Continuous wave lasing between Landau levels in graphene

We predict the general feasibility and demonstrate the specific design of the THz laser operating between Landau levels in graphene placed on a polar substrate in a magnetic field of order 1 T. Steady state operation under a continuous wave optical pumping is possible due to an interplay between Auger and surface-phonon mediated relaxation of carriers. The scheme is scalable to other materials with massless Dirac fermions, for example surface states in 3D topological insulators such as Bi$_2$Se$_3$ or Bi$_2$Te$_3$.Comment: 12 pages, 5 figure

Second-order nonlinear optical response of graphene

Although massless Dirac fermions in graphene constitute a centrosymmetric medium for in-plane excitations, their second-order nonlinear optical response is nonzero if the effects of spatial dispersion are taken into account. Here we present a rigorous quantum-mechanical theory of the second-order nonlinear response of graphene beyond the electric dipole approximation, which includes both intraband and interband transitions. The resulting nonlinear susceptibility tensor satisfies all symmetry and permutation properties, and can be applied to all three-wave mixing processes. We obtain useful analytic expressions in the limit of a degenerate electron distribution, which reveal quite strong second-order nonlinearity at long wavelengths, Fermi-edge resonances, and unusual polarization properties.Comment: The new version makes the bold font vector notation uniform throughout the paper and corrects a misprint in Eq. (34): replaces v_F^2 with v_F^3 in the common facto

Effective scheduling algorithm for on-demand XML data broadcasts in wireless environments

The organization of data on wireless channels, which aims to reduce the access time of mobile clients, is a key problem in data broadcasts. Many scheduling algorithms have been designed to organize flat data on air. However, how to effectively schedule semi-structured information such as XML data on wireless channels is still a challenge. In this paper, we firstly propose a novel method to greatly reduce the tuning time by splitting query results into XML snippets and to achieve better access efficiency by combining similar ones. Then we analyze the data broadcast scheduling problem of on-demand XML data broadcasts and define the efficiency of a data item. Based on the definition, a Least Efficient Last (LEL) scheduling algorithm is also devised to effectively organize XML data on wireless channels. Finally, we study the performance of our algorithms through extensive experiments. The results show that our scheduling algorithms can reduce both access time and tuning time signifcantly when compared with existing work

Nonlinear Optical Processes in Two-Dimensional Semiconductor Structures

The optical properties of two types of two-dimensional (2D) semiconductor structures are studied. One of them is for structures based on quantum wells (QWs), and the other is graphene. We study the dynamics of optically excited electron-hole plasma or magnetoplasma in uncoupled QW structure. Experimentally we have observed a delayed burst of optical pulse, or super fluorescence (SF). Time and energy-resolved measurement shows the center frequency of the pulse is red-shifting with time. We explain this by developing the generalized semiconductor Bloch equations (SBEs), where Coulomb interaction between electrons is taken into account. For electronhole plasma in quasi-equilibrium, the calculation shows the peak gain is near the Fermi-edge. So, the red-shifting is because of the decreasing of Fermi energy with time. The effect of Coulomb interaction in intersubband transitions is also studied, where we have developed equations similar to the SBEs, and show that the Coulomb effect could enhance particular second-order nonlinear optical processes. Quantum cascade lasers (QCLs) are well developed devices based on QWs. We study the active modulation in mid-infrared (mid-IR) QCLs. We show that QCLs with short gain recovery time can also generate short pulses by active modulation, while it is previously thought active modulation can only be applied for QCLs with long gain recovery time. Comparisons between the two cases show the performance of QCLs with short gain recovery time is more robust for active modulation. Also, mode-locking can be achieved by tuning the modulation period. As a natural 2D material, graphene has linear energy dispersion near the Dirac points. This leads to interesting electronic and optical properties. Under Landau quantization, we propose a scheme for achieving continuous-wave terahertz (THz) gain by mid-IR pumping. In this scheme, scattering of surface-optical (SO) phonons from the substrate is utilized to populate the upper laser state. All the important scattering processes are calculated to justify the design. We also study the properties of second-harmonic generation (SHG) in graphene without magnetic field. The experimental measurement shows peculiar relations between the polarizations of fundamental light and second-harmonic (SH) light. We develop a quantum theory to explain the observations

Dicke Superradiance in Solids

Recent advances in optical studies of condensed matter have led to the emergence 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 superradiance (SR), a profound quantum optical process 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 system. 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 $\propto N^2$. Such SF pulses have been observed in atomic and molecular gases, and their intriguing quantum nature has been unambiguously demonstrated. Here, we focus on the rapidly developing field of research on SR in solids, where not only photon-mediated coupling but also strong Coulomb interactions and ultrafast scattering 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 quantum wells in strong magnetic fields. These studies 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.Comment: 23 pages, 26 figure

The existence of positive periodic solutions of a class of lotka-volterra type impulsive systems with infinitely distributed delay

AbstractIn this paper, the existence of positive periodic solutions of a class of periodic Lotka-Volterra type impulsive systems with distributed delays is studied. By using the continuation theorem of coincidence degree theory, a set of easily verifiable sufficient conditions are obtained, which improve and generalize some existing results

When Things Matter: A Data-Centric View of the Internet of Things

With the recent advances in radio-frequency identification (RFID), low-cost wireless sensor devices, and Web technologies, the Internet of Things (IoT) approach has gained momentum in connecting everyday objects to the Internet and facilitating machine-to-human and machine-to-machine communication with the physical world. While IoT offers the capability to connect and integrate both digital and physical entities, enabling a whole new class of applications and services, several significant challenges need to be addressed before these applications and services can be fully realized. A fundamental challenge centers around managing IoT data, typically produced in dynamic and volatile environments, which is not only extremely large in scale and volume, but also noisy, and continuous. This article surveys the main techniques and state-of-the-art research efforts in IoT from data-centric perspectives, including data stream processing, data storage models, complex event processing, and searching in IoT. Open research issues for IoT data management are also discussed