S. A. Tereshchenko

A refined diffusion model of the interaction of laser radiation with biological tissue

A refined diffusion model of laser radiation propagation in a highly scattering medium that, in contrast to the conventional variant, takes into account the initial single directivity of the photon source is presented. The two variants of the diffusion model are numerically compared. For small thicknesses of the scattering layer, the refined model is shown to be more accurate than the classical model.

Investigation of nonlinear characteristics of intensity limiters of high-power laser radiation

The problem of determining nonlinear-optical characteristics of working substances of limiters of high-power laser radiation is analyzed. A new method of simultaneous calculation of the beam-waist radius and nonlinear absorption coefficient from the Z-scan data is proposed. The value of nonlinear optical characteristics of ZnSe, porphyrin-graphene (Graphene-TPP), fullerene-graphene (Graphene-60), polyethylene oxide containing multilayer carbon nanotubes (PEO/MWNT), along with polymethine dyes PD-792 and PD-7098 and dicyanomethylene-pyran dyes DCM-627 and DCM-684, are obtained. Different working substances are compared for the first time under the same conditions by plotting output characteristics of laser-radiation limiters. The change of the laser-pulse shape after propagation through the limiters with different working substances is demonstrated.

Determination of optical characteristics of biological tissues by temporal distribution of an ultrashort laser pulse passed through homogeneous scattering layer

The determination of optical characteristics of biological tissues by the temporal distribution of an ultrashort laser pulse passed through a homogeneous scattering layer is considered. Four nonstationary models used to describe passing of laser pulses through a highly scattering medium are compared for the first time. The domains of applicability for the diffusion models are determined. It has been shown that the determination of optical characteristics of highly scattering media by experimental temporal distributions leads to different numerical values depending on the used model. Nevertheless, any of these models can be used because the performed study does not permit one to give preference to any of them. However, in determining optical characteristics of the scattering samples, it is necessary to specify the used model, as well as the conditions under which the initial experimental data (in particular, the thickness of the layer of a studied sample) are obtained.

Laser-limiting materials for medical use

The important problem of modern laser medicine is the decrease of an exposure of biological tissues outside of an operational field and can be solved by optical radiation limiting. Organic dyes with reversibly darkening can be placed onto surfaces of irradiated tissues or can be introduced in solder for laser welding of vessels. The limiting properties of a set of nontoxic organic compounds were investigated. Nonlinear optical properties of dyes having reverse saturable absorption (pyran styryl derivatives, cyanine and porphyrine compounds) were studied under XeCl and YAG:Nd (II harmonics) lasers excitation. The effect of attenuation of a visible laser radiation is obtained for ethanol solutions of cyanines: radiation attenuation coefficient ( AC) = 25-35 at N/S = 100-250 MW/cm2. In water solutions of such compounds in UV spectrum range AC ≈ 10. The spectral characteristics of compounds appeared expedient enough to operational use in laser limiters (broad passband in visible range of a spectrum). Under the data of Z-scanning (the scheme F/10) value AC ≈ 70 was reached. The limiting of power laser radiation in visible (λ = 532 nm) and UV- (λ = 308 nm) spectral region and nanosecond pulse duration (7 -13 ns) across porphyrine solutions and their complexes with some metals (13 compounds) was investigated too. The comparative study of optical limiting dependence on intensity of laser radiation, solvent type and concentration of solutions was carried out for selecte wavelength. There was shown a possible use of pyran styryl derivatives DCM as limiters of visual laser radiation. To understand a mechanism of laser radiation limitation the light induced processes were experimentally and theoretically studied in organic molecules. The quantum-chemical investigation of one cyanine compound was carried out. There were noted the perspectives of laser radiation limiting by application of inverted schemes of traditional laser shutters. Usage of phenomena of light -induced opalescence in one-component liquids and spinodal decay in stratifying liquid solutions is proposed.

Nonlinear threshold effect in the Z-scan method of characterizing limiters for high-intensity laser light

A threshold model is described which permits one to determine the properties of limiters for high-powered laser light. It takes into account the threshold characteristics of the nonlinear optical interaction between the laser beam and the limiter working material. The traditional non-threshold model is a particular case of the threshold model when the limiting threshold is zero. The nonlinear characteristics of carbon nanotubes in liquid and solid media are obtained from experimental Z-scan data. Specifically, the nonlinear threshold effect was observed for aqueous dispersions of nanotubes, but not for nanotubes in solid polymethylmethacrylate. The threshold model fits the experimental Z-scan data better than the non-threshold model. Output characteristics were obtained that integrally describe the nonlinear properties of the optical limiters.

Research on limiting of high power laser radiation in nonlinear nanomaterials

Dispersive and composite nanomaterials based on multi-walled and single-walled carbon nanotubes and its conjugates with dye of zinc phthalocyanine were produced. The composition and the structure of dispersive and composite materials were investigated using analytical methods of spectroscopy and microscopy. Nonlinear characteristic of nanomaterials of limiters by direct nonlinear scanning and Z-scan method were investigated. Studies suggest the possibility of using such nanomaterials in laser intensity limiters. Proposed threshold model characterizing limiters of powerful laser radiation takes into account the threshold nature of nonlinear interaction of irradiation with the nonlinear material. Threshold effect of nonlinear interaction of laser irradiation with several nonlinear material based on multi-walled and single-walled carbon nanotubes was experimentally found. It was shown that threshold model fit better with experimental data of Z-scan.

Development of the experimental setup for determination of nanoparticle sizes by nanotracking

The experimental setup for determination of particle sizes by nanotracking is developed. It is shown the possibility to identify particles with different fluorescent labels using developed experimental setup. The first results of analysis for samples of polystyrene micro- and nanoparticles are obtained using developed experimental setup.

Threshold effect in properties of limiters for high-intensity laser radiation

Creation of limiters for intensive laser radiation requires the development of effective methods for testing materials to determine the nonlinear optical parameters characterizing their properties. The limiting threshold, linear and nonlinear absorption coefficients can be determined not only from data of Z-scan with open aperture, but also with the help of a fixed location of the limiter. The use of this method makes it possible to determine the output characteristic of the studied material from which nonlinear optical parameters can be calculated. Characteristics of carbon nanotubes and graphene oxide in water were obtained with the fixed location of the limiter. The experiments were performed using an Nd:YAG laser that generates pulses of 16 ns duration at a wavelength of 532 nm with the linearly polarized laser beam in the horizontal plane and a shape closed to Gaussian type. Theoretical curves for method of fixed location of the sample according to threshold model was calculated and compared with the experimental data. Normalized weakening coefficients, limiting threshold, linear and nonlinear absorption coefficients were found for studied dispersions and calculation of Z-scan with open aperture was made. The value of normalized weakening coefficient was higher in dispersed medium of SWСNTs with water (Knorm≈20) in comparison with oxide graphene in water (Knorm≈14). The dependences of normalized weakening coefficient bias input energy were approximately linear in both cases.

Development of the experimental setup for multispectral nanoparticle tracking analysis

The experimental setup for multispectral nanoparticle tracking analysis is developed. The optical system for sample illumination in nanoparticle tracking analysis is designed that makes it possible to obtain images of particles in suspensions with a high signal-to-noise ratio. The results of the comparison of particle size distributions obtained by nanoparticle tracking analysis using developed experimental setup, dynamic light scattering method and scanning electron microscope are shown.

Particular features of time-dependent diffusion models applied in optical methods for medical diagnostics

In the presented work the absorption coefficient and reduced scattering coefficient of the homogeneous tissue-like biological phantoms are determined using three various types of the diffusion model (classical, refined and extrapolated) in the case of the scattering slab geometry which is commonly used for the optical mammography. The nonlinear relationships of the scattering and absorption coefficients from the slab thickness were obtained for each of the examined models. We suggest that this effect was originated by the inadequate approximation of the scattering phase function features employed in the diffusion models. This interpretation was confirmed by Monte Carlo simulations, where almost the similar type relationships were obtained.