P. S. Kondratenko

On the Mechanism of the Absorption of Femtosecond Laser Pulses in the Melting and Ablation of Si and GaAs

The melting and ablation thresholds have been measured for Si and GaAs irradiated by 1240-nm femtosecond pulses of a chromium-forsterite laser, i.e., when the photon energy is lower than the bandgap width. A small difference of these quantities from the respective melting and ablation thresholds measured for the action of the 620-nm second-harmonic pulses with a photon energy higher than the bandgap width cannot be explained using available theoretical models. A new approach to the mechanism of the appearance of the electron-hole plasma and the formation of a thin, strongly absorbing surface layer in semiconductors irradiated by femtosecond laser pulses of the visible and infrared spectral ranges has been proposed on the basis of the avalanche mechanism of the filling of the conduction band.

Formation of periodic surface structures by ultrashort laser pulses

The formation of periodic surface structures by ultrashort laser pulses was observed experimentally and explained theoretically. The experiments were performed on graphite with picosecond laser pulses. The spatial period of the structures is of the order of the wavelength of the incident radiation, and the orientation of the structures is correlated with the direction of polarization of the light. The key point of the theoretical model proposed is resonance excitation of surface electromagnetic waves, which under conditions such that the temperature of the electronic subsystem is decoupled from the temperature of the crystal lattice causes a “temperature grating” to be written on the flat solid surface of the sample while the laser pulse is being applied on account of the temperature dependence of the surface impedance. The formation of a periodic surface profile from the temperature grating occurs by the volume expansion of a melted layer near the surface of the material. For typical values of the surface tension and viscosity for metals, there is not enough time for the periodic profile to be resorbed before the liquid layer solidifies. The formation of periodic surface structures is delayed in time relative to the laser pulse.

Ultrafast laser-induced phase transitions in tellurium

We present experimental investigations of ultrafast phase transitions in tellurium following excitation by an intense femtosecond laser pulse. Femtosecond time-resolved polarization-sensitive microscopy is used to monitor the temporal evolution of optical anisotropy (birefringence) of the irradiated material. The decay of optical anisotropy associated with the loss of order in crystalline tellurium is fluence-dependent and occurs within 0.5–3 ps.

Anomalous diffusion in generalized Dykhne model

Contaminant transport is investigated in the generalized Dykhne model differing from the original Dykhne model by the presence of advection in the high-permeability medium. An analysis is presented of transport regimes and concentration tail behavior in the high-permeability medium. It is found that the transport regimes include anomalous ones: subdiffusion and quasi-diffusion. A difference is revealed between longitudinal and transverse transport. Regime change over time leads to multiple-regime long-distance asymptotic behavior of concentration distributions. An analogy is drawn between the problems examined here and transport through comb structures.

Formation of amorphous carbon on melting of microcrystalline graphite by picosecond laser pulses

Observations of microcrystalline graphite subjected to picosecond laser pulses reveal the formation of a liquid phase with a subsequent transition to a uniform amorphous state of a surface layer upon solidification. This phenomenon is observed on a definite type of graphite and with the radiation incident on a plane parallel to the sixfold symmetry axis, and only for certain parameters of the laser pulse. A structural analysis of the amorphous phase is performed by electron microscopy and Raman scattering spectroscopy. A periodic structure with a period of the order of the wavelength of the heating pulse is formed in the heating region. The “rulings” of this periodic structure are oriented in the direction of polarization of the heating pulse. A study of the reflection kinetics of the probe laser pulse showed that the characteristic existence time of the liquid phase and of the solidification process is ∼10−10 s.

Impurity Transport in Percolation Media

An equation describing the impurity transport in a percolation medium is obtained and the inferences drawn from this equation are analyzed based on the scale invariance concept. A determining part in this analysis is allowance for the sinks inherent in such media. At distances shorter than the correlation length, the particles are transferred in the regime of subdiffusion; at large distances, the concentration asymptotics exhibits a characteristic “tail” shape. In the medium occurring in the state above the percolation threshold, the impurity transport over time periods longer than the characteristic time related to the correlation length is well described by the classical equation with a renormalized diffusion coefficient. In this case, the concentration tail has a Gaussian shape at moderate distances and tends to subdiffusion asymptotics at very long distances. A relation is established between the factor determining renormalization of the diffusion coefficient and the factor determining a decrease in the number of active impurity particles at large times.

Anomalous diffusion in regular heterogeneous media

Contaminant transport in homogeneous medium I restricted in one dimension (plane-parallel layer, PPL) or in two dimensions (straight cylinder, SC) and surrounded by another medium II filling the rest of the space has been analyzed. Both media are regular. Diffusivity of the medium I is assumed to be significantly greater than that for the medium II, D » d. It has been found that in the time range of t1 « t « t2 [where t 1 = a2/d, a is characteristic dimension along the restriction directions of the medium I; t 2 = (D/d)2 t 1 for PPL and t 2 = (D/d) ln(D/d)t 1 for SC] the contaminant transport is of subdiffusion behavior with variance for PPL and ˜Dt 1 ln(t/t1) for SC. Classical diffusion in the time ranges t « t 1 and t » t 2 takes place with effective diffusivities D and d, respectively. When the ratio D/d is extremely large (e.g. in case of rock fractures), the time t2 might practically not be accessible resulting in the regime of anomalous diffusion (subdiffusion) becoming asymptotic. Therefore, in connection with contaminant transport in fractured rocks, we must deal with the effect of dispersion suppression in fractures. The results obtained may be helpful for the development of methods to assess reliability of radioactive waste storage in rock massifs.

Anomalous transport regimes in a stochastic advection-diffusion model

A general solution to the stochastic advection-diffusion problem is obtained for a fractal medium with long-range correlated spatial fluctuations. A particular transport regime is determined by two basic parameters: the exponent 2h of power-law decay of the two-point velocity correlation function and the mean advection velocity u. The values of these parameters corresponding to anomalous diffusion are determined, and anomalous behavior of the tracer distribution is analyzed for various combinations of u and h. The tracer concentration is shown to decrease exponentially at large distances, whereas power-law decay is predicted by fractional differential equations. Equations that describe the essential characteristics of the solution are written in terms of coupled space-time fractional differential operators. The analysis relies on a diagrammatic technique and makes use of scale-invariant properties of the medium.

Ultrafast structural transformations in graphite

Femtosecond laser-induced structural transitions in graphite were studied by time-resolved optical anisotropy measurements. The decay of the anisotropic reflectivity seems to indicate a loss of long-range order on a subpicosecond time scale, which is much faster than the electron-phonon relaxation time. This observation confirms the nonthermal nature of the structural transition.

Dynamics of first- and second-order phase transitions in amorphous magnetooptic TbFeCo films

The dynamics of phase transformations in thin amorphous TbFeCo films under the action of ∼ 1 ps laser pulses is investigated. The films are heated to the Curie temperature in the amorphous state (TC1), to the crystallization temperature (Tac), and to the Curie temperature in the crystalline phase (TC2). The change in magnetization is detected by Faraday magnetooptic effect during and after the action of the heating pulse. A static external magnetic fieldH∼1−12 kOe, whose flux lines are directed perpendicular to the plane of the film, is used in the experiments. Amorphous TbFeCo films possess a perpendicular magnetic anisotropy, which on crystallization becomes reoriented in the plane of the film. It is observed that crystallization and magnetization reorientation occur during the heating pulse (within ∼ 1 ps). The spin subsystem is heated to the Curie temperature several picoseconds after the end of the laser pulse. The characteristic spin relaxation time is ∼ 10 ps. A model of the dynamics of the electronic, spin, and phonon subsystems that makes it possible to explain the experimental results is proposed on the basis of the data obtained.