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Physical Review B

Superfluorescence from photoexcited semiconductor quantum wells: Magnetic field, temperature, and excitation power dependence

Kankan Cong, Yongrui Wang, Ji‐Hee Kim, G. Timothy Noe, Stephen McGill, Alexey A. Belyanin, Junichiro Kono

Published Jun 29, 2015• 24 citations
Publisher

Abstract

Superfluorescence (SF) is a many-body process in which a macroscopic polarization spontaneously builds up from an initially incoherent ensemble of excited dipoles and then cooperatively decays, producing a delayed pulse of coherent radiation. SF arising from electron-hole recombination has recently been observed in ${\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{As}/\mathrm{GaAs}$ quantum wells [G. T. Noe et al., Nature Phys. 8, 219 (2012) and J.-H. Kim et al., Sci. Rep. 3, 3283 (2013)], but its observability conditions have not been fully established. Here, by performing magnetic field $(B)$, temperature $(T)$, and pump power $(P)$ dependent studies of SF intensity, linewidth, and delay time through time-integrated and time-resolved magnetophotoluminescence spectroscopy, we have mapped out the $B\ensuremath{-}T\ensuremath{-}P$ region in which SF is observable. In general, SF can be observed only at sufficiently low temperatures, sufficiently high magnetic fields, and sufficiently high laser powers with characteristic threshold behavior. We provide theoretical insights into these behaviors based primarily on considerations on how the growth rate of macroscopic coherence depends on these parameters. These results provide fundamental new insight into electron-hole SF, highlighting the importance of Coulomb interactions among photogenerated carriers as well as various scattering processes that are absent in SF phenomena in atomic and molecular systems.

DOI
10.1103/physrevb.91.235448

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Superfluorescence (SF) is a many-body process in which a macroscopic polarization spontaneously builds up from an initially incoherent ensemble of excited dipoles and then cooperatively decays, producing a delayed pulse of coherent radiation. SF arising from electron-hole recombination has recently been observed in In0.2Ga0.8As/GaAs{\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{As}/\mathrm{GaAs}In0.2​Ga0.8​As/GaAs quantum wells [G. T. Noe et al., Nature Phys. 8, 219 (2012) and J.-H. Kim et al., Sci. Rep. 3, 3283 (2013)], but its observability conditions have not been fully established. Here, by performing magnetic field (B)(B)(B), temperature (T)(T)(T), and pump power (P)(P)(P) dependent studies of SF intensity, linewidth, and delay time through time-integrated and time-resolved magnetophotoluminescence spectroscopy, we have mapped out the B\ensuremath−T\ensuremath−PB\ensuremath{-}T\ensuremath{-}PB\ensuremath−T\ensuremath−P region in which SF is observable. In general, SF can be observed only at sufficiently low temperatures, sufficiently high magnetic fields, and sufficiently high laser powers with characteristic threshold behavior. We provide theoretical insights into these behaviors based primarily on considerations on how the growth rate of macroscopic coherence depends on these parameters. These results provide fundamental new insight into electron-hole SF, highlighting the importance of Coulomb interactions among photogenerated carriers as well as various scattering processes that are absent in SF phenomena in atomic and molecular systems.
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.
Nonequilibrium can be a source of order. This rather counterintuitive statement has been proven to be true through a variety of fluctuation-driven, self-organization behaviors exhibited by out-of-equilibrium, many-body systems in nature (physical, chemical, and biological), resulting in the spontaneous appearance of macroscopic coherence. Here, we report on the observation of spontaneous bursts of coherent radiation from a quantum-degenerate gas of nonequilibrium electron-hole pairs in semiconductor quantum wells. Unlike typical spontaneous emission from semiconductors, which occurs at the band edge, the observed emission occurs at the quasi-Fermi edge of the carrier distribution. As the carriers are consumed by recombination, the quasi-Fermi energy goes down toward the band edge, and we observe a continuously red-shifting streak. We interpret this emission as cooperative spontaneous recombination of electron-hole pairs, or superfluorescence (SF), which is enhanced by Coulomb interactions near the Fermi edge. This novel many-body enhancement allows the magnitude of the spontaneously developed macroscopic polarization to exceed the maximum value for ordinary SF, making electron-hole SF even more "super" than atomic SF.
Superfluorescence (SF) originates from the spontaneous buildup of macroscopic coherence among initially incoherent emitters. Its formation is highly sensitive to dephasing and collective coupling in the system. Here, we observe room-temperature SF in CdS/CdSe/CdS spherical quantum wells. With increasing excitation fluence, the peak emission intensity shows a nearly quadratic increase, while the emission delay and pulse width decrease. Burnham-Chiao ringing is also observed, revealing the characteristic collective radiation dynamics of SF. Further experiments with stripe excitation show that shortening the excitation length L changes the dominant fast emission from amplified spontaneous emission to SF. More importantly, the threshold for SF under stripe excitation is only about 1/30 of that under spot excitation. This large reduction indicates that the propagating radiation field provided by spatially extended excitation favors the buildup of cooperative coherence. These findings provide direct experimental evidence for understanding ultrafast many-body coherence dynamics and offer a route to actively control collective emission at room temperature.
Abstract Quantum particles sometimes cooperate to develop a macroscopically ordered state with extraordinary properties. Superconductivity and Bose‐Einstein condensation are examples of such cooperative phenomena where macroscopic order appears spontaneously. Here, we demonstrate that such an ordered state can also be obtained in an optically excited semiconductor quantum well in a high magnetic field. When we create a dense electron‐hole plasma with an intense laser pulse, after a certain delay, an ultrashort burst of coherent radiation emerges. We interpret this striking phenomenon as a manifestation of superfluorescence (SF), in which a macroscopic polarization spontaneously builds up from an initially incoherent ensemble of excited quantum oscillators and then decays abruptly producing giant pulses of coherent radiation. SF has been observed in atomic gases, but the present work represents the first observation of SF in a solid‐state setting. While there is an analogy between the recombination of electron‐hole pairs and radiative transitions in atoms, there is no a priori reason for SF in semiconductors to be similar to atomic SF. This is a complex many‐body system with a variety of ultrafast interactions, where the decoherence rates are at least 1,000 times faster than the radiative decay rate, an unusual situation totally unexplored in previous atomic SF studies. We show, nonetheless, that collective many‐body coupling via a common radiation field does develop under certain conditions and leads to SF bursts. The solid‐state realization of SF resulted in an unprecedented degree of controllability in the generation of SF, opening up opportunities for both fundamental many‐body studies and device applications. We demonstrate that the intensity and delay time of SF bursts are fully tunable through an external magnetic field, temperature, and pump laser power.
Superfluorescence, a cooperative coherent spontaneous emission, is of great importance to the understanding of many-body correlation in optical processes. Even though superfluorescence has been demonstrated in many diverse systems, it is hard to observe in electron-hole plasma (EHP) due to its rapid dephasing and hence needs strong magnetic fields or complex microcavities. Herein, we report the first experimental observation of superfluorescence from EHP up to a moderate temperature of 175 K without external stimuli in a coupled metal halide perovskite quantum dots film. The EHP exhibits macroscopic quantum coherence through spontaneous synchronization. The coherence of the excited state decays by superfluorescence, which is redshifted 40 meV from the spontaneous emission with a ∼1700 times faster decay rate and exhibits quadratic fluence dependence. Notably, the excited state population's delayed growth and abrupt decay, which are strongly influenced by the pump fluence and the Burnham-Chiao ringing, are the characteristics of the superfluorescence. Our findings will open up a new frontier for cooperative emission and light beam-based technologies.