Browsing by Author "Zhang, Qi"
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Item Dicke superradiance in solids [Invited](The Optical Society, 2016) Cong, Kankan; Zhang, Qi; Wang, Yongrui; Noe, G. Timothy; Belyanin, Alexey; Kono, JunichiroRecent 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.Item Interplay between Point and Extended Defects and Their Effects on Jerky Domain-Wall Motion in Ferroelectric Thin Films(American Physical Society, 2024) Bulanadi, Ralph; Cordero-Edwards, Kumara; Tückmantel, Philippe; Saremi, Sahar; Morpurgo, Giacomo; Zhang, Qi; Martin, Lane W.; Nagarajan, Valanoor; Paruch, Patrycja; Rice Advanced Materials InstituteDefects have a significant influence on the polarization and electromechanical properties of ferroelectric materials. Statistically, they can be seen as random pinning centers acting on an elastic manifold, slowing domain-wall propagation and raising the energy required to switch polarization. Here we show that the “dressing” of defects can lead to unprecedented control of domain-wall dynamics. We engineer defects of two different dimensionalities in ferroelectric oxide thin films—point defects externally induced via He2+ bombardment, and extended quasi-one-dimensional 𝑎 domains formed in response to internal strains. The 𝑎 domains act as extended strong pinning sites (as expected) imposing highly localized directional constraints. Surprisingly, the induced point defects in the He2+ bombarded samples orient and align to impose further directional pinning, screening the effect of 𝑎 domains. This defect interplay produces more uniform and predictable domain-wall dynamics. Such engineered interactions between defects are crucial for advancements in ferroelectric devices.Item Nonequilibrium Dynamics of Quantum-Degenerate Fermionic and Bosonic Gases in Semiconductors Probed by Coherent Terahertz Magneto-optics(2015-10-21) Zhang, Qi; Kono, Junichiro; Du, Rui-rui; Natelson, DougQuantum-confined semiconductor structures are ideal systems in which to study non-equilibrium and coherent dynamics of interacting many particles in a highly controllable fashion. In particular, two-dimensional (2D) semiconductor systems in a strong perpendicular magnetic field provide one of the cleanest condensed matter systems with ultralong coherence times, allowing us to excite and control macroscopic coherent phenomena. When doped, either electrically or optically, such systems can accommodate quantum-degenerate fermions (electrons) and/or bosons (excitons). In this dissertation, we studied the coherent terahertz (THz) dynamics of 2D gases of electrons and excitons in GaAs quantum wells in magnetic fields with time-domain THz magneto-spectroscopy. In high-mobility 2D electron gases, we made the first observation of collective radiative decay, or superradiance, of cyclotron resonance (CR). The decay rate of coherent CR oscillations increased linearly with the electron density in a wide range, which is a hallmark of superradiant damping. Our fully quantum mechanical theory provided a universal formula for the decay rate. We further achieved ultrastrong coupling of coherent CR with THz photons in a high quality factor 1D photonic crystal cavity. We directly observed time-domain vacuum Rabi oscillations, and the square root of N dependence of collective Rabi splitting with respect to the carrier density. Superradiance decay of CR was significantly suppressed in the cavity, and an intrinsic CR linewidth as sharp as 5.6~GHz was resolved. In undoped GaAs quantum wells, we systematically investigated the nonequilibrium dynamics of electron-hole pairs using ultrafast optical-pump THz-probe spectroscopy. We simultaneously monitored the intraexcitonic 1s-2p transition, which splits into the 1s-2p+ and 1s-2p- transitions in a magnetic field, and the CR of unbound carriers as a function of pair density, temperature, magnetic field, and probe delay time. We found that the 1s-2p- feature is robust at high magnetic fields even under high excitation fluences, indicating magnetically enhanced stability of excitons. While mimicking some of the well-known phenomena in quantum optics of atomic and molecular gases, these results highlight some of the unique features of condensed matter systems due to strong many-body Coulomb interactions among carriers and open a door to the novel physics of THz many-body electrodynamics.Item 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, JunichiroWe 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.Item Superradiant Decay of Cyclotron Resonance of Two-Dimensional Electron Gases(American Physical Society, 2014) Zhang, Qi; Arikawa, Takashi; Kato, Eiji; Reno, John L.; Pan, Wei; Watson, John D.; Manfra, Michael J.; Zudov, Michael A.; Tokman, Mikhail; Erukhimova, Maria; Belyanin, Alexey; Kono, JunichiroWe report on the observation of collective radiative decay, or superradiance, of cyclotron resonance (CR) in high-mobility two-dimensional electron gases in GaAs quantum wells using time-domain terahertz magnetospectroscopy. The decay rate of coherent CR oscillations increases linearly with the electron density in a wide range, which is a hallmark of superradiant damping. Our fully quantum mechanical theory provides a universal formula for the decay rate, which reproduces our experimental data without any adjustable parameter. These results firmly establish the many-body nature of CR decoherence in this system, despite the fact that the CR frequency is immune to electron-electron interactions due to Kohnﾒs theorem.Item Terahertz and Infrared Spectroscopy of Gated Large-Area Graphene(American Chemical Society, 2012) Ren, Lei; Zhang, Qi; Yao, Jun; Sun, Zhengzong; Kaneko, Ryosuke; Yan, Zheng; Nanot, Sébastien L.; Jin, Zhong; Kawayama, Iwao; Tonouchi, Masayoshi; Tour, James M.; Kono, Junichiro; Applied Physics ProgramWe have fabricated a centimeter-size single-layer graphene device with a gate electrode, which can modulate the transmission of terahertz and infrared waves. Using time-domain terahertz spectroscopy and Fourier-transform infrared spectroscopy in a wide frequency range (10–10 000 cm–1), we measured the dynamic conductivity change induced by electrical gating and thermal annealing. Both methods were able to effectively tune the Fermi energy, EF, which in turn modified the Drude-like intraband absorption in the terahertz as well as the “2EF onset” for interband absorption in the mid-infrared. These results not only provide fundamental insight into the electromagnetic response of Dirac fermions in graphene but also demonstrate the key functionalities of large-area graphene devices that are desired for components in terahertz and infrared optoelectronics.Item Terahertz Dynamics of Quantum-Confined Electrons in Carbon Nanomaterials(Springer, 2012) Ren, Lei; Zhang, Qi; Nanot, Sébastien; Kawayama, Iwao; Tonouchi, Masayoshi; Kono, JunichiroLow-dimensional carbon nanostructures, such as single-wall carbon nanotubes (SWCNTs) and graphene, offer new opportunities for terahertz science and technology. Being zero-gap systems with a linear, photon-like energy dispersion, metallic SWCNTs and graphene exhibit a variety of extraordinary properties. Their DC and linear electrical properties have been extensively studied in the last decade, but their unusual finite-frequency, nonlinear, and/or non-equilibrium properties are largely unexplored, although they are predicted to be useful for new terahertz device applications. Terahertz dynamic conductivity measurements allow us to probe the dynamics of such photon-like electrons, or massless Dirac fermions. Here, we use terahertz time-domain spectroscopy and Fourier transform infrared spectroscopy to investigate terahertz conductivities of one-dimensional and two-dimensional electrons, respectively, in films of highly aligned SWCNTs and gated largearea graphene. In SWCNTs, we observe extremely anisotropic terahertz conductivities, promising for terahertz polarizer applications. In graphene, we demonstrate that terahertz and infrared properties sensitively change with the Fermi energy, which can be controlled by electrical gating and thermal annealing.Item Vacuum Bloch–Siegert shift in Landau polaritons with ultra-high cooperativity(Springer Nature, 2018) Li, Xinwei; Bamba, Motoaki; Zhang, Qi; Fallahi, Saeed; Gardner, Geoff C.; Gao, Weilu; Lou, Minhan; Yoshioka, Katsumasa; Manfra, Michael J.; Kono, JunichiroA two-level system resonantly interacting with an a.c. magnetic or electric field constitutes the physical basis of diverse phenomena and technologies. However, Schrödinger’s equation for this seemingly simple system can be solved exactly only under the rotating-wave approximation, which neglects the counter-rotating field component. When the a.c. field is sufficiently strong, this approximation fails, leading to a resonance-frequency shift known as the Bloch–Siegert shift. Here, we report the vacuum Bloch–Siegert shift, which is induced by the ultra-strong coupling of matter with the counter-rotating component of the vacuum fluctuation field in a cavity. Specifically, an ultra-high-mobility two-dimensional electron gas inside a high-Qterahertz cavity in a quantizing magnetic field revealed ultra-narrow Landau polaritons, which exhibited a vacuum Bloch–Siegert shift up to 40 GHz. This shift, clearly distinguishable from the photon-field self-interaction effect, represents a unique manifestation of a strong-field phenomenon without a strong field.Item Wafer-scale monodomain films of spontaneously aligned single-walled carbon nanotubes(Springer Nature, 2016) He, Xiaowei; Gao, Weilu; Xie, Lijuan; Li, Bo; Zhang, Qi; Lei, Sidong; Robinson, John M.; Hároz, Erik H.; Doorn, Stephen K.; Wang, Weipeng; Vajtai, Robert; Ajayan, Pulickel M.; Adams, W. Wade; Hauge, Robert H.; Kono, JunichiroThe one-dimensional character of electrons, phonons and excitons in individual single-walled carbon nanotubes leads to extremely anisotropic electronic, thermal and optical properties. However, despite significant efforts to develop ways to produce large-scale architectures of aligned nanotubes, macroscopic manifestations of such properties remain limited. Here, we show that large (>cm2) monodomain films of aligned single-walled carbon nanotubes can be prepared using slow vacuum filtration. The produced films are globally aligned within ±1.5° (a nematic order parameter of ∼1) and are highly packed, containing 1 × 106 nanotubes in a cross-sectional area of 1 μm2. The method works for nanotubes synthesized by various methods, and film thickness is controllable from a few nanometres to ∼100 nm. We use the approach to create ideal polarizers in the terahertz frequency range and, by combining the method with recently developed sorting techniques, highly aligned and chirality-enriched nanotube thin-film devices. Semiconductor-enriched devices exhibit polarized light emission and polarization-dependent photocurrent, as well as anisotropic conductivities and transistor action with high on/off ratios.