Optical phonon generation by photoexcited electrons

Optical-phonon generation rate of photoexcited electrons via polarization and deformation electron-phonon interaction is calculated. The electron distribution function at energies higher than the optical-phonon energy is found.     

Evolution of carrier and optical phonon distribution in photoexcited semiconductors

A study of the time evolution of a coupled photoexcited electron-LO phonon system is presented. Zubarev's method to deal with irreversible relaxation processes is used to derive the dynamic equation for the distribution functions. We report results which allow to remove some inconsistencies and controversies concerning early models proposed for the interpretation of measurements of the ratio Stokes to anti-Stokes intensities in the LO-phonon Raman spectrum.     

声子频率对同位素掺杂硅声子散射的影响

点缺陷对声子的散射是影响电绝缘体热导率的重要机制之一,其中声子频率是影响声子散射的重要因素．本文主要研究声子频率对同位素掺杂硅声子散射的影响。首先产生一个窄频率范围的声子波包,然后使用分子动力学(MD)模拟声子在同位素掺杂硅中的散射过程,在原子尺度下清晰展示了声子对同位素掺杂的散射过程,并对能量的透射率和反射率进行分析。将模拟结果和已发表的理论结果相比较,在单个同位素掺杂缺陷下,在临近共振频率区域内用改进的R.O.Pohl公式成功的拟合了MD结果,这一结果会对在较宽频率包括非色散和色散声子范围内构造声子热传导公式有帮助.对于在较高的掺杂浓度下,声子频率对声子散射特性的影响还需要更进一步的研究。     

Flow-induced beam steering in a single laser hot spot

The transmitted angular distribution of a 527 nm nearly diffraction-limited laser is measured after it propagates through a plasma with supersonic transverse flow. The laser beam is deflected by as much as 10 degrees and exhibits bowlike features in the flow direction, which is attributed to flow-induced beam steering. The finite interaction volume allows for direct comparison with a 3D hydrodynamic simulation, which is in good agreement with details of the experiment.     

Photoexcited hot phonons and phonon lifetimes in GaAs

A study of optical lifetimes as determined from surface conditions is made through Raman lineshape analysis and non-equilibrium phonon distributions induced by energetic photoelectrons. Results indicate that first-order Raman scattered linewidths are lifetime determined and that phonon lifetimes depend upon surface conditions. Our observations also give strong support to the mechanism and analysis proposed earlier for hot phonon generation through photoelectron relaxation.     

Acoustic timescale characterisation of a one-dimensional model hot spot

Hot spots have been shown to be the autoignition centre in reactive mixtures. Linear temperature gradients and thermal stratification are used to characterise their behaviour. In this work, a model hot spot is considered by combining a linear temperature gradient with a constant temperature plateau. This approach retains the simplicity of a linear temperature gradient, but captures the effects of a local temperature maximum of finite size. A one-step Arrhenius reaction for H₂–air is used to model the reactive mixture. Plateaus of three different initial sizes spanning two orders of magnitude are simulated. Each length corresponds to a different ratio of excitation time to acoustic time. It is shown that ratios less than unity react at nearly isochoric conditions while ratios greater than unity react at nearly isobaric conditions. Furthermore, it is demonstrated that the gasdynamic response is characterised by the a priori prescribed hot spot acoustic timescale ratio. Based upon the prescribed timescale ratio, it is shown that the plateau can have either a substantial or negligible impact on the reaction of a surrounding temperature gradient. This is explored further as the slope of the temperature gradient is varied. Based upon the heating-to-acoustic timescale ratio, plateaus of a particular size are shown to facilitate detonation formation inside gradients that would otherwise not detonate.     

Microscopic phonon theory of Si/Ge nanocrystals

Microscopic phonon theory of semiconductor nanocrystals (NCs) is reviewed in this paper. Phonon modes of Si and Ge NCs with various sizes of up to 7 nm are investigated by valence force field theory. Phonon modes in spherical SiGe alloy NCs approximately 3.6 nm (containing 1147 atoms) in size have been investigated as a function of the Si concentration. Phonon density-of-states, quantum confinement effects, as well as Raman intensities are discussed.      

Remote polar phonon scattering for hot electrons in Si-inversion layers

The effect of the remote interfacial phonon (R.I.P.) scattering on the carrier drift velocity v is evaluated in function of the effective mobility, i.e. in function of the surface roughness. A perturbation theory using the experimental ν?F relation as a zero order approximation is used to calculate the contribution of the R.I.P. scattering. The calculation shows that the influence of this phonon mode scattering on the transport properties in Si-inversion layers is dependent on the carrier low field mobility and is of the order of 10%. The R.I.P. scattering is particularly significant in the warm electron regime, having no consequence on the saturation velocity.     

Coherent storage of photoexcited triplet states using 29Si nuclear spins in silicon

Pulsed electron paramagnetic resonance spectroscopy of the photoexcited, metastable triplet state of the oxygen-vacancy center in silicon reveals that the lifetime of the m(s)=±1 sublevels differs significantly from that of the m(s)=0 state. We exploit this significant difference in decay rates to the ground singlet state to achieve nearly ~100% electron-spin polarization within the triplet. We further demonstrate the transfer of a coherent state of the triplet electron spin to, and from, a hyperfine-coupled, nearest-neighbor (29)Si nuclear spin. We measure the coherence time of the (29)Si nuclear spin employed in this operation and find it to be unaffected by the presence of the triplet electron spin and equal to the bulk value measured by nuclear magnetic resonance.     

Influence of recombination centers on the relaxation process of a 2D photoexcited hot electron plasma

The effect of recombination centers on the relaxation process of photoexcited 2D electron systems in polar semiconductors is studied theoretically. Our analysis takes into account the most important mechanisms of relaxation. We find that under certain conditions recombination to centers can drastically modify the e-ph interaction via reduction of screening. Consequently, the time behaviour of the energy exchange rate between the electronic system and the lattice can also be altered. Our results indicate that recombination to centers affects, in different ways, the time evolution of the carrier temperature and the rate of energy exchange rate. Qualitative agreement with some experimental data lends support to the model and allows us to make some predictions.     

Advanced aspects of electromagnetic SERS enhancement factors at a hot spot

In this paper, we discuss some advanced theoretical aspects of electromagnetic enhancement factors (EFs) in surface‐enhanced Raman scattering (SERS). We focus in particular on the influence of surface selection rules (SSRs) on SERS EFs at hot spots, and the determination of SERS depolarization ratios. Both aspects could be viewed as secondary (compared to the overall magnitude of the SERS EF), but are nevertheless observable experimentally and crucial for a fundamental understanding of SERS. They also share the property that they cannot be studied within the commonly used |E | ⁴ approximation to the SERS EFs, and appropriate tools are developed here to make predictions beyond this approximation in the case of a SERS hot spot. In addition, theoretical estimates of different types of (previously defined) EFs are provided, and their origins discussed for the typical example of a SERS substrate dominated by SERS hot spots. Finally, experimental measurements of SERS depolarization ratios are presented to support the theoretical predictions. Copyright © 2008 John Wiley & Sons, Ltd.     

The mechanism of hot spot formation in condensed explosives

The hypothesis of an electrical mechanism of formation of detonation initiation zones (hot spots) in condensed explosives is considered. The hypothesis is based on the generation of electric fields and the appearance of shock-induced conductivity in the propagation of a shock wave (compression wave) in condensed dielectric media, including explosives. These physical phenomena can cause a local electrical breakdown, whose channel is identified with a hot spot. The available experimental data are analyzed from the point of view of the hypothesis suggested, and a procedure for verifying this hypothesis is outlined.     

Probing phonon emission via hot carrier transport in suspended graphitic multilayers

We study hot carrier transport under magnetic fields up to 15 T in suspended graphitic multilayers through differential conductance () spectroscopy. Distinct high-energy anomalies have been observed and shown to be related to intrinsic phonon-emission processes in graphite. The evolution of such anomalies under magnetic fields is further understood as a consequence of inter-Landau level cyclotron-phonon resonance scattering. The observed magneto-phonon effects not only shed light on the physical mechanisms responsible for high-current transport in graphitic systems, but also offer new perspectives for optimizing performance in graphitic nano-electronic devices.     

Coulomb explosion of the hot spot of micropinches

It has been shown that the generation of hard X-ray radiation, electron beam, and high energy ions that have been detected in experiments on compressing pinches can be related to the Coulomb explosion of a micropinch hot spot, which is formed due to the outflow of the material. In the outflow process, the plasma temperature in the hot spot increases and conditions appear for the transition of electrons to the regime of continuous acceleration. The exit of runaway electrons from the hot spot region leads to the creation of a positive bulk charge, then to a Coulomb explosion. Conditions under which electrons pass to the continuous acceleration regime have been determined and estimates of the ion kinetic energy upon a Coulomb explosion have been obtained.