Photon-Assisted Conductance Structure for a Quantum Dot

We investigate theoretically the electron transport of a two-level quantum dot irradiated under a weak laser field at low temperatures in the rotating wave approximation. Using the method of the Keldysh equation of motion for nonequilibrium Green function, we examine the conductance for the system with photon polarization perpendicular to the tunnelling current direction. It is demonstrated that by analytic analysing and numerical examples, a feature of conductance peak splitting appears, and the dependence of conductance on the incident laser frequency and self-energy are discussed.     

Time-dependent electron transport in HgTe/CdTe quantum wells

Based on the Floquet theory and Keldysh's nonequilibrium Green's function methods, we study the electron transport through the HgTe/CdTe quantum wells (QWs) irradiated by a monochromatic laser field. We find that when the laser field is applied, the edge states are split into a series of sidebands. When the Fermi level lies among these sidebands, the quantized plateau of the conductance is destroyed. Instead, the conductance versus the radiation frequency exhibits the successive oscillation peaks corresponding to the resonant tunneling through the sidebands of the edge states. The resonant interaction between the quasiparticles and the radiation field opens the gaps in the crossing region of the sidebands, which can be tuned by the radiation strength and frequency. This leads to the shift of the oscillation peaks in the conductance. We also show that the amplitudes of the oscillation peaks in the conductance are governed by the radiation strength and frequency.     

Out of equilibrium Anderson model: Conductance and Kondo temperature

We calculate the conductance through a quantum dot weakly coupled to metallic contacts by means of the Keldysh out of equilibrium formalism. We model the quantum dot with the SU(2) Anderson model and consider the limit of infinite Coulomb repulsion. The interacting system is solved with the numerical diagrammatic Non-Crossing Approximation (NCA) and the conductance is obtained as a function of temperature and gate voltage from differential conductance (dI/dV) curves. We discuss the results in comparison with those from the linear response approach which can be performed directly in equilibrium conditions. Comparison shows that out of equilibrium results are in good agreement with the ones from linear response supporting reliability of the method employed. The last discussion becomes relevant when dealing with general transport models through interacting regions. We also analyze the evolution of conductance vs gate voltage with temperature. While at high temperatures the conductance is peaked, when the Fermi energy coincides with the localized level it presents a plateau at low temperatures as a consequence of the Kondo effect. We discuss different ways to determine Kondo's temperature.     

Electron transport for a laser-irradiated quantum channel with Rashba spin--orbit coupling

We investigate theoretically the electron transport for a two-level quantum channel (wire) with Rashba spin--orbit coupling under the irradiation of a longitudinally-polarized external laser field at low temperatures. Using the method of equation of motion for Keldysh nonequilibrium Green function, we examine the time-averaged spin polarized conductance for the system with photon polarization parallel to the wire direction. By analytical analysis and a few numerical examples, the interplay effects of the external laser field and the Rashba spin--orbit coupling on the spin-polarized conductance for the system are demonstrated and discussed. It is found that the longitudinally-polarized laser field can adjust the spin polarization rate and produce some photon sideband resonances of the conductance for the system.     

Recording ion channels across soy-extracted lecithin bilayer generated by water-soluble quantum dots

We report on the quantum dot (QD)-induced ion channels across a soya-derived lecithin bilayer supported on a laser drilled of ~100 μm aperture of cellulose acetate substrate that separates two electrolytic chambers. Adequate current bursts were observed when the bilayer was subjected to a gating voltage. The voltage-dependent current fluctuation, across the bilayer, was attributed to the insertion of ~20?nm sized water-soluble CdSe QDs, forming nanopores due to their spontaneous aggregation. Apart from a closed state, the first observable conductance levels were found as 6.3 and 11?nS, as for the respective biasing voltages of ?10 and ?20?mV. The highest observable conductance states, at corresponding voltages were ~14.3 and 21.1?nS. Considering two simplified models, we predict that the non-spherical pores (d_nspore) can be a better approximation over spherical nanopores (d_spore) for exhibiting a definite conductance level. At times, even d_nspore?≤?4d_spore and that the non-spherical nanopores were associated with a smaller No. of QDs than the case for spherical nanopores, for a definite conductance state. It seems like the current events are partly stochastic, possibly due to thermal effects on the aggregated QDs that would form nanopores. The dwell time of the states was predicted in the range of 384–411?μs. The ion channel mechanism in natural phospholipid bilayers over artificial ones will provide a closer account to understand ion transport mechanism in live cells and signaling activity including labelling with fluorescent QDs.     

Nanobubbles in solid-state nanopores

From conductance and noise studies, we infer that nanometer-sized gaseous bubbles (nanobubbles) are the dominant noise source in solid-state nanopores. We study the ionic conductance through solid-state nanopores as they are moved through the focus of an infrared laser beam. The resulting conductance profiles show strong variations in both the magnitude of the conductance and in the low-frequency noise when a single nanopore is measured multiple times. Differences up to 5 orders of magnitude are found in the current power spectral density. In addition, we measure an unexpected double-peak ionic conductance profile. A simple model of a cylindrical nanopore that contains a nanobubble explains the measured profile and accounts for the observed variations in the magnitude of the conductance.     

Diffractive scanning mechanism for laser marker

We present a new idea of marking based on spectral properties of CO₂ laser radiation. The idea has been illustrated on RF excited waveguide CO₂ lasers in different configurations: single channel, multi-waveguide and slab-waveguide ones. The pulse operation of the laser has been considered as well. The advantage of the presented diffraction marker is avoiding complicated and fallible mechanical elements. The only executive elements of the marker are a diffraction grating and a piezoceramic transducer. It has been shown that the slab-waveguide configuration of the RF excited CO₂ laser equipped with an unstable resonator is the most promising configuration for application to the diffraction marker.     

A time-dependent,ray-geometrical treatment of the interference fringes within a thermally self-defocused laser beam

A time-dependent theoretical treatment of the intensity distribution within a thermally self-defocused laser beam is presented, based on a ray-geometrical approach. Well-modulated interference fringes are predicted extending over most of the beam. This treatment is applicable to beam propagation in media for which the heat transfer by convection can be neglected when compared with that due to conduction. Numerically determined intensity distributions are given and compared with ones obtained experimentally using a laser beam of 488 nm wavelength and an absorbing medium of glycerol doped with iodine. Good agreement is obtained.     

Ageing in granular aluminium insulating thin films

We present a new set of electrical field effect measurements on granular aluminium insulating thin films. We have explored how the conductance relaxations induced by gate voltage changes depend on the age of the system, namely the time elapsed since its quench at low temperature. A clear age dependence of the relaxations is seen, qualitatively similar to ageing effects seen in other well studied glassy systems such as spin glasses or polymers. We explain how our results differ from the previous ones obtained with different protocols in indium oxide and granular aluminium thin films. Our experimental findings bring new information on the dynamics of the system and put new constraints on the theoretical models that may explain slow conductance relaxations in disordered insulators.     

Even-harmonic generation due to beyond-Born-Oppenheimer dynamics

We calculate the harmonic spectrum generated by a model HD molecule in a strong laser pulse. The unequal nuclear masses lead to the emission of even harmonics, i.e., photon frequencies which are even multiples of the laser frequency. The effect does not occur within the Born-Oppenheimer approximation. In the high-frequency region, the even harmonics are almost of the same order of magnitude as the odd ones.     

Scaling electron acceleration in the bubble regime for upcoming lasers

Electron acceleration in the laser-plasma bubble appeared to be the most successful regime of laser wake field acceleration in the last decade. The laser technology became mature enough to generate short and relativistically intense pulses required to reach the bubble regime naturally delivering quasi-monoenergetic bunches of relativistic electrons. The upcoming laser technology projects are promising short pulses with many times more energy than the existing ones. The natural question is how will the bubble regime scale with the available laser energy. We present here a parametric study of laser-plasma acceleration in the bubble regime using full three dimensional particle-in-cell simulations and compare numerical results with the analytical scalings from the relativistic laser-plasma similarity theory.     

亚纳秒光脉冲下GaAs光电开关行为的理论分析

本文对GaAs光电开关在亚纳秒光脉冲(650ps左右)触发下的性能作了理论分析。用计算机数值分析方法计算了光电开关的电导随光脉冲的变化过程,分析了光脉冲的能量、半宽度的涨落引起的光电开关的触发跳动、响应时间及幅度的变化规律。在合理选用参数时,用SPICE设计得到了上升沿为5kV/ns,触发跳动小于5ps的斜坡电压,从而证实了光电开关在亚纳秒光脉冲作用下的可行性。     

Change of surface properties of high-chromium cast iron under conditions of laser treatment and wear in a pellet stream

We investigate the influence of laser treatment on the formation of residual stresses relative to the changing structure-phase composition in the surface layers of high-chromium cast iron with 16% chromium. We show that appreciable tensile stresses are produced in the region of the laser action and that their distribution depends on whether the laser treatment was or was not accompanied by surface melting. The produced residual stresses are responsible for the formation of a large number of cracks. Preheating to 400°C lowers the level of the tensile residual stresses and prevents crack formation. A pellet stream acting on the surface produces cold-work hardening layers in which the tensile stresses change into compressive ones. The depth, hardness, and magnitude of the compressive residual stresses depend on the method used to work harden the cast iron and on the angle of attack of the pellet as it acts on the surface.Translation of Preprint No. 195, Lebedev Institute of Physics, Academy of Sciences of the USSR.     

Multiphoton free-free transitions in a spatially inhomogeneous laser field

Summary The results of an investigation are presented in which it is shown how the spatial inhomogeneity of a laser field modifies the multiphoton free-free transition cross-sections compared to the case of a homogeneous field. This kind of investigation is required to make more close contact with experiments in intense fields, as in these cases the inhomogeneity is produced by the focusing of the laser beam. Furthermore, taking into account the intensity spatial distribution allows us to achieve in an effective way the asymptotic decoupling of the scattered particles from the field, which is very important for theoretical models using asymptotic initial and final states embedded in the field. Differential and total cross-sections are calculated over a wide range of parameters as functions of the scattering angle, of the incoming-particle energy and of the laser intensity and frequency. The laser spatial inhomogeneity is found to wash out most of the oscillating behaviour of multiphoton differential cross-sections, derived within the model of a homogeneous laser. Little modifications are, instead, found in the total cross-sections which are simply scaled to slightly lower values. Crosssections with zero photon exchange are increased, while those with photon exchanges are lowered. This yields the result that the sum of all the ≪inhomogeneous≫ cross-sections is equal to the sum of all the ≪homogeneous≫ ones (sum rule). The multiphoton free-free transition differential cross-sections are found to be very sensitive quantities which may be used to get information on the laser properties and on their nonlinear behaviour, when these are not precisely known. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Effects of contact shape on ballistic phonon transport in semiconductor nanowires

We study the effects of contact shape on ballistic phonon transport in semiconductor nanowires at low temperatures using an approximative scalar model of continuum elasticity. Five different contacts connected to two semiconductor nanowires with different transverse widths are discussed. Numerical results show that the contact shape acts as an ‘acoustic impedance adaptor’, playing a crucial role on the ballistic phonon transmission and thermal conductance. The phonon coupling in the contacts with certain length facilitates ballistic phonon transmission compared to the abrupt interface, in which the phonon scattering is the strongest. It is found that the more the contact is abrupt, the smaller the thermal conductance is. The catenoidal contact rather than the abrupt interface is also the competitive candidate to obtain bigger thermal conductance. These results indicate that choosing an appropriate contact shape is one of the most critical factors to accurately measure the thermal conductance with a very high precision and reliability in different temperature ranges at low temperatures.     

Resonant Photon-Assisted Structure of Conductance through a Quantum Wire

We investigate theoretically the electron transport properties of a two-sublevel quantum wire irradiated by a strong laser field resonant with the quasiparticle transition at low temperatures. Using the method of the Keldysh equation of motion for nonequilibrium Green function, we examine the time-averaged conductance for the system with photon polarization parallel and perpendicular to the tunnelling current direction, respectively. We demonstrate that, by analysing some numerical examples, a feature of absolute negative conductance appears in the parallel case, while the conductance shows a symmetry distributed peaks in the Derpendicular case.     

Quantum interference in carbon-nanotube electron resonators

A new mechanism is proposed to explain the slow conductance fluctuations in the conductance-gate voltage plot observed in the nanotube electron resonators. It is found that the slow conductance fluctuation is an intrinsic quantum interference phenomenon and exists in all metallic nanotube resonators except zigzag ones. Analytical expressions for both slow and rapid oscillation periods of the conductance fluctuations have been derived, which are well consistent with the existing experiments. It is predicted that the ratio of the slow oscillation period to the rapid one is independent of the gate-voltage efficiency, and determined only by the nanotube length used in experiments.     

Deformation behavior of laser bending of circular sheet metal

The application of a thermal source in non-contact forming of sheet metal has long been used. However, the replacement of this thermal source with a laser beam promises much greater controllability of the process. This yields a process with strong potential for application in aerospace, shipbuilding, automobile, and manufacturing industries, as well as the rapid manufacturing of prototypes and adjustment of misaligned components. Forming is made possible through laser-induced non-uniform thermal stresses. In this letter, we use the geometrical transition from rectangular to circle-shaped specimen and ring-shaped specimen to observe the effect of geometry on deformation in laser forming. We conduct a series of experiments on a wide range of specimen geometries. The reasons for this behavior are also analyzed. Experimental results are compared with simulated values using the software ABAQUS. The utilization of line energy is found to be higher in the case of laser forming along linear irradiation than along curved ones. We also analyze the effect of strain hindrance. The findings of the study may be useful for the inverse problem, which involves acquiring the process parameters for a known target shape of a wide range of complex shape geometries.     

绝缘层上半导体多晶膜激光再结晶临界条件

从理论的角度分析了绝缘衬底对其上面半导体多晶膜激光熔化再结晶过程的影响,发现低导热的绝缘层使产生固-液相变的临界激光功率有明显的降低。用喇曼光谱测量了激光再结晶SOI层中的应力。应力的出现是多晶膜内曾经发生过固-液相变的佐证。从这一思想出发,对LPCVD方法制备的大量SOI样品进行激光再结晶临界条件的研究,证明了忽略绝缘层低热导影响的模型不能解释实验结果,而经过修正的公式则可以较好地拟合实验结果。 关键词：     

Output performance of thermally-insensitive passively Q-switched Nd:YAG laser

We study a Cr⁴⁺:YAG Q-switched Nd:YAG laser with a thermally-insensitive corner-cube-prism cavity where the corner cube prism is the key element. The corner cube prism is insensitive to misalignment in any direction. We demonstrate experimentally that a laser cavity with such a prism provides stable laser performance under violent changes of ambient temperature and the effect of the laser crystal thermal lens. The laser mode properties are analyzed by numerical simulations. We show that the numerically simulated results agree well with the experimental ones.