Photoacoustic Amplitude Research Articles

Dependence of photoacoustic amplitude on chopping frequency for porous silicon

The variation in the photoacoustic signal produced by porous silicon on a silicon substrate with chopping frequency was investigated using excitation energies of 1.22 and 2.75 eV, below and above the band gap of porous silicon, respectively. At 1.22 eV, the porous silicon is transparent, and photoacoustic amplitude follows a − 3 2 -power dependence with chopping frequency. At 2.75 eV, at which the porous silicon is opaque, an exact − 1 2 -power dependence was observed for lower chopping frequencies in addition to the ordinary −1-power dependence at higher frequencies. The transition frequency between these two dependencies decreases with anodization time.

Photoacoustic discrimination of viable and thermally coagulated blood using a two-wavelength method for burn injury monitoring

Discriminating viable from thermally coagulated blood in a burn wound can be used to profile burn depth, thus aiding the removal of necrotic tissue. In this study, we used a two-wavelength photoacoustic imaging method to discriminate coagulated and non-coagulated blood in a dermal burn phantom. Differences in the optical absorption spectra of coagulated and non-coagulated blood produce different values of the ratio of peak photoacoustic amplitude at 543 and 633 nm. The absorption values obtained from spectroscopic measurements indicate that the ratio of photoacoustic pressure for 543 and 633 nm for non-coagulated blood was 15.7:1 and 1.6:1 for coagulated blood. Using planar blood layers, we found the photoacoustic ratios to be 13.5:1 and 1.6:1, respectively. Using the differences in the ratios of coagulated and non-coagulated blood, we propose a scheme using statistical classification analysis to identify the different blood samples. Based upon these distinctly different ratios, we identified the planar blood samples with an error rate of 0%. Using a burn phantom with cylindrical vessels containing coagulated and non-coagulated blood, we achieved an error rate of 11.4%. These results have shown that photoacoustic imaging could prove to be a valuable tool in the diagnosis of burns.

Simulation of photoacoustic imaging of microcracks in silicon wafers using a structure-changeable multilayered thermal diffusion model

The detection characteristics for photoacoustic imaging of microcracks in silicon wafers were theoretically and quantitatively investigated using a numerical simulation. The simulation is based on a one-dimensional multilayered thermal diffusion model coupled with the thermal-wave impedance of each layer, the layer structures of which are constructed along the wafer surface and are variable according to the scanning position of the point heat source. As the modulation frequency was reduced, the spatial resolution of the temperature amplitude profile at the cracks decreased, showing good agreement with the experimentally obtained photoacoustic amplitude images. At a modulation frequency of 200 kHz, for cracks with narrow air gaps of up to 20 nm, which is much smaller than both the beam spot size of 1.5 microm and the thermal diffusion length of 12 microm, the temperature amplitude is twice that of regions without cracks, and the temperature contrast increased with an increase in the modulation frequency. These calculation results suggest the effectiveness of using a high modulation frequency, making it possible to detect microcracks of the order of 10 nm.

Photoacoustic spectroscopy investigation of sintered zinc-tin-oxide ceramics

In this paper the changes that occurred in differently activated ZnO-SnO2 and sintered samples were investigated using photoacoustic spectroscopy. ZnO and SnO2 powders, mixed in the molar ratio 2:1, were mechanically activated in a planetary ball mill for 10-160 min. The mixtures were pres?sed and isothermally sintered at 1300?C for two hours. X-ray diffraction analysis of the obtained sintered samples was performed in order to investigate changes of the phase composition and confirmed only the presence of a pure zinc stannate (Zn2SnO4) phase in all the sintered samples as a result of the solid state reaction and reaction sintering between the starting ZnO and SnO2 powders. The microstructure of the sintered sam?ples was examined by scanning electron microscopy and showed that mechanical activation leads to the formation of a structure with reduced particle size which accelerates spinel formation. Grain growth of the spinel phase slows down the densification process and together with the agglomerates formed during mechanical activation causes the appearance of a porous microstructure. The photoacoustic (PA) phase and amplitude spectra of the sintered samples were recorded as a function of the chopped frequency of the laser beam used (red laser with a power of 25 mW, ?=632 nm) in a thermal-transmission detection configuration. PA experimental data were analyzed using the Rosenzweig-Gersho thermal-piston model, which enabled determination of the thermal diffusivity, Z)T (m2s-1), diffusion coefficient of the minority free carriers D (m2s-1) and the optical absorption coefficient (m-1). The detected differences of the measured thermal-electrical properties of the obtained Zn2SnO4 ceramics indicate changes in the material induced by the different preparation procedure of the starting powders before the sintering process.

Investigation of zinc stannate synthesis using photoacoustic spectroscopy

Mixtures of ZnO and SnO2 powders, with molar ratio of 2:1, were mechanically activated for 40, 80 and 160 minutes in a planetary ball mill. The resulting powders were compacted into pellets and non-isothermally sintered up to 1200?C with a heating rate of 5?C/min. X-ray diffraction analysis of obtained powders and sintered samples was performed in order to investigate changes of the phase composition. The microstructure of sintered samples was examined by scanning electron microscopy. The photoacoustic phase and amplitude spectra of sintered samples were measured as a function of the laser beam modulating frequency using a transmission detection configuration. Fitting of experimental data enabled determination of photoacoustic properties including thermal diffusivity. Based on the results obtained a correlation between thermal diffusivity and experimental conditions as well the samples microstructure characteristics was discussed. .

Photoacoustic and luminescence spectra study on the effects of chlorine substituent on the energy transfer of Eu(III)–chlorobenzoic acid

The photoacoustic (PA) amplitude spectra of three complexes of Eu(III) combined with chlorobenzoic acid (Eu( o-ClC 6H 4CO 2) 3·H 2O, Eu( m-ClC 6H 4CO 2) 3·H 2O and Eu( p-ClC 6H 4CO 2) 3·H 2O) have been measured, and the PA phase of the different complexes have been calculated. Both the PA amplitude spectra and the luminescence spectra reflect the variation of the luminescent properties, and the PA phase is directly relative to the relaxation time. Since the relaxation is the process of the intramolecular energy transfer between the ligands and the central ion, the molecular structure of ligand is the important factor to decide the energy gap between the lowest triplet state of ligand and the resonance level of central ion. The effects of chlorine substituent on the molecular structure and energy gap of the complexes have been studied by PA phase and luminescence spectra.

Photoacoustic and Optical Properties of Zinc-Stannate Thin Films

Thin films of single-phase zinc-stannate (Zn2SnO4) were grown by rf magnetron sputtering onto glass substrates. Transmission in the visible range was measured allowing determination of the energy gap and thickness of analyzed thin film samples using interference fringes. The photoacoustic phase and amplitude spectra of all samples were measured as a function of the laser beam modulating frequency using a transmission detection configuration. Fitting of experimental data enabled calculation of thermal diffusivity, the coefficient of minority carrier diffusion, their mobility and lifetime.

Determination of Thermal Diffusivity of the Pb0.88Ln0.08Ti0.98Mn0.02O3 (Ln = La, Eu) Ferroelectric Ceramic System by Photoacoustic Technique

Thermal diffusivities of the Pb0.88(Ln)0.08Ti0.98Mn0.02O3 (Ln = La, Eu) ferroelectric ceramic system are investigated by using Open Photoacoustic Cell (OPC) technique. The preparation of the ceramics was performed in our laboratory by using the oxide-mixing conventional ceramic method. It should be noted that these ceramics were taken in an unpoled state. For the thermal characterization, measurements at room temperature, of the photoacoustic amplitude signals as a function of the light modulation frequency, were performed by using OPC technique. Experimental data were fitted to the OPC theoretical expression in the heat transmission configuration for optically opaque samples. The thermal diffusivity values obtained for these ceramics are close to those results reported for BaTiO 3 based ferroelectric ceramics.

Effect of thermal nonlinearity in high-absorption media on the parameters of the photoacoustic signal detected by the gas microphone method: The fundamental and second harmonics

A perturbation theory is put forward that describes the effect of thermal nonlinearity due to the temperature dependence of the thermophysical parameters of high-absorption systems with a low thermal conductivity on the parameters of the photoacoustic signal detected by the gas microphone technique. It is found that the dependence of the photoacoustic signal amplitude on incident beam intensity I0 stems from the dependence of the illuminated surface temperature on I0. This dependence is a complicated function instead of being a simple quadratic function as was expected. In the limiting cases (μsβ ≪ 1 and μsβ ≫ 1), this contribution to the photoacoustic signal amplitude is described by simple expressions, which are convenient for determining the thermal coefficients of the thermophysical parameters of the medium. It is found that the thermal nonlinearity significantly affects the photoacoustic signal phase in the frequency region meeting the condition μsβ ∼ 1. In the above limiting cases, its effect is insignificant. A theory of generation of the photoacoustic signal second harmonic is proposed. The second harmonic is related to the temperature dependence of the thermophysical parameters of the buffer gas and sample. It is shown that the amplitude of the signal is a quadratic function of the incident beam intensity and varies with its frequency as ω−3/2 for μsβ ≫ 1 and ω−5/2 for μsβ ≪ 1.

In vivo evaluation of drug delivery after ultrasound application: A new use for the photoacoustic technique

Ultrasound application is a therapeutical resource widely employed in physiotherapy. One of its applications is the phonophoresis, a technique in which the ultrasound radiation is utilized to deliver drugs through the skin to soft tissues. The proposal of our study was to employ the Photoacoustic Technique to evaluate the efficacy of such treatment, analyzing if phonophoresis could enhance drug delivery through skin when compared to the more traditional method of manual massage. The configuration of the system employed was such that it was possible to perform in vivo measurements, which is a pre-requisite for this kind of study. The changes observed in the photoacoustic signal amplitude after each form of drug application were attributed to changes in the thermal effusivity of the system, due to penetration of the drug. The technique was able to detect differences in drug delivery between the specified physiotherapy treatments, indicating that phonophoresis enhances drug absorption by tissue.

In vitro determination of the non radiative relaxation time of triplet state in protoporphyrin IX

In this work we used Photoacoustic Spectroscopy (PAS) to determine in vitro the non radiative relaxation time (NRRT) of protoporphyrin IX (PpIX) triplet state. In order to obtain this characteristic time, we have used PpIX disodium salt (DS) solutions of 25 % HCl. From each solution it was obtained its optical absorption spectrum, by using PAS, ranging from 300 to 800 nm. In the maximum absorption peak, at 410nm, we obtain the photoacoustic (PA) signal, amplitude and phase, as a function of the light modulation frequency, from 17 to 80 Hz. From the PA signal phase it was possible to determine, for each solution, the NRRT by using the Rosencwaig-Gesho theory, modified to include the effect of a finite nonradiative deexitation time. A NRRT average value, obtained from these solutions, is reported and compared with some NRRT of triplet states reported in the literature for molecules with the same tetrapyrrolic stricture.

Photoacoustic analysis of dental resin polymerization

In this work, we use the photoacoustic technique to monitor the curing process of diverse dental materials, as the resins chemically activated (RCA). The results obtained reveal that the composition of a determined RCA significantly alters its activation kinetics. Photoacoustic data also show that temperature is a significant parameter in the activation kinetics of resins. The photoacoustic technique was also applied to evaluate the polymerization kinetics of photoactivated resins. Such resins are photoactivated by incidence of continuous light from a photodiode. This leads to the polymerization of the resin, modifying its thermal properties and, consequently, the level of the photoacoustic signal. Measurements show that the polymerization of the resin changes the photoacoustic signal amplitude, indicating that photoacoustic measurements can be utilized to monitor the polymerization kinetic and the degree of polymerization of photoactivated dental resins.

Photoacoustic investigations of thermal and electronic properties of single crystal Ge doped with Cr

A high purity single crystal Ge with a (111) orientation was doped with Cr. A thin film of Cr was evaporated and then diffused into Ge substrate. The photoacoustic PA amplitude and phase spectra were measured and numerically analyzed. The substitution of Cr in Ge produces a compensation effect.

Anisotropy in thermal and electronic properties of single crystal GaSe determined by the photoacoustic method

Anisotropy in thermal and electronic properties for two directions of thermal wave propagation in the GaSe single crystal was investigated using photoacoustic (PA) frequency transmission techniques. First, the electric field of an incident polarized light beam was adjusted to be perpendicular to the c-axis (E ⊥c) and then parallel to it (E∥c). An obvious difference in the PA phase and amplitude spectra of the same sample for these two cases was shown. The thermal diffusivity D T obtained by the fitting procedure for these two orientations of the electric field were calculated as D T ∥=0.46×10 -5 m 2 /s and D T ⊥=0.62×10 -6 m 2 /s eg. Their ratio is about 7.4. The electronic transport parameters are also given. The carrier diffusion coefficient D II is 0.53x10 -3 m 2 /s and D⊥ is 0.3x10 -3 m 2 /s.

Investigation of the thermal diffusivity of thick film NTC layers obtained with the photoacoustic method

The thermal diffusivity of thick NTC layers based on a metal oxide powder mixture was measured at room temperature by the photoacoustic (PA) frequency transmission technique. The experimental PA amplitude and phase diagrams were numerically analyzed and the thermal diffusivity and electron transport parameters were calculated.

The model of a thin semiconductor layer on a thermally thick semiconductor backing for the photoacoustic use

In this paper the thermal model of a thin semitransparent layer on a semitransparent thermally thick backing material is presented. The formula for the temperature of the front surface of the structure that was derived in this paper allowed the computations of the photoacoustic amplitude and phase spectra of a thin GaAs layer on a silicon substrate and a porous silicon layer on a silicon substrate. This formula is a useful tool in the field of the photoacoustic spectroscopy of thin layers.

High-sensitivity photoacoustic leak testing

The photoacoustic effect may be exploited for detection and localization of gas leaks on the surface of otherwise sealed components. The technique involves filling the test component with a photoactive tracer gas, and irradiating the component to produce photoacoustic sound from any leak site where a tracer gas cloud forms. This presentation describes demonstration experiments utilizing 10.6-μm radiation from a nominally 145-W carbon-dioxide laser with sulfur hexafluoride as a tracer gas. Here, photoacoustic sounds from six NIST-traceable calibrated leak sources with leak rates between 1 cc in 4.6 h, and 1 cc in 6.3 years were recorded with 12 microphones in a bandwidth from 3 to 80 kHz. Bartlett matched-field processing of the microphone array measurements both detect and localize these leaks when the leak and the array are separated by 152 mm. These experiments suggest that the sensitivity of photoacoustic leak testing may reach or even exceed the capabilities of the most sensitive commercial leak test systems using helium mass-spectrometers. Comparison of the measured results and an analytical scaling law suggests that tracer cloud geometry influences the photoacoustic signal amplitude. [Work supported by the U.S. Dept. of Energy.]

Thermal diffusivity of NTC layers obtained with photoacoustic technique

The thermal diffusivity of thick film NTC layers based on a metal oxide powder mixture was measured at room temperature by the photoacoustic (PA) technique. The powder mixture was composed of MnO (60 percent), CoO (32 percent) and Fe2O3 (8 percent), which were ball milled to nanometer particle size. NTC layers of different thicknesses were made by sequentional screen‐printing followed by drying and co‐firing at 850, 900 and 1,000°C in a hybrid conveyor furnace. The experimental PA phase and amplitude diagrams were numerically analyzed and the thermal diffusivity and electron transport parameters were calculated. An increase of thermal diffusivity with sintering temperature was observed. The fractal structure model was in agreement with the experimental data for modulation frequencies in which the sample behaved as thermally thick.

Photoacoustic properties of sintered NiO

Abstract The thermal diffusivity of NiO pellets was determined by the photoacoustic technique. A NiO powder, with a purity of 99.95 %, was pressed into pellets 10 mm in diameter. The average agglomerate size of the NiO powder was 50 μm and the size of individual particles was between 1 and 10 μm. The pressing pressure was 1.5 GPa and the sintering temperature was 1100 °C. The sintering time was between 15 and 240 min. The photoacoustic phase and amplitude spectra of NiO were measured as a function of the modulating frequency, using a transmission detection configuration. Thermal diffusivity, DT, the coefficient of minority carrier diffusion, their mobility and life time, and front and rear recombinating velocities were calculated as a function of sintering time by comparing the experimental results with theoretical photoacoustic phase and amplitude signals. The thermal diffusivity increases from 0.67 · 10–5 to 0.97 · 10–5 m2/s when the sintering time was changed from 15 to 240 minutes.

High-sensitivity photoacoustic leak testing.

The photoacoustic effect may be exploited for the detection and localization of gas leaks from otherwise sealed components. The technique involves filling the test component with a tracer gas, and radiating the component to produce photoacoustic sound from any leak site where tracer gas is present. This paper describes demonstration experiments utilizing 10.6-micro radiation from a carbon-dioxide laser and sulfur hexafluoride as a tracer gas for photoacoustic leak testing at leak rates between 6 x 10(-5) cm3/s (1 cm3 in 4.6 h) and 5 x 10(-9) cm3/s (1 cm3 in 6.3 years). The technique may reach or exceed the capabilities of the most sensitive commercial leak test systems using helium mass-spectrometers. In addition, comparison of the measured results to a simple scaling law suggests that tracer gas cloud geometry influences the photoacoustic signal amplitude.