Terahertz generation in quantum cascade structures by two-color deriving fields: An approach to reach inversion population via optical pumping

In this paper, a quantum cascade laser (QCL) design is proposed based on GaAs/AlGaAs material system, which simultaneously operates at three widely separated wavelengths (${\lambda }_1=11.1\mu m,{\lambda }_2=14.1\mu m$ and ${\lambda }_{THz}=60\mu m$). In the design, all the wavelength radiations are achieved by the engineering of the electronic spectrum via the quantum-well widths and the applied electric field in a single active region within a same waveguide. The mid-infrared (mid-IR) wavelengths are obtained by adoption a dual-upper-state active region, and the proposed design aims to use both the mid-IR radiations as the coherent deriving fields to populate the upper THz lasing state to aid the THz-laser population inversion via optical pumping instead of direct electrical injection. A detailed analysis of electronic transport in the structure is carried out using a multi-level rate-equation model. The results show that the proposed structure offers an alternative approach to room temperature THz generation in QCLs.     

Dynamically Induced Excitonic Instability in Pumped Dirac Materials

Driven and non-equilibrium quantum states of matter have attracted growing interest in both theoretical and experimental studies in condensed matter physics. Recent progress in realizing transient collective states in driven or pumped Dirac materials (DMs) is reviewed herein. In particular, the focus is on optically pumped DMs which are a promising platform for transient excitonic instabilities. Optical pumping combined with the linear (Dirac) dispersion of the electronic spectrum offers a knob for tuning the effective interaction between the photoexcited electrons and holes, and thus provides a way of reducing the critical coupling for excitonic instability. As a result, a transient excitonic condensate could be achieved in a pumped DM while it is not feasible in equilibrium. A unifying theoretical framework is provided for describing transient collective states in 2D and 3D DMs. The experimental signatures are described and numerical estimates of the size of the dynamically induced excitonic gaps and the values of the critical temperatures for several specific systems, are summarized. In addition, general guidelines for identifying promising material candidates are discussed. Finally, comments are provided regarding recent experimental efforts in realizing transient excitonic condensate in pumped DMs, and outstanding issues and possible future directions are outlined.     

Redox- and protonation-state driven substrate-protein dynamics in respiratory complex I

Respiratory complex I is a key enzyme in the electron transport chains of mitochondria and bacteria. It transfers two electrons to quinone and couples this redox reaction to proton pumping to electrically charge the membrane it is embedded in. The charge and pH gradient across the membrane drives the synthesis of ATP. The redox reaction and proton pumping in complex I are separated in space and time, which raises the question of how the two reactions are coupled so efficiently. Here, we focus on the unique ～35 Å long tunnel of complex I, which houses the binding site of quinone reduction. We discuss the redox and protonation reactions that occur in this tunnel and how they influence the dynamics of protein and substrate. On the basis of recent structural data and results from molecular simulations, we review how quinone reduction and dynamics may be coupled to proton pumping in complex I.     

New results for the Liebau phenomenon via fixed point index

We prove new results regarding the existence of positive solutions for a nonlinear periodic boundary value problem related to the Liebau phenomenon. As a consequence we obtain new sufficient conditions for the existence of a pump in a simple model. Our methodology relies on the use of classical fixed point index. Some examples are provided to illustrate our theory. We improve and complement previous results in the literature.     

Energy Transfers in a System of Two Coupled Oscillators with Essential Nonlinearity: 1:1 Resonance Manifold and Transient Bridging Orbits

The purpose of this study is to highlight and explain the vigorous energy transfers that may take place in a linear oscillator weakly coupled to an essentially nonlinear attachment, termed a nonlinear energy sink. Although these energy exchanges are encountered during the transient dynamics of the damped system, it is shown that the dynamics can be interpreted mainly in terms of the periodic orbits of the underlying Hamiltonian system. To this end, a frequency-energy plot gathering the periodic orbits of the system is constructed which demonstrates that, thanks to a 1:1 resonance capture, energy can be irreversibly and almost completely transferred from the linear oscillator to the nonlinear attachment. Furthermore, it is observed that this nonlinear energy pumping is triggered by the excitation of transient bridging orbits compatible with the nonlinear attachment being initially at rest, a common feature in most practical applications. A parametric study of the energy exchanges is also performed to understand the influence of the parameters of the nonlinear energy sink. Finally, the results of experimental measurements supporting the theoretical developments are discussed. This study was carried out while the author was a postdoctoral fellow at the National Technical University of Athens and at the University of Illinois at Urbana-Champaign.     

Contribution to efficiency of irreversible passive energy pumping with a strong nonlinear attachment

The present study deals with nonlinear energy pumping which consists in passive irreversible transfer of energy from a linear structure to a nonlinear one. Various results (theoretical, numerical, and experimental) about energy pumping based on recent works are given. Thus, the phenomenon is studied for different excitations: transient and periodical. Moreover, advantages of such a system are carried out in particular efficiency of this phenomenon. That is why the robustness and comparison with classical tuned mass damper are analyzed. An application is considered with physical experiment using a reduced scale building.     

Hot carrier degradation mechanism interpretation by lateral distribution of interface and bulk trap density

We investigated the drain avalanche hot carrier effect (DAHC) of p-type metal-oxide-semiconductor field effect transistor of 0.14 μm channel length (PMOSFET) with SiON gate dielectric. Using three different stress conditions of substrate maximum current, the changes to threshold voltage, maximum transconductance, saturation current and channel leakage current was monitored. Concurrently, the lateral distribution of interface trap density (N_it) and bulk trapped charge density (N_ot) with stress time has been extracted along the 70 nm half channels from gate edge to drain junction, which is the first endeavor in describing charge traps along sub 100 nm short channels. The degradation of the PMOSFET was described by combining electrical property with N_it and N_ot profiles. Hot electron punch through (HEIP) effect was evidenced by negative Not distribution near the drain junction while more severe hot carrier degradation was successfully demonstrated by the empirical power law dependence of the electrical parameters N_it and N_ot. We have studied the evolution of degradation behavior along highly scaled tens of nanometer channel, and N_it and N_ot profile offers systematic study and interpretation of degradation mechanism of hot carrier effect in MOSFET devices.     

Heat transfer augmentation and pumping power in double-pipe heat exchangers

One of the criteria for evaluating the performance of a heat exchanger with extended surfaces is the pumping power required for a specified heat duty. The results of an experimental project to relate the pumping power to heat transfer augmentation in a double-pipe heat exchanger are reported. The inner, electrically heated pipe was provided with external, rectangular, axial extended surfaces with interruptions. Heat transfer augmentation and friction factors were determined for different configurations with air as the fluid. Starting with continuous fins, cuts were introduced in the fins to give four ratios of the finssegment length to the gap between the segments, and finally all the fins were removed, which resulted in smooth pipes. Five different mass flow rates in two different inner pipes were employed. Lengths, surface areas, and pumping powers for finned pipes are compared with those for smooth pipes. The average heat transfer coefficient increases with an increase in the frequency of the interruptions. For equal heat transfer rates a significant reduction in the lengths can be achieved by interrupted fins. In many cases the reduction in the length is also accompanied by a reduction in the pumping power.     

Designing a Linear Structure with a Local Nonlinear Attachment for Enhanced Energy Pumping

We present a design procedure for enhancing nonlinear energy pumping from a mode of a linear-damped substructure to a weakly coupled, essentially nonlinear oscillator. By this we denote the one way, irreversible passive transfer of vibrational energy from the mode to the nonlinear attachment. The design relies in the asymptotic expansion for large energies of a nonlinear normal mode of the underlying conservative system that provides an analytic estimate of the level of the amplitude reached by the nonlinear attachment in the energy pumping regime. The analytical findings are validated by direct numerical simulations.