Pulsed Laser Photoacoustic Research Articles

On-line thin film thickness monitor by pulsed laser photoacoustics

On-line thickness monitor of thin film deposition by thermal evaporation is demonstrated using femtosecond pump-probe photoacoustics. A Ti:sapphire femtosecond laser (wavelength 800 nm, pulse width ~110 fs, pulse energy ~1.3 nJ) is split into a pump and a probe beams. The pump beam is used to instantaneously heat the illuminated area of an aluminum thin film during its deposition on a silicon substrate, whereas the probe beam is used to investigate the change in the transient thermoreflectance ΔR/R. The thermal expansion, by the laser heating, creates stress in the aluminum surface region that generates an acoustic wave of ultrasonic frequency. The travel time of the optically-induced acoustic wave normal to the surface of the thin film is measured to evaluate the thickness of the aluminum in the range of ~32 to 388 nm during the film deposition.

Spectral analysis for photoacoustic pressure sensor designs: Theoretical model improvement and experimental validation

In the pulsed laser photoacoustic (PA) detection and spectroscopy applications, the fundamental frequency of the PA signal produced, and the sensor resonance frequency should be as close as possible to each other so that analyzes from the obtained signals can be performed effectively. In order to determine the fundamental frequency of the PA wave, a theoretical model approach based on the development of the frequency domain solution of the PA wave equation is presented for use in the PA pressure sensor designs. For the validation of the theoretical model approach, a PA experimental setup was established, and measurements were made in distilled water. The theoretical and the experimental PA frequency spectra were determined to be very compatible with each other. Thus, the theoretical model approach was experimentally validated. According to the theoretical model approach, fundamental frequency values obtained from the experimental measurement results were determined with an average accuracy of ∓ 4.212%. Furthermore, it has been determined that this value has fallen to ∓0.267% in the measurements. With the obtained results from the theoretical model approach, we propose that the PA pressure sensors with the more selective and narrower band can be designed for the more sensitive detection. Moreover, in this study the effects of different laser parameters such that pulse duration, and laser beam width, on the spectral content of the obtained PA signal are analyzed. These analyses will shed light on the vision of acoustic pressure sensor design by helping to select the most optimum parameters for the PA detection.

In situ study of porous silicon thin films thermal oxidation by pulsed laser photoacoustics

Passivation is of remarkable importance for porous silicon (PSi) applications to guarantee the chemical stability of its high surface area through time. Thermal oxidation is one of the most common methods to passivate the surface of PSi. In order to better understand the oxidation process, here we performed an in situ study by using pulsed laser photoacoustics. During the thermal oxidation, the photoacoustical signal was measured each 1 °C from 70 °C to 900 °C. The measurements were analyzed by standard correlation, and the significant signal changes were related to the presence of different surface species. We have found temperatures, where the photoacoustical signal changes drastically, that can be related to the absorption and desorption of chemical species in the surface. Species identification where made through Fourier-transform infrared spectroscopy-attenuated total reflection analysis of the PSi samples, at temperatures around where these notorious modifications on the surface were observed.

Determination of Tequila Quality by Photoacoustic Analysis

A pulsed laser photoacoustic (PLPA) technique is proposed to distinguish original from adulterated tequila. In fact, it brings a reliable cheaper and more sensible method in adulteration detection, in comparison with traditional techniques. The method proposed is comparative and non-destructive, and it is based on a correlation analysis of photoacoustic signals, obtained by exciting tequila samples with short laser pulses (7 ns), in the UV region (355 nm). Eleven samples of tequila were analyzed. From a reference sample, all other samples were classified.

Characterization of an innovative drug carrying photoacoustic contrast agent: fluorescent polymer microcapsules

Local drug delivery is studied to cross biological barriers and reduce the systemic side effects. Multimodal agents are being developed to combine step-response activation with the monitoring of its triggered release. The release can be triggered by acoustical or thermal means, e.g. using thermo-sensitive liposomes. Here we design an optically triggered microcarrier with well-controlled release precision and, in addition, a strong acoustic response in the far field, making the carrier a highly specific photoacoustic agent. The novel biocompatible microcapsules with a shell of fluorinated poly-L-lactic acid mixed with a fluorescent dye were produced with hexadecane oil core as drug-carrier reservoir. Single capsules were excited by a pulsed laser and their responses were monitored through combined ultra-high-speed imaging and sensitive acoustic detection. The experiments support a model where the polymer heats up through dye absorption thereby inducing the shell destruction and the vaporization of the surrounding water, resulting in the core release. Beyond the classical pulsed laser photoacoustics, capsules also respond to CW laser excitation by emitting a continuous signal, which offers promising opportunities for real-time photoacoustics. The subsequent study shows that the prolonged response results from repeated vaporization cycles and a complex interaction of the laser with the dyed polymer.

Optical Absorbance Measurements of Opaque Liquids by Pulsed Laser Photoacoustic Spectroscopy

In many relevant industrial applications, UV-vis online process monitoring is hampered by light scattering and opacity of the samples, whereas diluted and filtered samples are rarely available. Pulsed laser photoacoustic (PA) spectroscopy allows the measurement of both high and low absorptions without any need for sample preparation. An optimized detection geometry for absorption measurements in opaque liquids is described. The proposed PA sensor was realized by using two orthogonal detectors based on piezoelectric poly(vinylidene fluoride) (PVDF). Laser-induced pressure waves were sensed perpendicularly to (side-on mode) and along the axis of the laser beam (forward mode). Pressure waves generated by a single laser pulse, optical transmission and absorption, as well as the speed of sound in liquid samples were determined simultaneously using time-resolved detection. Evaluation of the PA signal permits the determination of absorption coefficients ranging from 0.1 to 1000 cm(-1). The influence of absorbing or scattering compounds on the signal was investigated in dye solutions and suspensions of TiO(2) particles.

Optical absorption photoacoustic measurements for determination of molecular symmetries in a dichroic organic-film

A novel approach to specify symmetries and main optical axes in anisotropic polymeric films is proposed. This method is based on the analysis of the optical absorption via the pulsed laser photoacoustic (PLPA)- technique in a common polarizer film, while rotating the polarizer axis at normal incidence. Since the PLPA-signals are directly proportional to the optical absorption, it is shown that a symmetric and complementary Malus's law can be obtained over full root mean square (RMS)- and correlation (CA)-analysis of the PLPA-signals. Such data processing reveals the main material directions of the constituting film molecules defining the symmetry structure of the sample. PLPA-results were compared to the pure optical transmission experiments and show unambiguous information, allowing this technique to be used in nonstandard and opaque polymeric films, where the analysis of the optical measurements represents a difficult task, and in general, in anisotropic media.

Photophysics of Rhodamine B Interacting with Silver Spheroids

Deactivation processes of Rhodamine B (RB) in water and interacting with silver sols were studied by using conventional fluorescence, laser-induced fluorescence, and pulsed laser photoacoustics. Results showed that the emission efficiencies, absorption cross sections, and nonradiative deactivation pathway of RB interacting with silver spheroids of different sizes are comparable to those obtained for the dye in aqueous solutions.

24] Deconvolution analysis for pulsed-laser photoacoustic

Publisher Summary This chapter describes time-resolved, pulsed-laser photoacoustics and the way its data can be deconvolved to give kinetic and volumetric information. The process uses two analysis methods common to time-resolved fluorescence decay—namely, nonlinear least-squares iterative reconvolution and the method of moments. The analysis techniques are illustrated in the chapter with experimental and synthetic data. The processes involved in the pulsed-laser photoacoustic experiment involve absorbing of a photon from a laser pulse by a molecule in solution. The purpose of photoacoustic waveform deconvolution is to obtain information on the rate and magnitude of volume changes (for example, owing to heat release) following the absorption of a photon. The chapter describes the features of photoacoustic waveform analysis. The preprocessing of photoacoustic data is necessary before waveform deconvolution is performed. The method of moments has been developed primarily for the deconvolution and analysis of fluorescence decay data. The method of moments is a transform method. Both nonlinear least-squares iterative reconvolution and the method of moments can be used to analyze time-resolved, pulsed-laser photoacoustic waveform data accurately. Both techniques adequately return the magnitude of fast components and the magnitude and lifetime of slower components. The nonlinear least-squares method is somewhat easier to conceptualize and interpret, but the method of moments has strength in being robust with respect to the nonrandom errors, undoubtedly present in experimental photoacoustic data.

Analysis of photoacoustic waveforms using the nonlinear least squares method

Pulsed-laser photoacoustics is a technique which measures photoinduced enthalpic and volumetric changes on the nano- and microsecond timescales. Analys

ChemInform Abstract: PULSED LASER PHOTOACOUSTIC CALORIMETRY OF METASTABLE SPECIES

Chemischer InformationsdienstVolume 14, Issue 36 Article ChemInform Abstract: PULSED LASER PHOTOACOUSTIC CALORIMETRY OF METASTABLE SPECIES L. J. ROTHBERG, L. J. ROTHBERGSearch for more papers by this authorJ. D. SIMON, J. D. SIMONSearch for more papers by this authorM. BERNSTEIN, M. BERNSTEINSearch for more papers by this authorK. S. PETERS, K. S. PETERSSearch for more papers by this author L. J. ROTHBERG, L. J. ROTHBERGSearch for more papers by this authorJ. D. SIMON, J. D. SIMONSearch for more papers by this authorM. BERNSTEIN, M. BERNSTEINSearch for more papers by this authorK. S. PETERS, K. S. PETERSSearch for more papers by this author First published: September 6, 1983 https://doi.org/10.1002/chin.198336098AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume14, Issue36September 6, 1983 RelatedInformation