Browsing by Author "Wang, Yongrui"

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    Dicke superradiance in solids [Invited]
    (The Optical Society, 2016) Cong, Kankan; Zhang, Qi; Wang, Yongrui; Noe, G. Timothy; Belyanin, Alexey; Kono, Junichiro
    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.
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    Fermi-edge superfluorescence from a quantum-degenerate electron-hole gas
    (Nature Publishing Group, 2013) Kim, Ji-Hee; Noe, G. Timothy II; McGill, Stephen A.; Wang, Yongrui; Wojcik, Aleksander K.; Belyanin, Alexey A.; Kono, Junichiro
    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 ﾑﾑsuperﾒﾒ than atomic SF.
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    Stability of High-Density Two-Dimensional Excitons against a Mott Transition in High Magnetic Fields Probed by Coherent Terahertz Spectroscopy
    (American Physical Society, 2016) Zhang, Qi; Wang, Yongrui; Gao, Weilu; Long, Zhongqu; Watson, John D.; Manfra, Michael J.; Belyanin, Alexey; Kono, Junichiro
    We have performed time-resolved terahertz absorption measurements on photoexcited electron-hole pairs in undoped GaAs quantum wells in magnetic fields. We probed both unbound- and bound-carrier responses via cyclotron resonance and intraexciton resonance, respectively. The stability of excitons, monitored as the pair density was systematically increased, was found to dramatically increase with increasing magnetic field. Specifically, the 1s−2p− intraexciton transition at 9 T persisted up to the highest density, whereas the 1s−2p feature at 0 T was quickly replaced by a free-carrier Drude response. Interestingly, at 9 T, the 1s−2p− peak was replaced by free-hole cyclotron resonance at high temperatures, indicating that 2D magnetoexcitons do dissociate under thermal excitation, even though they are stable against a density-driven Mott transition.
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    Superfluorescence from photoexcited semiconductor quantum wells: Magnetic field, temperature, and excitation power dependence
    (American Physical Society, 2015) Cong, Kankan; Wang, Yongrui; Kim, Ji-Hee; Noe, G. Timothy II; McGill, Stephen A.; Belyanin, Alexey; Kono, Junichiro
    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 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−T−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.