Summary form only given. The radiative behavior of an ensemble of correlated atoms can be dramatically different from that of a single atom. Correlation, for example, can give rise to sub- or superradiant emission. Experimental studies of the fascinating correlative effects expected have been severely constrained by difficulties in creating suitably correlated media and by propagation effects that perturb correlation during emission. We have found that optical cavities of moderate reflectivity and very short ring-down time can be employed to "accelerate" the superradiant process and bring it to dominance in media sufficiently dilute so that propagation effects both in excitation and emission are absent. For the first time, it is possible to observe strong optical superradiance under the pure and simple conditions that were originally envisioned. Note that superradiance here does not refer to the emission of initially inverted atomic samples (i.e. superfluorescence), but to cooperative emission from samples of atoms prepared in correlated superposition states. Moreover, the cavity accelerated superradiant system is found to display heretofore unknown nonlinear behavior in terms of its effect on the "area" of injected optical pulses.
Summary form only given. The radiative behavior of an ensemble of correlated atoms can be dramatically different from that of a single atom. Correlation, for example, can give rise to sub- or superradiant emission. Experimental studies of the fascinating correlative effects expected have been severely constrained by difficulties in creating suitably correlated media and by propagation effects that perturb correlation during emission. We have found that optical cavities of moderate reflectivity and very short ring-down time can be employed to "accelerate" the superradiant process and bring it to dominance in media sufficiently dilute so that propagation effects both in excitation and emission are absent. For the first time, it is possible to observe strong optical superradiance under the pure and simple conditions that were originally envisioned. Note that superradiance here does not refer to the emission of initially inverted atomic samples (i.e. superfluorescence), but to cooperative emission from samples of atoms prepared in correlated superposition states. Moreover, the cavity accelerated superradiant system is found to display heretofore unknown nonlinear behavior in terms of its effect on the "area" of injected optical pulses.
A theoretical model for the propagation of coherent light pulses in resonant media where a large number of atoms cooperate in spontaneous emission (superradiance) is discussed. The medium is supposedly free of inhomogeneous broadening, and no lossy relaxation mechanisms are taken into account. The conditions for the emission of all spontaneous radiation in the direction of pulse propagation are described. A semi-classical model is used and, at each point in the medium, the propagation field and the spontaneous damping field are considered separately. The equation governing the evolution of a superradiant system in the presence of an applied pulse is derived. A mechanism of energy transfer from the traveling pulse to the medium is described which includes superradiant spontaneous emission effects. As a result of the neglect of all lossy relaxation processes, all energy stored in the medium is eventually returned to the pulse by coherent spontaneous emission. Therefore, in this theory, all pulses travel through the medium without energy loss. Numerical results for square input pulses are discussed which show two possible types of propagation behavior: (a) Low input intensities produce a negative tail to the input (positive) pulse and sharpening of the tail with the advance in the medium; (b) higher input intensities produce, after sufficient penetration, a steady-state pulse envelope somewhat similar to the $2\ensuremath{\pi}$ pulses of self-induced transparency. The steady-state solutions of the equations of motion are found numerically and the pulse shapes are computed for different degrees of superradiance. In the limit of strong superradiance, very asymmetric pulse envelopes of large area are found. For negligible superradiance, approximately $2\ensuremath{\pi}$ hyperbolic-secant pulses are obtained. The effects of inhomogeneous broadening are briefly considered, and the consequences of important superradiant damping in photon-echo experiments are qualitatively discussed.
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.
We calculate the far-field intensity pattern of superradiant emission from an optically thin ensemble of atomic oscillators excited by a spatially nonuniform laser pulse of large Rabi area. As the excitation intensity is increased, the superradiant field develops ring structure and expands due to self-diffraction. This effect is made manifest by a two-pulse photon echo experiment where the second pulse is spatially much broader than the first and has a small Rabi area. As a function of the intensity of the first excitation pulse, we measure the on-axis and off-axis echo intensity and find, in agreement with our calculations, that the two behave differently. We also show that for a random, sufficiently smooth excitation profile, the total power of cooperative emission is proportional to the integral, over the excited volume, of the induced dipole moment squared.
When an inverted ensemble of atoms is tightly packed on the scale of its emission wavelength or when the atoms are collectively strongly coupled to a single cavity mode, their dipoles will align and decay rapidly via a superradiant burst. However, a spread-out dipole phase distribution theory predicts a required minimum threshold of atomic excitation for superradiance to occur. Here we experimentally confirm this predicted threshold for superradiant emission on a narrow optical transition when exciting the atoms transversely and show how to take advantage of the resulting sub- to superradiant transition. A π/2-pulse places the atoms in a subradiant state, protected from collective cavity decay, which we exploit during the free evolution period in a corresponding Ramsey pulse sequence. The final excited state population is read out via superradiant emission from the inverted atomic ensemble after a second π/2-pulse, and with minimal heating this allows for multiple Ramsey sequences within one experimental cycle. Our scheme is an innovative approach to atomic state readout characterized by its speed, simplicity, and highly directional emission of signal photons. It demonstrates the potential of sensors using collective effects in cavity-coupled quantum emitters.
Large atomic ensembles coupled to a single optical resonator mode can be steered to strongly enhanced or suppressed collective emission via phase controlled excitation. Employing the Tavis-Cummings model we find so far unreported phenomena. Using a second order cumulant expansion we predict that a homogeneously excited ensemble equally distributed between odd and even sites along the cavity mode is extremely subradiant as long as the average excitation remains below 50%, but shows pulsed emission for inversion. The combination of these two properties enables the implementation of an efficient cavity-enhanced Ramsey probing featuring a fast readout and minimal heating with particular advantages for atomic clock transitions. For continuous illumination the nonlinear atom-field interaction induces regular superradiant self-pulsing. Additionally, we observe an increased pulse delay time in comparison to an excitation through the cavity.
We report observations of optical superradiant emission and the atomic evolution it drives under conditions closely approximating those originally envisioned in the classic work of Dicke [Phys. Rev. 93, 99 (1954)]. Our experiment involves an optically thin solid sample in a short-lifetime optical cavity whose homogeneous coherence is cryogenically stabilized. Pulsed coherent excitation initiates superradiant emission which subsequently drives the sample to higher or lower states of coherence. Suppression of dephasing via cryogenics and propagation effects through use of an optically thin sample and cavity provides one of the clearest and cleanest examples of Dicke superradiance yet reported.