Impulsive stimulated Raman scattering: comparison between phase-sensitive and spectrally filtered techniques

Impulsive stimulated Raman scattering is used to generate and control coherent phonons and other low-frequency modes. In transparent materials, pump-probe experiments are usually performed by spectrally resolving the probe beam and measuring the spectral shift as a function of pump-probe time delay. By measuring the optical phase of the probe pulse as a function of time delay, we find that the phonon signal can be increased by a factor alpha(omegadelta)(-1), where omega is the phonon frequency and delta is the pulse duration.     

Phonons in Ge/Si superlattices with Ge quantum dots

Ge/Si superlattices containing Ge quantum dots were prepared by molecular beam epitaxy and studied by resonant Raman scattering. It is shown that these structures possess vibrational properties of both two-and zero-dimensional objects. The folded acoustic phonons observed in the low-frequency region of the spectrum (up to 15th order) are typical for planar superlattices. The acoustic phonon lines overlap with a broad emission continuum that is due to the violation of the wave-vector conservation law by the quantum dots. An analysis of the Ge and Ge-Si optical phonons indicates that the Ge quantum dots are pseudoamorphous and that mixing of the Ge and Si atoms is insignificant. The longitudinal optical phonons undergo a low-frequency shift upon increasing laser excitation energy (2.54–2.71 eV) because of the confinement effect in small-sized quantum dots, which dominate resonant Raman scattering.     

多空穴错位分布对石墨纳米带中热输运的影响

利用非平衡格林函数方法研究了石墨纳米带中三空穴错位分布对热输运性质的影响.研究结果发现:三空穴竖直并排结构对低频声子的散射较小,导致低温区域三空穴竖直并排时热导最大,而在高频区域,三空穴竖直并排结构对高频声子的散射较大,导致较高温度区域三空穴竖直并排时热导最小;三空穴的相对错位分布仅能较大幅度地调节面内声学模高频声子的透射概率,而三空穴的相对错位分布能较大幅度地调节垂直振动膜高频声子和低频声子的透射概率,导致三空穴的相对错位分布不仅能大幅调节面内声学模和垂直振动模的高温热导,也能大幅调节垂直振动模的低温热导.研究结果阐明了空穴位置不同的石墨纳米带的热导特性,为设计基于石墨纳米带的热输运量子器件提供了有效的理论依据.     

长骨中振动声激发超声导波的方法

为了实现一定频段内任意低频下在长骨中激励导波信号,本文提出一种采用聚焦高频(5 MHz)超声换能器在长骨皮质骨中激发低频(150 kHz)超声导波的振动声方法.首先介绍了板状超声导波理论和双声束共聚焦法与单声束调幅法激发振动声的基本原理;进而采用三维有限元仿真方法分析振动声激发低频超声导波的基本现象,然后结合牛胫骨板离体实验,验证振动声激发低频超声导波的可行性.结果均表明,双声束共焦与单声束振动超声均可在骨板中激发低频超声导波.相关研究方法有助于提高空间域长骨中超声导波测量精度,以及在一定频段内实现任意频率激励等,对发展低频超声导波在体测量长骨皮质骨的新技术具有一定的指导意义.     

Pressure Dependence of Acoustic Modes in AgGaSe 2

The pressure dependence of the transverse acoustic TA[0 0 1] and TA[1 0 0] phonons and the low-frequency optical v 3 phonons of AgGaSe 2 has been measured at pressures up to 4.3 GPa by inelastic neutron scattering. The strong frequency decreases in the whole Brillouin zone of the TA modes show generally a weak nonlinearity. Depending on the wavevector, the softening prior to the phase transition at 2.6 GPa also affects the optical mode to a less extent.     

Spin-Hall conductivity in electron-phonon coupled systems

We derive the ac spin-Hall conductivity sigmasH(omega) of two-dimensional spin-orbit coupled systems interacting with dispersionless phonons of frequency omega0. For the linear Rashba model, we show that the electron-phonon contribution to the spin-vertex corrections breaks the universality of sigmasH(omega) at low frequencies and provides a nontrivial renormalization of the interband resonance. On the contrary, in a generalized Rashba model for which the spin-vertex contributions are absent, the coupling to the phonons enters only through the self-energy, leaving the low-frequency behavior of sigmasH(omega) unaffected by the electron-phonon interaction.     

Upconversion of whistler waves by gyrating ion beams in a plasma

A gyrating ion beam, with a ring-shaped distribution in velocity, supports negative energy beam modes near the harmonics of beam gyro-frequency. An investigation of the non-linear interaction of high-frequency whistler waves with the negative energy beam cyclotron mode is made. A non-linear dispersion relation is derived for the coupled modes. It is shown that a gyrating ion-beam frequency upconverts the whistler waves separated by harmonics of beam gyro-frequency. The expression for the growth rate of whistler mode waves has been derived. In Case 1, a high-amplitude whistler wave decays into two lower frequency waves, called a low-frequency mode and a side band of frequency lower than that of pump wave. In Case 2 a high-amplitude whistler wave decays into two lower frequency daughter waves, called the low-frequency mode and whistler waves. Generation mechanism of these waves has application in space and laboratory plasmas.     

Electric field modulated Raman scattering in CdS

We have observed electric field modulated Raman scattering by A₁ LO phonons in CdS. The field induced scattering is observed with a geometry in which Raman scattering by A₁ LO phonons is normally allowed. The interference of the field induced and allowed terms in the transition susceptibility leads to a modulated Raman scattering intensity proportional to the applied field. This is contrasted with data previously reported on field induced Raman scattering by E₁ LO phonons in a configuration in which the Raman scattering is normally forbidden and in which there is no interference between linear wavevector dependent and field induced terms in the transition susceptibility. Electric field effects on Raman scattering by TO phonons and by 2 LO phonons is also discussed.     

Parametric generation of whistler waves due to the interaction of high-frequency wave beams with a magnetoplasma

The parametric generation of low-frequency whistler waves by a pump wave beam formed by high-frequency whistler waves with close frequencies is studied experimentally. The electromagnetic fields excited by the beats of two co- or counterpropagating high-frequency waves, or by an amplitude-modulated pump are studied. It is shown that the nonlinear currents at the beat (modulation) frequency are generated by a transverse ponderomotive force arising due to the finite width of the high-frequency beam. In this case, the nonlinear azimuthal drift currents enclose the pump beam and can radiate low-frequency whistler waves to the surrounding plasma.     

Perturbat ion Approach for the Localization Transition in the Dissipative Two-State System

Analysis of a dissipative two-state system at zero temperature shows that the model Hamiltonian may be exactly reduced to a modified quantum sine-Gordon model, which describes the effective interactions between the low-frequency phonons under coupling with a tunneling system. Directly considering the infrared divergence encountered in the conventional perturbation treatment, we have developed a new perturbation approach for the effective Hamiltonian, and derived the exact critical conditions for the localization transition. In the critical regime, a gap will be opened near zero momentum in the elementary excitation spectrum of the low-frequency phonons, and the corresponding ground state wave function is found to be a pairing quasi-particle state, analogous to the BCS superconducting state.     

Lattice dynamics of random and quasiperiodic heterostructures

We report on the quantum-mechanical displacement form factor in quasiperiodic and random heterostructures. A one-dimensional treatment is adopted to describe the longitudinal displacement along the growth axis. Elastic properties are assumed to be homogeneous, while the inhomogeneous mass density characterizes the heterostructure. In the low-frequency limit, the peak structure can be attributed to acoustic phonons, whereas for higher frequencies the quasiperiodic and random cases differ markedly. In the quasiperiodic case and constant momentum transfer, resonances separated by gaps occur and their number depends on the resolution in the frequency domain. The random case is dominated by an acoustic resonance becoming broader with increasing frequency.     

抛物量子线中束缚磁极化子的性质

研究抛物量子线中束缚磁极化子的性质,采用线性组合算符和幺正变换方法导出了强、弱耦合两种情况下的基态能量、振动频率和光学声子平均数.结果表明,无论是强耦合还是弱耦合情况,抛物量子线中束缚磁极化子的振动频率λ、基态能量E₀和光学声子平均数N都随约束强度ω₀的增大而迅速增大.     

Impact of thermostat on interfacial thermal conductance prediction from non-equilibrium molecular dynamics simulations

The knowledge of interfacial thermal conductance (ITC) is key to understand thermal transport in nanostructures. The non-equilibrium molecular dynamics (NEMD) simulation is a useful tool to calculate the ITC. In this study, we investigate the impact of thermostat on the prediction of the ITC. The Langevin thermostat is found to result in larger ITC than the Nose-Hoover thermostat. In addition, the results from NEMD simulations with the Nose-Hoover thermostat exhibit strong size effect of thermal reservoirs. Detailed spectral heat flux decomposition and modal temperature calculation reveal that the acoustic phonons in hot and cold thermal reservoirs are of smaller temperature difference than optical phonons when using the Nose-Hoover thermostat, while phonons in the Langevin thermostat are of identical temperatures. Such a non-equilibrium state of phonons in the case of the Nose-Hoover thermostat reduces the heat flux of low-to-middle-frequency phonons. We also discuss how enlarging the reservoirs or adding an epitaxial rough wall to the reservoirs affects the predicted ITC, and find that these attempts could help to thermalize the phonons, but still underestimate the heat flux from low-frequency phonons.     

Raman scattering of Ge dot superlattices

Self-organised Ge dot superlattices grown by molecular beam epitaxy of Ge and Si layers utilizing Stranski-Krastanov growth mode were investigated by Raman spectroscopy. An average size of Ge quantum dots was obtained from transmission electron microscopy measurements. The strain and interdiffusion of Ge and Si atoms in Ge quantum dots were estimated from the analysis of frequency positions of optical phonons observed in the Raman spectra. Raman scattering by folded longitudinal acoustic phonons in the Ge dot superlattices was observed and explained using of elastic continuum theory. Received 25 January 2000     

Scheme for teleportation of unknown states of trapped ion

A scheme is presented for teleporting an unknown state in a trapped ion system. The scheme only requires a single laser beam. It allows the trap to be in any state with a few phonons, e.g. a thermal motion. Furthermore, it works in the regime, where the Rabi frequency of the laser is on the order of the trap frequency. Thus, the teleportation speed is greatly increased, which is important for decreasing the decoherence effect. This idea can also be used to teleport an unknown ionic entangled state.     

Creating a collimated ultrasound beam in highly attenuating fluids

We have devised a method, based on a parametric array concept, to create a low-frequency (300-500 kHz) collimated ultrasound beam in fluids highly attenuating to sound. This collimated beam serves as the basis for designing an ultrasound visualization system that can be used in the oil exploration industry for down-hole imaging in drilling fluids. We present the results of two different approaches to generating a collimated beam in three types of highly attenuating drilling mud. In the first approach, the drilling mud itself was used as a nonlinear mixing medium to create a parametric array. However, the short absorption length in mud limits the mixing length and, consequently, the resulting beam is weak and broad. In the second improved approach, the beam generation process was confined to a separate “frequency mixing tube” that contained an acoustically non-linear, low attenuation medium (e.g., water) that allowed establishing a usable parametric array in the mixing tube. A low-frequency collimated beam was thus created prior to its propagation into the drilling fluid. Using the latter technique, the penetration depth of the low frequency ultrasound beam in the drilling fluid was significantly extended. We also present measurements of acoustic nonlinearity in various types of drilling mud.     

Generation of low-frequency vibration using a cantilever beam for calibration of accelerometers

At low frequencies (below 10 Hz), performance of a conventional shaker is limited by small acceleration amplitudes and a high level of total harmonic distortion. The present article describes a low-frequency vibration generator that overcomes these limitations. The vibration generator consists of a cantilever beam excited by a conventional shaker. The cantilever beam is tuned to resonate at the desired excitation frequency, which leads to a relatively large vibratory motion at the beam tip with very small harmonic distortion. Analysis of the system is performed by means of model equations describing both the flexural and longitudinal components of vibration. A comprehensive measurement of the generator's performance confirms that it can serve as an economically attractive alternative to existing low-frequency vibration generators used in vibration measurement and calibration.     

Change of zonal flow spectra in the JIPP T-IIU tokamak plasmas

When Ohmically heated low-density plasmas are additionally heated by higher-harmonics ion-cyclotron-range-of frequency heating, heated by neutral beam injection, or strongly gas puffed, the intensity of zonal flows in the geodesic acoustic mode frequency range in the tokamak core plasma decreases sharply and that of low-frequency zonal flow grows drastically. This is accompanied by a damping of the drift wave propagating in the electron diamagnetic drift direction, turbulence by trapped electron mode (TEM), and the increase of the mode propagating to ion diamagnetic drift direction (ITG). In the half-radius region, TEM and high-frequency zonal flows remain intense in both OH and heated phases. ITG and low-frequency zonal flows grow in heated plasmas, suggesting a strong coupling between ITG and low-frequency zonal flow.     

The interaction of high- and low-frequency waves in a free electronlaser

When a free electron laser (FEL) interaction occurs inside a waveguide, there are two possible operating frequencies at the two intersections of the beam line with the waveguide dispersion curve. Using a one-dimensional model valid for a low-gain oscillator, the mode competition between the high- and low-frequency waves is studied. It is found that the low-frequency wave can be nonlinearly suppressed, even when its linear growth rate is positive. When the frequency ratio is two, phase locking occurs and the nonlinear gain of the low-frequency wave can exceed its linear growth rate