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Coherent Control of Nuclear Excited State Population Using X-ray Free Electron Lasers

André Junker

Published Jan 1, 2011• 0 citations
OpenAlex

Abstract

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.

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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 interaction between super-intense coherent x-ray light and nuclei is studied theoretically. One of the main difficulties in driving nuclear transitions arises from the very narrow nuclear excited state widths, which limit the coupling between lasers and nuclei. In the context of direct laser–nucleus interaction, we consider the nuclear width broadening that occurs when in solid targets, when the excitation caused by a single photon is shared by a large number of nuclei, forming a collective excited state. Our results show that cooperative effects mostly contribute with a modest increase to the nuclear excited state population except in the case of 57 Fe, where the enhancement can reach almost two orders of magnitude. Additionally, an update is given of previous estimates of the nuclear excited state population and signal photons for x-ray lasers interacting with solid-state and ion beam nuclear targets taking into account the experimental advances of x-ray coherent light sources. The presented values are an improvement by orders of magnitude and are encouraging as to the future prospects of nuclear quantum optics.
The controlled transfer of nuclear state population using two x-ray laser pulses is investigated theoretically. The laser pulses drive two nuclear transitions in a nuclear three-level system facilitating coherent population transfer via the quantum optics technique of stimulated Raman adiabatic passage. To overcome present limitations of the x-ray laser frequency, we envisage accelerated nuclei interacting with two copropagating or crossed x-ray laser pulses in a three-beam setup. We present a systematic study of this setup providing both pulse temporal sequence and laser pulse intensity for optimized control of the nuclear state population. The tolerance for geometrical parameters such as laser beam divergence of the three-beam setup as well as for the velocity spread of the nuclear beam are studied and a two-photon resonance condition to account for experimental uncertainties is deduced. This additional condition gives a less strict requirement for the experimental implementation of the three-beam setup. Present experimental state of the art and future prospects are discussed.
Coherent control of nuclear pumping in a three-level system driven by x-ray light is investigated. In single nuclei the pumping performance is determined by the branching ratio of the excited state populated by the x-ray pulse. Our results are based on the observation that in ensembles of nuclei, cooperative excitation and decay leads to a greatly modified nuclear dynamics which we characterize by a time-dependent cooperative branching ratio. We discuss prospects of steering the x-ray pumping by coherently controlling the cooperative decay. First, we study an ideal case with purely superradiant decay and perfect control of the cooperative emission. A numerical analysis of x-ray pumping in nuclear forward scattering with coherent control of the cooperative decay via externally applied magnetic fields is presented. Next, we provide an extended survey of nuclei suitable for our scheme, and propose proof-of-principle implementations already possible with typical M\"ossbauer nuclei such as 57Fe^{57}\mathrm{Fe}57Fe. Finally, we discuss the application of such control techniques to the population or depletion of long-lived nuclear states.
Population transfer of three-level $\mathrm{\ensuremath{\Lambda}}$-like nuclei interacting with x-ray free electron lasers (XFEL) via a train of coincident pulses has been investigated theoretically. This study uses copropagating beams in which the frequency of the pump laser is different from that of the Stokes laser. We calculate the required laser intensities for each step which satisfy the condition of coincident-pulse technique in different nuclei. By employing the master equation and considering the effect of spontaneous emission, we show that complete nuclear population transfer occurs by choosing the appropriate number of pulse pairs. It is shown that the effect of laser intensity fluctuation is suppressed by increasing the number of pulse pairs. In this scheme, after each step, the populations of system are in stable states and the time delays between the neighboring pulses do not affect the transmission efficiency.
The advent of x-ray free electron lasers (XFELs) has opened the door to exploring the x-ray-nuclei interaction in the multiexcitation regime. Here, we show the dependence of emission properties of x-ray photons from an ensemble of identical resonant nuclei on the number of excitations in the system, employing the theoretical methods we have introduced. Under the multiexcitation condition, the detection of x-ray quanta is emission-order selective. Our work presents an efficient approach to investigating the multiphoton emission of x-rays from cooperative resonant nuclei within and beyond weak-excitation regimes, offering valuable insights into nonlinear x-ray optics and many-body nuclear physics.
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.