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arXiv:0805.2619 · Revista Mexicana de Física

Refraction and absorption of x rays by laser-dressed atoms

Christian Buth, Robin Santra, Young, Linda

Published May 16, 2008• 1 citation
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Abstract

X-ray refraction and absorption by neon atoms under the influence of an 800 nm laser with an intensity of 10^13 W/cm^2 is investigated. For this purpose, we use an ab initio theory suitable for optical strong-field problems. Its results are interpreted in terms of a three-level model. On the Ne 1s --> 3p resonance, we find electromagnetically induced transparency (EIT) for x rays. Our work opens novel perspectives for ultrafast x-ray pulse shaping.

DOI
10.48550/arxiv.0805.2619

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•Apr 14, 2008•0 citations
X-ray refraction and absorption by neon atoms under the influence of an 800 nm laser with an intensity of 10^13 W/cm^2 is investigated. For this purpose, we use an ab initio theory suitable for optical strong-field problems. Its results are interpreted in terms of a three-level model. On the Ne 1s --> 3p resonance, we find electromagnetically induced transparency (EIT) for x rays. Our work opens novel perspectives for ultrafast x-ray pulse shaping.
We investigated the complex index of refraction in the x-ray regime of atoms in laser light. The laser (intensity up to 1013W∕cm2{10}^{13}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}1013W∕cm2, wavelength 800nm800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}800nm) modifies the atomic states but, by assumption, does not excite or ionize the atoms in their electronic ground state. Using quantum electrodynamics, we devise an ab initio theory to calculate the dynamic dipole polarizability and the photoabsorption cross section, which are subsequently used to determine the real and imaginary part, respectively, of the refractive index. The interaction with the laser is treated nonperturbatively; the x-ray interaction is described in terms of a one-photon process. We numerically solve the resolvents involved using a single-vector Lanczos algorithm. Finally, we formulate rate equations to copropagate a laser and an x-ray pulse through a gas cell. Our theory is applied to argon. We study the x-ray polarizability and absorption near the argon KKK edge over a large range of dressing-laser intensities. We find electromagnetically induced transparency (EIT) for x rays on the Ar $1s\ensuremath{\rightarrow}4p$ pre-edge resonance. We demonstrate that EIT in Ar allows one to imprint the shape of an ultrafast laser pulse on a broader x-ray pulse (duration 100ps100\phantom{\rule{0.3em}{0ex}}\mathrm{ps}100ps, photon energy 3.2keV3.2\phantom{\rule{0.3em}{0ex}}\mathrm{keV}3.2keV). Our work thus opens new opportunities for research with hard x-ray sources.
Electromagnetically-induced-transparency-like behavior in the extreme-ultraviolet region is studied theoretically, including the effect of intense 800 nm laser dressing of He 2s2p(1Po{}^{1}\phantom{\rule{-0.16em}{0ex}}{P}^{o}1Po) and 2p2{}^{2}2(1Se){(}^{1}\phantom{\rule{-0.16em}{0ex}}{S}^{e})(1Se) autoionizing states. We present an ab initio solution of the time-dependent Schr\"odinger equation in an LSLSLS-coupling configuration-interaction basis set. The method enables a rigorous treatment of optical field ionization of these coupled autoionizing states into the N=2N=2N=2 continuum in addition to N=1N=1N=1. Our calculated transient absorption spectra show encouraging agreement with experiment.
The spectrum of resonance fluorescence is calculated for a two-level system excited by an intense, ultrashort x-ray pulse made available, for instance, by free-electron lasers such as the Linac Coherent Light Source. We allow for inner-shell hole decay widths and destruction of the system by further photoionization. This two-level description is employed to model neon cations strongly driven by x rays tuned to the $1s2{p}^{\ensuremath{-}1}\ensuremath{\rightarrow}1{s}^{\ensuremath{-}1}2p$ transition at 848eV848\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{eV}848eV; the x rays induce Rabi oscillations which are so fast that they compete with Ne 1s1s1s-hole decay. We predict resonance fluorescence spectra for two different scenarios: first, chaotic pulses based on the self-amplified spontaneous emission principle, like those presently generated at x-ray free-electron-laser facilities and, second, Gaussian pulses which will become available in the foreseeable future with self-seeding techniques. As an example of the exciting opportunities derived from the use of seeding methods, we predict, in spite of the above obstacles, the possibility to distinguish at x-ray frequencies a clear signature of Rabi flopping in the spectrum of resonance fluorescence.
We develop a large scale quantum master equation approach to describe dynamical processes of practical open quantum systems driven by both coherent and stochastic interactions by including more than one thousand true states of the systems, motivated by the development of highly bright and fully coherent lasers in the X-ray wavelength regime. The method combines the processes of coherent dynamics induced by the X-ray laser and incoherent relaxations due to spontaneous emissions, Auger decays, and electronic collisions. As examples, theoretical investigation of {\it real} coherent dynamics of inner-shell electrons of a neon gas, irradiated by a high-intensity X-ray laser with a full temporal coherence, is carried out with the approach. In contrast to the rate equation treatment, we find that coherence can suppress the multiphoton absorptions of a neon gas in the ultra-intense X-ray pulse, due to coherence-induced Rabi oscillations and power broadening effects. We study the influence of coherence on ionization processes of neon, and directly prove that sequential single-photon processes for both outer- and inner-shell electrons dominate the ionizations for the recently typical experiments with a laser intensity of ≈1018\approx10^{18}≈1018 W/cm2{\rm W/cm^2}W/cm2. We discuss possible experimental implementations such as signatures for coherent evolution of inner-shell electrons via resonance fluorescence processes. The approach can also be applied to many different practical open quantum systems in atomic, quantum optical, and cold matter systems, which are treated qualitatively by a few-level master equation model before.
The project is focused on theoretical and experimental studies of the quantum coherence effects at the atomic and nuclear transitions in solids. The main results are the following. 1. Electromagnetically Induced Transparency (EIT) and all-optical writing and probing of quantum coherence in room temperature solids are demonstrated experimentally in the first time. 2. The method for coherent suppression of excited state absorption in laser crystals is suggested. It opens the prospects for realization of the variety tunable solid state lasers including UV and VUV ranges. 3. High-efficient THz generation by means of resonant three-level mixing based on EIT in solids is predicted. 4. EIT in gamma-ray range of frequencies at the nuclear transitions via the nuclear level crossing is demonstrated experimentally in the first time. 5. Slowing of the gamma-ray photons due to the absorption line doublet is demonstrated in the first time. 6. Modifications of the Mossbauer absorption spectra under the action of laser radiation demonstrated experimentally in the first time. 7. The method for suppression of the inhomogeneous line broadening of the Mossbauer spectra is suggested.