vaporization of a liquid layer on an opaque solid surface heated by an ultraviolet excimer pulsed laser is studied by a photoacoustic probe-beam deflection method. The probe beam traverses the liquid in the vicinity of the laser-heated liquid-solid interface. Below the explosion threshold, photoacoustic generation in the solid occurs only through a thermoelastic mechanism, which results mainly in shear waves that do not couple well into the liquid. Above the explosion threshold, photoacoustic pulses in the solid are also produced by explosive recoil, hence producing longitudinal pulses in the solid that couple well into the liquid after reflections. By setting the probe-beam refraction to detect longitudinal pulse echoes coupled back into the liquid, a sensitive detection of the explosive threshold can be established.

Effect of an external constraint on the photoacoustic spectrum of Nd_2O_3 powders

The photoacoustic spectrum of Nd(2)O(3) powder has been studied to understand the mechanism of the generation of the photoacoustic signal in the constrained and unconstrained powder samples. It is found that the photoacoustic spectrum is qualitatively similar to the transmission spectrum for the unconstrained sample, and the photoacoustic signal intensity is approximately four times less than that in the constrained case. The variation of the photoacoustic signal has been studied by increasing the degree of external constraint on the sample to determine the boundary between constrained and unconstrained situations. The role of air trapped in between powdered microcrystallites in photoacoustic spectrum generation has been also investigated.

Photoacoustic generation of anisotropic pressure waves through photodissociation of Cl_2

Photodissociation of diatomic molecules by linearly polarized radiation in general produces an anisotropic distribution of recoiling atomic fragments. When a short burst of radiation dissociates a gas at low pressure, the collective linear momenta of the individual recoiling photofragments add at any point in space to act as a macroscopic pressure wave, the impulse corresponding to which can be detected by a sensitive microphone. We report here detection of anisotropic pressure waves from photodissociation of Cl(2) with 355-nm radiation. The experimental data show the pressure wave to be peaked in a direction perpendicular to the polarization direction of the incident radiation, indicating a predominantly perpendicular transition moment in Cl(2) at this wavelength.

On the precursor in laser-generated ultrasound waveforms in metals

A laser pulse, when focused on a metal sample, produces characteristic elastic waveforms, which depend on whether thermoelastic or ablative/evaporative mechanisms dominate the generation process. In the thermoelastic regime, with an unconstrained surface, the predominant axial displacement is opposite to the direction of propagation (negative), but there is typically a small transient positive displacement. This precursor is not predicted by elastic point source models, but is predicted by models including thermal diffusion. A recent formulation of pulsed photoacoustic generation is used to show how the precursor arises from interaction of the thermal and elastic modes at the illuminated surface.

Practical quantitative theory of photoacoustic pulse generation

The problem of photoacoustic pulse generation is treated using generalized thermoelastic equations, specifically incorporating the hyperbolic heat conduction equation to avoid an infinite thermal propagation velocity. The assumption of equality of longitudinal and thermal velocities leads to a simplification of the solution in certain limiting cases, enabling insights into the character of the solution, without appreciably affecting the numerical results. The effect(s) of approximations made by previous authors may also be assessed. Numerical Hankel–Laplace transform inversion is shown to be practical for the general case, allowing such calculations to be duplicated by others.