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Journal of Physics B Atomic Molecular and Optical Physics

Suppression of resonant auger effect with chirped x-ray free-electron laser pulse

Yuping Sun, Quan Miao, Aiping Zhou, Ruijin Liu, Bo Liu, Faris Gel’mukhanov

Published Nov 1, 2017• 2 citations
Publisher

Abstract

Abstract We study the Auger effect in the presence of strong x-ray free-electron lasers (XFELs) propagating through resonant argon vapors by solving the Maxwell–Bloch equations numerically. The simulations are based on the three-level system with the carrier frequency tuned in the 2 p 3/2 –4 s resonance. It is shown that the Auger branching is sensitive to the pulse area and duration. The relative Auger yield can be suppressed in the course of pulse propagation due to the interplay between the Auger decay and stimulated emission. Further suppression can be achieved by chirping the initial pulse, which is more effective for the long-pulse case. In addition, the sign and magnitude of the chirp rate play important roles in pulse reshaping and Auger emission.

DOI
10.1088/1361-6455/aa9784

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Abstract We study the Auger effect in the presence of strong x-ray free-electron lasers (XFELs) propagating through resonant argon vapors by solving the Maxwell–Bloch equations numerically. The simulations are based on the three-level system with the carrier frequency tuned in the 2 p 3/2 –4 s resonance. It is shown that the Auger branching is sensitive to the pulse area and duration. The relative Auger yield can be suppressed in the course of pulse propagation due to the interplay between the Auger decay and stimulated emission. Further suppression can be achieved by chirping the initial pulse, which is more effective for the long-pulse case. In addition, the sign and magnitude of the chirp rate play important roles in pulse reshaping and Auger emission.
We study the compression of strong x-ray pulses from x-ray free-electron lasers (XFELs) propagating through the resonant medium of atomic argon. The simulations are based on the three-level model with the frequency of the incident x-ray pulse tuned in the $2{p}_{3/2}\text{\ensuremath{-}}4s$ resonance. The pulse propagation is accompanied by the self-seeded stimulated resonant Raman scattering (SRRS). The SRRS starts from two channels of amplified spontaneous emission (ASE), $4s\text{\ensuremath{-}}2{p}_{3/2}$ and $3s\text{\ensuremath{-}}2{p}_{3/2}$, which form the extensive ringing pattern and widen the power spectrum. The produced seed field triggers the Stokes ASE channel $3s\text{\ensuremath{-}}2{p}_{3/2}$. The population inversion is quenched for longer propagation distances where the ASE is followed by the lasing without inversion (LWI), which amplifies the Stokes component. Both ASE and LWI reshape the input pulse: The compressed front part of the pulse (up to 100 as) is followed by the long tail of the ringing and beating between the pump and Stokes frequencies. The pump pulse also generates weaker Stokes and anti-Stokes fields caused by four-wave mixing. These four spectral bands have fine structures caused by the dynamical Stark effect. A slowdown of the XFEL pulse up to 78% of the speed of light in vacuum is found because of a large nonlinear refractive index.
We study self-seeded stimulated resonant x-ray Raman scattering and show a 20-fold compression of the strong XFEL pulse propagating through the resonant medium of atomic argon with the frequency (244.3 eV) tuned to the 2p 3/2 –4s resonance. The strong x-ray pulse inverts the medium and produces an extensive ringing tail which widens the power spectrum. Newly created seed field triggers the Stokes channel 3s–2p 3/2 of amplified spontaneous emission. The population inversions are quenched for longer propagation distances where lasing without inversion enhances the Stokes component. The pump pulse also generates weaker Stokes and anti-Stokes fields caused by four-wave mixing. The group velocity is decreased up to 78% of the speed of light in vacuum.
A computer simulation code to treat atomic excitation and laser beam propagation simultaneously in an atomic laser isotope separation system has been developed. The three- level Bloch-Maxwell equations are solved numerically to analyze the change of pulse shapes, the modification of laser frequencies and the time-varying atomic populations. The near- resonant effects on the propagation of frequency chirped and non-chirped laser pulses have been analyzed. It was found that there are serious differences in pulse shapes, frequency modifications and propagation velocities between laser pulses with and without chirping.
We investigate propagation of femtosecond chirped Gaussian laser pulse in a dense three-level Λ-type atomic medium by using the numerical solution of the full Maxwell-Bloch equations without the slowly varying envelope and the rotating-wave approximations, and the solution is obtained by PC-FDTD method. It is shown that, variation of the sign and size of the chirp coefficient has considerable effect on pulse propagation property, and the effect is closely relative to size of the pulse area. When the area of chirped pulse is smaller than 4π, splitting doesn’t occur and the chirped pulse evolves gradually to an approximate normal Gaussian pulse (C=0), and this characteristic doesn’t vary with the chirp coefficient varying. However, variation of the chirp coefficient will changed the amplitude and group velocity of the pulse. For the positive chirp(C>0), amplitude and group velocity of the pulse decrease with chirp coefficient increasing, for the negative chirp(CC increasing. Both the chirped pulses with area equal to larger than 4π will split into sub-pulses of different numbers and shapes, the time and number of the pulse splitting will be determined by the sign and size of the chirp coefficient. But in the two cases, the pulse splitting patterns are much different, and the effects of the coefficient are also different. When the pulse area equals 4π, larger chirp coefficient will lead to increased sub-pulse number, but when the pulse area is larger than 4π, larger chirp coefficient will lead to decreased sub-pulse number. In addition, regardless of pulse area being larger or smaller, changing sign and size of the chirp coefficient always produces obvious effect on the atomic population.
The direct interaction between nuclei and strong electromagnetic fields generated by novel coherent x-ray light sources such as the x-ray free electron laser is studied. We investigate the case of various stable or long-lived isotopes with energetically low-lying excited states, where the laser drives the nuclear transition from the ground to the first excited state. We therefore consider the nucleus as a two-level system including the corresponding hyperfine levels in our theoretical description. The performance of present and upcoming x-ray free electron laser facilities in coherently controlling the nuclear excited state population is analyzed for setups involving both solid state as well as accelerated nuclear targets to bridge the gap between photon and transition frequency. Current and predicted laser parameters are applied to calculate excited state populations and signal photon rates of nuclei after a radiation pulse, finding significant improvements compared to former calculations. Furthermore, we determine the parameters necessary to transfer substantial amounts of the nuclear ground state population to the excited state, and therefore to bring nuclear Rabi-oscillation within reach. Additionally, we take into account for the first time collective effects that take place in solid state targets and show that they may provide a significant effect in broadening natural linewidths of nuclear levels depending on the experimental setup. Our findings support the case of nuclear quantum optics for XFEL applications. Therefore, one hopes that eventually coherent control of nuclear transitions can be successfully applied in a number of areas in nuclear physics including preparation, control and detection methods.
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.