Yurii A. Tolmachev

Transient Cherenkov radiation from an inhomogeneous string excited by an ultrashort laser pulse at superluminal velocity

The optical response of a one-dimensional string made of dipoles with a periodically varying density excited by a spot of light moving along the string at superluminal (subluminal) velocity is studied theoretically. The Cherenkov radiation in such a system is rather unusual, possessing both transient and resonant character. We show that, under certain conditions, in addition to the resonant Cherenkov peak, another Doppler-like frequency appears in the radiation spectrum. Both linear (small-signal) and nonlinear regimes as well as different string topologies are considered.

Nondispersive methods of ultrashort pulses of light encoding, recording and transformation

A new idea of the holography process realization is developed. The method opens a way to the direct recording of information simultaneously on the spatial and temporal (spectral) characteristics of the field of light using the femtosecond pulses. The analysis of 1D- and 2D-cases under the conditions of an infinitely short (delta-shaped) pulse illumination is performed. The process of the hologram recording is similar to that in the conventional holography. The reference delta-wave interferes with the wave scattered by an object. No stationary interference pattern is formed, but the trace of the two-wave crossing point or line is recorded inside the photosensitive medium. The illumination of the hologram formed in such a process by the reference wave reconstructs the object wave, and vice versa. New property of the capability for the precise recording and reconstruction of the scatterer color is predicted. The time-encoding system for the holography is proposed, which facilitates the hologram recording and enables the femtosecond pulse amplification. Some fundamental problems of the ultrashort pulse diffraction are also considered.

Asymptotic relations for the diffraction of the ultrashort pulse from the circular aperture

The solution of the problem of ultrashort pulse diffraction is considered using a method of linear systems theory. The diffraction of the ultrashort pulse from the circular aperture is studied both analytically and in computer simulation. Asymptotic relations for the Fraunhofer diffraction of the monochrome wave obtained with the method under consideration are shown to be equivalent to the well- known results of the conventional approach.

Problem of ultrashort-pulse diffraction

The problem of delta-shaped pulse of light diffraction from the aperture in the plane screen is formulated and solved in general form for the plane wave. Two examples are considered: diffraction from the slit and from the circular aperture. The analytical solution is given for the illuminated zone of the space behind the aperture and for the shadow. The temporal response is shown to consist of the direct wave and that scattered from the aperture edges. At large distances from the screen the Fourier transform of the solution coincide with well-known results for the plane monochrome wave.

Spectral and temporal characteristics of a transient Cherenkov radiation from a periodic resonant medium excited by an ultrashort laser pulse at superluminal velocity

We consider Cherenkov-type radiation from inhomogeneous periodic resonant medium excited by an ultrashort pulses of light. It is shown that if the velocity of the propagating excitation is greater (lower) then the velocity of light in vacuum c, a new Doppler-like frequency appears in the spectrum of the medium response. This frequency depends on the medium density distribution and on the observation angle. Possible applications of the effect are discussed.

Comments on the metrology properties of FROG method

Process of ultrashort pulse time structure reconstruction using FROG method and/or its variants is analyzed from two starting positions. The first one is in using of some general properties of pulses in a far zone of diffraction. This way permits for the increasing of stability of the inverse mathematical problem solution by reduction of the set of test functions. The possibility of reconstruction of the form of pulse emitted by laser itself from the results obtained in the far zone of diffraction is shown. The second position is in the analysis of space-and-time structure of the interference pattern formed in the nonlinear transparent matter used to produce the signal to be measured. To explain the formation of self-diffraction signal, the hypothesis of Cherenkov emission from this pattern is proposed. Special effects of spectrum transformation are discussed and some indirect confirmation of the idea is demonstrated.

Formation of excited helium molecules in a low temperature helium plasma

Two different channels for He2 excited molecule formation in a low temperature plasma are studied: recombinations of molecular ion He2+ and collisions of excited Helium atom with two normal atoms. Rate constants for the second processes is estimated as k3≈0.5 10-30 cm6s-1. Different role of two processes in the active phase of plasma excitation and in afterglow is shown.

New approach to the analysis of ultrashort pulse diffraction (Invited Paper)

Progress in optics led to the point when scientists and engineers operate the pulses of the electromagnetic energy containing only a few (in fact, sometime less than one, see examples in [1]) oscillations of the field instead of monochrome or quasimonochrome waves. Even omitting physical problems of interaction between field and matter, including those associated with essential nonlinearity due to extremely high amplitudes of the field, one encounters a lot of problems caused by extraordinary large homogeneous spectral bandwidth of ultrashort pulses. Linear optics of ultrashort pulses essentially differs from that of monochrome waves; hence, the development of appropriate methods for description of such pulses propagation through an optical system is necessary. It is common to our perception to deal with monochrome wave transformation, not wide-spectrum signal one, because in the traditional optics there is the set of solutions for the typical problems. The purpose of this presentation is to show that the equivalent set may be developed also for the optics of ultrashort pulses. As a result, the qualitative study of the diffraction and interference ofultrashort pulses becomes as simple as the ray optics. There are different methods of the analysis of diffraction and interference phenomena. The first possible approach is based on Huygens representation of the diffraction phenomena. Traditional procedure involves Fourier transform of the initial pulse process to get a set of monochrome waves. Then one analyzes the diffraction of each of those waves and finally the resulting monochrome diffraction fields sum back to form a scattered pulse. Rather widely used [2, 3, 4], this method implies reconsideration of developed solutions of classical diffraction problems to take into account some second-order components. Thrown away in the conventional theory as contributing only small phase correction for rather law frequencies, they become significant for high frequency components of the field. Another method suggests decomposition of the initial pulse into a set of wavelets [5]. Being convenient to those researchers who operate with an equal ease with this class of functions and with monochrome waves, this method does not possess the clearness of first one. Moreover, some of them does not meet the causality principle. At the same time, there is another group of methods based on one of principal results of general theory of diffraction [6]. Their formulation is closer to Young interpretation of diffraction and considers the interaction of waves scattered by the edge of aperture and through-passing one [7]. In acoustics and electro-engineering, the method based on the formalism of Green function for the wave equation [8, 9] is used. The fact that this method performs well for linear system analysis in electronics supported us in our attempt. In optics, it was successfully used by J. Connes [10] and L. Mertz [1 1], for example. Both of them studied the propagation of pulses through some most simple devices. Moreover, their analysis was based only on intuitive concepts that led to some mistakes, as it becomes clear today. Nevertheless, the general conclusion on the possibility of using the Dirac transform for the description of the waveform conversion in the process of propagation through the linear optical system became obvious. The technique based on 8-wave ideas proposed in our previous publications [12, 13, 14]. Those papers demonstrated the simplicity of interpretation of spatial and temporal form of the diffracted wave transformation using the pulse approach. The present paper extends this concept and demonstrates complete agreement of its results with classical theory for monochrome waves; the method opens a way to the development of specific software for the analysis of propagation of ultrashort pulses. All methods mentioned above give identical results. To our knowledge, the S -wave approach was not used directly for the solution of diffraction problems in the Kirchhoff approximation. However, it seems to be handy for the description of short pulses propagation. Moreover, it was shown in [12, 14] that the method can clarify some features of diffracted wave that can give a key to a fine experiment realization

New channel for alpha-particles transformation in a low temperature Helium plasma

For the first time, the process of charge exchange of Helium mucleus on the metastable Helium atom was observed. The process was detected by the analysis of HeII line afterglow excited in recombination of doubly ionized atoms with slow electrons in plasma formed by electron beam. The kinetic equations set for the most populated states of atom and ion was formulated. Solution of the set provides the satisfactory accuracy of the description of population dependence on time in the afterglow and permitted to determine the absolute value of the charge exchange rate constant. The measured value was compared with theoretical estimates and a good agreement between them was found.

Femtosecond pulse of light in optical correlator: new capabilities of the holographic Fourier spectroscopy

A two-beam interferometer as a correlator of signals in its branches is considered. An interpretation of the process of the interference pattern formation in terms of ultrashort [(delta) (t)-shaped] pulses is given. A method of observation of the cross-correlation between ultrashort pulse [or any (delta) (t)-correlated process] and complex signal is proposed as a way to the holographic registration of the full amplitude/phase spectrum of such a signal.