Defect modes in one-dimensional photonic lattices

Linear defect modes in one-dimensional photonic lattices are studied theoretically. For negative (repulsive) defects, various localized defect modes are found. The strongest confinement of the defect modes appears when the lattice intensity at the defect site is nonzero rather than zero. When launched at small angles into such a defect site of the lattice, a Gaussian beam can be trapped and undergo snake oscillations under appropriate conditions.     

Defect modes supported by optical lattices in photovoltaic-photorefractive crystals

We study defect modes in optically induced one-dimensional lattices in photovoltaic-photorefractive crystals. These defect modes exist in different bandgaps due to the change of defect intensity. For a positive defect, defect mode branches exist not only in the semi-infinite bandgap, but also in the first and second bandgaps. When the defect mode branch is fixed, the confinement of defect modes increases with the defect strength parameter. For a negative defect, defect mode branches exist only in the first and second bandgaps. For a given defect mode branch, the strongest confinement of the defect modes appears when the lattice intensity at the defect site is not the smallest in its branch. On the other hand, when the defect strength parameter is fixed, the most localized defect modes arise in the semi-infinite bandgap for the positive defect and in the first bandgap for the negative defect.     

基于布里渊区谱的二维光子晶格线性缺陷模式分析

结合均匀光子晶格的衍射关系和晶格中正负缺陷的布里渊区谱特性,探讨了光诱导二维光子晶格中线性缺陷模式的形成条件,对布里渊区中各点的缺陷模式进行了模拟分析,并得到了光子带隙结构中“嵌入点”X₁点的正、负缺陷模式.结果表明：对于二维缺陷,只有当衍射关系曲面中沿两个正交的横向波矢方向同时为正常（反常）衍射的区域才能存在正（负）缺陷模式;而对于一维缺陷,只要在一个横向波矢方向上存在正常（反常）衍射的区域就可以支持正（负）缺陷模式,因此在某些特殊点处,正和负的缺陷模式可以同时存在.X₁点正缺陷模式的存在预示着自聚焦非线性同样可以支持带内孤子.研究结果有助于对光子晶格中晶格孤子（特别是带内孤子）的理解和进一步研究. 关键词： 光诱导光子晶格 光子带隙 线性缺陷态     

Gaussian profile laser intensification by nodular defects in mid-infrared high reflectance coatings

Electric field modeling of nodular defects is performed to investigate the interaction between defective multilayer coatings and Gaussian profile laser beam. Light-intensity is significantly enhanced as large as six times in areas adjacent to the defect boundary as well as within the defects. Different geometry of defects irradiated by a range of 0-45° incident laser is analyzed. Nodules with large but shallow seed or in the case of 45° oblique incidence tend to have the greatest enhancement effect.     

Angular tuning of defect modes spectrum in the one-dimensional photonic crystal with liquid-crystal layer

A one-dimensional ZrO₂/SiO₂ photonic crystal with a 4-n -pentyl-4' -cyanobiphenyl (5CB) nematic defect layer was used to investigate the transmission spectra of light polarized parallel and perpendicular to the liquid-crystal director at different angles of incidence. The spectra of the photonic crystal were shown to split into four polarized components T_ij at oblique incidence. When the incident angle increased, the bandgap edges and the defect modes shifted towards short wavelengths, while the amplitudes of the defect modes increased for the transverse magnetic polarization and decreased for the transverse electric polarization. The observed discrepancy between the defect mode amplitudes in the center and near the edges of the photonic bandgap was found to be related to the radiation losses inside the defect layer of a non-ideal photonic crystal. The simulated transmission spectra obtained using recurrence relations and taking into account the decay of defect modes are in good agreement with the experimental data.     

非线性介质表面缺陷对激光光场的调制

以高斯光束为例,研究了非线性介质表面振幅调制型缺陷对光场的调制。基于广义惠更斯-菲涅尔衍射积分并采用泰勒级数展开方法,建立了高斯光束经过表面存在缺陷的非线性介质后的传输模型,得到了受缺陷调制光束经非线性介质后的光强分布解析式,研究了缺陷尺寸和光束在非线性介质中产生的附加相移大小对光强分布的影响,结果表明介质表面的缺陷尺寸越大,介质内产生的附加相移越大,光场受到的调制越严重。分别考虑了非线性折射率为正值和为负值的情况,研究发现相应的折射率值导致光束发生会聚或发散现象,并由于缺陷调制的作用,在光束传输过程中始终存在一光强极值点,且随着附加相移绝对值的增大光强极值点的峰值也随之增强。     

Propagation of Single-Mode Fibre Laser Beams through an Optical ABCD System with Circular Aperture at the Fibre Output End

Based on the expansion expression of the fundamental mode of a single-mode fibre in terms of Laguerr-Gauss modes, the propagation of a beam of a weakly guiding fibre laser through an opticM ABCD system with a circular aperture at the fibre end is studied. The results show that there is much difference between the propagation of the laser beam described by the expansion expression and by the Gaussian mode approximation. The depth of focus of the laser beam is longer than that of the Gaussian modes.     

The symmetry properties of stable polynomial Gaussian beam profiles

We consider the symmetry properties of polynomial Gaussian beam profiles (intensity distributions) that remain stable during propagation, apart from being scaled and possibly rotated. These beams are expressed as special linear combinations of the Laguerre-Gaussian modes. Two kinds of symmetries are present: discreet rotational symmetries and mirror symmetries. The symmetry properties are shown to depend on the particular subset of Laguerre-Gaussian modes that is used to construct the stable beam. We demonstrate the symmetry properties of a few examples of stable beams through numerical simulations.     

Nonlinear localized modes in bandgap microcavities

We study experimentally an electrically pumped GaAs-based bandgap structure based on a vertical cavity surface emitting laser (VCSEL). We demonstrate that a microcavity embedded into this bandgap VCSEL structure supports localized optical modes without any holding beam. We propose a model of surface-structured VCSELs based on a reduced dissipative wave equation for describing electromagnetic modes in such semiconductor cavities and analyze a crossover between linear and nonlinear solitonlike cavity modes.     

各向异性焊缝中超声相控阵瞬态声场仿真及缺陷定位方法

针对多层各向异性奥氏体不锈钢焊缝中超声相控阵瞬态声场的仿真问题,提出应用高斯声束等效点源模型计算宽带离散化的多个单频稳态声场,通过傅里叶变换将其拓展为瞬态声场,并分析了声场转换过程的主要影响参数.该方法可快速计算焊缝内部超声相控阵聚焦声场的瞬态能量分布和任意一点的时域波形信号.在此基础上针对多层奥氏体不锈钢焊缝内部缺陷...     

Hybrid Transmission Line for ECRH in the Helically Symmetric Experiment

The HSX oversized, mode-converting ECRH transmission line has been upgraded to a hybrid system to increase launched microwave power and reduce electrical arcing. Filtering of high-order, spurious modes ensures efficient coupling to a Gaussian beam for optimal electron heating. A Vlasov mode converter and two phase-correcting ellipsoidal mirrors convert the TE₀₂ gyrotron output mode to a symmetric, linearly polarized, microwave beam. A swappable twist reflector plate rotates beam polarization for 2nd-harmonic X-mode or fundamental O-mode ECRH. Long distances are traversed by coupling the beam to a dual-mode (TE₁₁ + TM₁₁), smooth, circular cross-section waveguide. This system has been successfully tested without arcing for 50 ms pulses and over 100 kW of launched power. Analysis of the microwave beam for 50 kW, 2 ms microwave pulses reveals agreement with predicted beam shapes at two beam locations. The new system has also demonstrated increased plasma stored energy for ECRH plasmas with equal launched power.     

Fresnel filtering of Gaussian beams in microcavities

We study the output from the modes described by the superposition of Gaussian beams confined in the quasi-stadium microcavities. We experimentally observe the deviation from Snell's law in the output when the incident angle of the Gaussian beam at the cavity interface is near the critical angle for total internal reflection, providing direct experimental evidence on the Fresnel filtering. The theory of the Fresnel filtering for a planar interface qualitatively reproduces experimental data, and a discussion is given on small deviation between the measured data and the theory.     

Enhancement of cross-polarized beam components at a metamaterial surface

Beam-interface interactions under incidence of a TE polarized elegant Hermite–Gaussian beam are considered in order to obtain efficient excitation of higher-order beam modes at a homogeneous, isotropic metamaterial surface. It is shown by numerical simulations that two transmitted TE and TM polarization beam components are enhanced more efficiently at a metamaterial surface than in the case of a dielectric interface. The mixed Poynting vector contribution to the incident energy flux is evaluated to explain this enhancement. The numerical results obtained are verified against energy balance at the interface.     

二维八重准晶有机光子晶体平板的光学传播特性

采用时域有限差分法研究了二维八重准晶有机光子晶体的光传输特性,重点分析了光束在聚苯乙烯空气柱平板结构和聚苯乙烯介质柱结构中的透射特性与光局域特性。研究结果表明,即使在低折射率对比度的情况下,两种完整八重准晶平板结构中均出现了可见光波段的光子带隙和本征模,且光子带隙中心位置随着平板厚度的增大而红移。当在两种准晶结构中引入缺陷微腔时,带隙内的缺陷模产生位置和波长红移特性随着微腔结构的变化规律明显不同,这种差异性是由两种物理机制(即光子晶体缺陷能级变化与微腔所支持的驻波条件)共同作用的结果。这一研究结果将为实验制备有机准晶发光器件提供一定的理论基础。     

Propagation and interaction of beams with initial phase-front curvature in highly nonlocal media

In this paper, an exact Gaussian solution with initial phase-front curvature for the 1 + 2D Snyder–Mitchell linear model is presented. It is found that the propagation of the beam in bulk strongly nonlocal media is deeply affected by the initial phase-front curvature, which can cause the pulsating behavior of Gaussian beam, and lead to the occurrence of the periodic interference pattern during the interaction of two Gaussian beams. It is useful for further understanding the properties of optical beam in strongly nonlocal media and may be applied to precision measurement.     

Waveguide lenses with multimode interference for low-loss slab propagation

It is shown that a nearly ideal two-dimensional focusing Gaussian beam can be synthesized by use of a linear combination of the two lowest-order even modes of an optical waveguide. This property can be used to couple laterally guided modes across slab waveguide regions with low loss. The technique is illustrated by use of a conventional multimode interference (MMI) geometry, in which the MMI coupler transforms the fundamental mode of an initial waveguide into a focusing Gaussian beam, which is then fed to a slab region. Two-dimensional beam propagation simulations show that the beam does not initially diverge in the slab region, but rather comes to a focus. A second MMI coupler then transforms the diverging beam back to the initial mode. A structure is designed that can couple the fundamental mode of a 9-microm-wide waveguide across an 88-microm-long slab region with only a 0.036-dB loss. This technique can be applied to improve the performance of small-angle waveguide crossings and integrated turning mirrors.     

Surface defect gap solitons in superlattices with self-defocusing nonlinearity

We put forward the existence and stability of defect surface gap solitons at the interface between uniform media and an superlattice with self-defocusing nonlinearity. We reveal that the defect plays the significant role in controlling the region of solitons existing. Various solitons are found to be existed in different gaps for different defects. For positive defects, fundamental solitons can exist stably in the semi-infinite gap, and dipole solitons can exist stably in the first gap but they are unstable in the second gap. For zero or negative defects, fundamental and dipole solitons can exist stably in the first gap and the second gap, respectively.     

Stone-Wales defects can cause a metal-semiconductor transition in carbon nanotubes depending on their orientation

It has been shown that the two different orientations of Stone-Wales (SW) defects, i.e. longitudinal and circumferential SW defects, on carbon nanotubes (CNTs) result in two different electronic structures. Based on density functional theory we have shown that the longitudinal SW defects do not open a bandgap near the Fermi energy, while a relatively small bandgap emerges in tubes with circumferential defects. We argue that the bandgap opening in the presence of circumferential SW defects is a consequence of long-range symmetry breaking which can spread all the way along the tube. Specifically, the distribution of contracted and stretched bond lengths due to the presence of defects, and hopping energies for low-energy electrons, i.e. the 2p(z) electrons, show two different patterns for the two types of defects. Interplay between the geometric features and the electronic properties of the tubes have also been studied for different defect concentrations. Considering π-orbital charge density, it has also been shown that the deviations of bond lengths from their relaxed length result in different doping for two defect orientations around the defects-electron-rich for a circumferential defect and hole-rich for a longitudinal one. We have also shown that, in the tubes having both types of defects, circumferential defects would dominate and impose their electronic properties.