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.     

Pore formation in thin nickel films under laser irradiation

Porous nickel films have been prepared using irradiation with nanosecond laser pulses, and their microstructure has been studied by transmission electron microscopy. The pores produced in the nickel films by nanosecond laser pulses are shown to be unfaceted and to be located in the grain bulk.     

Effect of laser irradiation on the oxidizer transport and self-ogranization of the interfacial layer during thermal oxidation of silicon

The oxidation of silicon under mid and near-IR laser irradiation was compared with thermal and combined (thermal + laser-enhanced) oxidation processes. The average rate of oxidizer transport through the oxide layer and the time taken for the formation and self-organization of the interfacial layer between SiO₂ and Si were evaluated from kinetic data for the three regimes studied. The processes influenced by laser irradiation were identified.     

Light distribution in the erythrocyte under laser irradiation: a finite-difference time-domain calculation

In medical applications of low power laser irradiations, safety is one of the most concerning problems since the light focused by the biological object itself may cause damage of living organisms. The light distributions in an erythrocyte with the shape of native biconcave, oblate spheroid, or disk sphere under the irradiation of a plane light of 632.8 nm were studied with a numerical calculation method of finite-difference time domain. The focusing effect by either the biconcave erythrocyte, oblate spheroid, or disk sphere erythrocyte was found to be so remarkable that the light intensities at the focused areas close to the erythrocyte membrane were about 10 times higher than that of the incident light when the light irradiated along the erythrocyte plane. This focusing effect became weak and even disappeared when the irradiation direction deviated from the erythrocyte plane for more than an angle of 15 degrees. Because the highest light intensity in the erythrocyte can be about one order of magnitude higher than that of the incident light, this factor should be taken into account for laser safety in medical applications.     

Stability of photodiodes under irradiation with a 157-nm pulsed excimer laser

We have measured the stability of a variety of photodiodes exposed to 157-nm light from a pulsed excimer laser by using a radiometry beamline at the Synchrotron Ultraviolet Radiation Facility at the National Institute of Standards and Technology. The intense, pulsed laser light exposed the photodiodes, whereas the low-intensity, continuously tunable light from the synchrotron source measured changes in the characteristics of the photodiodes, such as in the responsivity and the reflectance from the surface of a photodiode. Photodiodes studied include both silicon pn-junction and Schottky-barrier types. Among these photodiodes, we found that the damage mechanism for photodiodes with SiO2-based passivating layers is mainly the buildup of SiO2-Si interface trap states. The interface trap state buildup is well known for other semiconductor devices and is generally recognized as a product induced by radiation with an energy more than the 9-eV SiO2 bandgap energy rather than the 7.9-eV energy of the 157-nm radiation. Based on the generation of interface trap states, a model is proposed to describe the dependence of detector responsivity on exposure to 157-nm radiation. We also observed slow recovery in some of the damaged photodiodes, confirming that some of the interface trap states are only semipermanent. Radiation damage induced by low-power continuous 157-nm synchrotron light was also studied. As for the other photodiodes with no SiO2 layers, measurement results support the assumption that the changes in responsivity are due mainly to the deposition of thin layers on the tops of the detectors during laser irradiation.     

The influence of a phase change on the thermal history of a sample undergoing laser irradiation

This paper presents a comparison of the shapes of the thermal history curves for the rear surface of a sample undergoing laser irradiation. The curves are obtained experimentally and also calculated using the quasi-implicit technique of finite differences. The position and the size of the plateau observed on the theoretical and experimental curves has a significant impact on the accuracy of the determination of the thermal diffusivity obtained using the axial laser flash method. The experimental measurements were performed on phase-change composite materials and also on moist porous material samples (a gypsum slurry). This revised version was published online in November 2006 with corrections to the Cover Date.     

Interaction between metal nanotip and substrate under terahertz wave radiation

In this paper, the interaction between a gold nanotip and a silver substrate under the terahertz (THz) wave radiation is simulated by a finite difference time domain method and theoretically analyzed from the quasi-static charges model. Results from simulations and theoretical analysis show that with the increase in the distance between the nanotip and the substrate, the maximum electric field intensity on the substrate prominently decreases. With the increase in radius of the nanotip, diameter of the focused THz spot almost linearly increases and the intensity of THz spot also has a slight increase. These results are in agreement with the physical principles of the tip-enhanced near-field optical microscopy (TENOM), which indicates the TENOM technique is also suitable for the THz frequency range.     

Fracture analysis of a surface through-thickness crack in PZT thin film under a continuous laser irradiation

In this paper, the surface crack problem in PZT thin films under a continuous laser irradiation has been investigated by the superposition principle. Using commercial (FEM) software ANSYS 9.0, the piezoelectric fields near the crack tip were solved for surface crack in the finite PZT thin film. The SIFs for crack-tip fields were obtained by using the limited stress extrapolation technique (LSET) and then the energy release rates were calculated by the relation to the intensity factors. When the irradiation time and crack location were changed, the energy release rates G, G_I, and G_e for total, mechanical terms (mode I) and electric contribution were investigated. The results show that the mechanical opening mode I is the main mode for the surface crack under a continuous laser irradiation. However, electric mode IV has inhibiting effect on crack growth. At the beginning of laser irradiation, the surface tiny crack which is close to the centre of film will propagate more easily. During the laser irradiation, the crack which is far from the centre of film will propagate more easily.     

Dynamics of melting and crystallization processes in CdTe under laser irradiation

Numerical modeling results indicate that cadmium vaporization caused by nanosecond pulses of a ruby laser has a significant effect on the dynamics of phase transitions in the near-surface region of CdTe and leads to surface cooling of the material, resulting in a nonmonotonic temperature profile, with a maximum at a depth of about 20 nm. At incident energy densities above the threshold for CdTe melting, the molten zone forming below the surface layer extends both toward the surface and into the bulk of the semiconductor. Cd vaporization and the diffusion of Cd and Te in the melt give rise to tellurium enrichment in the near-surface region. Taking into account the dependences of the crystallization temperature and the latent heat of the phase transition on the Cd and Te concentrations in the melt, we achieved reasonable agreement with experimental data on the effect of incident energy density on the time during which a molten layer is present in CdTe.     

Isolated concentration waves of point defects under pulsed laser irradiation

A model is proposed for the first time for the propagation of an isolated (soliton-like) concentration wave of point defects in a crystal exposed to laser pulses. It is shown that this isolated concentration wave is formed as a result of a nonlinear concentration dependence of the defect source function which is caused by a reduction in the activation energy for defect formation near clusters when the elastic stress field is taken into account. Conditions for the excitation of an isolated concentration wave, its profile, and propagation velocity are determined. Pis’ma Zh. Tekh. Fiz. 25, 90–94 (August 26, 1999)     

Permissible heat loads on cooled laser mirrors under nonuniform irradiation

The results of a study are presented pertaining to a theoretical analysis of the temperature field and the field of thermal strains and stresses in cooled laser mirrors produced by nonuniform heating. The limiting thermal fluxes have also been calculated.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 2, pp. 314–319, August, 1984.     

Secondary recrystallization in thin films of FCC metals under pulsed laser irradiation

Mathematical modeling was used to examine vacancy migration in Ag and Al films during pulsed laser annealing at different pulse durations and incident power densities. The results demonstrate that, during pulsed laser processing of fcc-metal films containing nonequilibrium initial vacancy concentrations (10^-7 to 10^-3), secondary recrystallization proceeds in the presence of a high vacancy concentration, accelerating grainboundary migration.     

Optical features of noble-metal-oxide thin films under irradiation of blue laser

The optical reflectance and transmittance of Platinum oxide (PtO(x)) and palladium oxide (PdO(x)) thin films for ultrahigh-density optical storage are investigated using Z-scan technique under irradiation of blue laser (442 nm). The power thresholds of the PtO(x) and PdO(x) decomposition, 3.1 mW and 2.6 mW, are obtained respectively; the PtO(x) (PdO(x)) thin film is reversible and irreversible when input power is less than and more than 3.1 mW (2.6 mW). Deformation analyses by using an atom force microscope (AFM), which is formed in the micro irradiation region of surface on the thin film samples due to decomposition of the PtO(x) or PdO(x) driving the Z-scan, agree well with the Z-scan results. The optical features obtained at 442-nm wavelength are compared with those at 532-nm wavelength, and the threshold difference between the two wavelengths is also analysed in detail based on wavelength-dependent average power density and wavelength-selected absorption of the thin films.     

Modification of the LSMPS method for the conservation of the thermal energy in laser irradiation processes

In this work, new least-square moving particle semi-implicit (LSMPS) formulations for the modeling of the heat conduction in laser irradiation processes for both thick blocks and thin plates are developed. These new LSMPS formulations guarantee the conservation of the total thermal energy during the heat exchange between particles. The conservation of the thermal energy in the LSMPS method was implemented together with multiresolution techniques for the discretization of the domain with particles of different sizes so that a better characterization of the thermal gradients in the vicinity of the laser beam can be obtained. The simulation of laser irradiation processes for thin plates is still very challenging for particle methods with spherical particles and this is essentially because it is difficult to accommodate a minimum number of particles along the thickness direction without increasing considerably the resolution or the number of particles in the entire plate. In order to overcome this difficulty, a new multiresolution method based on particles with ellipsoidal shapes was also developed for a more efficient modeling of the laser irradiation in thin plates. By conducting the heat conduction simulations, in which the standard LSMPS method can provide accurate temperature distribution and by comparing the results with an analytical solution, it was confirmed that the proposed method is as accurate as the standard LSMPS method. Moreover, the heat conduction with an external heat source, in which the total thermal energy is not conserved by using the standard LSMPS method, was successfully simulated by using the proposed method. The simulations of laser irradiations were also conducted, and the validity of the proposed method has been confirmed by comparing numerical results with experimental data.     

Behavior of GeSbTeBi phase-change optical recording media under subnanosecond pulsed laser irradiation

We investigated the variations in reflectivity during the phase transition between amorphous and crystalline states of a Bi-doped GeTe-Sb2Te3 pseudobinary compound film with subnanosecond laser pulses, using a pump-and-probe technique. We also used a two-laser static tester to estimate the onset time of crystallization under 2.0-micros pulse excitation. Experimental results indicate that the formation of a melt-quenched amorphous mark is completed in approximately 1 ns, but that crystalline mark formation on an as-deposited amorphous region requires several hundred nanoseconds. Simple arguments based on heat diffusion are used to explain the time scale of amorphization and the threshold for creation of a burned-out hole in the phase-change film.     

Effect of nitrogen-containing substituents on fragmentation of perylene derivatives under laser irradiation

The effect of nitrogen-containing substituents such as phenyl imide and actyl imide on the character of fragmentation of perylene derivative molecules under laser irradiation has been studied by laser desorption (LD) mass spectrometry. Replacement of the central oxygen atom by a nitrogen atom in anhydride carboxy groups in perylene tetracarboxylic dianhydride (PTCDA) molecule results in the suppression of CO2 desorption and predominant desorption of CO. The LD mass spectrum exhibits peaks that correspond to fragments of the perylene nucleus and those of the aromatic and aliphatic substituents. Intact PTCDA molecules are present in the desorbed flux in insignificant amounts.