Numerical modeling of the thermal lensing effect in a grazing-incidence laser

The numerical modeling of thermal lensing effect is investigated in a grazing-incidence laser. The deformation of the bounce face is introduced into the modeling for the first time, and the Gaussian distribution of the pump light and the anisotropic heat conduction are considered. The results indicate that the proportion of the deformation on the bounce face to the thermal lensing effect is as high as 80% for small grazing-incident angle of 5°. The thermal lensing effect sensitively depends on the pump power, grazing-incident angle and the pump distribution in a grazing-incidence bounce geometry laser.     

Spatial distributions of electron density in microplasmas produced by laser ablation of solids

The spatial distribution of electron density in argon microplasmas produced by laser ablation of solids has been investigated by time-resolved emission spectroscopy. The electron density was derived from Stark broadening and shift of spectral lines. It was found that the radial gradient of the electron density is much smaller than the gradient of the atomic number density of atoms ablated by the laser into the plasma. The almost homogeneous plasma conditions in the centre of the microplasmas are essential for quantitative element analysis of solid samples by laser ablation. On the other hand, because of the homogeneous conditions microplasmas are excellent sources for measurements of reliable Stark broadening and shift parameters of atomic and ionic spectral lines of all elements which can be ablated by lasers from solid samples.     

Surface electron states produced by a Rayleigh wave

The electronic properties of a semiconductor bounded by an uneven surface representing an infinitely high potential barrier are investigated. The surface irregularities are produced by a Rayleigh acoustic wave. It is shown that, on the boundary of a semiconductor, surface electron states (waves) may arise whose dispersion laws are obtained under the conditions when conduction electrons are located either in or outside the field of the acoustic wave. Existence domains of surface electron states are found that are distinguished by their physical properties. These domains are separated by a band gap whose width is determined by the height of irregularities.     

Study on current density distribution of electron beam spots by wave optics theory

The image theory of wave electron optics is used to study the current density distribution of electron beam spots for high-resolution electron optical systems. Using the wave aberration concept and paraxial expansion of charge density, the space charge effect is taken into consideration by modification of paraxial rays and spherical aberration coefficient of the lens. The point spreading function is obtained by Fourier-Bessel transform of the optical transfer function considering the space charge and partial coherence of the electron source.     

Formation of ultrashort electron pulses on scattering of an electron beam by a standing laser wave of ultrashort duration

The scattering of a well collimated electron beam by a strong standing laser wave of ultrashort duration, giving rise to a great number of scattered photons, is considered. This type of scattering is found to cause an electron beam to divide effectively into two parts. Ultrashort laser pulses are shown to be capable of forming ultrashort electron bunches whose length is governed by the laser beam diameter.     

Metastable surface alloys produced by ion implantation,laser and electron beam treatment

Ion Implantation, Laser and Electron-beam Treatment (LET) of metals have been employed extensively to produce metastable surface alloys. Recent published work on implanted alloys is reviewed first. The dilute implanted alloys (solute concentration <10 at. %) are shown to lead to crystalline metastable solid solutions. At higher solute concentrations, an amorphous phase has been observed for several binary systems and recently for a ternary system. The physical mechanisms at play, are discussed in detail. A review of the surface alloys produced by LET of metals is then presented—with an emphasis on the mechanisms involved. In particular, general criteria governing formation of metastable solid solutions under LET are proposed and shown to have excellent agreement with available data on metals and Si.     

Size and aerial density distributions of Ge nanocrystals in a SiO2 layer produced by molecular beam epitaxy and rapid thermal processing

We discuss the distribution of size and aerial density of Ge nanocrystals in a metal-oxide-semiconductor (MOS) memory structure fabricated by molecular beam epitaxy combined with rapid thermal processing; the size and aerial density of Ge nanocrystals are controlled by varying the thickness of the deposited Ge layer and the processing time. Variation of tunnel oxide thickness is demonstrated with the extension of the processing time. The effect of processing time and tunnel oxide thickness on the electrical properties of the MOS structures is investigated by high frequency capacitance–voltage measurements. PACS  61.46.+w; 81.07.-b; 81.07.Bc; 81.07.Ta     

Spatial patterns induced in a laser beam by thermal nonlinearities

Stable spatial laser patterns were observed in a high-finesse Fabry-Perot cavity containing up to 2 atm of CO(2) and O(2). The gases displayed the same sequence of patterns that obey a scaling law of the form P(beta)p(2), where P is the power stored in the cavity, p is the pressure of the gas, and beta is a material-dependent parameter.     

Study of electron density profile in the CASTOR tokamak using a lithium beam produced by diagnostic impulse arc source

A diagnostic impulse arc source of a beam of lithium atoms was used for the measurement of electron density profile in front of the lower hybrid grill on the CASTOR tokamak. It was shown that this profile is modified significantly during lower hybrid wave application if the regime with improved particle confinement is achieved.The work was performed under Grants of the Czechoslovak Academy of Sciences No. 14308 and 14310.     

Numerical modeling of thermal effects in a high-power injection-locked cw Ti:sapphire laser

With the FEA and matrix methods, the thermal lenses induced by the thermooptic effect, thermally induced mechanical strain and end face bulging in a Ti:sapphire crystal are analyzed. The results show that there is an unignorable strain-induced lens in a high pump power region. With the thermal lens effect, an injection-locked cw Ti:sapphire laser is numerically simulated. We find that the poor mode matching between the lasing and pump beams due to thermal lens will cause the slope efficiency decreasing. The calculated results agree with the reported experimental data, and we also predict that the thermal lens is obviously active at a certain pump power level.     

Trapping, compression, and acceleration of an electron beam by the laser wake wave

The scheme of laser wake-field acceleration in plasma is proposed and considered for the case where a relatively rare nonrelativistic or weakly relativistic electron beam is initially situated ahead of the intense laser pulse. It is shown that an electron beam is trapped in the region of the first accelerating wake maximum; then it is strongly compressed and accelerated to ultrarelativistic energies.     

Numerical modeling of the action of pulse laser radiation on small absorbing targets

The features of acoustic wave generation in a dense medium due to the absorption of laser pulse radiation were analyzed numerically, based on solutions to the equations of motion in Lagrange form. The influence of thermal and acoustic mechanisms in changes of physical parameters in a dense medium was studied over a wide range of temporal and energetic characteristics typical of laser systems.     

Numerical model of Ar-N2 laser with pumping by a high-current beam of charged particles

A model of a Ar-N₂ laser with pumping by an electron beam is investigated. The calculation is made with additional allowance for the kinetic processes of formation of excimer levels of the argon molecule. The obtained results are in good quantitative agreement with the experimental data. The results obtained with the model are used to estimate the possibility of pumping a Ar-N₂ mixture by a high-current ion beam.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 10–14, December, 1980.     

Observation of a backward ion-cyclotron wave parametrically excited by a modulated electron beam

The lower-hybrid parametric resonant decay instability (decay into electrostatic lower-hybrid waves and backward ioncyclotron waves) driven by a modulated electron beam is investigated experimentally. Plasma heating and the possibility of wave-enhanced particle loss associated with this instability are discussed.     

Study of color centers produced in thulium doped YLF crystals irradiated by electron beam and femtosecond laser pulses

In this work, we report the influence of the presence of photochromic and color centers in the photobleaching of thulium ions blue emission in YLF (YLiF₄) crystals doped with 1 mol% Tm (3+). The samples were irradiated at room temperature both with electron beam and high intensity ultrashort pulses from a Ti:Sapphire CPA laser system. In both irradiations the production of photochromic and color centers was observed via the absorption bands in the UV and visible ranges. Pure LiF and pure and oxygen doped YLF crystals were used to identify the color centers produced and their optical properties. From a phenomenological model it was possible to study the interaction between color centers and thulium ions, and their effect in photobleaching and photodarkening behaviors. Finally, the blue up laser level population was computed using a rate equation analysis.