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 also introduced aspects of many-body correlations inherent in optical processes in condensed matter systems. This article 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 ∝ N 2 . 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 two-dimensional 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.

Light polarization is used in the animal kingdom for communication, navigation, and enhanced scene interpretation and also plays an important role in astronomy, remote sensing, and military applications. To date, there have been few photodetector materials demonstrated to have direct polarization sensitivity, as is usually the case in nature. Here, we report the realization of a carbon-based broadband photodetector, where the polarimetry is intrinsic to the active photodetector material. The detector is based on p-n junctions formed between two macroscopic films of single-wall carbon nanotubes. A responsivity up to ~1 V/W was observed in these devices, with a broadband spectral response spanning the visible to the mid-infrared. This responsivity is about 35 times larger than previous devices without p-n junctions. A combination of experiment and theory is used to demonstrate the photothermoelectric origin of the responsivity and to discuss the performance attributes of such devices.

Next-generation terahertz (THz) sources demand lightweight, low-cost, defect-tolerant, and robust components with synergistic, tunable capabilities. However, a paucity of materials systems simultaneously possessing these desirable attributes and functionalities has made device realization difficult. Here we report the observation of asymmetric spintronic-THz radiation in Two-Dimensional Hybrid Metal Halides (2D-HMH) interfaced with a ferromagnetic metal, produced by ultrafast spin current under femtosecond laser excitation. The generated THz radiation exhibits an asymmetric intensity toward forward and backward emission direction whose directionality can be mutually controlled by the direction of applied magnetic field and linear polarization of the laser pulse. Our work demonstrates the capability for the coherent control of THz emission from 2D-HMHs, enabling their promising applications on the ultrafast timescale as solution-processed material candidates for future THz emitters.

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. Nat. 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 timeresolved magneto-photoluminescence 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.

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Optical nonlinearities can be engineered into high-speed optical gates when a probe signal is switched coherently and fast by an optical control pulse through various nonlinear effects for switching. In semiconductors, a strong light–matter interaction can also excite many electrons to interact with each other, which can deteriorate switching through Coulomb-induced dephasing. Here, we demonstrate that optical transmission of carbon nanotubes can be switched reversibly hundreds of times via detuned Rabi splitting, faster than 200 fs via nonresonant but strong control pulses. Our detailed experiment–theory analysis identifies that quantum memory in Coulombic scattering restores reversibility while simultaneously reducing undesirable pure dephasing of coherences. This capability creates new possibilities for ultrafast quantum optoelectronic processing in quantum materials.

We report experimental results on current–voltage characteristics of Au-Ti/n-Si/Au-Ti devices made on unintentionally n-doped silicon, which depend dramatically on temperature and external magnetic field. While such devices show Ohmic behavior in the temperature range 154–300 K, space-charge effect dominates at high bias voltages in the temperature range 77–154 K. In the later case, external magnetic field may increase its breakdown voltage, thus inducing large positive magnetoresistance similar to those reported by other groups.

Reversible, 200 fs optical switching of carbon nanotubes is demonstrated by utilizing Coulombic many-body effects, strong exciton binding and quantum memory in Coulombic scattering which together eliminate pure dephasing of coherences.