Browsing by Author "Cong, Kankan"

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    Coherent Light-Matter Coupling and Nonequilibrium Carrier Dynamics in Single-Chirality Carbon Nanotubes
    (2018-03-02) Cong, Kankan; Kono, Junichiro
    Single-wall carbon nanotubes (SWCNTs) are unique one-dimensional (1D) condensed matter systems in which strongly enhanced Coulomb interactions are combined with unusual band structure. There are metallic and semiconducting SWCNTs, in both of which electron-electron interactions have significant impact on their electronic and optical properties. In this dissertation work, we used ultrafast optical pump-probe spectroscopy to investigate nonequilibrium dynamics of photogenerated electron-hole pairs, or excitons, in a sample in which a particular species, or chirality, of semiconducting SWCNTs was enriched. Specifically, we studied both an aqueous suspension and an aligned film of (6,5) SWCNTs. Depending on the pump photon energy, intensity, and polarization, different physical processes ensue after ultrafast pumping, including coherent light-matter interactions and incoherent relaxation of carriers/excitons. For example, under below-gap pumping, a transient blueshift of the exciton peak occurred, only during the pump pulse duration, a hallmark of the optical Stark effect. Under resonant pumping, transient splitting of the exciton peak was observed within the pulse duration, which is a manifestation of the Rabi doublet due to coherent light-matter interaction in the strong coupling regime. The Rabi doublet was observed only under resonant or near-resonant pumping conditions. In the case of a macroscopically aligned (6,5) SWCNT film sample, an anisotropic Rabi doublet of the exciton peak was observed under resonant pumping. In the case of above-gap excitation, incoherent relaxation processes dominated the dynamics of excitons. Analysis of these ultrafast, nonequilibrium, and strongly driven phenomena provided considerable new insight into the states and dynamics of electrons in the presence of extreme quantum confinement and strong many-body interactions.
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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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    Superfluorescence from a Two-Dimensional Electron-Hole System: Magnetic Field, Temperature, and Density Dependence
    (2014-10-30) Cong, Kankan; Kono, Junichiro; Mittleman, Daniel; Natelson, Douglas
    In the phenomenon of superfluorescence (SF), a macroscopic polarization spontaneously builds up from an initially incoherent ensemble of excited dipoles and then cooperatively decays, producing giant pulses of coherent radiation. SF arising from electron-hole recombination has recently been observed in semiconductor quantum wells, but its observability conditions have not been fully understood. Here, by fully mapping out the magnetic field (B), temperature (T), and electron-hole pair density (n) dependence of SF intensity and linewidth, we have constructed a ‘phase’ diagram, showing the B-T-n region in which SF is observable. In general, SF can be observed only at low enough temperatures, high enough magnetic fields, and high enough laser powers with characteristic threshold behaviors. These results lay the foundation of our understanding of electron-hole SF and provide guidelines for our search for a Bardeen-Cooper-Schrieffer state of excitons.
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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.