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Physical Review A

Investigation of radiative coupling and of enlarged decay rates of nuclear oscillators

W. Potzel, Uwe van Bürck, P. Schindelmann, G. M. Kalvius, G. V. Smirnov, Erich Gerdau, Yuri V. Shvyd'ko, H. D. Rüter, O. Leupold

Published Mar 14, 2001• 11 citations
Publisher

Abstract

The nuclear forward scattering of synchrotron radiation (SR) of two spatially separated stainless-steel foils has been investigated. Immediately after the SR pulse a nuclear exciton extending over both targets is formed, which then decays under the influence of radiative coupling of the nuclear oscillators both within and between the targets. Cooling one target to low temperatures causes the M\"ossbauer linewidth to increase by inhomogeneous broadening, which leads to a rapid dephasing of the nuclear oscillators of this system. In general, the influence of the radiation field of the upstream oscillators on the oscillators of the downstream target causes the latter to radiate at a high decay rate (steep slope) at early times. Surprisingly, for the combined targets, the initial decay can be less steep than that of the inhomogeneously broadened sample alone if this broadening is sufficiently large. In addition, when the upstream oscillators are broadened in energy they may cause the downstream oscillators to emit only very little intensity at late times. In the experiments, this influence on the downstream sample was changed by varying the inhomogeneous resonance broadening and the resonance frequencies of the targets. Drastic changes of the time evolution of the nuclear forward scattering were observed, which are discussed within the dynamical theory.

DOI
10.1103/physreva.63.043810

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Pulse propagation of synchrotron radiation through a nuclear two-resonance system

Uwe van Bürck, W. Potzel, P. Schindelmann +4 more

Pulse propagation through a two-resonance medium was investigated in nuclear forward scattering (NFS) of synchrotron radiation (SR). The SR pulse was coherently transmitted through a target system exhibiting two equivalent resonances, the energy separation of which was varied via the Doppler effect. This allowed a continuous change from a single-resonance medium to a two-resonance system with adjustable splitting. For increasing splitting, the observed time evolutions of NFS changed from a dynamical beat (DB) via hybrid forms of beating into a DB-modulated quantum beat. The experimental results were consistently fitted using a dynamical theory for NFS of SR.

Physical Review A•Dec 8, 1999•11 citations

Ultrasound-induced echoes in the nuclear-exciton decay utilising synchrotron radiation Bragg-scattered by a quartz crystal

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The time evolution of the coherent decay of the nuclear exciton induced by 14.4 keV synchrotron radiation was observed with a system of two stainless-steel resonance absorbers. In between these foils, the radiation was Bragg-scattered from a vibrating quartz crystal. The ultrasound-modulated radiation disrupts the coherent superposition of the radiation from both targets. However, at times corresponding to multiples of the ultrasound period, spikes or dips in the decay were observed which are interpreted as ultrasound-induced echoes.

Europhysics Letters (EPL)•Nov 1, 1997•14 citations

Superradiance and subradiance in systems of excited nuclei

Tian-Xiang Shen

This thesis studies superradiance and subradiance in the normal modes of excitation of systems of resonant nuclei. The nuclei are treated as classical electric dipole oscillators interacting through their radiation fields. This coupling radically alters the radiative decay of the system relative to the decay of an isolated nucleus, and leads to both superradiant and subradiant states with strongly enhanced or suppressed radiative decay rates, and with normal mode frequencies which can be strongly shifted from the natural resonance frequency. This issue is of considerable current interest because it is now possible to create spatially coherent single exciton nuclear states by illuminating crystals containing resonant Mossbauer nuclei with synchrotron radiation pulses. The purpose of this thesis is to solve for the radiative eigenmodes in a system of nuclei and to investigate superradiance and subradiance both in the eigenmode solutions and in the synchrotron radiation produced nuclear exciton state.

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Abstract Recently, an observation of acoustically induced transparency (AIT) of a stainless-steel foil for resonant 14.4-keV photons from a radioactive 57Co Mössbauer source due to collective uniform oscillations of atomic nuclei was reported [Phys Rev Lett 124,163602, 2020]. In this paper, we propose to use the steep resonant dispersion of the absorber within the AIT spectral window to dramatically reduce a propagation velocity of γ-ray and x-ray photons. In particular, we show that a significant fraction (more than 40%) of a 97-ns γ-ray single-photon wave packet from a 57Co radioactive source can be slowed down up to 3 m/s and delayed by 144 ns in a 57Fe-enriched stainless-steel foil at room temperature. We also show that a similarly significant slowing down up to 24 m/s and a delay by 42 ns can be achieved for more than 70% of the 100-ns 14.4-keV x-ray single-photon pulse from a synchrotron Mössbauer source available at European Synchrotron Radiation Facility (ESRF) and Spring-8 facility. The propagation velocity can be widely controlled by changing the absorber vibration frequency. Achieving the propagation velocity on the order of 1–50 m/s would set a record in the hard x-ray range, comparable to what was obtained in the optical range.

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Excitation- and Disorder-induced Reduction of the Superradiant Excitonic Decay in GaAs Quantum Well Bragg Structures

J. Kühl, Matthias Hübner, T. Stroucken +5 more

The theoretically predicted rapid superradiant decay of coherent 2D excitons on a time scale of approximately 10 ps [1] is expected to be strongly enhanced in multiple QW (MQW) Bragg structures consisting of N identical QWs separated by (Ga,Al)As barriers the thickness of which was adjusted so that the QWs had a distance d equal to one half of the heavy-hole exciton resonance wavelength λ in the medium [2]. This enhancement originates from efficient radiative coupling of excitons excited in different wells via the exchange of transverse photons. Recently we have published [3] degenerate-four-wave-mixing experiments with fs optical pulses on GaAs QW Bragg structures which demonstrate that the radiative lifetime in high-quality GaAs single QWs (SQW) is approximately 5 ps and decreases to less than 2 ps in Bragg samples with N=10. These studies revealed that radiative recombination provides the dominant contribution to the homogenous linewidth of the 2D exciton in high-quality samples and exceeds nonradiative contributions caused by scattering with sample imperfections in contrast to previous assumptions.

Quantum Optoelectronics•Jan 1, 1997•0 citations
The nuclear forward scattering of synchrotron radiation (SR) of two spatially separated stainless-steel foils has been investigated. Immediately after the SR pulse a nuclear exciton extending over both targets is formed, which then decays under the influence of radiative coupling of the nuclear oscillators both within and between the targets. Cooling one target to low temperatures causes the M"ossbauer linewidth to increase by inhomogeneous broadening, which leads to a rapid dephasing of the nuclear oscillators of this system. In general, the influence of the radiation field of the upstream oscillators on the oscillators of the downstream target causes the latter to radiate at a high decay rate (steep slope) at early times. Surprisingly, for the combined targets, the initial decay can be less steep than that of the inhomogeneously broadened sample alone if this broadening is sufficiently large. In addition, when the upstream oscillators are broadened in energy they may cause the downstream oscillators to emit only very little intensity at late times. In the experiments, this influence on the downstream sample was changed by varying the inhomogeneous resonance broadening and the resonance frequencies of the targets. Drastic changes of the time evolution of the nuclear forward scattering were observed, which are discussed within the dynamical theory.
Pulse propagation through a two-resonance medium was investigated in nuclear forward scattering (NFS) of synchrotron radiation (SR). The SR pulse was coherently transmitted through a target system exhibiting two equivalent resonances, the energy separation of which was varied via the Doppler effect. This allowed a continuous change from a single-resonance medium to a two-resonance system with adjustable splitting. For increasing splitting, the observed time evolutions of NFS changed from a dynamical beat (DB) via hybrid forms of beating into a DB-modulated quantum beat. The experimental results were consistently fitted using a dynamical theory for NFS of SR.
The time evolution of the coherent decay of the nuclear exciton induced by 14.4 keV synchrotron radiation was observed with a system of two stainless-steel resonance absorbers. In between these foils, the radiation was Bragg-scattered from a vibrating quartz crystal. The ultrasound-modulated radiation disrupts the coherent superposition of the radiation from both targets. However, at times corresponding to multiples of the ultrasound period, spikes or dips in the decay were observed which are interpreted as ultrasound-induced echoes.
This thesis studies superradiance and subradiance in the normal modes of excitation of systems of resonant nuclei. The nuclei are treated as classical electric dipole oscillators interacting through their radiation fields. This coupling radically alters the radiative decay of the system relative to the decay of an isolated nucleus, and leads to both superradiant and subradiant states with strongly enhanced or suppressed radiative decay rates, and with normal mode frequencies which can be strongly shifted from the natural resonance frequency. This issue is of considerable current interest because it is now possible to create spatially coherent single exciton nuclear states by illuminating crystals containing resonant Mossbauer nuclei with synchrotron radiation pulses. The purpose of this thesis is to solve for the radiative eigenmodes in a system of nuclei and to investigate superradiance and subradiance both in the eigenmode solutions and in the synchrotron radiation produced nuclear exciton state.
Abstract Recently, an observation of acoustically induced transparency (AIT) of a stainless-steel foil for resonant 14.4-keV photons from a radioactive 57Co Mössbauer source due to collective uniform oscillations of atomic nuclei was reported [Phys Rev Lett 124,163602, 2020]. In this paper, we propose to use the steep resonant dispersion of the absorber within the AIT spectral window to dramatically reduce a propagation velocity of γ-ray and x-ray photons. In particular, we show that a significant fraction (more than 40%) of a 97-ns γ-ray single-photon wave packet from a 57Co radioactive source can be slowed down up to 3 m/s and delayed by 144 ns in a 57Fe-enriched stainless-steel foil at room temperature. We also show that a similarly significant slowing down up to 24 m/s and a delay by 42 ns can be achieved for more than 70% of the 100-ns 14.4-keV x-ray single-photon pulse from a synchrotron Mössbauer source available at European Synchrotron Radiation Facility (ESRF) and Spring-8 facility. The propagation velocity can be widely controlled by changing the absorber vibration frequency. Achieving the propagation velocity on the order of 1–50 m/s would set a record in the hard x-ray range, comparable to what was obtained in the optical range.
Large-amplitude oscillations are predicted in the free-induction decay (FID) of atomic or spin two-level quantum systems that are coherently prepared by a resonant electromagnetic field in the form of a square amplitude pulse of duration TTT. The effect should be detectable in nuclear magnetic resonance (NMR) or in optical resonance experiments when the transition line shape is inhomogeneously broadened (${T}_{2}^{*}\ensuremath{\ll}{T}_{2}$) and the applied field is intense enough that the pulse area $\ensuremath{\chi}T\ensuremath{\ge}2\ensuremath{\pi}$, where $\ensuremath{\chi}$ is the Rabi frequency. Physically, the effect is due to Rabi oscillations which are generated in the preparative stage and are then reproduced because of atomic memory in the radiative period which follows. The atomic polarization derived from the Bloch equations assumes the form of an integral over the inhomogeneous line shape, and can be evaluated numerically for arbitrary linewidth $\ensuremath{\sigma}$ or analytically when $\ensuremath{\sigma}\ensuremath{\rightarrow}\ensuremath{\infty}$. The characteristics of the oscillation are unusual since the oscillation frequency, which is of order $\ensuremath{\chi}$, increases with time while the oscillation envelope vanishes identically for times $t\ensuremath{\ge}2T$. Coherent emission is therefore confined to one pulse width TTT immediately following the pulse. Previous NMR and infrared FID experiments with small-area pulses ($\ensuremath{\chi}T\ensuremath{\le}4\ensuremath{\pi}$), interpreted erroneously in terms of an edge echo effect, support the oscillatory FID behavior described here. For the case $\ensuremath{\chi}T>4\ensuremath{\pi}$, an experimental test would be even more decisive, as the oscillations are more pronounced.
The theoretically predicted rapid superradiant decay of coherent 2D excitons on a time scale of approximately 10 ps [1] is expected to be strongly enhanced in multiple QW (MQW) Bragg structures consisting of N identical QWs separated by (Ga,Al)As barriers the thickness of which was adjusted so that the QWs had a distance d equal to one half of the heavy-hole exciton resonance wavelength λ in the medium [2]. This enhancement originates from efficient radiative coupling of excitons excited in different wells via the exchange of transverse photons. Recently we have published [3] degenerate-four-wave-mixing experiments with fs optical pulses on GaAs QW Bragg structures which demonstrate that the radiative lifetime in high-quality GaAs single QWs (SQW) is approximately 5 ps and decreases to less than 2 ps in Bragg samples with N=10. These studies revealed that radiative recombination provides the dominant contribution to the homogenous linewidth of the 2D exciton in high-quality samples and exceeds nonradiative contributions caused by scattering with sample imperfections in contrast to previous assumptions.