The Generation of a Dense Hot Plasma by Intense Subpicosecond Laser Pulses

A simple model is developed for describing the interaction of intense femtosecond laser pulses with solid-state targets which is based on a set of equations of two-temperature hydrodynamics for electrons and ions of a plasma formed upon ionization of the target matter, equations describing the variation of ion composition of plasma upon ionization and the heat energy expenditure on thermal ionization, and equations defining the energy contribution by laser radiation to the target matter. A self-similar solution is suggested which well describes the heating of plasma electrons during the time of effect of a femtosecond laser pulse in a wide range of its parameters. Lagrangian computer codes developed for this purpose are used to derive, in a one-dimensional approximation, a numerical solution for the set of equations for the parameters corresponding to femtosecond experimental facilities under development in Germany and Russia. Profiles of hydrodynamic quantities (electron and ion temperature, plasma pressure and density, mean ion charge) obtained in the numerical solution at different moments of time may be used for preliminary assessment of the results of future experiments with a view to optimizing their parameters.     

Harmonics Generation in 4HeII by Femtosecond Laser Excitation

Non-resonant laser excitation of superfluid He by means of sub-picosecond pulses (=776 nm, 160 fs, 1 kHz) at intensities below the optical breakdown (I<4.5×10¹³ W/cm²) results in a strong emission at 258.7 nm corresponding to the third harmonic of the pump beam. Above a threshold of 0.75×10¹³ W/cm² the intensity and halfwidth of this collinear emission show a nearly linear dependence on the input pulse energy. Absolute UV energies up to 10 nJ and a conversion efficiency of about 10^–4 are observed. The emission intensity shows a significantg instability around 2.17 K.     

Generation of Steady-state Ultrashort Laser Pulses Based on a Novel Nonlinear Michelson Interferometer

Abstract

A pulsed neodymium-doped yttrium aluminium garnet laser mode locked by a novel nonlinear Michelson interferometer, where one branch contains a frequency-doubling potassium titanyl orthophosphate crystal, is reported for the first time. Under the optimal experimental condition, shorter than 10 ps average pulse width at 1·06 μm and up to 95% reproducibility of mode locking can be obtained, and the stability of output energy is about 7·5%. Finally, the reason why its mode-locking output characteristics have been significantly improved is also analysed in this paper.     

Action of High-Energy Neodymium Laser Radiation Pulses Having a Different Space-Time Shape on Metals

The role of different factors favorable for the arrival of the liquid-drop phase of the target material at the erosion flame has been elucidated. The crushing of particles of the liquid-drop phase in the process of their motion toward the laser beam has been revealed. The conditions under which the liquid-drop phase influences a laser-radiation loss have been determined. The methods of control of the parameters of the liquid-drop phase in erosion plasma flames have been found.     

Interaction of Intense Heat Pulses and Vortices in He II

Dynamics of intense heat pulses propagating in He II and interacting with quantum vortices induced by these pulses has been investigated using numerical methods based on equations of superfluid turbulence hydrodynamics. Investigation has been carried out for the plane geometry up to the second order in amplitude of pulses. The initial generation of vortex filaments has been simulated by the generating term in the Vinen equation. The results on pulse evolution for various heat fluxes are presented for different temperatures. It has been shown that the Feynman–Vinen theory is applicable in the phase-transition region. The results obtained have been compared to experimental data.     

Spectrum-preserving Amplification of Ultra-broadband Optical Pulses

Abstract

An amplifier scheme which allows control of the spectrum of the amplified pulse is proposed. The spectral components of the initial pulse are sent into laterally shifted regions of the amplifying medium, which enables control of the gain of particular frequency components. By the use of a properly formed pump, spectral narrowing during amplification can be avoided. A ‘spectrum preserving’ amplification of ultra-broadband (f.w.h.m. of about 30 nm) nanosecond pulses is demonstrated.     

Shortening of Free-running XeCl Laser Pulses by Stimulated Brillouin Scattering

Abstract

A stimulated Brillouin scattering mirror has been used to ‘temporally tune’ from nanoseconds to subnanoseconds the width of laser pulses generated by a free running XeCl oscillator, equipped with a super-Gaussian reflectivity unstable resonator. Pulse shortening techniques by stimulated Brillouin scattering (SBS) and by truncated SBS have been investigated. A maximum pulse shortening of about fifty has been achieved by truncated SBS.     

Non-uniform Transient Gratings Induced by Laser Pulses in Nonlinear Media

The excitation of transient gratings induced by the interference of two equal parts of a short laser pulse in nonlinear media with one- or two-photon absorption is considered. The pulse-induced grating displays completely the pulse self-interference pattern; it is not periodic but has the form of a spatially non-uniform permittivity stratification. The grating amplitudes are described by pulse correlation functions. They contain information on the pulse shape and are sensitive to its phase modulation. The diffraction efficiency of the induced grating is calculated using electrodynamic perturbation theory. It is shown that the measurement of the spectral or angular distribution of the diffracted probe light enables the grating amplitude to be computed. The correlation functions of individual pulses can be determined in this way. Some applications of the pulse-induced gratings are proposed, as is a simplified technique for measuring the pulse width. The practicability of the grating in real nonlinear media is estimated.     

色散渐减光纤压缩光脉冲

利用色散补偿光纤（ＤＣＦ）及色散位移光纤（ＣＳＦ）压缩重复频率为２．５ＧＨｚ的增益开关量子阱ＤＦＢ半导体激光器出射的光脉冲，将光脉冲由５３．８ｐｓ压缩至２．５ｐｓ，而后利用色散渐减光纤将２．５ｐｓ的光脉冲进一步压缩至０．８３ｐｓ。     

Optical Double-resonance Spectra with Time-dependent Pulses

Abstract

The optical double-resonance spectrum for a three-level atom interacting with two monochromatic laser pulses with arbitrary temporal profiles is calculated by use of Light's perturbation theory. Unlike some earlier work, the theory is not restricted to exact resonance of the pump laser, or to pulse areas which are exact multiples of 2π. A variety of pulse profiles are investigated, and the spectra are found to depend strongly on the detuning of the pump laser.     

Transitions in Presence of Short Laser Pulses

Abstract

Accurate numerical calculations are carried out to investigate the validity of the two-state approximation in the case of resonant interactions between electromagnetic radiation and atoms. Short pulses are considered and the presence of the atomic spectrum is modelled by introducing a third, non-resonant, state. We show that the harmonics of the pulse profile may play a significant role in the dynamics of the process and may cause energy non-conserving transitions between the atomic states.     

Modification of Relaxation Dynamics of a Quantum System by Intense Laser Fields

The modification of the relaxation dynamics of a quantum system interacting with a strong laser field are investigated by modelling the frequency modulation by a dichotomic Markov process. For this model various physical quantities such as mean polarization, inversion, the spectrum of spontaneous emission and absorption from a probe beam are calculated exactly for all values of the laser field strength and the correlation time of the modulation. Numerical results for the spectra for a wide range of parameters are given. The long correlation time of the modulation leads to considerable narrowing of the spectra, the width of which is governed by the correlation time itself, and also to the further splitting of the spectral peaks at moderate strengths.     

Above-threshold Ionization Photoelectron Spectrum Induced by a Train of Ultra-short Laser Pulses

We discuss an atomic system comprising a single discrete, ground level and two orthogonal continua. We assume that the continua are structured m we discuss two cases: that of Lorentzian continua, and the case of Gaussian structures. Our system is irradiated by an external laser field in the form of a train of ultra-short pulses. The analytic formulas for long-time photoelectron spectra are found and discussed for various values of the parameters describing our system.     

Semiconductor Plasma Antennas Formed by Laser Radiation

Technical Physics Letters - Efficiency of the transmission of high-frequency signals by semiconductor plasma antennas based on Ge and Si single crystals with surface nonequilibrium electron-hole...