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Sensor Electronics and Microsystem Technologies

STUDY OF ATOMIC SYSTEMS IN STRONG LASER FIELDS: SPECTRAL HIERARCHY, DYNAMICAL STABILISATION AND GENERATION OF ULTRA-SHORT VUV AND X-RAY PULSES

Alexander V. Glushkov, I. M. Shpinareva, V. M. Ignatenko, V. I. Gura

Published Nov 30, 2014• 5 citations
Publisher

Abstract

A new method for sensing a spectral hierarchy and dynamical stabilization in atomic systems in the intense laser field is presented. It is carried out modeling generation of the atto-second VUV and X-ray pulses under ionization of atomic systems by femto-second optical pulse. The theory of studied phenomena is the physical basis for construction of the new nano-atomic elements and devices, including sensors, sources of VUV and X-ray radiation, quantum Carnot engine, single-atomic lasers, quantum computers elements etc.

DOI
10.18524/1815-7459.2006.1.116891

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A new method for sensing a spectral hierarchy and dynamical stabilization in atomic systems in the intense laser field is presented. It is carried out modeling generation of the atto-second VUV and X-ray pulses under ionization of atomic systems by femto-second optical pulse. The theory of studied phenomena is the physical basis for construction of the new nano-atomic elements and devices, including sensors, sources of VUV and X-ray radiation, quantum Carnot engine, single-atomic lasers, quantum computers elements etc.
New generations of ultrashort and intense laser pulses as well ashigh power synchrotron radiation sources and x-ray free electronlasers have promoted fast developments in nonlinear optics andx-ray spectroscopy.The new experimental achievements and the appearance of varieties of novelnonlinear phenomena call for further development of theories. The objective of this thesis is to develop and apply thetheories to explain existing experimental data and to suggest new experiments. The first part of the thesis is devoted to nonlinear propagation of optical pulses. It is shown that the vibrational levels can be selectively populated by varying the duration, shape and intensity of the pump pulse. We obtained a strict analytical solution for the resonant two-photon interaction in a multilevel system beyond rotating wave approximation. Simulations show that the polarization anisotropy of the two-photon excitation affects strongly the anisotropy of photobleaching.The two-photon area theorem is reformulated with taking into account the dynamical Stark shift and the contribution from the permanent dipole moments. In general the dynamical Stark shift does not allow complete population of the excited state, but it can be compensated by detunings in atoms. A dynamical theory of the sequential two-photon absorption of microsecond pulses is developed to explore the role of transverse inhomogeneity of the light beam on optical limiting properties. The propagation of ultrashort laser pulses in nondipolar and dipolar media is investigated with special attention to the generation of superfluorescence and supercontinuum and the formation of attosecond pulses. The second part of the thesis addresses the interaction of molecules with x-ray radiation. We explore here the role of nuclear dynamics in resonant Auger scattering. Multimode simulations of the Auger spectra of ethylene molecule explain the main spectral features of the experimental spectra and show that the spectral profiles are formed mainly due to six vibrational modes. We predict the Doppler splitting of the atomic peak in resonant Auger scattering from SF6 molecule for circularly polarized x-rays. This effect is confirmed by the recent experiment. A new scheme of x-ray pump-probe spectroscopy, namely, resonant inelastic x-ray scattering accompanied by core-hole hopping induced by strong laser fields is suggested. The laser-induced promotion of core holes opens the symmetry forbidden scattering channels and gives rise to new spectral lines in the x-ray scattering spectrum. The strength of the symmetry forbidden lines becomes strong when the time of Rabi flopping is shorter than the lifetime of the core-excited state. We study the role of propagation of femtosecond x-ray free-electron pulses on the Auger process. Simulations show that there exists a strong competition between Auger decay and stimulated emission. The Auger yield and Auger branching ratio are strongly suppressed in the course of pulse propagation.
The resonant multiphoton resonances shifts and widths and ionization cross-sections for multielectron atoms in a intense laser radiation field are studied. It is carried out a new consistent approach to atom in a strong realistic laser field, based on the relativistic energy formalism (S-matrix adiabatic formalism), relativistic Dirac equation Green function method and relativistic many-body perturbation theory with the Dirac-Kohn-Sham zeroth approximation. In relativistic theory, the Gell-Mann and Low adiabatic formula for energy shift is connected with electrodynamical scattering matrice, which includes interaction with as a laser field as a photon vacuum field (radiative decay). The optimized basis of the relativistic orbitals is generated with using a minimization procedure for the gauge-non-invariant contribution (the fourth-order of the QED perturbation theory) to the radiation width of atomic state. An approach to treating the multiphoton atomic processes is outlined on example of H, Cs, Kr,Mg etc. Analysis shows that the shift and width of the multi-photon resonance line for the interaction “atom- multimode laser pulse” is greater than the corresponding shift and width for a case of the “atom- single-mode pulse” (the Lorenz pulse model) interaction. From the physical point of view it is naturally provided by action of the photon-correlation effects and influence of the laser pulse multi-modity.
A new method for sensing a structure of the stochastic, multi-mode laser pulse field and photon-correlation, chaotic features of atomic and nano-optical systems in this field is developed and a dynamics of multi-photon ionization in intense field is studied. As a basis for developing a new method for sensing stochasticity and chaotic features we use new quantum approach to calculation of the multi-photon resonances spectra characteristics for atomic systems in a stochastic laser field and the same field plus DC electric field. Some numerical illustrations regarding the multi-photon resonance spectra and stochasticity sensing for a number of atomic systems are presented.
The past thirty years witnessed rapid developing of strong field physics that leads to the first generation of attosecond (10−18 s) pulses. The duration of available pulses has recently been reduced to less than 100 as, approaching the Kepler period of a classical electron revolving around the nucleus. For the first time, electron motions inside atoms, molecules and solids can be traced, probed and even steered in real time. It pushed the study on ultrafast dynamic processes into a new frontier, giving birth to attosecond physics. The development of attosecond pulses provides unprecedented coherent ultrashort XUV (extreme ultra-violet) of soft X-ray sources that enable imaging the atomic structure and resolving the related quantum dynamics for matter science. In particular, it allows the probing and monitoring of electron dynamics in the natural time scale of attoseconds with atomic spatial resolution. The generation and applications of attosecond pulses are directly related to the control and manipulation of the sub-cycle electron dynamics. Recent studies on the correlated multielectron dynamics have provided insight into the optical and dynamical properties of matter in the perspectives of time, phase and entanglement. It deepens our understanding on some of the fundamental questions, e.g., how the single or multiple photons are absorbed and how short are the quantum processes such as photoionization, tunneling ionization and charge migration. Although currently attosecond pulses are limited by the flux and pulse duration, the combination of attosecond pulses with intense infrared laser pulses paves new ways of controlling and probing electron dynamics in the domains of energy, time and space that lays the foundation for petahertz optoelectronics and attosecond pulse based transient spectroscopy. In this review, we will first briefly summarize the advancement of strong field and ultrafast physics including the relevant experimental findings, the theoretical explorations, the related technologies and the possible applications. Then we focus on the sub-cycle electron or multielectron dynamics and their manifestation on the ionization and coherent radiation properties. We start with the time behavior of Fano resonance and how it can be probed in time domain. In section 2, the correlated electron dynamics is discussed in term of anti-screening effect and Pauli effect that influence strong field ionization and high harmonic generation processes. In section 3, the coherent emission with frequency from terahertz to soft XUV and their joint measurement are discussed by emphasizing the generation mechanism of terahertz wave in two-color laser pulses. The developed high-harmonic and terahertz wave spectroscopy (HATS) is shown capable of imaging the molecular structure and dynamics. In section 4, the time-resolving of electron and hole dynamics and their coherence is proposed based on attosecond transient absorption technique. The coherence of ionization and the correlated electron dynamics is hinted crucial for electron-hole interaction. The topics are very limited and mainly selected from the perspectives of our group.
The interaction of intense laser fields with matter is accompanied by a multitude of phenomena, ranging from the generation of coherent radiation across various spectral ranges (from terahertz to soft X-rays) to the creation of beams of accelerated charged particles and micromodification of the volume and surface of various samples.To describe this diversity of phenomena, several theoretical approaches have been developed to date.These approaches utilize different approximations to address this complex nonlinear problem, allowing for the explanation of specific aspects of such interactions.The developed theoretical frameworks consider the interaction at both microscopic and macroscopic levels; at the microscopic level, they investigate the characteristics of single atoms interacting with laser fields, while at the macroscopic level, they analyze nonlinear susceptibilities that describe the nonlinear polarization of the medium during the propagation of laser radiation through it.This work presents an overview of methods describing some of the most commonly employed theoretical approaches to model the response of a medium at a microscopic (quantum-mechanical) level, as well as various methods for controlling the properties of generated coherent radiation.A model for the gas medium is described that accounts for both the response characteristics of individual atoms and allows for consideration of phase and quasiphase matching effects during radiation generation.The influence of medium parameters on the efficiency of generation and polarization properties of generated radiation is analyzed under both phase and quasi-phase matching conditions.A detailed description of an analytical method for calculating the nonlinear susceptibility of a gas medium is provided.The obtained values of nonlinear susceptibility for arbitrary orders may be useful in numerical calculations concerning the propagation of laser radiation in matter.Various channels for radiation generation are analyzed as parameters of the laser field are varied.
This work aims at development of new dynamical methods to control a resonant light-matter interaction as well as at their application for producing new sources of coherent intense sub-fs radiation in x-ray range. The dynamical control is based on modulation in time and in space of the parameters of the atomic transition (coupled to the resonant high-frequency field) by means of sufficiently strong off-resonant low-frequency control field. In particular, it may result in efficient transfer of quasi-monochromatic VUV or XUV radiation into ultrashort pulses via its resonant interaction with atomic hydrogen gas or plasma of hydrogen-like ions respectively by means of two different techniques: time and space dependent linear Stark effect or interruption of resonant interaction by the tunneling ionization, as shown in previous works of our group. \n\nIn this thesis, I further develop these ideas and demonstrate their important potential applications. First of all, both techniques are extended to arbitrary (non-hydrogen-like) atomic medium. Furthermore, advanced analytical and numerical solutions describing a process of pulses formation are found and shown to be in excellent agreement with each other. A deep physical analogy between the processes of coherent forward scattering of γ-ray radiation in the vibrated quasi-resonant nuclear absorber and the XUV field propagation in the quasi-resonant atomic medium in the presence of the moderately strong IR field is established. \n\nFinally, the application of the developed techniques for production of intense coherent attosecond sources of soft X-ray radiation, including so-called “water window” range (2.2-4.3nm) (especially promising for dynamical imaging of the proteins in a living cell) is proposed. Two different paths towards production of intense coherent attosecond pulses in a soft X-ray range are suggested: (i) via efficient transformation of the picosecond radiation of the high energy pulses of the existing x-ray plasma lasers into the trains of attosecond pulses in the resonant passive plasma, and (ii) via amplification of the low energy pulses of the existing high harmonic sources in the resonant active plasma of x-ray lasers.