Magnon propagation in a nanometric magnetic cluster chain: Effects of additional clusters near the chain

A fundamental understanding of nanoscaled materials has become an important challenge for any technical applications. For magnetic nanoparticles, the investigations are in particular stimulated by the magnetic storage devices. In this paper we present a theory of the magnon propagation in a nanometric chain and of the effects of neighbor magnetic clusters. We show that with an appropriate choice of the additional clusters, it is possible to control the transmission spectrum of the magnons. The additional clusters can also create a resonator with zero of transmission. This model could be useful to construct device to transmit or stop the magnon signal.     

Magnon nanometric filters in quasi-one-dimensional cluster chains

In the frame of the discrete dipole approximation, the propagation of magnons in quasi-one-dimensional resonant structures made of nanometric magnetic clusters is examined theoretically. These resonant structures, composed of a nanometric magnetic cluster chain and adsorbed clusters near the chain, may exhibit sharp peaks (dips) in the magnon transmission spectrum. Stop bands and sharp resonant magnonic states in the gaps can be created with an appropriate choice of the geometrical or magnetic parameters of the structure. These resonant states result from an internal resonance of the structure when such a resonance is situated in the vicinity of a zero of transmission or placed between two zeros of transmission. This model may have potential applications in constructing magnon filter devices.     

Tunable Fano resonances in the ballistic transmission and tunneling lifetime in a biased bilayer graphene nanostructure

Dynamics of the Dirac fermions, in particular the transmission coefficient and the resonant tunneling lifetime are studied across a bilayer graphene electrostatic double barrier structure modulated by an in plane homogeneous electric field. Asymmetric Fano type resonances are noted for the first time in the transmission spectrum of the bilayer graphene nanostructures and are found to be highly sensitive to the direction of incidence of the charge carriers and the applied homogeneous electric field. The effect of the external field on the extended and the evanescent modes is also analysed. Resonant tunneling lifetime is found to be highly anisotropic in nature. The chiral carriers are either accelerated or decelerated by the electric field depending on the energy of the quasi-bound states, the angle of incidence and the magnitude of the applied field. Effects of the external field on the localization of the chiral carriers are also discussed.     

XRD, HRTEM, magnetic and Mössbauer studies on chemically prepared Fe-doped nanoparticles of cerium oxide

Nanocrystalline samples of Fe-doped cerium oxide (Ce_0.90Fe_0.1O₂) are prepared by sol-gel method. The precursor materials used for the synthesis are ferric nitrate and cerium nitrate. The as-prepared samples is annealed at different temperatures to obtain the sample with different particle sizes. The crystallographic phases of the nanocrystalline materials have been confirmed by X-ray diffractograms (XRD). The sizes of the nanoparticles estimated from the peaks of the XRD patterns using Debye-Scherrer equation are in the range 6-58 nm. Results extracted from the high-resolution transmission electron microscopy (HRTEM) are in agreement with the findings obtained from XRD. The average magnetic susceptibilities of all the samples with different particle sizes are measured in the temperature range 300-14 K. The average susceptibilities of the samples annealed below ∼740 °C show paramagnetic behaviour. The susceptibilities of the samples annealed at and above ∼740 °C sharply decrease at ∼240 K and this sharp transition is quite likely due to the anti-parallel alignment of Fe³⁺ spins and is attributed to Morin transition of α-Fe₂O₃. Mössbauer spectra of the samples annealed at and above ∼740 °C give sextet patterns indicating the presence of exchange interaction among the Fe³⁺ ions of these samples and these sextets are also of typical nature of the α-Fe₂O₃ phase. The Mössbauer spectra of the samples annealed below ∼740 °C are doublets which may be attributed to either superparamagnetic and/or paramagnetic type nanoparticles.     

枝节直波导与开口方环耦合谐振腔的Fano谐振特性研究

本文构建了一个枝节直波导与带开口方环耦合谐振腔.基于金属-绝缘体-金属结构的Fano谐振原理,采用有限元法分析了该波导结构的透射谱线对谐振腔结构参数的依赖关系.在开口方环与直波导间距G=90 nm,枝节高度H=140 nm时,该谐振腔结构中可以产生中心波长分别为λ=746 nm和1521 nm具有反对称线型的双重Fano共振透射峰.研究表明：左侧谐振峰值先随H增高而升高,至H=140 nm达到最大值,之后随H增高峰值缓慢减弱,直至消失.右侧谐振峰值则几乎不受H变化影响.传输谷受H影响最大,随着H增大,枝节谐振腔的传输谷向长波长方向移动,而右侧两个传输谷基本保持不变.当H=210 nm时,左边Fano谐振完全消失,只剩下右边的谐振,且谐振峰基本保持不变.设置参数化扫描,观测Fano谐振的变化,在波长1500 nm～1700 nm之间,设置最佳参数,经拟合计算得到该谐振耦合腔可以作为一个灵敏度S为1496 nm/RIU,FOM=60.1的折射率传感器.该结构可以为纳米级折射率传感器设计提供有效依据.     

Young's double-slit experiment in photonic crystals

We present an experimental and numerical study of the transmission of a photonic crystal perforated by two subwavelength slits, separated by two wavelengths. The experimental near-field image of the double-slit design of the photonic crystal shows an interference pattern, which is analogous to Young's experiment. This interference arises as a consequence of the excitation of surface states of the photonic crystals and agrees very well with the simulations.     

Fano resonance sensing based on coupled sub-wavelength dielectric grating and periodic photonic crystal

Based on the diffraction effect of sub-wavelength dielectric grating and the optical property of periodic photonic crystal, a hybrid structure of sub-wavelength grating all-dielectric multilayer thin film containing periodic photonic crystal is proposed. The transmission property of the structure is simulated by finite element method (FEM). The result shows that the discrete state generated by the sub-wavelength waveguide grating will be coupled with the continuous state generated by the photonic crystal cavity and the Fano resonance can be formed. The Fano resonance sensing model based on structural parameters and resonance wavelength are established, the influence of structural parameters on the Fano resonance spectral curve is quantitatively analyzed by numerical simulations, and the dynamic detection of the refractive index of samples is realized. The above structure can realize the optical refractive index sensing with high figure of merit (FOM) value and provide an effective theoretical reference for the formation of Fano resonance in the all-dielectric hybrid structure.     

Spin-dependent Breit-Wigner and Fano resonances in photon-assisted electron transport through a semiconductor heterostructure

We theoretically investigate the electron transmission through a seven-layer semiconductor heterostructure with the Dresselhaus spin-orbit coupling under two applied oscillating fields. Numerical results show that both of the spin-dependent symmetric Breit-Wigner and the asymmetric Fano resonances appear and that the properties of these two types of resonance peaks are dependent on the amplitude and the relative phases of the two applied oscillating fields. The modulation of the spin-polarization efficiency of transmitted electrons by the relative phase is also discussed.     

2维光子晶体中的掺杂效应数值研究

 把平面波展开方法(PWM)用于2维光子晶体掺杂情况下透射特性的研究,计算得到了粗细不同的空气柱掺杂、相同半径不同介质柱掺杂、不同柱体形状掺杂情况下2维光子晶体的透射系数与入射光频率的关系曲线。结果表明2维光子晶体的禁带的宽度、位置、透过率与掺杂体半径、掺杂体介电常数、掺杂体柱体几何形状等因素有关,掺杂因素相差越大透射曲线变化越明显,特别是损耗介质的掺入更使得禁带可调要素增加。     

聚苯乙烯微粒光子晶体的反常透过特性

在实验上发现了3μm聚苯乙烯(PS)小球光子晶体在350nm至900nm波长上的透过率的周期扰动.根据透过率曲线的周期扰动的峰谷值判断，这种周期性透过率特性是基于PS小球的whispering-gallery-mode(WGM)共振，而不是源于米氏散射或布拉格衍射.实验表明，周期性排列 小球在非带隙区域对光的传输同样有一定的影响.这一结果对光子晶体薄膜的应用提供了新 的可能性. 关键词： 光子晶体 透过率 whispering-gallery-mode共振     

Mixed Fano-SP resonant absorption of THz electromagnetic waves in a photonic resonator contacting with a metal film

In terahertz frequency region, we have investigated theoretically the correlation between spectra of a free photonic structure and that deposited on a metal for several models of metal. It was found that for quasi-normal incidence of p-polarized electromagnetic field the presence of metal generates narrow spectral wells in the middle of reflection windows existing in a free photonic crystal. Quite another manifestation of metal-resonator inter-influence takes place at incident angles exceeding the Brewster angle when reflection spikes coincide with modes of photonic crystal resonator and they are absent throughout the stopband areas. The effects are strongly depended on polarization, number of periods and angle of incidence.     

一维Au/MgF2光子晶体的透射性质

理论上研究了一维金属/电介质光子晶体的透射率性质,用传输矩阵方法数值计算了Au/MgF2光子带隙晶体的透射率．发现周期性结构产生的透射共振会大幅度增加光通过层状金属的透射率．通过调节一维Au/MgF2光子晶体中金和氟化镁的厚度以及周期数,得到了可见波段的高透射允带,透射率在0.4以上,而对于可见波段以外的频率几乎是完全不能传输的．这种结构在保持了高透射率的前提下还具有金属的优点,这在传感器、护目装置、反射窗、液晶显示等诸多领域都有很大的实用价值．     

Band structure of collective modes and effective properties of binary magnonic crystals

In this paper a theoretical study of the band structure of collective modes of binary ferromagnetic systems formed by a submicrometric periodic array of cylindrical cobalt nanodots partially or completely embedded into a permalloy ferromagnetic film is performed. The binary ferromagnetic systems studied are two-dimensional periodic, but they can be regarded as three-dimensional, since the magnetization is non uniform also along the z direction due to the contrast between the saturation magnetizations of the two ferromagnetic materials along the thickness. The dynamical matrix method, a finite-difference micromagnetic approach, formulated for studying the dynamics in one-component periodic ferromagnetic systems is generalized to ferromagnetic systems composed by F ferromagnetic materials. It is then applied to investigate the spin dynamics in four periodic binary ferromagnetic systems differing each other for the volume of cobalt dots and for the relative position of cobalt dots within the primitive cell. The dispersion curves of the most representative frequency modes are calculated for each system for an in-plane applied magnetic field perpendicular to the Bloch wave vector. The dependence of the dispersion curves on the cobalt quantity and position is discussed in terms of distribution of effective “surface magnetic charges” at the interface between the two ferromagnetic materials. The metamaterial properties in the propagative regime are also studied (1) by introducing an effective magnetization and effective “surface magnetic charges” (2) by describing the metamaterial wave dispersion of the most representative mode in each system within an effective medium approximation and in the dipole-exchange regime. It is also shown that the interchange between cobalt and permalloy does not necessarily lead to an interchange of the corresponding mode dispersion. Analogously to the case of electromagnetic waves in two-dimensional photonic crystals, the degree of localization of the localized collective modes is expressed in terms of an energy concentration factor.     

Instabilities and quasi-localized states in nonlinear Fano-like systems

We study the dynamical scattering in one-dimensional systems with a nonlinear side-coupled defect. Such structures exhibit the nonlinear Fano resonances, where nothing can propagate through. We developed a numerical model to study dynamical scattering. According to our analysis the scattering waves become dynamically unstable in the vicinity of the nonlinear Fano resonances, due to modulational instability caused by the presence of nonlinearity. It results in a time-growing amplitude of the nonlinear defect. We also demonstrate the existence of the nonlinear quasi-localized state, supported by such structures.     

Fano resonance and spectral compression in a ring resonator drop filter with feedback

We propose a ring resonator drop filter with feedback loop which can form the other ring cavity to produce an asymmetric Fano-resonance line shape and mode compression. By properly adjusting the feedback loop, the asymmetric line shapes of the transmission spectra can be controlled and selectively mode compression of the circuiting intensity in the feedback loop is achieved. Such characteristics are useful for applications in ring resonator-based photonic devices such as all-optical switches, sensors and intracavity frequency selection single-mode laser.     

不同压电分流电路对声子晶体梁带隙的影响

通过在匀质基体梁上周期性对贴连有分流电路的压电片,可以形成一维声子晶体结构.不同分流电路将决定梁的带隙特性.电阻分流电路能够对梁的模态共振有一定的抑制作用,而且可以通过改变压电效应的强弱使Bragg带隙结构发生变化.LC分流电路会使梁产生局域共振带隙,其带隙是由压电片激励LC电路产生电磁振荡形成.引入电阻后会使LC电路的共振作用变弱,使得带隙衰减幅值变小,但可以扩宽带隙频率范围.     

Fano resonance and wave transmission through a chain structure with an isolated ring composed of defects

We consider a discrete model that describes a linear chain of particles coupled to an isolated ring composed of N defects.This simple system can be regarded as a generalization of the familiar Fano-Anderson model.It can be used to model discrete networks of coupled defect modes in photonic crystals and simple waveguide arrays in two-dimensional lattices.The analytical result of the transmission coefficient is obtained,along with the conditions for perfect reflections and transmissions due to either destructive or constructive interferences.Using a simple example,we further investigate the relationship between the resonant frequencies and the number of defects N,and study how to affect the numbers of perfect reflections and transmissions.In addition,we demonstrate how these resonance transmissions and refections can be tuned by one nonlinear defect of the network that possesses a nonlinear Kerr-like response.     

Fano resonance and wave transmission through a chain structure with an isolated ring composed of defects

We consider a discrete model that describes a linear chain of particles coupled to an isolated ring composed of N defects. This simple system can be regarded as a generalization of the familiar Fano-Anderson model. It can be used to model discrete networks of coupled defect modes in photonic crystals and simple waveguide arrays in two-dimensional lattices. The analytical result of the transmission coefficient is obtained, along with the conditions for perfect reflections and transmissions due to either destructive or constructive interferences. Using a simple example, we further investigate the relationship between the resonant frequencies and the number of defects N, and study how to affect the numbers of perfect reflections and transmissions. In addition, we demonstrate how these resonance transmissions and refections can be tuned by one nonlinear defect of the network that possesses a nonlinear Kerr-like response.     

Nonlinearities in the reflection and transmission spectra of the photonic bandgap heterostructures with n–i–p–i crystals

Nonlinear optical properties of photonic crystal heterostructures with embedded n–i–p–i superlattices are investigated. Self-consistent calculations of the transmission and reflection spectra near the defect mode are performed using the transfer-matrix method and taking into account the gain saturation. Analysis of features and output characteristics is carried out for one-dimensional photonic crystal heterostructure amplifiers in the GaAs–GaInP system having at the central part an active “defect” from doubled GaAs n–i–p–i crystal layers. The gain saturation in the active layers in the vicinity of the defect changes the index contrast of the photonic structure and makes worse the emission at the defect mode. Spectral bistability effect, which can be exhibited in photonic crystal heterostructure amplifiers, is predicted and the hysteresis loop and other attending phenomena are described. The bistability behavior and modulation response efficiency demonstrate the potential possibilities of the photonic crystal heterostructures with n–i–p–i layers as high-speed optical amplifiers and switches.      

Tunable Fano resonances and plasmonic hybridization of gold triangle–rod dimer nanostructure

A gold dimer structure consisting of a notched triangle nanoslice and a rectangle nanorod is proposed to produce distinct Fano resonance. Owing to the coupling between the dipole plasmon mode of the nanorod and the dipole or quadrupole plasmon mode of the nanoslice, the extinction spectrum with a deep Fano dip is formed and can be well fitted by the Fano interference model for different geometry parameters. In addition, Fano resonance of the gold dimer nanostructure also intensely depends on the polarization direction of incident light. Moreover, Fano resonance of the triangle–rod trimer is also analyzed by adding another nanorod into the former dimer and exhibits the splitting of plasmonic resonant peak in high order coupling modes. The plasmonic hybridizations in these nanostructures have been analyzed for revealing the physical origin of the Fano resonance.