Showing papers on "Photoacoustic spectroscopy published in 1996"

Photothermal science and techniques

Abstract: Introduction. History. Overview. Thermal waves. Optical generation of thermal waves - photothermal effect. Instrumentation and detection techniques. Transient thermal phenomena. Photoacoustic spectroscopy and its applications. Thermal wave non-destructive evaluation. Characterization of semiconducting materials. Thermal property measurement. Monitoring and measurement of gases and the atmosphere.

Optical determination of the direct bandgap energy of lead iodide crystals

Abstract: The optical bandgap energy of lead iodide PbI2, grown by Bridgman’s method, is obtained by photoacoustic spectroscopy. Due to its potential applications, as a room temperature, semiconductor material detector, which may be used as a photocell, or as a x‐ and γ‐ray radiation detector, the physical properties of PbI2 have attracted much attention. We computed, by different methods the bandgap energy. We found the energy in a range 2.301±0.038⩽〈Eg〉⩽2.359±0.037 eV.

Pressure generation and measurement in the rapid vaporization of water on a pulsed-laser-heated surface

Abstract: The transient pressure generated by the interaction of short‐pulsed laser light with the liquid–solid interface is studied quantitatively. A KrF excimer laser beam of tens of nanoseconds pulse duration irradiates water on a solid surface and induces rapid thermal expansion and explosive vaporization. The pressure pulses launched into water by such processes are detected experimentally by the photoacoustic probe beam deflection method and a broadband piezoelectric transducer. The peak intensities of the traveling pressure wave measured by these two methods are compared with the theoretical thermoelastic predictions. The measurements show that a compressional pressure wave packet is radiated from the water‐solid interface with the peak intensity of the order of 1 MPa at laser fluences up to about 100 mJ/cm2. Simultaneous monitoring of the bubble growth kinetics by the optical specular reflectance probe has been performed. It is observed that the pressure generation is enhanced by the bubble expansion in the superheated water for laser fluences exceeding the bubble nucleation thresholds.

Photoacoustic spectroscopy on trace gases with continuously tunable CO(2) laser.

Abstract: A novel photoacoustic (PA) system that uses a continuously tunable high-pressure CO(2) laser as radiation source is presented. A minimum detectable absorption coefficient of 10(-6) cm(-1) that is limited mainly by the desorption of absorbing species from the cell walls and by residual electromagnetic perturbation of the microphone electronics has currently been achieved. Although a linear dependence of the PA signal on the gas concentration has been observed over 4 orders of magnitude, the dependence on energy exhibits a nonlinear behavior owing to saturation effects in excellent agreement with a theoretical model. The calibration of the laser wavelength is performed by PA measurements on low-pressure CO(2) gas, resulting in an absolute accuracy of ± 10(-2) cm(-1). PA spectra are presented for carbon dioxide (CO(2)), ammonia (NH(3)), ozone (O(3)), ethylene (C(2)H(4)), methanol (CH(3)OH), ethanol (C(2)H(5)OH), and toluene (C(7)H(8)) in large parts of the laser emission range. The expected improvement in detection selectivity compared with that of studies with line-tunable CO(2) lasers is demonstrated with the aid of multicomponent trace-gas mixtures prepared with a gas-mixing unit. Good agreement is obtained between the known concentrations and the concentrations calculated on the basis of a fit with calibration spectra. Finally, the perspectives of the system concerning air analyses are discussed.

Thermal diffusivity of solids determination by photoacoustic piezoelectric technique

Abstract: A simple approach for the calculation of photoacoustic piezoelectric response in the case of low modulation frequencies has been proposed. The analytical expressions have been obtained for the amplitude and phase of the photoacoustic signal and their experimental verification has been carried out. The new method for the thermal diffusivity determination from the phase versus frequency characteristics has been developed.

Noninvasive determination of blood constituents using an array of modulated laser diodes and a photoacoustic sensor head.

Abstract: Using photoacoustic laser spectroscopy, the noninvasive determination of blood constituents like hemoglobin and glucose is feasible. The aim of our investigations is the development of a sensor which is suitable for continuously noninvasive monitoring of blood glucose concentrations in diabetic patients. For this purpose a photoacoustic sensor head was developed and coupled via an optical fiber bundle to an array of 8 laser diodes emitting at various wavelengths in the near infrared region. Applying a special modulation scheme, the tiny changes of the absorption coefficient of whole blood caused by the variations of blood glucose concentrations could be measured. A resolution of 70 mg/dl was achieved, a value which is already close to the clinical requirements for a continuously working glucose sensor.

Quantitative Depth Profiling of Layered Samples Using Phase-Modulation FT-IR Photoacoustic Spectroscopy:

Abstract: In phase-modulation FT-IR spectroscopy, all wavelengths in a spectrum are modulated at the same frequency and in phase. This factor makes the use of photoacoustic phase data for depth profiling samples much easier in phase modulation than in rapid scan. A method to quantitatively measure layer thickness by using the phase of a substrate spectrum peak is demonstrated with a series of samples consisting of thin polymer films on substrates. Additions to the basic method are demonstrated that extend its application to cases where the substrate peak is overlapped by a spectrum peak of the surface film. A linear relationship between phase angle and layer thickness extending to thicknesses greater than twice the thermal diffusion length is demonstrated. Representations of phase modulation data as a family of angle-specific spectra, as magnitude vs. phase curves, and as a power spectrum and phase spectrum pair, each of which is useful for different aspects of depth profiling, are discussed. Calculating these representations from a single pair of orthogonal interferograms is described.

Temperature-dependent photoacoustic spectroscopy with a Helmholtz resonator

Abstract: CO-laser photoacoustic spectroscopy (PAS) is introduced to monitor the absorption of fatty acid vapors in the temperature range between 280 and 350 K. Since, the absorption as well as the photoacoustic (PA) signal depend on temperature, simultaneous transmission measurements have been performed to calibrate the PA signal at every temperature. A special PA cell acting as a Helmholtz resonator has been constructed fulfilling the requirements for temperature-dependent PA measurements. The results of numerical calculations of the acoustic properties of the PA cell are compared with experimental measurements with respect to the frequency response, the quality factor of the cell and the temperature dependence of the cell constant. As an example, the analysis of the obtained absorption spectra of deuterated acetic acid, which were recorded with this set-up, revealed the dimerization enthalpyΔH and entropyAS as well as the integrated band absorbances of the C=O-vibration of the monomer and dimer molecules at CO-laser wavelengths.

Optical and photoelectrical properties of ReS2 single crystals

Abstract: Optical absorption, photoacoustic spectroscopy and photoconductivity were investigated in ReS2 single crystals in the temperature range 50–300 K. The energy gap (1.55 eV at 80 K) and its temperature dependence, the value of the average phonon energy (17 meV) and the electron phonon coupling parameter (S=2.40) as well as the electron‐hole mobility due to lattice scattering were determined.

Image reconstruction from photoacoustic pressure signals

Abstract: Photoacoustic imaging theory is presented based on fundamental physical processes involved in photoacoustic phenomena. Excessive pressure is used to characterize the photoacoustic signal. A solution to photoacoustic pressure derived for liquids is investigated under a rapid heating condition. Photoacoustic image reconstruction theory is proposed for short, square heating pulses. The limitation of the reconstruction theory is discussed. Relationship of the photoacoustic image reconstruction and the Radon transform is presented. The local radiation absorption energy density is determined by the reconstruction theory for three- and two- dimensional cases and for complete or partial data acquisition. The accurate solution is an infinite series. The general formulas of the zero- and first order approximations for three- and two-dimensional cases are provided. A computer simulation for a two-dimensional case exhibits good agreement between the first order approximation and true value.

Detection of NO and NO(2) by (2 + 2) resonance-enhanced multiphoton ionization and photoacoustic spectroscopy near 454 nm.

Abstract: Trace concentrations of NO and NO(2) are detected with a dye laser operating near 454 nm. NO is detected by a (2 + 2) resonance-enhanced multiphoton ionization process by means of NO A(2)Σ+-X(2)Π(0, 0) transitions with miniature electrodes, and NO(2) is detected by a one-photon absorption photoacoustic process by means of NO(2)A¯' (2)B(1)(0, 8, 0)- X¯ (2)A(1)(0, 0, 0) transitions with a miniature microphone. Rotationally resolved excitation spectra show that the spectral resolution is sufficiently high to identify these species at 1 atm. The technique's analytical merits are evaluated as functions of concentration, pressure, and laser intensities. Low laser intensities favor NO(2) photoacoustic detection whereas high laser intensities favor NO ionization. Limits of detection (signal-to-noise ratio 3) of 160 parts in 10(9) for NO and 400 parts in 10(9) for NO(2) are determined at 1 atm for a 10-s integration time. Signal response and noise analyses show that three decades of NO/NO(2) mixtures can be measured with a computational relative error in concentration that is three times the relative error in measuring the NO and NO(2) signals.

Photoacoustic thermal characterization of spark‐processed porous silicon

Abstract: A non‐separation approach to determine the spark‐processed porous silicon thermal parameters is presented. This thermal characterization was performed through application of the photoacoustic technique, in combination with compositional models for spark‐processed porous silicon samples. The thermal parameters obtained are in agreement with existing studies about the composition of this material. This approach opens the possibility of performing the thermal characterization of other porous semiconductors and analogous materials.

Thermal diffusivity of conducting polypyrrole

Abstract: We have used the photoacoustic technique in an open photoacoustic cell configuration to measure the thermal diffusivity of conducting polypyrrole films. The samples were obtained by electrochemical deposition from solutions of pyrrole dissolved in 0.08 M p‐toluene sulfonic acid and their thicknesses were in the 50–125 μm range. The thermal diffusivity α was found to be very dependent on the synthesis conditions.

Direct evidence of the liquid-solid transition of a sol-gel material using photoacoustic spectroscopy

Abstract: The gelation process of a sol−gel material is monitored for the first time in a detailed manner by use of a photoacoustic spectroscopy derived methodology. Measurements of the acoustic response spectrum of a sol−gel material during the transition permits a clear monitoring of the sol−gel evolution as well before macroscopic gelation as afterward. Different phases are inferred corresponding to the three first proceeding steps of the material formation.

Fourier Transform Infrared Photoacoustic Spectroscopy of Surface-Treated Wool

Abstract: Infrared photoacoustic spectroscopy (PAS) is a useful technique for examining sur face-treated wool samples. Sample preparation is simple, and reproducible spectra of adequate signal-to-noise ratio are obtained in a few minutes. The ability of PAS to vary the penetration depth by varying the optical path difference velocity is used to obtain spectra of the near surface region of the samples. These spectra are compared with those taken with a greater penetration depth, which therefore represent more of the bulk wool sample. Samples are chlorinated and chlorinated/neutralized wool from a shrinkproofing process, and wool treated with a proprietary fluoropolymer.

Photoacoustic Spectra of CdInGaS4 with a DC Electric Field

Abstract: Photoacoustic (PA) spectroscopy is used to study the heat generated by nonradiative processes of CdInGaS4 with an external dc electric field transverse to the light path in order to study the correlation between carrier diffusion effects and nonradiative processes in the wavelength range of 310 to 650 nm. Maxima are observed in PA spectra at 2.7 and 2.3 eV, the amplitudes of which values are proportional to the square of the applied voltage. The peak at 2.7 eV shifted toward higher energy when the modulation frequency increased. That at 2.3 eV did not shift. Therefore, there is a possibility that the peaks at 2.7 and 2.3 eV are related to the electron diffusion length and the impurity or deep levels of CdInGaS4, respectively. PA phase spectra showed minima at 2.4 eV, which shifted toward lower energy when the dc electric field strength increased.

Photoacoustic spectroscopy of diluted magnetic semiconductors

Abstract: The absorption coefficient of several diluted magnetic semiconductors was measured below the fundamental absorption edge of the pure host compound at room temperature using the photoacoustic technique. Several absorption bands have been detected and assigned to electronic transitions between the crystal field split 3d levels of the transition metal ion. In addition, weak absorption thresholds have been observed and assigned to the ionization of the transition metal d states. An energy level diagram of the d states has been discussed.The absorption coefficient of several diluted magnetic semiconductors was measured below the fundamental absorption edge of the pure host compound at room temperature using the photoacoustic technique. Several absorption bands have been detected and assigned to electronic transitions between the crystal field split 3d levels of the transition metal ion. In addition, weak absorption thresholds have been observed and assigned to the ionization of the transition metal d states. An energy level diagram of the d states has been discussed.

Investigation of cavitation bubble dynamics using particle image velocimetry: implications for photoacoustic drug delivery

Abstract: Photoacoustic drug delivery is a technique for delivering drugs to localized areas in the body. In cardiovascular applications, it uses a laser pulse to generate a cavitation bubble in a blood vessel due to the absorption of laser energy by targets (e.g., blood clots) or surrounding liquids (e.g., blood or injected saline). The hydrodynamic pressure arising from the expansion and collapse of the cavitation bubble can force the drug into the clots and tissue wall tissue. Time-resolved particle image velocimetry was used to investigate the flow of liquids during the expansion and collapse of cavitation bubbles near a soft boundary. A gelatin-based thrombus model was used to simulate the blood clot present during laser thrombolysis. An argon laser chopped by an acousto-optic modulator was used for illumination and photography was achieved using a CCD camera. The implications of this phenomenon on practical photoacoustic drug delivery implementation are discussed.

Transport properties of carriers in GaAs obtained using the photoacoustic method with the transmission detection configuration

Abstract: Electron transport properties of single-crystal GaAs were determined using the photoacoustic method with the transmission detection configuration. The excess-carrier lifetime, the front and the rear recombination velocity, and the coefficients of the carrier diffusion and the thermal diffusivity were determined by comparing experimental results and theoretical photoacoustic signals.

Detection of Ambient NO by Laser-Induced Photoacoustic Spectroscopy Using A(2)Sigma(+)- X(2)Pi (0,0) Transitions Near 226 nm,

Abstract: Trace concentrations of NO are detected under ambient conditions by laser-induced photoacoustic spectroscopy. NO is excited via its A2S+ - X2P (0,0) band with radiation near 226 nm, and the subsequent heat released is monitored by a microphone. Rotationally resolved photoacoustic spectra are recorded and fit with the use of a multiparameter computer simulation based on a Boltzmaun distribution. Transition probabilities and rotational energies are used as input parameters. The effect of buffer gas pressure, buffer gas, laser energy, and NO concentration on the PA signal is investigated both experimentally and by model calculations. Limits of detection (LODs) of 1.2, 2.8, and 4.9 ppm are obtained for NO in Ar, N2, and air, respectively. The ultimate sensitivity of this approach is greater with LODs projected in the low-ppb range by utilizing higher laser energies and an improved system design. The results are compared with those of previous studies using complementary laser-based spectroscopic techniques.

Photoacoustic spectroscopic estimation of optical absorption and thermal diffusivity of amorphous Ge15AsxSe85−x

Abstract: Photoacoustic spectroscopy (PAS) has been applied to measure the composition dependent optical energy gap (Eo), optical absorption coefficient (α), and thermal diffusivity (σs) of glassy Ge15AsxSe85−x (0≤x≤25) alloys. The energy gap is found to decrease with increase of As concentration and shows a threshold behavior around x∼15, which corresponds to the average coordination number 〈r〉∼2.45. This behavior is found to be consistent with the Phillips‐Thorpe theory. The variation of the optical gap (Eo) with composition (x) is analyzed on the basis of the Kastner’s model of bond energies. The observed exponential edge (Urbach edge) can be considered as an internal Franz‐Keldysh effect arising due to the charged impurity generated, as well as ‘‘frozen‐in’’ optical phonon‐generated, electric microfields. It could be described in the framework of Tauc and Dow‐Redfield model which ascribes the Urbach rule to the ionization of the exciton as an extension of the stark shift. The concentration (x) dependent thermal diffusivity (σs) estimated from the PAS studies also showed a similar critical behavior at the same concentration x (≊15) which arises due to the threshold percolation of rigidity of the system. The measurements of glass transition temperatures (Tg) and the magnetic susceptibilities (χ) of the samples also support this critical behavior around x=15.

Photoacoustic location of optical absorbers in phantom tissue

Abstract: Pulsed photoacoustic (PA) signals may be used for the detection and imaging of blood vessels in tissue. A relatively strong absorption by red blood cells and low absorption by the surrounding tissue, combined with a reasonable penetration depth of the light is found at a wavelength of ca. 577 nm. Experiments were performed with a pulsed frequency doubled Nd:YAG laser which delivered 10 ns pulses at 532 nm wavelength. Ten percent dilutions of India ink and 50% suspensions of red blood cells in PBS were used as optical absorbers. Blood vessels were simulated by hollow nylon fibers with an inner diameter of ca. 250 micrometer through which these suspensions flow. The optical scattering of the surrounding tissue was simulated by a 12% dilution of Intralipid-10% to get a solution with a reduced scattering coefficient of 1.8 mm-1. The PA signals were detected with a hydrophone that contained four wide band piezoelectric transducers made of 9 micrometer thick PVdF film with an effective diameter of 200 micrometers. Laser pulses with energies up to 8 microjoules were delivered to the sample by a 50 or 100 micrometer core diameter glass fiber. Pulsed optical heating of red blood cells up to 30 - 35 degrees for more than 12,000 times did not affect the photoacoustic response of the cells. If a single fiber is used to illuminate the sample, then even at a depth of 1 mm the PA signals show that the volume that is effectively illuminated is laterally restricted to a diameter of ca. 1 mm. Vessels with blood or ink dilutions were detected up to a depth of more than 1 mm in the scattering medium. Monte- Carlo (MC) simulations were used to simulate the spatial distribution of light absorption in phantom tissue. From this distribution the PA response of blood vessels was simulated. A delay-and-sum beam forming algorithm was developed for 3-D near field configurations and applied to a PA image reconstruction program. The images based on MC simulations as well as experimental data show that the side of larger vessels that is facing the illuminating fiber can be located with a resolution that depends on the configuration and varies between 0.1 and 1 time the inner vessel diameter. This shows the principle and the feasibility of three dimensional photoacoustic dermal tissue imaging.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Simulation of photoacoustic signal production in human breast phantoms at 1064 nm

Abstract: Photoacoustic signals generated by breasts irradiated with short microwave, infrared or optical pulses could be used to detect breast cancer. Since radiation at this spectrum is non-ionizing, the photoacoustic approach provides a special safety feature. The purpose of the paper is to present a means to predict photoacoustic pressure signals for different breast phantoms and irradiation conditions. The photoacoustic wave equation was derived for linear, non-viscous liquid media. The equation was solved assuming uniform acoustic properties in an infinite medium. Compressed breast phantoms were used as the objects of simulation. The spatial dependence of electromagnetic energy absorption was given by another research paper of this conference. The time dependence of the absorption was assumed to be either uniform or bell- shaped. Photoacoustic pressure signals received by transducers at different locations were calculated numerically.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.