Liquid Crystals and Polymer-Based Photonic Crystal Fibers

Experimental results of polymer photonic liquid crystal fibers based on commercially available (Kiriama) PMMA and cyclo-olefin polymer (Zeonex 480R) microstructured polymer fibers infiltrated with nematic liquid crystals (2CHBT/8CHBT and PCB) are presented and thermally-tuned photonic band-gap propagation mechanism is observed. These preliminary results suggest, that polymers binding to liquid crystals much easier than silica, can offer new opportunities while using polymer-based photonic crystal fibers.     

Photo- and thermo-induced variation of photonic properties of cholesteric liquid crystal containing azobenzene-based chiral dopant

Photooptical properties of a new cholesteric mixture containing azobenzene-based chiral dopant were investigated. Presence of a photosensitive dopant in the investigated liquid crystal enabled to effectively manipulate optical characteristics of the photonic band upon temperature change and illumination. Temperature dependence of the optical anisotropy and the orientational order parameter of the liquid crystal were determined. The obtained results are compared with existing theories. Discrete multistable change of the spectral position of the photonic band on illumination is realized.     

Photonics of liquid-crystal structures: A review

The original results of studies of the electro-optical and laser effects which have been performed at the Laboratory of Liquid Crystals of the Institute of Crystallography, Russian Academy of Sciences, over the last few years are reviewed. Cholesteric liquid crystals as vivid representatives of photonic structures and their behavior in an electric field are considered in detail. The formation of higher harmonics in the periodic distribution of the director field in a helical liquid crystal structure and, correspondingly, the new (anharmonic) mode of electro-optical effects are discussed. Another group of studies is devoted to bistable light switching by an electric field in chiral nematics. Polarization diffraction gratings controlled by an electric field are also considered. The results of studies devoted to microlasers on various photonic structures with cholesteric and nematic liquid crystals are considered in detail. Particular attention is given to the new regime: leaky-mode lasing. Designs of liquid crystal light amplifiers and their polarization, field, and spectral characteristics are considered in the last section.     

Functionalisation of cholesteric liquid crystals by direct laser writing

Cholesteric liquid crystals selectively reflect circularly polarised light with the same handedness as the helix. Because of their sensitivity to external stimuli, such as heat and electrical fields, various applications utilising their tunability have been proposed. Tuning is usually performed in the bulk, meaning that cholesteric liquid crystals usually possess a single pitch throughout the medium. However, when the helical structure is locally modulated, different optical properties arise, such as tunable photonic defect modes and multiple reflection bands. Here we show a technique to locally modulate the helical structure of cholesteric liquid crystals on a submicron scale, based on two-photon excitation direct laser lithography. Two examples of cholesteric liquid crystal structures with modulated helical structures will be presented.     

A new method of modeling cladding band structure of air-guiding photonic crystal fibers

Cladding band structure of air-guiding photonic crystal fibers with high air-filling fraction is calculated in terms of fiber shape variation. The fundamental photonic band gap dependence on structure parameters, air-filling fraction and spacing, is also investigated. The numerical results show that the band gap edges shift toward longer wavelength as the air-filling fraction is increased, whereas the relative band gap width increases linearly. For a fixed air-filling fraction, the band gap edges with respect to spacing keep constant. With this method, the simulation results agree well with the reported data.     

Topological Soft Matter for Optics and Photonics

Liquid crystal colloids are interesting for a variety of mechanisms—including self-assembly, optical-tweezers assisted assembly, topology, and material flow—that can be used to create various complex optical and photonic structures. Here, we present a brief overview of liquid crystal colloidal structures, as recently achieved by numerical modeling and experiments. Central to the structures are complex conformations of topological defects, as they can bind, stabilize, or distort the structure. Using topological and geometrical arguments, we show that the defects can be controllably rewired and imprinted, for example by using optical tweezers. We show that 3D colloidal crystals can be assembled from elastic dipoles of spherical beads in nematic liquid crystals or via inherently inhomogeneous order profiles in bulk and confined cholesteric blue phases. Colloidal crystals are generalized to close-packed colloidal lattices, which we show can serve as natural templates for defect networks. Finally, photonic bands are calculated for selected structures and possible defects in the structure are discussed.     

Sapphire waveguides and fibers for terahertz applications

Sapphire shaped crystals are considered as a favorable material platform of the terahertz (THz) waveguide and fiber optics. Unique physical properties of sapphire, along with advantages of the Edge-defined Film-fed Growth (EFG) technique, yield fabrication of the THz waveguides and fibers with a complex cross-section geometry directly from the Al₂O₃-melt, where no labour-intensive mechanical processing is required. Wide variability of the as-grown sapphire shaped crystal geometries yields different physical mechanisms of electromagnetic waveguidance. In this review, recent advantages in the THz waveguides and fibers based on the EFG-grown sapphire shaped crystals are discussed. While possessing moderate THz-wave absorbtion and quite high dispersion, flexible sapphire fibers with a simple step-index cross-section geometry yield strong confinement of guided modes in a fiber core due to a high refractive index of sapphire in the THz range. This effect opens novel opportunities of sapphire fibers in high-resolution THz imaging, using the principles of either scanning-probe near-field optical microscopy or optical fiber bundles. In turn, antiresonant and photonic crystal hard hollow-core waveguides demonstrate advanced optical performance, along with wide capabilities in THz endoscopy and sensing in harsh environments. This review highlights that the EFG-grown sapphire shaped crystals hold strong potential in different branches of THz optics.     

Study of liquid crystals at the institute of crystallography

The history of studying liquid crystal at the Shubnikov Institute of Crystallography is briefly reviewed. The scientific results of researchers from the institute (starting from the 1960s and leading to the formation of a new field of science: physics of liquid crystals) are presented. In particular, it is shown how the physics of electro-hydrodynamic instability was developed in the 1970s. The origin of the physics of photonic crystals is considered. The discovery of the ferroelectric properties of ultrathin films of polar molecules is described. A breakthrough in the molecular theory of ferroelectricity in liquid crystals in the new century is discussed. The entire abundance of liquid-crystalline phases and processes occurring in them is estimated. An analysis of the participation of researchers from the Institute of Crystallography in the development of liquid-crystal science demonstrates their valuable contribution, which offers perspectives of new studies in this field.     

光在介电常数正弦调制的一维光子晶体中的传输

本文用平面波展开法和时域有限差分法研究了光在介电常数受正弦函数调制的一维光子晶体传输规律,发现该光子晶体同样具有一般光子晶体的带隙结构.带隙宽度随调制深度的增加而增加,随周期长度的增加而减小.还用时域有限差分法计算得到源频率分别在禁带和通带时电场在介质中的分布,禁带情况下电场在介质中受到抑制,通带情况下不受抑制.说明不论介质的介电常数如何分布,只要满足周期性的条件,介质都会具有一般光子晶体共有的本质特征.     

An alternative method to obtain direct opal photonic crystal structures

We describe the protocol that we have elaborated in order to obtain monosize polystyrene spheres. Starting from these spheres a simple and effective method, based on spin-coating technique, was developed to realize colloidal photonic crystal structures. The process produces compact 3D arrays of polystyrene microspheres (opals) that are organized into crystalline lattices. This process offers the ability to rapidly form 3D photonic crystals using inexpensive instrumentation, which makes it attractive for an array of applications. Process parameters, fabricated structures, and their experimental characterization are presented.     

Particle swarm optimization for the design and characterization of silica-based photonic crystal fiber amplifiers

In recent years, the development of photonic crystal fibers has allowed novel opportunities for enhancing optical amplifier characteristics. In this field, accurate numerical modeling is a significant need to predict the device behavior. Conventional approaches perform this task by using methods which could yield solutions characterized by divergent or unstably convergent algorithms. Global optimization methods can be considered as efficient tools to face this problem.In this paper, the application of particle swarm optimization to perform the design and characterization of photonic crystal fiber amplifiers is proposed. The employment of this technique shows different attractive features. In particular, solutions are found quickly and the implementation of the algorithm does not require complicated evolutionary operators. Numerical results show the effectiveness of the approach for both the design and characterization of a fiber amplifier. In fact, if considered as a design tool, the obtained numerical results are in good accordance with respect to the ones yielded by a conventional approach. If considered as a characterization tool, the algorithm performs a forecasting, allowing to determine parameters, such as homogeneous upconversion coefficients, whose computation could present difficulties when it is obtained via direct or indirect measurements.     

Development of thermally stable tellurite glasses designed for fabrication of microstructured optical fibers

We optimize the composition of tellurite glass for the manufacture of photonic crystal fibers with a large spectrum of transparency. The glasses, synthesized in four and five component (TeO₂-WO₃-Na₂O-Nb₂O₅ and TeO₂-WO₃-PbO-Na₂O-Nb₂O₅) oxide systems with variable contents of WO₃ (5-38 mol%) and PbO (0-18 mol%), are designed and manufactured, and the transmission properties of the obtained glasses for the spectral range of 200 nm-7 μm have been determined. Thermal expansion coefficients and glass characteristic temperatures are determined by the dilatometer and Leitz heat microscope methods. Differential Scanning Calorimetry (DSC) measurements as well as crystallization tests by isothermal heat treatment are used to measure the thermal stability of the glasses and their ability to crystallize. Diffractive X-ray (XRD) measurements are used to determine the crystalline phases of the glass samples and the glasses with the highest resistance to recrystallization during thermal treatment were selected and used for the manufacture of photonic crystal fibers.     

Enhanced light extraction in Er3+ doped SiO2–TiO2 microcavity embedded in one-dimensional photonic crystal

Erbium doped silica–titania one-dimensional photonic crystal microcavities have been analyzed by solving the erbium rate equations and the Maxwell equations by means of an auxiliary differential equation finite difference time domain technique. The interaction time between light and matter is enhanced by the presence of microcavity, used to localize the light inside the PC and allows to achieve a significant gain. The investigation treats the influence of the pump signal power, the device length and the dopant concentration, on the optical amplification in correspondence of the defect resonance. A comparison between the erbium doped photonic crystal microcavities and erbium doped photonic crystal band edge amplifiers is reported as well, which shows a higher compactness of the erbium doped photonic crystal microcavities respect with the erbium doped photonic crystals.     

异质结构光子晶体的制备与带隙特性研究

本文采用多次垂直沉积法制备出了可见光范围的多重异质结构光子晶体,并对其形貌特征和带隙特性进行了分析.扫描电镜图像表明:所制备的异质结构光子晶体排列规整、不同结构间界面明显.光学吸收光谱分析结果表明:异质结构光子晶体与各单一结构光子晶体带隙相比,带隙明显增大,为各单一结构光子晶体带隙的叠加,且与不同结构的叠加顺序无关.     

Fabrication and optical properties of two‐dimensional photonic crystal of ZnO pillars

Two‐dimensional (2D) photonic crystal of ZnO pillars was synthesized on silicon substrate by the combination of template method and vapor‐phase transport method. The microstructure and morphology of the ZnO photonic crystal was evaluated by using scanning electron microscope (SEM) and X‐ray diffraction (XRD). Large‐area specular reflectance measurements showed the presence of photonic stop band. The effect of the photonic band gap and the special structure on the photoluminescence (PL) properties of ZnO photonic crystal has been investigated. Both suppression and enhancement in the PL were observed. Raman scattering analyses demonstrated that the defect of ZnO photonic crystal exists in this experiment. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

一维光子晶体结构参数的随机扰动对其光学特性的影响

用特征矩阵法研究结构参数存在随机扰动的情况下一维光子晶体的光学特性, 无论是加工过程中介质层几何厚度的误差, 还是介质层折射率的随机波动都会影响一维光子晶体的光学特性.随机扰动对一维光子晶体带结构高频部分影响较大, 造成带结构消失, 甚至全部变成禁带; 随机扰动对光子晶体缺陷模式的影响是使缺陷模的位置发生随机平移, 平移的程度与随机度有关, 介质层折射率的随机变化要比介质层厚度的随机变化对缺陷模位置的影响要大; 周期数目的增加可以部分地减小缺陷模的平移, 但同时会使缺陷模透射率减小,增加缺陷层厚度可以有效降低随机扰动对光子晶体缺陷模式的影响. 本文的研究将对一维光子晶体的设计工作提供有价值的参考.     

二维异质结光子晶体弯曲波导的传输特性研究

应用时域有限差分(FDTD)方法,通过数值模拟研究光在二维异质结光子晶体L型弯曲波导的传输特性.结果表明:在异质结光子晶体波导中能更好地对光进行控制;并且异质结光子晶体L型弯曲波导的传输效率较同质结光子晶体波导提高了6;~7;.计算结果为高度集成化的光子晶体波导器件,提供了一条新的设计思路和依据.     

Microscopic modeling of the optical properties of semiconductor nanostructures

A brief overview of a consistent microscopic approach to model the optical and electronic properties of semiconductor nanostructures is presented. Coupled semiconductor Bloch and Maxwell equations are used to investigate the performance of semiconductor microcavity structures, photonic band gap systems, and lasers. The predictive potential of the microscopic theory is demonstrated for several examples of practical importance. Optical gain and output characteristics are computed for modern vertical external cavity surface emitting laser structures. It is shown how design flexibilities can be used to optimize the device performance. Nanostructures are proposed where semiconductor quantum wells are embedded in one-dimensional photonic crystals. For field modes spectrally below the photonic band edge it is shown that the optical gain and absorption can be enhanced by more than one order of magnitude over the value of the homogeneous medium. The increased gain can be used for laser action by placing quantum wells and a suitably designed photonic crystal structure inside a microcavity.     

反蛋白石结构光子晶体制备技术

光子晶体是一种具有光子带隙的新型材料,由于其可以控制和抑制光子运动的特性,在光通讯领域具有广阔的应用前景.反蛋白石结构是光子晶体一种重要的结构,由于其制备方法简便、成本低廉而受到人们的普遍关注.本文在介绍目前常用的几种制备光子晶体技术的基础上,详细阐述了制备反蛋白石结构光子晶体的各种技术和方法、以及利用这些制备技术和方法在反蛋白石结构光子晶体上制备一维和二维缺陷的最新进展.     

多孔氧化铝/氧化锌组装体的光子带隙和发光性能

从光子晶体的角度研究了AAO/ZnO组装体的光子带隙和发光性能,并解释了该组装体中ZnO发光增强的原因.利用平面波法对AAO及AAO/ZnO组装体的带隙结构进行了研究,得到了该组装体在可见光范围内TE波和TM波的不同的禁带结构特征,讨论了晶格常数、填充比及填充介质的介电常数对带隙结构和禁戒波长的影响,找到了使AAO/ZnO组装体产生与ZnO的缺陷发光峰匹配的完全光子禁带的条件.