Biological tissue destruction under laser irradiation

Physical processes proceeding in a biological tissue under laser irradiation are described. Based on evaluation of all heat-mass transfer processes, a model of biological tissue destruction is proposed as the multiboundaryvalue Stefan problem with stepwise variation of thermophysical parameters at phase boundaries. To check the model adequacy, a one-dimensional problem has been numerically solved which makes it possible to determine the dimensions of carbonization zones and the boundary velocities. Analysis of the results obtained allows a conclusion to be made about the model adequacy to real processes of biological tissue destruction.Institute of Precision Mechanics and Optics, St. Petersburg, Russia. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 64, No. 5, pp. 598–603, May, 1993.     

Random laser action of ZnO@mesoporous silicas

ZnO@mesoporous silica nanocomposite was prepared by the impregnation method, and very efficient laser action was highlighted. As revealed by high-resolution transmission electron microscopy (HR-TEM), nanometric ZnO particles are confined inside the mesochannels of CMI-1 mesoporous silicas. Upon excitation at 3.6?eV of a femtosecond pulsed laser and at low pumping intensity, the ZnO@mesoporous silica showed a broad photoluminescence (PL) band corresponding to the excitonic recombination of ZnO. When the pumping intensity is increased up to a threshold (2.5?mJ?cm(-2)), the excitonic emission turns to stimulated emission through a mechanism which will be discussed. The same threshold value was obtained with another excitation source and nanocomposites with different ZnO loadings inside the CMI-1 mesoporous silica. These results allow a better understanding of the random laser effect in ZnO@mesoporous silica and, consequently, a model has been proposed to explain this phenomenon. Based on these new observations, many new applications can be considered since short-wavelength devices are required by industry to design new information storage supports.     

Defects and laser action in GaAs diodes

Zn-diffused, Te-doped GaAs injection lasers were examined by means of infrared microscopy, etching, transmission electron microscopy and scanning electron probe microanalysis.Bands of high infrared absorption, i.e., striations, were present in many diodes and the lines of intersection of the striations with the p-n junction were minimum emission filaments. Misfit dislocation networks were present in the p-n junction region but did not influence the emission pattern. Precipitates and diffusion-induced dislocations were found in the zinc-diffused region. All the properties of the material that enter into the expression for the threshold current for laser action were considered. It was concluded that the striations affect laser emission patterns mainly through the variation of the infra-red absorption coefficient.     

Random laser action from ceramic-doped polymer films

We report random lasing emission from polymer films doped with ceramic particles in a gain medium. The Al₂O₃ particles and Rh6G laser dye-doped PVP films were fabricated by a spin-coating technique and they were exposed to a pulsed laser at 526 nm, by which we collected intensive feedback random laser (IFRL) emission when the pump energy reached thresholds. The threshold depended on particle size, film thickness and particle contents in solution. A model with randomly distributed scatterers was established to confirm that the scattering properties could strongly effect the thresholds and lasing spatial distributions.     

Random laser action from a natural flexible biomembrane-based device

Abstract

Incoherent random lasing action in flexible eggshell membranes infiltrated with rhodamine 6G laser dyes is demonstrated. Laser radiation is achieved by exciting samples with 1-ns pulses at 526 nm. A threshold of 58 μJ/pulse is measured for the samples. The minimum threshold decreases to 35 μJ/pulse after the sample is coated with gold nanoparticles using a magnetron sputtering technique. The peaks of emission spectra are observed to redshift from 576 to 596 nm as dye concentration rises from 0.03 to 0.6 wt%. A linewidth of approximately 5 nm is obtained for most samples. This study is expected to offer a new way to induce lasing emission using biological microfibrils, and enriches basic knowledge of biophotonics.     

Thermal effects in amorphous YBaCuO layers under high-power laser action

We have studied the effect of pulsed UV laser radiation (KrF, ħω = 5.01 eV, τ = 25 ns) on the optical transmission and temperature distribution in amorphous YBaCuO layers. The interaction of laser radiation with such layers exhibits an adiabatic character, with an exponential decrease in the temperature in depth of the sample. The slope of the temperature profile is determined by the optical absorption coefficient for the radiation employed. The specific heat capacity of amorphous YBaCuO is c = 1.15 J/(g K).     

Metal fusion by laser radiation action in an oxidizing medium

Results are presented from a calculation of fusion of a massive zirconium plate under the action of laser radiation with consideration of simultaneous surface oxidation.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 61, No. 1, pp. 11–14, July, 1991.     

On the growth of neodymium pentaphosphate crystals for laser action

The growth of Nd_xLa_1?xP₅O₁₄ (0 ≤ x ≤ 1) crystals from phosphoric acid solution has been studied. Crystals have been obtained at temperatures between 300°C and 750°C. Vitreous graphite is a suitable container for the crystal growth. Changes in crystal morphology and crystal quality with growth temperature have been observed. Evaporation at high temperatures (>550°C) in an open system is too rapid to allow the growth of high quality crystals, although large (> 1 cm) crystals may be prepared, and the best quality crystals with respect to optical inhomogeneities are prepared at 450–550°C. Both pulsed and cw laser action have been observed. The Nd³⁺${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ fluorescent lifetime increases from 120 μsec (x = 1) to μsec (sec ≤ 0.05). These values indicate a seven-fold reduction in fluorescence quenching with respect to YAG:Nd. For crystals grown at temperatures below 600°C, however shorter lifetimes are observed and the quenching becomes more severe as the temperature of preparation is lowered. The quenching may be removed by appealing low lifetime crystals at 600–700°C in a P₂O₅ atmosphere. It is proposed on the basis of infrared absorption measurements that the lifetime shortening is related to the presence of hydrogen in crystals grown at lower temperatures. The vibrational characteristics of pentaphosphate crystals have also been investigated by Raman spectroscopy.     

Thermoelastic stresses in a multilayer plate upon action of laser radiation

The problem of determining thermal stress in a multilayer plate upon action of a laser flux with Gaussian distribution is solved.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 55, No. 5, pp. 816–821, November, 1988.     

Effective random laser action in Rhodamine 6G solution with Al nanoparticles

We have studied the random laser action in Rhodamine 6G (Rh6G) ethylene glycol solution with Al nanoparticles. The experiment results are obtained by pumping with a nanosecond (7 ns) laser pulse, which demonstrated the existence of effective random laser emission. It is found that the threshold of the random laser depends on the concentration of the Rh6G and the concentration of Al nanoparticles. The concentration and diameter of Al nanoparticles have effects on the optical path; a higher concentration or a larger diameter results in a shorter optical path length. Also multimode survival and mode competition have been observed at a relatively high concentration (0.08 M) of Rh6G, where the concentration of Al nanoparticles is 0.0015 M.     

Possibilities of obtaining nickel nanoparticles in an aqueous medium using laser action

An aqueous suspension of nickel nanoparticles has been obtained by the laser-erosion method. The diameters and concentrations of nickel nanoparticles in the aqueous medium have been measured using the laser-probing method. The obtained suspension of nanosize particles has been analyzed with the use of atomic-force and electron microscopy. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 2, pp. 206–210, March–April, 2008.     

Mechanical fracture in transparent dielectrics under the action of a focused laser radiation

A model is proposed for the process of laser-induced fracture in transparent dielectrics. The fracture occurs when mechanical stresses created in the crystal lattice by a spatially inhomogeneous cloud of nonequilibrium electrons exceed a threshold value. Relationships between of the mechanical stresses, the laser beam parameters and the crystal properties are established. Regions of the most probable primary crack formation are determined.     

Evaluation of temperature fields in corrosion-resistant steels under the action of laser radiation

We construct a mathematical model for the theoretical investigation of nonstationary three-dimensional temperature fields in bodies with plane boundary surfaces subjected to the surface laser treatment. The model takes into account the local character and the mode of action of laser beams, the shape of the spot of heating, the path of its translational motion, the sizes of a product, and its heat exchange with the ambient medium. A procedure aimed at the prediction of the modes of laser treatment is proposed. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 6, pp. 43–48, November–December, 2007.     

Multiwavelength laser action of Nd:YAlO3 single crystals grown by the laser heated pedestal growth method

We describe the growth of high quality transparent fibers of pure and Nd-doped YAlO₃ from unreacted pedestals (green rods) by the laser heated pedestal growth technique. The X-ray diffraction analysis shows that the fibers are monocrystalline, with a distorted Pnma perovskite structure and grow along 0 1 0 direction (b axis). In this work, multi-wavelength laser action is demonstrated for these fibers in the two main laser channels of Nd³⁺ ions. In the ⁴F_3/2 → ⁴I_11/2 laser transition, low threshold multi-wavelength laser emission is obtained at wavelengths of 1065, 1073 and 1081 nm. Additionally, for the first time in LHPG grown YAlO₃ fibers, laser action in the ⁴F_3/2 → ⁴I_13/2 transition is demonstrated at a wavelength of 1341 nm.     

Phase transitions in thin titanium oxide films under the action of excimer laser radiation

We have studied the action of pulsed vacuum ultraviolet (VUV) radiation of an ArF excimer laser (wavelength, 193 nm; pulse duration, 20 ns; pulse energy density, 40–50 J/cm²) on thin amorphous films of titanium oxide (TiO₂) grown by light-assisted molecular beam epitaxy. A comparison of the Raman spectra of samples measured before and after exposure shows that VUV irradiation at increasing fluence induces the crystallization of amorphous TiO₂ to anatase, which is further transformed into rutile.     

Photoablation of thin indium oxide films under the action of a pulsed arf excimer laser

The laser-induced ablation threshold of indium oxide (In₂O₃) films was studied in order to evaluate the possibility of using this material as a photoresist for vacuum ultraviolet lithography. In₂O₃ films with a thickness of about 30 nm were prepared by electron beam cathode sputtering with deposition onto quartz substrates in a rarefied oxygen-containing atmosphere. Then the films were irradiated by 20-ns pulses of an ArF excimer laser operating at a wavelength of λ=193 nm and a variable pulse intensity E _p. For a laser intensity below 30 mJ/cm², the oxide etching rate is negligibly small. As the laser radiation intensity increases above this threshold, the etching becomes more effective due to the development of a thermal ablation component.     

Start of condensation in erosional jets of metals subjected to highly intense submicrosecond laser action

Using the technique of laser probing, the time dependences of the transparency factor, integral glow, and of the scattered component of probing radiation for erosional laser jets of metals exposed to intense submicrosecond laser pulses have been determined. Based on the results of laser probing, a conclusion on the condensation nature of the process of formation of a liquid-droplet phase of the target-material under the given conditions of laser action has been drawn.     

On possibility of high-power terahertz emission from target under the action of powerful laser pulses

The possibility of terahertz (THz) emission from a target irradiated by short (∼0.1 ns) high-intensity (I ∼ 10¹⁸–10¹⁹ W/cm²) laser pulses has been studied by numerical simulations using a relativistic electromagnetic PIC code. The laser pulse action on the target generates plasma and the runaway electrons form a virtual cathode, which oscillates in the intrinsic field of electrons and the field of plasma ions. These oscillations account for the emission of radiation in a THz range. The generation efficiency is about three times as high as that in the absence of ions (according to the conventional reditron mechanism). Explanation of the observed phenomena is proposed.     

Formation of a low-resistivity penetrating metal-Si layer under the action of CO2 laser radiation

Some characteristic features observed as a result of the action of CO₂ laser radiation on a metal-silicon layer are described, especially the formation of a low-resistivity penetrating layer. The assumption is put forward that this low-resistivity penetrating layer forms as a result of the solid-phase dissolution of metal in Si or by diffusion of the metal into defects formed by laser radiation. Pis’ma Zh. Tekh. Fiz. 24, 60–63 (February 12, 1998)