2,438 research outputs found
Terahertz cascades from nanoparticles
In this article we propose a system capable of THz radiation with quantum
yield above unity. The system consists of nanoparticles where the material
composition varies along the radial direction of each nanoparticle in such a
way that a ladder of equidistant energy levels emerges. By then exciting the
highest level of this ladder we produce multiple photons of the same frequency
in the THz range. We demonstrate how we can calculate a continuous material
composition profile that achieves a high quantum yield and then show that a
more experimentally friendly design of a multishell nanoparticle can still
result in a high quantum yield.Comment: 5 pages, 4 figure
All-to-all connected networks by multi-frequency excitation of polaritons
We analyze theoretically a network of all-to-all coupled polariton modes,
realized by a trapped polariton condensate excited by a comb of different
frequencies. In the low-density regime the system dynamically finds a state
with maximal gain defined by the average intensities (weights) of the
excitation beams, analogous to active mode locking in lasers, and thus solves a
maximum eigenvalue problem set by the matrix of weights. The method opens the
possibility to tailor a superposition of populated bosonic modes in the trapped
condensate by appropriate choice of drive
Continuous THz emission from dipolaritons
We propose a scheme of continuous tunable THz emission based on dipolaritons
--- mixtures of strongly interacting cavity photons and direct excitons, where
the latter are coupled to indirect excitons via tunnelling. We investigate the
property of multistability under continuous wave (CW) pumping, and the
stability of the solutions. We establish the conditions of parametric
instability, giving rise to oscillations in density between the direct exciton
and indirect modes under CW pumping. In this way we achieve continuous and
tunable emission in the THz range in a compact single-crystal device. We show
that the emission frequency can be tuned in a certain range by varying an
applied electric field and pumping conditions. Finally, we demonstrate the
dynamic switching between different phases in our system, allowing rapid
control of THz radiation.Comment: Main article 6 pages and 5 figures, two appendices 8 pages and 2
figure
Spontaneous Pattern Formation in a Polariton Condensate
Polariton condensation can be regarded as a self-organization phenomenon,
where phase ordering is established among particles in the system. In such
condensed systems, further ordering can possibly occur in the particle density
distribution, under particular experimental conditions. In this work we report
on spontaneous pattern formation in a polariton condensate under non-resonant
optical pumping. The slightly elliptical ring-shaped excitation laser we employ
is such to force condensation to occur in a single-energy state with periodic
boundary conditions, giving rise to a multi-lobe standing wave patterned state
Spontaneous self-ordered states of vortex-antivortex pairs in a Polariton Condensate
Polariton condensates have proved to be model systems to investigate
topological defects, as they allow for direct and non-destructive imaging of
the condensate complex order parameter. The fundamental topological excitations
of such systems are quantized vortices. In specific configurations, further
ordering can bring the formation of vortex lattices. In this work we
demonstrate the spontaneous formation of ordered vortical states, consisting in
geometrically self-arranged vortex-antivortex pairs. A mean-field generalized
Gross-Pitaevskii model reproduces and supports the physics of the observed
phenomenology
Origins of ferromagnetism in transition-metal doped Si
We present results of the magnetic, structural and chemical characterizations of Mn<sup>+</sup>-implanted Si displaying <i>n</i>-type semiconducting behavior and ferromagnetic ordering with Curie temperature,T<sub>C</sub> well above room temperature. The temperature-dependent magnetization measured by superconducting quantum device interference (SQUID) from 5 K to 800 K was characterized by three different critical temperatures (T*<sub>C</sub>~45 K, T<sub>C1</sub>~630-650 K and T<sub>C2</sub>~805-825 K). Their origins were investigated using dynamic secondary mass ion spectroscopy (SIMS) and transmission electron microscopy (TEM) techniques, including electron energy loss spectroscopy (EELS), Z-contrast STEM (scanning TEM) imaging and electron diffraction. We provided direct evidences of the presence of a small amount of Fe and Cr impurities which were unintentionally doped into the samples together with the Mn<sup>+</sup> ions, as well as the formation of Mn-rich precipitates embedded in a Mn-poor matrix. The observed T*<sub>C</sub> is attributed to the Mn<sub>4</sub>Si<sub>7</sub> precipitates identified by electron diffraction. Possible origins of and are also discussed. Our findings raise questions regarding the origin of the high ferromagnetism reported in many material systems without a careful chemical analysis
A Parametric Cycle Analysis of a Separate-Flow Turbofan with Interstage Turbine Burner
Today's modern aircraft is based on air-breathing jet propulsion systems, which use moving fluids as substances to transform energy carried by the fluids into power. Throughout aero-vehicle evolution, improvements have been made to the engine efficiency and pollutants reduction. This study focuses on a parametric cycle analysis of a dual-spool, separate-flow turbofan engine with an Interstage Turbine Burner (ITB). The ITB considered in this paper is a relatively new concept in modern jet engine propulsion. The JTB serves as a secondary combustor and is located between the high- and the low-pressure turbine, i.e., the transition duct. The objective of this study is to use design parameters, such as flight Mach number, compressor pressure ratio, fan pressure ratio, fan bypass ratio, linear relation between high- and low-pressure turbines, and high-pressure turbine inlet temperature to obtain engine performance parameters, such as specific thrust and thrust specific fuel consumption. Results of this study can provide guidance in identifying the performance characteristics of various engine components, which can then be used to develop, analyze, integrate, and optimize the system performance of turbofan engines with an ITB
Parametric (On-Design) Cycle Analysis for a Separate-Exhaust Turbofan Engine With Interstage Turbine Burner
Today s modern aircraft is based on air-breathing jet propulsion systems, which use moving fluids as substances to transform energy carried by the fluids into power. Throughout aero-vehicle evolution, improvements have been made to the engine efficiency and pollutants reduction. The major advantages associated with the addition of ITB are an increase in thermal efficiency and reduction in NOx emission. Lower temperature peak in the main combustor results in lower thermal NOx emission and lower amount of cooling air required. This study focuses on a parametric (on-design) cycle analysis of a dual-spool, separate-flow turbofan engine with an Interstage Turbine Burner (ITB). The ITB considered in this paper is a relatively new concept in modern jet engine propulsion. The ITB serves as a secondary combustor and is located between the high- and the low-pressure turbine, i.e., the transition duct. The objective of this study is to use design parameters, such as flight Mach number, compressor pressure ratio, fan pressure ratio, fan bypass ratio, and high-pressure turbine inlet temperature to obtain engine performance parameters, such as specific thrust and thrust specific fuel consumption. Results of this study can provide guidance in identifying the performance characteristics of various engine components, which can then be used to develop, analyze, integrate, and optimize the system performance of turbofan engines with an ITB. Visual Basic program, Microsoft Excel macrocode, and Microsoft Excel neuron code are used to facilitate Microsoft Excel software to plot engine performance versus engine design parameters. This program computes and plots the data sequentially without forcing users to open other types of plotting programs. A user s manual on how to use the program is also included in this report. Furthermore, this stand-alone program is written in conjunction with an off-design program which is an extension of this study. The computed result of a selected design-point engine will be exported to an engine reference data file that is required in off-design calculation
Performance Cycle Analysis of a Two-Spool, Separate-Exhaust Turbofan With Interstage Turbine Burner
This paper presents the performance cycle analysis of a dual-spool, separate-exhaust turbofan engine, with an Interstage Turbine Burner serving as a secondary combustor. The ITB, which is located at the transition duct between the high- and the low-pressure turbines, is a relatively new concept for increasing specific thrust and lowering pollutant emissions in modern jet engine propulsion. A detailed performance analysis of this engine has been conducted for steady-state engine performance prediction. A code is written and is capable of predicting engine performances (i.e., thrust and thrust specific fuel consumption) at varying flight conditions and throttle settings. Two design-point engines were studied to reveal trends in performance at both full and partial throttle operations. A mission analysis is also presented to assure the advantage of saving fuel by adding ITB
Bounds for the second Hankel determinant of certain univalent functions
The estimates for the second Hankel determinant a_2 a_4-a_3^2 of the analytic function f(z)=z+a_2 z^2+a_3 z^3+⋯, for which either zf^' (z)/f(z) or 1+zf^'' (z)/f^' (z) is subordinate to a certain analytic function, are investigated. The estimates for the Hankel determinant for two other classes are also obtained. In particular, the estimates for the Hankel determinant of strongly starlike, parabolic starlike and lemniscate starlike functions are obtained
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