Hypergeometric-Gaussian modes

We studied a novel family of paraxial laser beams forming an overcomplete yet nonorthogonal set of modes. These modes have a singular phase profile and are eigenfunctions of the photon orbital angular momentum. The intensity profile is characterized by a single brilliant ring with the singularity at its center, where the field amplitude vanishes. The complex amplitude is proportional to the degenerate (confluent) hypergeometric function, and therefore we term such beams hypergeometric-Gaussian (HyGG) modes. Unlike the recently introduced hypergeometric modes [Opt. Lett. 32, 742 (2007)], the HyGG modes carry a finite power and have been generated in this work with a liquid-crystal spatial light modulator. We briefly consider some subfamilies of the HyGG modes as the modified Bessel Gaussian modes, the modified exponential Gaussian modes, and the modified Laguerre-Gaussian modes.     

半导体中定域模,准定域模晶格振动的光谱研究

本文讨论了用光学方法研究杂质诱发定域化振动模的理论模型,分门别类地论述了半导体中杂质诱发的定域模、禁带模、共振模和准定域模晶格振动及非晶态半导体的晶格振动谱,给出了它们的综合结果。半导体中杂质诱发定域模已被人们广为研究,作者主要以自己的结果阐述了禁带模、共振模、准定域模和非晶态半导体振动谱研究在近年来取得的重要进展。除以较多篇幅研究各种定域化振动模的位置外,本文还从键—电荷模型出发讨论了杂质诱发振动吸收带的强度。     

Spectral signatures of critical charge and spin fluctuations in cuprates

We discuss how Raman spectra of high temperature superconducting cuprates are affected by nearly critical spin and charge collective modes, which are coupled to charge carriers near a stripe quantum critical point. We find that specific fingerprints of nearly critical collective modes can be observed and that the selectivity of Raman spectroscopy in momentum space may be exploited to distinguish the spin and charge contribution. We apply our results to discuss the spectra of high-T_c superconducting cuprates finding that the collective modes should have masses with substantial temperature dependence in agreement with their nearly critical character. Moreover spin modes have larger masses and are more diffusive than charge modes indicating that in stripes the charge is nearly ordered, while spin modes are strongly overdamped and fluctuating with high frequency.     

Acoustic waves of GaN nitride nanowires

Acoustic waves of GaN nanowires are studied. The materials have a wurtzite structure and thus the elastic anisotropy is included. We use the xyz-algorithm originally employed in the analysis of resonant ultrasound spectroscopy results. We have considered hexagonal cross-section nanowires as those experimentally grown. We have studied also circular cross-section nanowires (nanotubes) for comparison. It was found that the lower frequency modes have a very similar dispersion relation both in shape and frequency value, but higher frequency modes exhibit more important differences. The nanowires are solid or hollow and the influence of the thickness on the dispersion relation and the elastic displacement pattern is considered. Nanowires with a zinc-blende structure have also been considered because of theoretical studies allowing for their existence. We have studied also the squared displacement vectors to see the spatial distribution of the different modes in the nanowires. It was found that there are many modes whose elastic displacement components concentrate around the borders of the nanowire and they do not penetrate almost in the inner part of the nanowire.     

Characteristics of guided waves in indefinite-medium waveguides

We have presented a discussion on the guided waves in the indefinite-medium waveguides. The characteristic equations describing TE and TM guided waves have been derived. Aiming mainly at the TE guided waves, we have analyzed the existence conditions for the guided and surface modes for four distinct cases, respectively. A number of exotic properties have been revealed in such waveguides, such as the coexistence of forward and backward modes, the absence of fundamental modes, the existence of surface modes and the relaxed requirements for waveguide core constitutive parameters. Numerical results have confirmed our analyses.     

Calculation of cladding modes using both two layer and three layer geometry in LPFG

In this paper we have calculated cladding modes of long period fiber grating. We have calculated cladding modes using both two layer (Graphical solution method) geometry and three layer geometry proposed by Erdogan. We have compared both these techniques, discussed the limitations of Graphical solution method and benefits of three layer geometry.     

Electromagnetic cavity to explore disordered systems

In the course of characterizing an electromagnetic cavity we have come to understand details of transmission and reflection traces, some of which may be useful as tools to investigate local modes in random media. We have demonstrated quantitative agreement of frequency shift observations with theory, have demonstrated that the spatial distribution of electric and magnetic fields can be measured using insulating and conducting spheres, and have used perturbation due to wires and disks to demonstrate the local direction of electric and magnetic fields. We have observed that not only frequency shifts but also spatial shifts of modes can be used to observe the intensity of interaction of modes with extended objects in the cavity.     

Analytical investigation of one-dimensional photonic crystals with a dielectric-superconducting pair defect

We have studied the dielectric-superconducting pair defect embedded into one-dimensional photonic crystals by an analytical method based on the Kronig-Penny model. The superconducting defect has been considered by using of two-fluid model whose permittivity depends on the frequency and superconductor parameters. We have showed that in contrast to the usual defect modes, superconducting defect modes are nearly invariant upon the change of defect size and with increasing defect sizes, the enhancement behavior is observed in the peak of electric field profiles. In addition, the physical parameters have less effect on the defect modes, which is placed under cutoff frequency of superconductor.     

On the contribution of nearly critical spin and charge collective modes to the Raman spectra of high-Tc cuprates

We discuss how Raman spectra are affected by nearly critical spin and charge collective modes, which are coupled to charge carriers near a stripe quantum critical point. We show that specific fingerprints of nearly critical collective modes can indeed be observed in Raman spectra and that the selectivity of Raman spectroscopy in momentum space may also be exploited to distinguish the spin and charge contribution. We apply our results to discuss the spectra of high-T_c superconducting cuprates finding that the collective modes should have masses with substantial temperature dependence in agreement with their nearly critical character. Moreover, spin modes should be more diffusive than charge modes indicating that in stripes the charge is nearly ordered, while spin modes are strongly overdamped and fluctuate with high frequency.     

Periodic Driving Induced Anomalous End Modes in a Superconducting Wire with the Second-Neighbor Pairing Potential

The anomalous end modes are generated in a one-dimensional p-wave superconducting wire with second-neighbor couplings and a periodic driving in pairing potential. We show that the driving on the amplitude or the phase of the first-neighbor and second-neighbor pairing potential can both generate conventional end modes and anomalous end modes, with corresponding Floquet eigenvalues equal to ± 1 or appear in complex conjugate pairs. We have numerically studied the driving of the first-neighbor and second-neighbor pairing potential, and also analyzed the end modes, Floquet eigenvalues and the Fourier transform of these end modes.

     

High-frequency modes in a two-dimensional rectangular room with windows

We examine a two-dimensional model problem of architectural acoustics on sound propagation in a rectangular room with windows. It is supposed that the walls are ideally flat and hard; the windows absorb all energy that falls upon them. We search for the modes of such a room having minimal attenuation indices, which have the expressed structure of billiard trajectories. The main attenuation mechanism for such modes is diffraction at the edges of the windows. We construct estimates for the attenuation indices of the given modes based on the solution to the Weinstein problem. We formulate diffraction problems similar to the statement of the Weinstein problem that describe the attenuation of billiard modes in complex situations.     

Energy Levels of Coupled Plasmonic Cavities

We demonstrate the hybridization of the plasmonic modes in directly coupled whispering gallery cavities fabricated on silver films and present the mode patterns and energy levels using cathodoluminescence spectroscopy.Although the energy of the most antisymmetrically coupled modes is higher than that of the corresponding symmetrically coupled ones,the contrary cases happen for small quantum number modes.We attribute the phenomenon to the different surface plasmon polariton paths between the symmetrically and antisymmetrically coupled modes.These results provide an understanding of the resonant properties in coupled plasmonic cavities,which have potential applications in nanophotonic devices.     

On the interpretation of IETS spectra of a small organic molecule

We have investigated vibrational spectra of nitrobenzene molecules adsorbed on Cu(111) by low temperature inelastic electron tunneling spectroscopy. This molecule, which should support 39 internal modes, only gives rise to seven peaks in the spectra. We outline a comparison with ensemble IR data and interpret the small number of vibrational peaks by the superposition of a multitude of almost isoenergetic vibrational modes. The non-detectability of further modes cannot be understood in terms of symmetry considerations. Additional modes in the spectra are attributed to external molecular-metal vibrations.     

Temperature‐dependent Raman scattering study of multiferroic MnWO4

We have measured polarized Raman spectra of MnWO₄ single crystals at low temperatures, and studied the temperature dependence of the various phonon modes. From our Raman studies of the MnWO₄, a new transition temperature, ∼180 K, was found. We have completely assigned the symmetries of the 18 observed Raman modes of the MnWO₄, as expected from a group theoretical analysis. These Raman modes have been classified into three groups according to weak, intermediate and strong temperature dependence of the modes in each group. Six internal modes have been identified by their weak temperature dependence of the Raman wavenumbers. The temperature dependence of the wavenumbers of the B_g modes in Mg O bonds, modes of intermediate temperature dependence group, shows an anomalous behavior under 50 K. The phonon modes of strong temperature dependence show an anomalous change at ∼180 K in the linewidths. This is believed to be a new transition temperature which involves the changes in the inter‐WO₆ octahedra structure. Copyright © 2009 John Wiley & Sons, Ltd.     

二维随机增益介质中局域模的模面积和放大

随机激光形成于随机介质中局域模的放大,其放大特性依赖于局域模的空间分布。通过复折射率的虚部将光学增益引入系统,采用时域有限差分(FDTD)法,分析了增益介质中局域模的放大特性,并用模面积描述了局域模空间分布,着重对比分析了模面积对局域模放大特性的影响。模拟结果显示,局域模具有不同的空间分布,直接影响了局域模的放大特性,模面积较小的模式具有较低的损耗,能够获得更多能量,在介质中被优先放大,而模面积较大的模式,由于损耗较大,将获得较少的能量。     

Faceting and branching in 2D crystal growth

Using atomic scale time-dependent density functional calculations we confirm that both diffusion-controlled and diffusionless crystallization modes exist in simple 2D systems. We provide theoretical evidence that a faceted to nonfaceted transition is coupled to these crystallization modes, and faceting is governed by the local supersaturation at the fluid-crystalline interface. We also show that competing modes of crystallization have a major influence on mesopattern formation. Irregularly branched and porous structures are emerging at the crossover of the crystallization modes. The proposed branching mechanism differs essentially from dendritic fingering driven by diffusive instability.     

二维胶体玻璃中玻色峰与结构无序度的关联

本文从实验上研究了胶体玻璃在相同面密度下随着体系结构无序程度的增加, 振动态密度和玻色峰的变化规律. 通过调制两种不同粒径的温敏性水凝胶的数量比来改变体系的无序程度. 通过分析无序体系的声子模式得到体系的振动特性. 研究发现, 随着无序度的增加, 态密度在低频区域增强、玻色峰增高、玻色峰的峰值向低频区域移动. 不同无序程度的样品引起玻色峰的低频声子模式都表现出准局域的特点, 且低频准局域声子模式与样品中无序结构存在关联.     

Modified method of surface plasmons in metal superlattices

We present a modified method to solve the surface plasmons(SPs) of semi-infinite metal/dielectric superlattices and predicted new SP modes in physics. We find that four dispersion-equation sets and all possible SP modes are determined by them. Our analysis and numerical calculations indicate that besides the SP mode obtained in the original theory, the other two SP modes are predicted, which have either a positive group velocity or a negative group velocity. We also point out the possible defect in the previous theoretical method in accordance to the linear algebra principle.     

Experimental investigation of low-frequency edge magnetoplasma modes in two-dimensional sheets of ions trapped below the surface of superfluid helium

We report the experimental observation of unconventional low-frequency magnetoplasma modes in 2D sheets of ions. Some of them are multipole edge modes of the type predicted to exist by Nazin & Shikin; others have a dependence of frequency on magnetic field similar to that for conventional edge modes and may be related to satellite modes discussed by Ye & Zaremba.     

TWO-DIMENSIONAL LOCALIZED VIBRATIONAL MODES AROUND A POLARON WITH THE NEXT-NEAREST-NEIGHBOR HOPPING INTERACTIONS

We have calculated the vibrational modes around a polaron to test the effects of the next-nearest-neighbor hopping interactions on the localized medes within a framework of the two-dimensional mode. The results show that: (1) For a negative polaron, eleven localized modes have been found, compared with ten modes obtained on a previous work without non-nearest neighbor hopping. (2) For a positive polaron, two additional localized modes occur. (3) The localization of localized modes are strengthened and their frequencies move after turning on the next-nearest-neighbor hopping interactions.