Recent progress in chiral photonic band‐gap liquid crystals for laser applications

This article describes a brief review of recent research advances in chiral liquid crystals (CLCs) for laser applications. The CLC molecules have an intrinsic capability to spontaneously organize supramolecular helical assemblages consisting of liquid crystalline layers through their helical twisting power. Such CLC supramolecular helical structures can be regarded as one‐dimensional photonic crystals (PhCs). Owing to their supramolecular helical structures, the CLCs show negative birefringence along the helical axis. Selective reflection of circularly polarized light is the most unique and important optical property in order to generate internal distributed feedback effect for optically‐excited laser emission. When a fluorescent dye is embedded in the CLC medium, optical excitation gives rise to stimulated laser emission peak(s) at the band edge(s) and/or within the CLC selective reflection. Furthermore, the optically‐excited laser emission peaks can be controlled by external stimuli through the self‐organization of CLC molecules. This review introduces the research background of CLCs carried out on the PhC realm, and highlights intriguing precedents of various CLC materials for laser applications. It would be greatly advantageous to fabricate active CLC laser devices by controlling the supramolecular helical structures. Taking account of the peculiar features, we can envisage that a wide variety of supramolecular helical structures of CLC materials will play leading roles in next‐generation optoelectronic molecular devices. © 2010 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.201000013     

Anchoring effects on the electrically controlled optical band gap in twisted photonic liquid crystals

We study a one-dimensional twisted photonic liquid crystal (TPLC), consisting of various nematic liquid crystal cells adopting a twisted configuration intercalated by isotropic dielectric layers, submitted to a dc electric field (E^dc ) aligned along the periodicity axis. We write the corresponding Euler–Lagrange equations describing the nematic layer configuration. By assuming arbitrary anchoring quasi-planar boundary conditions, we calculate the equilibrium textures for the nematic, parametrized by the two types of strength of its interaction (polar and azimuthal) with the plane walls. We write the electromagnetic equations in a 4?×?4 matrix representation and using the transfer matrix formalism, we obtain the transmittance and reflectance coefficients for normal incidence as functions of the external electric field and anchoring strengths. We have observed a remarkable dependence of the electric field on the transmission and reflection spectra in opening and closing band gaps.     

功能型聚合物光子晶体的制备及应用

光子晶体因其特殊的光调控性能,在各类高性能光学器件方面具有重要的应用前景.本文主要阐述了功能型聚合物光子晶体的制备方法及其在防护涂层、高效发光、高灵敏检测和高性能光信息存储等方面的应用.     

Opal and inverse opal photonic crystals: Fabrication and characterization

Three-dimensional photonic crystals made of close-packed polymethylmethacrylate (PMMA) spheres or air spheres in silica, titania and ceria matrices have been fabricated and characterized using SEM, XRD, Raman spectroscopy and UV–Vis transmittance measurements. The PMMA colloidal crystals (opals) were grown by self-assembly from aqueous suspensions of monodisperse PMMA spheres with diameters between 280 and 415 nm. SEM confirmed the PMMA spheres crystallized uniformly in a face-centred cubic (fcc) array, and UV–Vis measurements show that the colloidal crystals possess pseudo photonic band gaps in the visible and near-IR regions. Inverse opals were prepared by depositing silica (SiO₂), titania (TiO₂) or ceria (CeO₂) in the voids of the PMMA colloidal crystals using sol-gel procedures, then calcining the resulting structure at 550 °C to remove the polymer template. The resulting macroporous materials showed fcc ordering of air spheres separated by thin frameworks of amorphous silica, nanocrystalline titania or nanocrystalline ceria particles, respectively. Optical measurements confirmed the photonic nature of the inverse opal arrays. UV–Vis data collected for the opals and inverse opals obeyed a modified Bragg’s law expression that considers both diffraction and refraction of light by the photonic crystal architectures. The versatility of the colloidal crystal template approach for the fabrication of macroporous oxide structures is demonstrated.     

Fabrication of 3D polymeric photonic arrays and related applications

This review article describes the state-of-art methodologies, mainly self-assembly routes, which are in practice to fabricate photonic crystals (PCs) for advanced applications. The self-assembly of colloidal building blocks is an effective, affordable, and tunable approach to fabricate varieties of photonic materials of desired shapes and surface areas. Because of easy fabrication and controlled performance factors, PCs emerged as a potential platform for designing and developing optical devices with desired features such as photonic bandgap, high reflectance/transmittance, low loss, and lasing in the visible range of wavelengths. To develop next-generation optoelectronics and optical system, significant efforts are being made to explore novel and cost-effective fabrication methods to design and develop 3D-PCs platform, which is covered in this mini-review. The challenges, potential alternatives, and prospects of self-assembled 3D PCs are also discussed in this review.     

光子晶体的制备与应用进展

光子晶体概念的提出到现在已有20多年,由于其特有的带隙和缺陷结构可实现对光子传播的控制,与传统的基于半导体晶体的电子技术相比更具优势,是未来光子计算机和光子通讯技术研发的核心。近年来,基于新材料、新方法制备的具有复杂结构的光子晶体不断涌现,其应用范围也从较早的波导、光开关、反射镜拓展到传感器、超棱镜、微透镜、太阳能电池、增强LED发光、光存储、生物芯片、仿生等新兴领域。本文结合了最新的研究状况,探讨了光子晶体的制备与应用进展。     

蛋白石及反蛋白石結構光子晶體

光子晶体是由不同介电常数的材料构成的一种空间周期性结构，它能够在特定方向上禁阻、控制和操纵光子的运动。目前，已制备的光子晶体具有几种不同的结构类型，本文主要综述了蛋白石、反蛋白石结构光子晶体的制备方法及其光子带隙的影响因素。     

Anomalous light scattering in photonic cholesteric liquid crystals

ABSTRACT

We have carried out polarisation and angle-resolved measurements of the light scattered from photonic cholesteric liquid crystals. Both in samples doped with laser dyes and in inactive (non-doped) samples we have observed pronounced directional dependences of the scattered light, finding angular ranges where the scattering is greatly enhanced and regions where the effect is almost suppressed. Moreover, the total amount of scattered light has also been found to depend strongly on the polarisation and direction of the incident beam. All the results have been interpreted successfully in terms of a simple expression proposed for the scattering cross section, in which the density of states of the ingoing and outgoing beams plays a major role. The expression would be applicable not only to cholesteric liquid crystals but to any one-dimensional photonic material.     

Temperature-dependent optical band structure and defect mode in a one-dimensional photonic liquid crystal

We analysed the propagation of an electromagnetic wave in a one-dimensional periodic system composed of a finite set of E7 liquid crystal mixture slabs in a twisted configuration alternated by homogeneous and isotropic dielectric layers. For different incident angles of the circularly polarised wave, we studied the optical band structure for reflectance and transmittance considering that the dielectric matrix of the device depends on temperature and wavelength. We demonstrated that the position of the band can be moved from visible to short-infrared spectrum region by increasing the thickness of the layers. We found that for a fixed incident angle, the band spectrum shifts towards the short-wavelength region as the temperature gets increasing, whereas, for a constant temperature, such a spectrum moves towards larger frequencies as the incident angle increases. We show that when one of the homogeneous and isotropic slabs has a different size compared with the remaining ones, a defect mode is induced in the band structure whose frequency can be thermally tuned.     

Fabrication and characterization of core-shell photonic crystals via a dipping process

High-quality polystyrene (PS) colloidal photonic crystals in large area were fabricated in 24 h via a capillary-enhanced process. Then, the photonic crystals with core-shell structure were obtained by incorporating silica nanoparticles into the interstitial space of opal template via a dipping process. The filling ratio (V_silica) of interstitial space could be manipulated by dipping colloidal crystals into suspensions with different concentrations of silica nanoparticles, which in turn renders the obtained core-shell photonic crystals. The absorptive peak of opal without dipping process is at 445 nm as measured by UV–vis spectrometry. The filling ratios of 0.130, 0.167 and 0.253 can be calculated according to the modified Bragg's Law, which corresponds to the absorptive peaks for core-shell opals at 453, 463 and 469 nm obtained from suspensions with silica nanoparticles of 0.017, 0.122, and 0.244 wt%, respectively. Therefore, by using this dipping process, the characteristic absorption wavelength for photonic crystal will be varied easily, efficiently and cost effectively than that by traditional methods for constructing opal from monodispersed colloids of different diameters.     

Effects of polymer network on electrically induced reflection band broadening of cholesteric liquid crystals

The reflection band of polymer stabilized cholesteric liquid crystals with negative dielectric anisotropy can be broadened by DC electric fields, which was ascribed to the pitch gradient caused by the motion of the structural chirality, that is, the polymer network. They systematically varied the mixture components, such as the photo‐initiator concentration, the monomer functionality, and the chiral dopant, to explore their influences on the reflection band broadening behavior. They learned how to control the polymer network morphology and ion density, which in turn determined the reflection bandwidth. By optimizing the mixture, they have greatly enhanced the broadening effect and achieved large bandwidth at low voltages. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 835–846     

Focused ion beam micromachining and computation of polymeric photonic band gap

This article introduces a new approach to the achievement of low cost devices for optical telecommunications. For this purpose, the implementation of polymer components and of the WDM method show several advantages. A polymeric photonic band gap lattice can effectively assume demultiplexing at the wavelength of 1.55 μm without energy dissipation. First, the computation of the geometrical parameters designing a polymer photonic crystal by means of the finite‐difference‐time‐domain method, implemented with Bloch functions, is presented here. Second, the achievement of 2D polymer lattices by a dry process, the focused ion beam milling, is exposed. The experimental conditions for the achievement of a master are discussed on the basis of the milling performances and of the polymers physical properties. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2993–3002, 2007     

Enhancement of the optical absorption in cholesteric liquid crystals due to photonic effects: an experimental study

An anomalous strong optical absorption was measured in a cholesteric liquid crystal (CLC) at both edges of its photonic band gap. The experiment was carried out by studying the luminescence generated by the CLC sample doped with a small amount of fluorescent dye. The material was excited with monochromatic light at different angles of incidence and polarisations. Clear peaks were found in the luminescence response at angles for which the pumping wavelength coincides with the positions of the gap edges. The effect is especially noticeable for excitation under circularly polarised light of the same handedness as that of the CLC helix, and it is the highest at the long-wavelength edge. The modification of the absorption is originated by the helicoidal (photonic) structure of the material, which drastically influences the propagation of electromagnetic waves at certain frequencies and polarisations. The results were analysed numerically using an extension of the Berreman method that incorporates absorption effects. Good agreement with the experiment was found.     

Transmission and filtering in photonic circuits: effects of absorption and amplification

We study the effect of absorption and amplification on transmission and filtering in one-dimensional dielectric photonic structures made of dangling side branches grafted periodically along an infinite monomode waveguide with defective branches. The loss and gain are introduced by adding an imaginary part ″ to the dielectric constant. We show that for reasonable values of ″, loss and gain affect essentially the transmission at the frequencies of localized modes associated with the defect branch introduced into the periodic structure. The amplitude and the phase of the transmission and reflection coefficients associated with defect modes are discussed as function of ″ and the size of the structure. The properties of the defect modes in presence of loss and gain can be used as an on/off switching device in a demultiplexer made of the above photonic structures.     

Recent advances in fabrication of monolayer colloidal crystals and their inverse replicas

Monolayer colloidal crystals(MCCs)are two-dimensional(2D)colloidal crystals consisting of a monolayer of monodisperse colloidal particles arrayed with a 2D periodic order.In recent years,MCCs have attracted intensive interest because they can act as 2D photonic crystals and be used as versatile templates for fabrication of various 2D nanostructure arrays.In this review,we provide an overview of the recent progress in the controllable fabrication of MCCs and their inverse replicas.First,some newly-developed methods for the self-assembly of MCCs based on different strategies including interfacial assembly and convective assembly are introduced.Second,some representative novel methods regarding the fabrication of various functional2D inverse replicas of MCCs,such as 2D arrays of nanobowls,nanocaps,and hollow spheres,as well as 2D monolayer inverse opals(MIOs),are described.In addition,the potential applications of MCCs and their inverse replicas are discussed.     

Recent advances in short peptide self-assembly: from rational design to novel applications

Owing to their structural simplicity and robust self-assembled nanostructures, short peptides prove to be an ideal system to explore the physical processes of self-assembly, hydrogels, semi-flexible polymers, quenched disorder, and reptation. Rational design in peptide sequences facilitates cost-effective manufacturing, but the huge number of possible peptides has imposed obstacles for their characterization to establish functional connections to the primary, secondary, and tertiary structures. This review aims to cover recent advances in the self-assembly of designed short peptides, with a focus on physical driving forces, design rules, characterization methods, and exemplar applications. Super-resolution microscopy in combination with modern image analysis have been applied to quantify the structure and dynamics of peptide hydrogels, while small-angle neutron scattering and solid-state nuclear magnetic resonance continue to provide valuable information on structures over complementary length scales. Short peptides are attractive in biomedicine and nanotechnology, e.g., as antimicrobials, anticancer agents, vehicles for controlled drug release, peptide bioelectronics, and responsive cell culture materials.     

Recent advances of hexaazatriphenylene (HAT) derivatives: Their applications in self-assembly and porous organic materials

As a rigid and planar aza-based heteroaromatic scaffold, hexaazatriphenylene (HAT) exhibits excellent electron-deficient property and high π-π stacking tendency, which makes it an ideal building block in the construction of supramolecular architectures and functional materials. In addition, HATs have also been picked out as building blocks for the construction of novel porous organic polymers, one of the most attractive fields of porous materials in the past decade, which includes intrinsic microporosity (PIMs), π-conjugated microporous polymers (CMPs), and covalent organic frameworks (COFs). In this digest paper, the synthetic methods of HAT derivatives have been briefly introduced and some recent advances of HATs in the applications of supramolecular self-assembly and porous organic materials have been highlighted.     

Optical and physical properties of iridescent photonic crystals obtained by self‐assembled polymethyl methacrylate nanospheres within graphene oxide nanoplatelets

Opal photonic crystals prepared by vertical templating of polymethyl methacrylate (PMMA) nanospheres in aqueous graphene oxide (GO) solutions were successfully obtained. The results show that increasing the PMMA nanospheres' size leads to the modification of the d‐spacing in GO nanoplatelets, inducing brilliant iridescence colors that span the entire visible electromagnetic spectrum. Scanning electron microscopy study shows a uniform distribution of GO nanoplatelets on the surface and the bulk of the opal photonic crystals. The reflectance spectra exhibit a significant red shift from 385 to 660 nm when the nanospheres' size increases from 160 to 306 nm, respctively. The Raman spectra show a systematic decrease in the intensity ratio of the D to G bands of GO (I_D/I_G), suggesting a partial reduction of graphene oxide with decrasing the extent of defects in the partially reduced GO nanoplatelets. This finding is confirmed by the significant decrease observed in the intensity of the hydroxyl band in the attenuated total reflectance mode‐Fourier transform infrared spectra of the photonic crystals. The results provide the first demonstrated example of intercalated assemblies of polymer nanospheres within GO nanosheets, leading to photonic crystals with brilliant iridescence colors that span the entire visible electromagnetic spectrum and can be tuned only by varying the size of the PMMA nanospheres.     

Recent advances of lead-free metal halide perovskite single crystals and nanocrystals: synthesis,crystal structure,optical properties,and their diverse applications

Lead halide hybrid perovskites have received massive research attention because of their unique inherent photophysical properties that driven them for potential application in the fields of photovoltaics, light-emitting devices, lasing, X-ray detector, and so on. Perovskite single crystals and nanocrystals are generally synthesized via various low-cost solution-processed techniques. The emergence of simple growth approaches of perovskite structures enable to fabricate low-cost and highly efficient devices. However, toxicity of Pb atoms and instability of perovskite structures obstruct further commercialization of these technologies. Recent efforts have been shifted to discover novel, eco-friendly, and stable lead-free metal halide perovskite (LFHP) materials and exploring their different growth processes for various device applications. This review aims to provide an up-to-date analysis of recent progress report on LFHPs and will mainly focus on their growth processes in the single crystalline and nanocrystalline forms. This review also tries to understand how the perovskite crystal structure impacts on their fundamental properties. In addition, we discuss the current progress in various field of applications and their future aspects.