Control of tissue mechanics upon the repetitive-pulse laser heating of cartilage

The viscoelastic properties of laser-irradiated cartilages are studied with optoacoustic methods upon the thermal excitation of mechanical oscillations by repetitive-pulse laser radiation. The effect of laser power, pulse duration, repetition rate, and irradiation time on the shape of the optoacoustic signal is analyzed. It is demonstrated that the optoacoustic response of the cartilage to the repetitive-pulse radiation of a fiber laser depends on the softening of the tissue upon the variation in its shape. Under repetitive-pulse laser irradiation, the optoacoustic response of the cartilage depends on the mechanical characteristics of the biotissue (elastic modulus, hydraulic permeability, and thickness). A simple model that makes it possible to estimate the contribution of the viscoelastic properties to the formation of the optoacoustic response at various laser repetition rates is proposed.     

Optoacoustic cross-section measurements for ir pulses: CO2 laser absorption by CF3I

The pulsed optoacoustic method is studied to measure absolute infrared multiphoton absorption cross sections. The influence of the thermalisation by the walls of the cell is shown to be very important at low pressure. This influence is analysed both experimentally and theoretically by solving the coupled diffusion and relaxation equations for vibrational and translational energies. The sensitivity of the method is limited by a spurious pressure signal present even with non-absorbing gases. This parasitic signal is attributed to an absorption located on the inner surface of the windows of the cell. For instance, using KCl windows, the observed spurious signal corresponds to about 10⁻⁴ of the total laser energy, transmitted from the windows to the gas. This proportion is independent on the energy fluence. Taking care of these limitations, we have measured the absorption cross section of CF₃I at the different wavelengths of the (001–020) transitions of the CO₂ laser. The typical energy fluences in these experiments were varied from 10⁻³ to 1J/cm². To be in collision free conditions, the CF₃I pressure was made equal to 0.2 Torr.     

Optoacoustic analysis of the laser-cleaning process

Laser cleaning is an optodynamic process in which the optically induced removal of a liquid or a solid contaminant from a substrate is accompanied by a optoacoustic wave in the surrounding air. In our experiments we used both dry and steam laser-cleaning techniques for various samples. Optoacoustic wave, produced by the abrupt heating and detachment of the contaminants, was observed with a probe-beam deflection technique. We determined two characteristic parameters of the optoacoustic wave: the amplitude and the time-of-flight of the acoustic signal. With an analysis of these waves we also determined possible generating mechanisms. The decrease of the amplitude and the velocity of propagation, which approaches sonic speed, of the consecutive waves indicate that the dynamics during the laser-cleaning process is progressively weakened. The cleaning process is over when both the parameters reach a constant value, so with measuring optoacoustic waves the progress of the cleaning process could be observed on-line.     

Nonlinear optoacoustic transformation in the system of dielectric substrate/submicron metal coating/liquid

The optoacoustic method has been shown to be an accurate technique for the measurement of the properties of submicron metal coatings deposited on a dielectric substrate, i.e., mirrors. The method has been previously theoretically described in terms of a linear model of optoacoustic transformation in a system substrate/coating/liquid. The goal of the present work was to determine the limits at which the linear model is still applicable. The modification of the laser induced acoustic signal profiles and transfer functions of optoacoustic transformation versus the laser fluence was studied for two liquids: ethanol and water.     

Doppler-free optoacoustic spectroscopy

The first observation of Doppler-free optoacoustic spectroscopy is reported. As a first example the P (193) line of the 11-0 band of the B←X transition of ¹²⁷I₂ is used. The output of cw single mode dye laser is split into two equal intensity beams chopped at frequencies ω₁ and ω₂. The nonlinear compoment of the optoacoustic signal at the frequency (ω₁ + ω₂) is detected and Doppler-free resolution is obtained. Comparing the Doppler-free optoacoustic and fluorescence spectra of iodine measured under similar conditions, good agreement is found. Since optoacoustic and fluorescence methods complement each other, this opens up new possibilities for weakly or nonfluorescing molecules.     

Time-resolved laser optoacoustic tomography of inhomogeneous media

The methods of time-resolved laser optoacoustic tomography of inhomogeneous media and related problems are reviewed. Time-resolved laser optoacoustic tomography allows one to measure the distribution of light absorption in turbid media with depth resolution up to several microns in real time. The theory of laser excitation of acoustic waves by absorbing of light in particles, dispersed in transparent, light-absorbing or scattering media, is developed. The distribution of light absorption can be obtained from the temporal course of acoustic pressure. Two schemes of acoustic wave detection — in the medium under testing (direct detection) and in transparent medium, coupled to the investigated one (indirect detection) — are discussed. In both cases the reconstruction of light absorption can be made by simple calculations. Test experiments with homogeneous and layered media confirm the proposed theoretical models and the possibility of using the proposed experimental schemes. Light absorption in homogeneous, inhomogeneous media and in absorbing particles dispersed in turbid media was investigated. The experimental setup allows one to measure the absorption coefficients over the range 1-500 cm^–1 with the depth resolution 10–15 m over the depth 1–1.5 mm.     

Phase Analysis in Solid-Sample Optoacoustic Spectrometry

The optoacoustic effect was discovered by Bell in 1881¹ and since the early investigations of this phenomena many workers have employed the effect for gas analysis^2,3 and the study of gaseous reaction intermediates⁴. The corresponding applications of optoacoustic spectrometry (OAS) to solid samples however, has received little attention, and it is only in recent years that analytical data for solid samples have been reported. Harshbarger and Robin⁵, Rosencwaig⁶ and Adams et al ⁷ have published the results of studies demonstrating the application of OAS to solid samples and have discussed the advantages of this technique over more conventional optical spectroscopic methods of analysis. In each of the systems described for the examination of solids by OAS, UV/visible radiation from a high power continuum source is modulated using a rotating sector and, after dispersion using a monochromator, is incident upon the sample mounted within a suitable closed cell which also contains a microphone transducer. Energy absorbed from the source may appear as heat in the sample and give rise to a periodic pressure rise in the gas which surrounds the sample. This periodic pressure rise is monitored by the microphone. When the wavelength of the incident radiation is scanned a spectrum of the sample is obtained which is the product of the optical electronic absorption spectrum of the sample and the emitted power spectrum of the source employed. To correct for the variation in incident radiant power with wavelength it is necessary to normalize the spectra obtained by reference to a blackbody absorber. This normalization may be achieved manually⁷, with a stored pre-recorded reference spectrum⁶ or simultaneously during the wavelength scan using a double-beam spectrometer.     

Optoacoustic monitoring of blood hemoglobin concentration: a pilot clinical study

The optoacoustic technique is noninvasive, has high spatial resolution, and potentially can be used to measure the total hemoglobin concentration ([THb]) continuously and accurately. We performed in vitro measurements in blood and in vivo tests in healthy volunteers. Our clinical protocol included rapid infusion of intravenous saline to simulate rapid change in the [THb] during fluid therapy or surgery. Optoacoustic measurements were made from the wrist area overlying the radial artery for more than 1 h. The amplitude of the optoacoustic signal generated in the radial artery closely followed the [THb] measured directly in concurrently collected blood samples.     

Frequency Stabilization of Waveguide CO2 Laser by a Digital Technique

In this work we present a simple technique for laser frequency stabilization, based on Digital signal processing. The technique is used to stabilize a waveguide CO₂ laser of wide tunability by using three kinds of reference signals: the CO₂ laser ouptut power, an infrared absorption optoacoustic signal and the output power of a Far-Infrared optically pumped molecular laser.     

Optoacoustic detection of nonlinear absorption in glasses

We report what we believe to be the first measurement of two photon absorption in a solid using the optoacoustic effect. A number of different glasses were studied. The dispersion of the optoacoustic signal of holmium oxide doped glass is compared with the corresponding absorption spectrum at twice the frequency of the incident light.     

Experimental and theoretical investigation of photoacoustic-signal generation by wavelength-modulated diode lasers

The dependence of photoacoustic spectra on different experimental parameters was investigated by both theoretical and experimental means. The experiments were carried out with an inexpensive resonant optoacoustic system based on near-infrared laser diodes, which allowed photoacoustic and direct absorption spectra to be recorded simultaneously. The experimental observations were compared to theoretical predictions. It was also demonstrated that source-frequency (wavelength) modulation at the resonance frequency of the cell provides superior signal to noise ratio compared to amplitude modulation and eliminates background drifts and fluctuations.     

The role of defects in chemical sensing properties of carbon nanotube films

The electrical resistance of 24 different carbon nanotube (CNT) thin film samples in blowing ambient air and 10 different analyte vapor environments was measured. The effects of the CNT growth method, different chemical treatments, ball milling, sample preparation conditions and Ar⁺-ion irradiation are compared. Significant differences in the response signal curves as a function of time in the case of the studied sensor/vapor combinations show the important role of the defect structure and attached functional groups in the chemical sensing properties of CNTs.     

Collisionless infrared energy deposition in OCS

We report experiments which show that OCS does not undergo collisionless infrared multiphoton dissociation at high fluences (250 J/cm²) and intensities (3 x 10¹¹ W/cm²), at variance with previously published results at lower fluence and intensity. Under these same conditions optoacoustic measurements, furthermore, show that an average of less than one photon per molecule is absorbed.     

Optoacoustic Effects in Pentaerythritol Tetranitrate with Ultrafine Aluminum-Particle Inclusions under Pulsed-Laser Action

Results of studying optoacoustic characteristics of pentaerythritol tetranitrate (PETN) specimens with inclusions of ultrafine aluminum particles (100 nm) are presented. Regularities of the increase of extinction coefficient k_ef and signal amplitude U on a piezodetector by increasing laser-pulse fluence H have been established. Estimates have been made, and it has been concluded that, during a laser pulse, heating of aluminum inclusions and a shell surrounding it occurs up to a gasification temperature and appear craters on a specimen’s surface appear at H > 0.1 J/cm².     

Nonlinear optical response of Ge nanocrystals in silica matrix with excitation of femtosecond pulses

We report an investigation of third-order optical nonlinearities in Ge nanocrystals (∼6 nm radius) embedded in silica matrix using the Z-scan and pump-probe techniques with femtosecond laser pulses at 780-nm wavelength. The nanocrystallite Ge samples were prepared using magnetron co-sputtering and post-thermal annealing at 800 °C. The nonlinear absorption coefficient and refractive index of the Ge nanocrystals were determined to be in the range from 1.8×10^-7 to 6.8×10^-7 cm/W and 1.5×10^-12 to 8.0×10^-12 cm²/W, respectively, which are proportional to the Ge atomic fraction in the matrix. Relaxation of the nonlinear response was found to have two characteristic time constants, 1.8 ps and 65 ps. The mechanisms responsible for the observed nonlinear response are discussed. Received: 21 August 2000 / Revised version: 17 January 2001 / Published online: 30 March 2001     

Application of the optoacoustic effect in optically pumped submillimeter lasers

The dependence of the optoacoustic signal generated in an optically pumped submillimetre laser system on gas pressure and modulation frequency has been studied experimentally and the results accounted for theoretically. With care an extracavity optoacoustic cell containing the laser gas diluted with air can be used to stablize the pump laser frequency to maximum absorption but this does not always optimise laser output.     

The effect of doping and processing conditions on the optical performance of Rh:BaTiO3

We demonstrate the advantages of using high levels of rhodium (2000–3200 ppm) to dope barium titanate for achieving finite absorption coefficients (0.36 cm^-1), high two-beam coupling gain (11.5 cm^-1), and acceptable response time (7 s) at 1.06 μm. We also report on the mass spectroscopy measurements on Rh:BaTiO₃ samples indicating a small segregation coefficient for rhodium (below 0.01) and the presence of a relatively large concentration (6000 ppm) of unintentionally added strontium. Received: 20 November 1998 / Revised version: 25 January 1999 / Published online: 12 April 1999     

Nonlinear absorption of ultraviolet femtosecond laser pulses in argon

The optogalvanic and optoacoustic methods are used to determine the mechanisms of the nonlinear absorption of intense ultraviolet femtosecond laser pulses in gaseous argon. An increase in the nonlinearity of the absorption process from 3 to 4 with an increase in the laser radiation intensity in the range 0.03–12 TW/cm² is revealed. The possible physical mechanisms of this phenomenon are discussed.     

Defect formation in oxygen- and boron- implanted MOS structures after gamma irradiation

The effect of gamma irradiation on the interface states of ion-implanted MOS structures is studied by means of the thermally stimulated charge method. 10-keV oxygen- or boron- (O⁺ or B⁺) implanted samples are gamma-irradiated with ⁶⁰Co. Gamma irradiation creates electron levels at the SiSiO₂ interface of the samples in a different way depending on the type of the previously implanted atoms (O⁺ or B⁺). The results demonstrate that the concentration of the shallower levels (in the silicon band gap) of oxygen-implanted samples increases more effectively after gamma irradiation. The same irradiation conditions increase more intensively the concentration of the deeper levels (in the silicon band gap) of boron-implanted samples. Received: 17 June 2002 / Accepted: 31 August 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +359-2/975-3236, E-mail: kaschiev@issp.bas.bg     

A mobile positron-lifetime spectrometer for field applications based on β+-γ coincidence

+ -γ coincidence measurement, which has been applied to in-situ examinations of microstructural evolution processes during the fatigue of copper single crystals, is described. Since no sandwich-type geometry is required, it is applicable to all specimen geometries commonly used in materials testing and to the non-destructive testing of engineering parts in service. As a radioactive source ⁷²Se generates the positron-emitting ⁷²As, which provides two positron spectra with maximum energies of 2.5 MeV and 3.3 MeV and a prompt γ quantum of 835 keV. The positrons emitted in the direction towards the specimen pass through a fast plastic scintillator and produce a scintillation signal, thereby losing about 150 keV of their energy. This signal serves as a start signal for the positron-lifetime measurement and is measured in coincidence with the subsequent 511 keV annihilation quantum. After passage through the plastic scintillator the remaining positron energy is still high enough to penetrate deep into the material and to allow for real bulk examinations. The prompt γ quantum may serve as an on-line control of the stability of the electronic system which will be useful under non-constant service conditions in a proposed field application. Received: 8 January 1997/Accepted: 14 March 1997