M. D. Rabasović

Low-cost, portable photoacoustic setup for solid samples

We have developed a low-cost, portable photoacoustic instrument. The device consists of a detection unit comprising a photoacoustic cell with an embedded laser diode or a light-emitting diode, a photodiode, an electret microphone (60 ? 40 ? 40 mm3), and a signal processing and power supply unit in a box containing batteries and electronics (160 ? 140 ? 60 mm3). A PC or portable computer is required to operate the device and for data processing. The weight of the instrument without the computer is 1.70 kg. The computer, or more precisely, its sound card is the essential part of the apparatus because it generates the signal for the laser diode or light-emitting diode modulation and processes signals from the microphone and photodiode. The computer sound card is used as a dual-phase lock-in amplifier. The software used for the control of the setup was also developed in course of this work. The photoacoustic instrument presented here allows measurements and quantitative analysis of numerous solid-state samples. It is simple in design and use, having a reasonable weight and portability.

Pulsed photoacoustic system calibration for highly excited molecules

The proposal of a simple method for pulsed photoacoustic system calibration is presented. Analysis of the photoacoustic signal shape is performed with different types of absorbing molecules (SF6 and C2H4) and the same buffer gas (Ar). Different total pressures (ptotal) of such gas mixtures and different laser fluences are used to obtain experimental results and compare them with theory. Results from such a comparison are used directly for the calculation of a calibration curve. A single experimental point calibration procedure for the case of highly vibrationally excited absorbing SF6 molecules is confirmed.

Pulsed photoacoustic system calibration for highly excited molecules: II. Influence of the laser beam profile and the excitation energy decay

Previously, we developed a simple semi-theoretical calibration procedure for a pulsed photoacoustic set-up. Also, we analysed the simplest, idealized case: top hat spatial profile of the laser beam and an exponential decay of excitation energy. The spatial profile of the laser beam is usually considered to be top hat or Gaussian in the photoacoustic measurements. In reality, there are always small discrepancies. Also, in these measurements the excitation energy decay is usually considered to be an exponential one. This assumption is commonly valid. Still, a non-exponential decay can exist as well. In this paper, we have examined the influence of these discrepancies on the measurement of the vibrational-to-translational relaxation time, as well as on the calibration of the photoacoustic set-up. We have theoretically examined gas mixtures in the case of pulsed excitation (multiphoton regime). Then, we have verified theoretical conclusions in one experimental example. It has been shown that the non-ideal profile and excitation energy decay can significantly influence the measurement of the vibrational-to-translational relaxation time. Also, it has been shown that they do not significantly influence the calibration of the photoacoustic set-up.

Effect of the absorption coefficient of aluminium plates on their thermoelastic bending in photoacoustic experiments

The open-cell photoacoustic signal measured in the transmission configuration for aluminum thin plates with thicknesses of 280 μm, 197 μm, and 112 μm is experimentally and theoretically analyzed, in the 20 Hz–7 kHz modulation frequency range. It is shown that the observed differences between the predictions of the standard thermoelastic model and the experiment data of both the amplitude and phase of the photoacoustic signal can be overcome by considering the aluminum samples coated with a thin layer of black paint as volume-absorber materials. This new approach provides a quite good agreement with the obtained experimental data, in the whole frequency range, and yields an effective absorption coefficient of (16 ± 2) mm−1, for a 280 μm-thick sample. The introduction of the finite absorption coefficient led to the correct ratio between the thermal diffusion and thermoelastic components of the photoacoustic signal. Furthermore, it is found that the “volume-absorber” approach accurately describes the behavior of the amplitude, but not that of the phase recorded for a 112 μm-thick sample, due to its relatively strong thermoelastic bending, which is not considered by this theory. Within the approximation of the small bending, the proposed “volume-absorber” model provides a reliable description of the photoacoustic signal for Al samples thicker than 112 μm, and extends the applicability of the classical “opaque” approach.

Laser beam spatial profile determination by pulsed photoacoustics: exact solution

The pulsed photoacoustic spectroscopy technique is one of the oldest but respectable techniques which offer some unique features that are relevant to trace gas monitoring. Despite obvious advantages there are some drawbacks that have so far alienated this spectroscopy technique from routine air monitoring. One of them is the additional instrumentation necessity to fulfill the requirement for an accurate knowledge of the spatial and temporal profiles of the excitation light source. Here we present one method for a laser beam spatial profile determination and simultaneous vibrational–translational relaxation time calculation. It is based on the temporal shape analysis of the photoacoustic signals. Symmetric laser beam spatial profiles are obtained with a high level of accuracy. Even though this method does not work in real time, it gives a good basis for future beam profile investigations. Experimental signals used in our analysis are obtained after infrared multiphoton absorption in the SF6–Ar mixture.

Nonlinear microscopy of chitin and chitinous structures: a case study of two cave-dwelling insects

Abstract. We performed a study of the nonlinear optical properties of chemically purified chitin and insect cuticle using two-photon excited autofluorescence (TPEF) and second-harmonic generation (SHG) microscopy. Excitation spectrum, fluorescence time, polarization sensitivity, and bleaching speed were measured. We have found that the maximum autofluorescence signal requires an excitation wavelength below 850 nm. At longer wavelengths, we were able to penetrate more than 150-μm deep into the sample through the chitinous structures. The excitation power was kept below 10 mW (at the sample) in order to diminish bleaching. The SHG from the purified chitin was confirmed by spectral- and time-resolved measurements. Two cave-dwelling, depigmented, insect species were analyzed and three-dimensional images of the cuticular structures were obtained.

Mapping of hemoglobin in erythrocytes and erythrocyte ghosts using two photon excitation fluorescence microscopy

Abstract. The present study describes utilization of two photon excitation fluorescence (2PE) microscopy for visualization of the hemoglobin in human and porcine erythrocytes and their empty membranes (i.e., ghosts). High-quality, label- and fixation-free visualization of hemoglobin was achieved at excitation wavelength 730 nm by detecting visible autofluorescence. Localization in the suspension and spatial distribution (i.e., mapping) of residual hemoglobin in erythrocyte ghosts has been resolved by 2PE. Prior to the 2PE mapping, the presence of residual hemoglobin in the bulk suspension of erythrocyte ghosts was confirmed by cyanmethemoglobin assay. 2PE analysis revealed that the distribution of hemoglobin in intact erythrocytes follows the cells’ shape. Two types of erythrocytes, human and porcine, characterized with discocyte and echinocyte morphology, respectively, showed significant differences in hemoglobin distribution. The 2PE images have revealed that despite an extensive washing out procedure after gradual hypotonic hemolysis, a certain amount of hemoglobin localized on the intracellular side always remains bound to the membrane and cannot be eliminated. The obtained results open the possibility to use 2PE microscopy to examine hemoglobin distribution in erythrocytes and estimate the purity level of erythrocyte ghosts in biotechnological processes.

Luminescence thermometry via the two-dopant intensity ratio of Y2O3:Er3+, Eu3+

In this work we investigated the photoluminescence properties of Y2O3:Er3+, Eu3+ as a function of temperature and the possibility to use this material as a temperature sensor. Photoluminescence emission measurements with 532 nm laser excitation were recorded in the temperature range from 303 up to 573 K. The measured intensity ratio of erbium 4S3/2 → 4I15/2 and europium 5D0 → 7F2 emission lines was used for determination of the temperature calibration curve. These emission lines are intense, narrow and well defined. The distance between the lines, being 47 nm, can be easily measured even with a low-resolution spectrometer. The calculated relative sensitivity of the temperature sensor was 1.4% K−1 at 303 K, in the physiological temperature range, meaning that it could be successfully applied in biological studies.

Photoacoustic elastic bending in thin film–substrate system: Experimental determination of the thin film parameters

The aim of this work is the development of photoacoustic (PA) method for the measurement and determination of parametres of thin films (with a thickness of less than 1 μm). Experimental study of the optical, thermal, and elastic characteristics of the thin film on Si substrate by PA elastic bending method was given. Thin film–semiconductor (Si) sample is modeled by simultaneous analysis of the plasma, thermal, and elastic wave equations. Two normalization procedures of the PA elastic bending signal in function of the modulation frequency of the optical excitation were established. The experimental PA elastic bending signals were measured and analysed. Without loss of generality, the TiO2 thin film (with a thickness of 0.5 μm) on Si substrate (circular plate) was experimentaly studied. We have studied the PA elastic bending signals in order to obtain the values of optical, thermal, and elastic parameters of TiO2 film. The analysis shows that it is possible to develop noncontact and nondestructive experimental method—PA elastic bending method for thin film study, with possibility to obtain the optical, thermal, and elastic parameters of the film thinner than 1 μm.

Photoacoustic elastic bending in thin film—Substrate system

Theoretical model for optically excited two-layer elastic plate, which includes plasmaelastic, thermoelastic, and thermodiffusion mechanisms, is given in order to study the dependence of the photoacoustic (PA) elastic bending signal on the optical, thermal, and elastic properties of thin film—substrate system. Thin film-semiconductor sample (in our case Silicon) is modeled by simultaneous analysis of the plasma, thermal, and elastic wave equations. Multireflection effects in thin film are included in theoretical model and analyzed. Relations for the amplitude and phase of electronic and thermal elastic bending in the optically excited two-layer mechanically-supported circular plate are derived. Theoretical analysis of the thermodiffusion, plasmaelastic, and thermoelastic effects in a sample-gas-microphone photoacoustic detection configuration is given. Two normalization procedures of the photoacoustic elastic bending signal in function of the modulation frequency of the optical excitation are established....