G. Timothy Noe

Giant superfluorescent bursts from a semiconductor magneto-plasma

Superfluorescence—the emission of coherent light from an initially incoherent collection of excited dipoles—is now identified in a semiconductor. Laser-excited electron–hole pairs spontaneously polarize and then abruptly decay to produce intense pulses of light.

Dicke superradiance in solids [Invited]

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.

Fermi-edge superfluorescence from a quantum-degenerate electron-hole gas

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.

A table-top, repetitive pulsed magnet for nonlinear and ultrafast spectroscopy in high magnetic fields up to 30 T.

We have developed a mini-coil pulsed magnet system with direct optical access, ideally suited for nonlinear and ultrafast spectroscopy studies of materials in high magnetic fields up to 30 T. The apparatus consists of a small coil in a liquid nitrogen cryostat coupled with a helium flow cryostat to provide sample temperatures down to below 10 K. Direct optical access to the sample is achieved with the use of easily interchangeable windows separated by a short distance of ~135 mm on either side of the coupled cryostats with numerical apertures of 0.20 and 0.03 for measurements employing the Faraday geometry. As a demonstration, we performed time-resolved and time-integrated photoluminescence measurements as well as transmission measurements on InGaAs quantum wells.

Single-shot terahertz time-domain spectroscopy in pulsed high magnetic fields

We have developed a single-shot terahertz time-domain spectrometer to perform optical-pump/terahertz-probe experiments in pulsed, high magnetic fields up to 30 T. The single-shot detection scheme for measuring a terahertz waveform incorporates a reflective echelon to create time-delayed beamlets across the intensity profile of the optical gate beam before it spatially and temporally overlaps with the terahertz radiation in a ZnTe detection crystal. After imaging the gate beam onto a camera, we can retrieve the terahertz time-domain waveform by analyzing the resulting image. To demonstrate the utility of our technique, we measured cyclotron resonance absorption of optically excited carriers in the terahertz frequency range in intrinsic silicon at high magnetic fields, with results that agree well with published values.

Superfluorescent laser sources based on single-wall carbon nanotube films (Conference Presentation)

High-quality thin films of highly aligned semiconducting single-wall carbon nanotubes have been recently demonstrated. They have excellent absorption and photoluminescence properties; however, fast nonradiative recombination of carriers prevents their use as a gain medium in lasers. Here we predict that such films can operate as efficient sources of ultrashort radiation pulses under the conditions of superfluorescence, i.e. cooperative interband recombination of injected electrons and holes. Superfluorescence develops much faster than nonradiative recombination and leads to high-intensity, coherent pulses of near/mid-infrared radiation.

Excitons in Magnetic Fields

Abstract An applied magnetic field greatly modifies the states and properties of excitons, or bound electron-hole pairs, in photoexcited semiconductors. Depending on the strength of the magnetic field, as well as the density and temperature of the excitons, a variety of physical phenomena occur, providing a rich and controllable environment for not only determining characteristic exciton parameters but also probing complex many-body interactions spectroscopically. In this paper, we review this important sub-field of semiconductor optics. After summarizing the basic theory of excitons in magnetic fields, with a special emphasis on the importance of dimensionality, we describe experimental studies of interband and intraband magneto-optical processes in various semiconductors, including bulk germanium and silicon, III-V semiconductor quantum wells, wires and dots, monolayer transition metal dichalcogenides, and single-wall carbon nanotubes. In addition to linear and continuous-wave optical absorption and photoluminescence spectroscopy studies, we consider nonlinear and ultrafast spectroscopy experiments performed at high magnetic fields. Finally, we discuss recent experiments revealing the extreme stability of two-dimensional excitons against ionization and the cooperative radiative recombination, or superfluorescence, of electron-hole pairs in high magnetic fields.

Sequential superfluorescent bursts from a dense electron-hole plasma via fermi-edge gain enhancement

A high-density electron-hole plasma in InGaAs/GaAs quantum wells emits a series of sequential bursts of intense superfluorescent radiation with photon energies corresponding to the separation between the electron and hole quasi-Fermi energies.

Cooperative phenomena in an ultradense electron-hole Magneto-plasma

We have performed ultrafast pump-probe and time-resolved photoluminescence experiments on highly excited semiconductor quantum wells in a high magnetic field, observing time-delayed suprefluorescence bursts of coherent radiation together with a sudden population drop from full inversion to zero.