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Europhysics Letters (EPL)

Forward-Backward Synchronization in Solid-State Superfluorescence

Richard Bausch, P Borgs, Reiner Kree, Fritz Haake

Published Nov 1, 1989• 18 citations
Publisher

Abstract

Recent experiments on superfluorescence from O 2 - centres in KCl host crystals, with quasi-one-dimensional active volumes, have shown identically shaped and perfectly synchronized forward and backward pulses in each run. This is in blatant contrast to previous experiments on atomic beams and gases in which no forward-backward correlation beyond energy conservation had been found. We argue that a linear coupling due to backscattering from the bulk or the surface of the active volume, a phenomenon much weaker in gases than in solids, is responsible for the effect.

DOI
10.1209/0295-5075/10/5/010

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Time-resolving experiments on Dicke superfluorescence of O2− centers in KCl. Two-color superfluorescence

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Single crystals of KCl containing $\mathrm{O}_{2}^{}{}_{}{}^{\ensuremath{-}}$ centers (8\ifmmode\times\else\texttimes\fi{}${10}^{16}$-7\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) were excited at low temperatures ($T<30$ K) using frequency-quadrupled single pulses of a mode-locked Nd-doped yttrium aluminum garnet laser. At excitation intensities above 20 GW/${\mathrm{cm}}^{2}$ a spontaneous emission of coherent light was observed which could be identified as Dicke superfluorescence. This superfluorescence was observed on two of the zero-phonon lines in the vibrational progression of the $\mathrm{O}_{2}^{}{}_{}{}^{\ensuremath{-}}$ fluorescence spectrum (0-10 and 0-11 at 592.8 and 629.1 nm, respectively). We have studied the time dependence, the statistics of the delay times, and the intensities. The observed delay times vary between 0.5 and 10 ns, the observed pulse widths between 0.5 and 6 ns, and the coherence time was found to be longer than 100 ps. The observed linewidths of the superfluorescence ($0.016\mathrm{nm}\ensuremath{\cong}14\mathrm{GHz}$) is a factor of 2.5 smaller than that of the spontaneous fluorescence (0.042 nm at 4.2 K). The superfluorescence is observed only below a critical temperature which depends on the $\mathrm{O}_{2}^{}{}_{}{}^{\ensuremath{-}}$ concentration (8 to 28 K for concentrations between 8\ifmmode\times\else\texttimes\fi{}${10}^{16}$ and 7\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$).

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L.O. Schwan, P. Schwendimann, E. Sigmund

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Physical Review A•Dec 1, 1989•19 citations
Recent experiments on superfluorescence from O 2 - centres in KCl host crystals, with quasi-one-dimensional active volumes, have shown identically shaped and perfectly synchronized forward and backward pulses in each run. This is in blatant contrast to previous experiments on atomic beams and gases in which no forward-backward correlation beyond energy conservation had been found. We argue that a linear coupling due to backscattering from the bulk or the surface of the active volume, a phenomenon much weaker in gases than in solids, is responsible for the effect.
Recent advances in optical studies of condensed matter systems have led to the emergence of a variety of phenomena that have conventionally been studied in the realm of quantum optics. These studies have not only deepened our understanding of light–matter interactions but have also introduced aspects of many-body correlations inherent in optical processes in condensed matter systems. This paper is concerned with the phenomenon of superradiance (SR), a profound quantum optical process originally predicted by Dicke in 1954. The basic concept of SR applies to a general N body system, where constituent oscillating dipoles couple together through interaction with a common light field and accelerate the radiative decay of the whole system. Hence, the term SR ubiquitously appears in order to describe radiative coupling of an arbitrary number of oscillators in many situations in modern science of both classical and quantum description. In the most fascinating manifestation of SR, known as superfluorescence (SF), an incoherently prepared system of N inverted atoms spontaneously develops macroscopic coherence from vacuum fluctuations and produces a delayed pulse of coherent light whose peak intensity ∝N2. Such SF pulses have been observed in atomic and molecular gases, and their intriguing quantum nature has been unambiguously demonstrated. In this review, we focus on the rapidly developing field of research on SR phenomena in solids, where not only photon-mediated coupling (as in atoms) but also strong Coulomb interactions and ultrafast scattering processes exist. We describe SR and SF in molecular centers in solids, molecular aggregates and crystals, quantum dots, and quantum wells. In particular, we will summarize a series of studies we have recently performed on semiconductor quantum wells in the presence of a strong magnetic field. In one type of experiment, electron-hole pairs were incoherently prepared, but a macroscopic polarization spontaneously emerged and cooperatively decayed, emitting an intense SF burst. In another type of experiment, we observed the SR decay of coherent cyclotron resonance of ultrahigh-mobility 2D electron gases, leading to a decay rate that is proportional to the electron density. These results show that cooperative effects in solid-state systems are not merely small corrections that require exotic conditions to be observed; rather, they can dominate the nonequilibrium dynamics and light emission processes of the entire system of interacting electrons.
Single crystals of KCl containing $\mathrm{O}_{2}^{}{}_{}{}^{\ensuremath{-}}$ centers (8\ifmmode\times\else\texttimes\fi{}1016{10}^{16}1016-7\ifmmode\times\else\texttimes\fi{}1017{10}^{17}1017 ${\mathrm{cm}}^{\ensuremath{-}3}$) were excited at low temperatures (T<30T<30T<30 K) using frequency-quadrupled single pulses of a mode-locked Nd-doped yttrium aluminum garnet laser. At excitation intensities above 20 GW/cm2{\mathrm{cm}}^{2}cm2 a spontaneous emission of coherent light was observed which could be identified as Dicke superfluorescence. This superfluorescence was observed on two of the zero-phonon lines in the vibrational progression of the $\mathrm{O}_{2}^{}{}_{}{}^{\ensuremath{-}}$ fluorescence spectrum (0-10 and 0-11 at 592.8 and 629.1 nm, respectively). We have studied the time dependence, the statistics of the delay times, and the intensities. The observed delay times vary between 0.5 and 10 ns, the observed pulse widths between 0.5 and 6 ns, and the coherence time was found to be longer than 100 ps. The observed linewidths of the superfluorescence ($0.016\mathrm{nm}\ensuremath{\cong}14\mathrm{GHz}$) is a factor of 2.5 smaller than that of the spontaneous fluorescence (0.042 nm at 4.2 K). The superfluorescence is observed only below a critical temperature which depends on the $\mathrm{O}_{2}^{}{}_{}{}^{\ensuremath{-}}$ concentration (8 to 28 K for concentrations between 8\ifmmode\times\else\texttimes\fi{}1016{10}^{16}1016 and 7\ifmmode\times\else\texttimes\fi{}1017{10}^{17}1017 ${\mathrm{cm}}^{\ensuremath{-}3}$).