四轮驱动全向底盘的控制方法研究

为提高机器人移动的灵活性和机动性,使机器人到任意位置均沿直线行走并改变姿态,需在传统的两轮驱动底盘上增加驱动轮,本文主要针对四轮驱动全向底盘的控制方法进行研究。本文对四轮驱动全向底盘进行运动学分析并建模,得到了控制系统传递函数,并进仿真验证;对四轮驱动全向底盘进行了动力学建模,分析了如何避免出现“打滑”现象;导航方式采用了经济、精度较高的全向码盘导航方式;控制上采用闭环控制,加入前置探测点和超前校正环节,并且考虑了实际过程中机体的加减速问题。实验结果表明,提出的控制方法可以使四轮驱动全向底盘实现预设功能,并且具有较高的控制精度,研究内容对机器人的灵活移动有着十分重要的意义。     

Photonic Bandgap Properties of Atom-lattice Photonic Crystals in Polymer

The present paper covers the various photonic crystals(PhCs) structures mimicking real atom-lattice structures in electronic crystals by using the femtosecond laser-induced two-photon photopolymerization of SU-8 resin. The bandgap properties were investigated by varying the crystal orientations in <111>, <110> and <100> of diamond-lattice PhCs. The photonic stop gaps were present at λ=3.88 μm in <111> direction, λ=4.01 μm in <110> direction and λ=5.30 μm in <100> direction, respectively. In addition, defect...     

CdxHg(1−x)Te Alloy Colloidal Quantum Dots: Tuning Optical Properties from the Visible to Near‐Infrared by Ion Exchange

The energy gap between valence and conduction levels in colloidal semiconductor quantum dots can be tuned via the nanoparticle diameter when this is comparable to or less than the Bohr radius. In materials such as cadmium mercury telluride, which readily forms a single phase ternary alloy, this quantum confinement tuning can also be augmented by compositional tuning, which brings a further degree of freedom in the bandgap engineering. Here it is shown that compositional control of 2.3 nm diameter Cd_xHg_(1?x)Te nanocrystals by exchange of Hg²⁺ in place of Cd²⁺ ions can be used to tune their optical properties across a technologically useful range, from 500 nm to almost 1200 nm. Data on composition‐dependent changes in the optical properties are provided, including bandgap, extinction coefficient, emission energy and spectral shape, Stokes shift, quantum efficiency, and radiative lifetimes as the exchange process occurs, which are highly relevant for those seeking to use these technologically important QD materials.     

Coherent control of stimulated emission inside one dimensional photonic crystals: strong coupling regime

The present paper discusses the stimulated emission, in strong coupling regime, of an atom embedded inside a one dimensional (1D) Photonic Band Gap (PBG) cavity which is pumped by two counter-propagating laser beams. Quantum electrodynamics is applied to model the atom-field interaction, by considering the atom as a two level system, the e.m. field as a superposition of normal modes, the coupling in dipole approximation, and the equations of motion in Wigner-Weisskopf and rotating wave approximations. In addition, the Quasi Normal Mode (QNM) approach for an open cavity is adopted, interpreting the local density of states (LDOS) as the local density of probability to excite one QNM of the cavity; and therefore rendering this LDOS dependent on the phase difference of the two laser beams. In this paper we demonstrate that the strong coupling regime occurs at high values of the LDOS. In accordance with the results of the literature, the emission probability of the atom decays with an oscillatory behaviour, so that the atomic emission spectrum exhibits two peaks (Rabi splitting). The novelty of this work is that the phase difference of the two laser beams can produce a coherent control of both the oscillations for the atomic emission probability and, as a consequence, of the Rabi splitting in the emission spectrum. Possible criteria to design active delay lines are finally discussed.     

级联电磁带隙结构对双频微带天线互耦的影响

一种均匀面片尺寸的图钉型电磁带隙结构只能实现一个表面波带隙,而将不同参数的图钉型电磁带隙结构级联可以实现多个或宽带的表面波带隙。利用不同金属面片尺寸的图钉型电磁带隙结构级联的方法实现双带的表面波带隙。设计两种均匀的电磁带隙结构,使其带隙范围分别覆盖天线的两个工作频带,然后将这两种结构级联起来,加载于两个缝隙天线单元之间。仿真分析和实验结果表明,级联型的电磁带隙结构可以有效抑制表面波,在两个频率带内同时减小互耦。     

二维光子晶体T型波导分支器数值模拟研究

采用平面波展开法和时域有限差分法,对GaAs二维正方晶格光子晶体的色散特性和带隙结构进行了数值模拟研究,并对GaAs二维光子晶体线缺陷构成的T型波导分支器的微波传输特性进行了模拟和优化。数值模拟结果表明,光子晶体填充比对带隙结构有显著地影响。通过在拐角处插入额外的电介质棒对GaAs二维正方光子晶体T型波导分支器进行优化,数值模拟的结果表明,优化的GaAs二维正方光子晶体T型波导分支器在一阶带隙范围内透射系数将提高到0.96以上。     

Self‐organized colloidal crystals for photonics and laser applications

We present an overview of recent developments in the fabrication and uses of colloidal crystals (CCs) for photonics and laser applications. Microparticles with a diameter in the range from 10 nm to 10 μm often have an intrinsic capability to spontaneously organize themselves from a colloidal suspension into 3D lattice structures. Such highly ordered 3D architectures of microparticles are called colloidal crystals (CCs). The CC structures have received tremendous attention as one of the facile and high‐throughput fabrication techniques of photonic crystals (PCs). We introduce here interesting precedents not only of diverse techniques of high‐quality CC structures, but also of their versatile applications in optical sensors responding to various external stimuli. This review also highlights a new potential use of the CCs as low‐threshold laser devices. We believe that a wide variety of CC architectures will play leading roles in the next generation of optoelectronic devices.     

Effect of high temperature band gap of GaN annealing on strain and nanoparticles

<正>Black-coloured GaN nanoparticles with an average grain size of 50 nm have been obtained by annealing GaN nanoparticles under flowing nitrogen at 1200℃for 30 min.XRD measurement result indicates an increase in the lattice parameter of the GaN nanoparticles annealed at 1200℃,and HRTEM image shows that the increase cannot be ascribed to other ions in the interstitial positions.If the as-synthesised GaN nanoparticles at 950℃are regarded as standard,the thermal expansion changes nonlinearly with temperature and is anisotropic;the expansion below 1000℃is smaller than that above 1000℃.This study provides an experimental demonstration for selecting the proper annealing temperature of GaN.In addition,a large blueshift in optical bandgap of the annealed GaN nanoparticles at 1200℃is observed,which can be ascribed to the dominant transitions from the C（Γ₇） with the peak energy at 3.532 eV.     

Proposal for a New Dense Wavelength Division Multiplexing Filter Based on Two-Dimensional Photonic Crystal Ring Resonator

Abstract

A new optical filter design based on a two-dimensional photonic crystal ring resonator structure with an N-channel model is proposed in this article. This study also shows that modifying the scatter radius and the waveguide width can significantly improve the performance of the original structure, which can solve the mode mismatch problem for output waveguide. Here, an example of a 16-channel photonic crystal ring resonator is provided; wavelength spacing of 1.6 nm and a high quality factor Q of 6,000 were achieved. The optical filter would be a potential key component in the application of dense wavelength division multiplexer devices.     

Analysis of photonic band gaps in a two-dimensional photonic crystal with centrifugal core-shell rods

The absolute photonic band gap (PBG) in two-dimensional (2D) photonic crystal with excentric core-shell rods is studied in this paper. The core rod shifts away from the core-shell rod center, and its position is decided by two new introduced parameters — the shift angle θ and the offset ρ. We use the FDTD algorithm to calculate the photonic bands of the photonic crystal, and analyze how the offset and shift angle affect the photonic bang gap of excentric core-shell photonic crystal for different core rod size. It has been shown that the variation of the photonic band gap is quite peculiar.