Space-frequency coupling, conical waves, and small-scale filamentation in water

Numerical simulations of fs laser propagation in water have been made to explain the small-scale filaments in water we have observed by a nonlinear fluorescence technique. Some analytical descriptions combined with numerical simulations show that a space-frequency coupling mainly from the interplay among self-phase modulation, dispersion and phase mismatching will reshape the laser beam into a conical wave which plays a major role of energy redistribution and can prevent laser beam from self-guiding over a long distance. An effective group velocity dispersion is introduced to explain the pulse broadening and compression in the filamentation. (c) 2005 American Institute of Physics.

Topology of toroidal helical fields in non-circular cross-sectional tokamaks

The ordinary differential magnetic field line equations are solved numerically; the tokamak magnetic structure is studied on Hefei Tokamak-7 Upgrade (HT-7U) when the equilibrium field with a monotonic q-profile is perturbed by a helical magnetic field. We find that a single mode (m, n) helical perturbation can cause the formation of islands on rational surfaces with q = m/n and q = (m +/- 1, +/- 2, +/- 3,...)/n due to the toroidicity and plasma shape (i.e. elongation and triangularity), while there are many undestroyed magnetic surfaces called Kolmogorov-Arnold-Moser (KAM) barriers on irrational surfaces. The islands on the same rational surface do not have the same size. When the ratio between the perturbing magnetic field B-r(r) and the toroidal magnetic field amplitude B(phi)0 is large enough, the magnetic island chains on different rational surfaces will overlap and chaotic orbits appear in the overlapping area, and the magnetic field becomes stochastic. It is remarkable that the stochastic layer appears first in the plasma edge region.

Hot spot formation outside of the fusion-fuel core

An alternative fast-ignition method is proposed involving the formation of a hot spot outside the precompressed fusion-fuel core by a series of shocks driven directly by the light pressure of laser pulses of increasing intensities. It is shown that a hot spot, which can be of different material from that of the fuel core, with temperature similar to 10 keV and density similar to 200 g/cm(2), can be formed. Being an electrically neutral plasma, the hot spot can easily be sent into the fuel core. (c) 2005 American Institute of Physics.

Ion acceleration in laser-excited plasma wake fields

The dynamics of the plasma ions in the wake fields of short, ultraintense laser pulses in underdense plasmas are investigated analytically and numerically. Owing to the large ion-to-electron mass ratio, the motion of plasma ions in-such wake fields has often been assumed to be neglectable. It is shown that when the laser intensity exceeds 10(20) W/cm(2), the ion motion can no longer be ignored. In this case, ion momentum peaks appear behind the laser pulse, which correspond with the ion density peaks. The laser-excited wake field appears to be effective for ion acceleration, in particular to ions with high-charge numbers. The dependence of ion acceleration on the laser intensity, pulse width, and background plasma density is discussed. (c) 2006 Optical Society of America.

Ion cascade acceleration from the interaction of a relativistic femtosecond laser pulse with a narrow thin target

Particle-in-cell simulations are performed to study the acceleration of ions due to the interaction of a relativistic femtosecond laser pulse with a narrow thin target. The numerical results show that ions can be accelerated in a cascade by two electrostatic fields if the width of the target is smaller than the laser beam waist. The first field is formed in front of the target by the central part of the laser beam, which pushes the electron layer inward. The major part of the abaxial laser energy propagates along the edges to the rear side of the target and pulls out some hot electrons from the edges of the target, which form another electrostatic field at the rear side of the target. The ions from the front surface are accelerated stepwise by these two electrostatic fields to high energies at the rear side of the target. The simulations show that the largest ion energy gain for a narrow target is about four times higher than in the case of a wide target. (c) 2006 American Institute of Physics.

Neutron source from thin foil confined by two laser pulses

Neutron production from a thin deuterium-tritium (D-T) foil irradiated by two intense femtosecond laser pulses from opposite sides with zero phase difference is studied analytically and numerically. For the interaction of a laser pulse of amplitude a = 7, focal area 100 mu m(2) and areal density 4.4 x 10(18) cm(-2) with a D-T plasma foil, about 1.17 x 10(21) neutron s(-1) can be obtained, much more than from other methods. The profiles of the ion and electron densities are also calculated.

Earthquake geology of Myanmar

This thesis describes the active structures of Myanmar and its surrounding regions, and the earthquake geology of the major active structures. Such investigation is needed urgently for this rapidly developing country that has suffered from destructive earthquakes in its long history. To archive a better understanding of the regional active tectonics and the seismic potential in the future, we utilized a global digital elevation model and optical satellite imagery to describe geomorphologic evidence for the principal neotectonic features of the western half of the Southeast Asia mainland. Our investigation shows three distinct active structural systems that accommodate the oblique convergence between the Indian plate and Southeast Asia and the extrusion of Asian territory around the eastern syntaxis of the Himalayan mountain range. Each of these active deformation belts can be further separated into several neotectonic domains, in which structures show distinctive active behaviors from one to another.

In order to better understand the behaviors of active structures, we focused on the active characteristics of the right-lateral Sagaing fault and the oblique subducting northern Sunda megathrust in the second part of this thesis. The detailed geomorphic investigations along these two major plate-interface faults revealed the recent slip behavior of these structures, and plausible recurrence intervals of major seismic events. We also documented the ground deformation of the 2011 Tarlay earthquake in remote eastern Myanmar from remote sensing datasets and post-earthquake field investigations. The field observation and the remote sensing measurements of surface ruptures of the Tarlay earthquake are the first study of this kind in the Myanmar region.

Temperature controlled filamentation in argon gas

Temperature controlled filamentation is experimentally demonstrated in a temperature gradient gas-filled tube. The proper position of the tube is heated by a furnace and two ends of the tube are cooled by air. The experimental results show that multiple filaments are shrunken into a single filament or no filament only by increasing the temperature at the beginning of the filament. This technique offers another degree of freedom of controlling the filamentation and opens a new way for intense monocycle pulse generation through gradient temperature in a noble gas.

Energy transfer between silicon-oxygen-related defects and Yb3+ in transparent glass ceramics containing Ba2TiSi2O 8 nanocrystals

We report on the conversion of near-ultraviolet radiation of 250-350 nm into near-infrared emission of 970-1100 nm in Yb3+-doped transparent glass ceramics containing Ba2TiSi2O8 nanocrystals due to the energy transfer from the silicon-oxygen-related defects to Yb3+ ions. Efficient Yb3+ emission (F-2(5/2)-> F-2(7/2)) was detected under the excitation of defects absorption at 314 nm. The occurrence of energy transfer is proven by both steady state and time-resolved emission spectra, respectively, at 15 K. The Yb2O3 concentration dependent energy transfer efficiency has also been evaluated, and the maximum value is 65% for 8 mol % Yb2O3 doped glass ceramic. These materials are promising for the enhancement of photovoltaic conversion efficiency of silicon solar cells via spectra modification.

Optimization of multi-cycle two-color laser fields for the generation of an isolated attosecond pulse

The effect of the mixing of pulsed two color fields on the generation of an isolated attosecond pulse has been systematically investigated. One main color is 800 nm and the other color (or secondary color) is varied from 1.2 to 2.4 mu m. This work shows that the continuum length behaves in a similar way to the behavior of the difference in the square of the amplitude of the strongest and next strongest cycle. As the mixing ratio is increased, the optimal wavelength for the extended continuum shifts toward shorter wavelength side. There is a certain mixing ratio of intensities at which the continuum length bifurcates, i.e., the existence of two optimal wavelengths. As the mixing ratio is further increased, each branch bifurcates again into two sub-branches. This 2D map analysis of the mixing ratio and the wavelength of the secondary field easily allows one to select a proper wavelength and the mixing ratio for a given pulse duration of the primary field. The study shows that an isolated sub-100 attosecond pulse can be generated mixing an 11 fs full-width-half-maximum (FWHM), 800 laser pulse with an 1840 nm FWHM pulse. Furthermore the result reveals that a 33 fs FWHM, 800 nm pulse can produce an isolated pulse below 200 as, when properly mixed. (c) 2008 Optical Society of America.

Comparison of laser performances of 5at.% Yb : Gd2xY2(1-x)SiO5 crystals between different cutting directions

We investigate the laser actions of 5at.% Yb:Gd2xY2(1-x)SiO5 (Yb:GYSO; x = 0.1) crystals with different cutting directions, parallel and vertical to the growth axis. Our results show that the cutting direction of the sample plays an astonished role in the laser operation. The sample cut vertically to the growth axis possesses the favourable lasing characteristics. Its output power reaches 3.13W at 1060nm with a slope efficiency of 44.68% when the absorbed pump power is 8.9 W. In contrast, the sample cut parallel reaches only 1.65 W at 1044 nm with a slope elLiciency of 33.76% with absorbed pump power of 7.99 W. The absorption and emission spectra of the two samples are examined and the merit factor M is calculated. Our analysis is in agreement well with the experimental results. The wavelength tuning range of the superior sample covers from 1013.68 nm to 1084.82 nm.

Acceleration of initially moving electrons by a copropagation intense laser pulse

Acceleration of an initially moving electron by a copropagation ultra-short ultra-intense laser pulse in vacuum is studied. It is shown that when appropriate laser pulse parameters and focusing conditions are imposed, the acceleration of electron by ascending front of laser pulse can be much stronger compared to the deceleration by descending part. Consequently, the electron can obtain significantly high net energy gain. We also report the results of the new scheme that enables a second-step acceleration of electron using laser pulses of peak intensity in the range of 10(19)-10(20) W mu m(2)/cm(2). In the first step the electron acceleration from rest is limited to energies of a few MeV, while in the second step the electron acceleration can be considerably enhanced to about 100 MeV energy.

Femtosecond laser-induced periodic surface structure on ZnO

Three different ZnO nanostructures include nanoparticles, ripples and regular nanogratings were successfully prepared by femtosecond laser irradiation under different experimental conditions. The in-situ observation of the second harmonic generation (SHG) excited in ZnO crystals before, during, and after the formation of the nanostructures was investigated. The obtained results show that the formed nanostructures contribute to the enhancement of the SHG. We propose that the second harmonics in the sample surface plays an important role in the formation of nanostructures. (c) 2007 Published by Elsevier B.V.

Generation of periodic ultrashort electron bunches and strongly asymmetric ion Coulomb explosion in nanometer foils interacting with ultra-intense laser pulse

The interaction of a linearly polarized intense laser pulse with an ultrathin nanometer plasma layer is investigated to understand the physics of the ion acceleration. It is shown by the computer simulation that the plasma response to the laser pulse comprises two steps. First, due to the vxB effect, electrons in the plasma layer are extracted and periodic ultrashort relativistic electron bunches are generated every half of a laser period. Second, strongly asymmetric Coulomb explosion of ions in the foil occurs due to the strong electrostatic charge separation, once the foil is burnt through. Followed by the laser accelerated electron bunch, the ion expansion in the forward direction occurs along the laser beam that is much stronger as compared to the backward direction. (c) 2008 American Institute of Physics.

Influence of target thickness on the generation of high-density ion bunches by ultrashort circularly polarized laser pulses

Ion acceleration by ultrashort circularly polarized laser pulse in a solid-density target is investigated using two-dimensional particle-in-cell simulation. The ions are accelerated and compressed by the continuously extending space-charge field created by the evacuation and compression of the target electrons by the laser light pressure. For a sufficiently thin target, the accelerated and compressed ions can reach and exit from the rear surface as a high-density high-energy ion bunch. The peak ion energy depends on the target thickness and reaches maximum when the compressed ion layer can just reach the rear target surface. The compressed ion layer exhibits lateral striation which can be suppressed by using a sharp-rising laser pulse. (c) 2008 American Institute of Physics.