Distributed feedback lasers in chiral media

The combined effects of chirality and gain (or loss) on wave propagation and coupling in periodic structures is investigated here. The focus is on distributed feedback (DFB) lasers in a transversely unbounded periodic slab with spatially modulated electromagnetic parameters. The analysis uses a coupled-mode approach employing a canonical physical model of chiral materials to predict the effects of modulated chirality admittance on DFB lasers. Results for DFB laser behavior in chiral media are compared and contrasted to that in achiral media. It is found that, under certain circumstances, the electric and magnetic field coupling, which is characteristic of chiral materials, results in a lower threshold gain for DFB lasers in media with a given index of refraction and characteristic impedance. It is also found that chiral index-coupled or gain-coupled DFB lasers exhibit the same spectral mode properties as achiral DFB lasers     

一种光学状态可由电场控制的液晶复合材料的制备与表征

液晶材料对电、热、磁、光等外界物理量的变化具有不同的响应特性,向其中添加具有一定特性的化合物,可以得到具备一定响应特性的液晶复合材料。利用所制备的液晶复合材料对于电场变化具有响应特性的特点,制备出具有信息记录功能的复合材料。合成手性离子液体,按一定配比将其加入手征向列相液晶(N~*-LC)中,得到反射波段可电控的手征向列相液晶/手性离子液体复合材料。实验结果表明:材料初始状态为光透射状态;对材料施加直流电压40V时,样品表现为光散射状态,透过率低于10%;施加高频交流电压40V时,样品表现为半透明镜面反射,反射范围覆盖400～750nm,透射率为45%左右;撤去电场后,可恢复至初始状态,并且每种状态都具有一定的记忆效应。该种液晶复合材料制备简单,无需紫外辐射工艺,且具有电场响应特性,可以通过电场控制在可见光范围内表现出光透射、强烈光散射、半透明镜面反射3种不同状态,具有记忆效应,操作简单方便。     

Radio wave propagation characteristics in lossy circular waveguidessuch as tunnels, mine shafts, and boreholes

We present the characteristics of radio propagation in a circular lossy waveguide whose walls are composed of earth soil materials with frequency-dependent properties. This type of structure is used to represent a radio link for an underground wireless communication channel such as a tunnel, mine shaft, or borehole. We present calculated results of the attenuation constant for various propagation modes in the soil waveguide structure for various soil constituents and moisture levels. Transverse field plots of the various modes for different soil types are also presented. Finally, it is shown that for small |k₂a| (where k₂ is the wavenumber in the soil and a is the radius of the waveguide) some modes in the waveguide disappear and a discussion of this behavior and how it relates to excitation problems is given     

Review on Polarization Selective Terahertz Metamaterials: from Chiral Metamaterials to Stereometamaterials

In this article, recent progress and development of terahertz chiral metamaterials including stereometamaterials are thoroughly reviewed. This review mainly focuses on the fundamental principles of design and arrangement of meta-atoms in metamaterials exhibiting chirality with various asymmetry and symmetry and 2D and 3D configuration. Related optical and propagation properties in chiral metamaterials, such as optical activity, circular dichroism, and negative refraction for each different chiral metamaterials, are compared and investigated. Finally, comparison between chiral metamaterials with stereometamaterials in terms of the polarization selective operation along with the similarity and the distinction is addressed as well.     

Supramolecular Chiral Structures: Smart Polymer Organization Guided by 2D Polarization Light Patterns

Chiral periodic structures and photonic crystals have attracted a great deal of interest due to their applications in advanced photonics. As a consequence, the design and the fabrication of periodic microstructures in photosensitive materials have been widely investigated. Many achievements in the fabrication of such devices have been made over the last decade, but most of the established methods are still restricted to light intensity distribution with different structural shapes and minor attention has been devoted to exploit the vectorial nature of the light coupled with the response of polarization sensitive materials. Here, supramolecular chiral structuring in an amorphous azo‐polymer is demonstrated, coupling the strong and diversified photoresponse of the material with the holographic recording of 2D polarization patterns. The smart polymer organization guided by the complex light field induces periodic chiral microstructures, with spiral‐ or ribbon‐like shape and identical or opposite helicity, characterized by high stability and complete reconfigurability. The holographic structures are theoretically described by means of the Jones matrix method and experimentally investigated, confirming the simultaneous presence of both linear and circular photoinduced anisotropies. These results prove an alternative approach to design a new class of materials with periodic chiral arrangement.     

Assembly of Molecular Building Blocks into Integrated Complex Functional Molecular Systems: Structuring Matter Made to Order

Function‐inspired design of molecular building blocks for their assembly into complex systems has been an objective in engineering nanostructures and materials modulation at nanoscale. This article summarizes recent research and inspiring progress in the design/synthesis of various custom‐made chiral, switchable, and highly responsive molecular building blocks for the construction of diverse covalent/noncovalent assemblies with tailored topologies, properties, and functions. Illustrating the judicious selection of building blocks, orthogonal functionalities, and innate physical/chemical properties that bring diversity and complex functions once reticulated into materials, special focus is given to their assembly into porous crystalline networks such as metal/covalent–organic frameworks (MOFs/COFs), surface‐mounted frameworks (SURMOFs), metal–organic cages/rings (MOCs), cross‐linked polymer gels, porous organic polymers (POPs), and related architectures that find diverse applications in life science and various other functional materials. Smart and stimuli‐responsive or dynamic building blocks, once embedded into materials, can be remotely modulated by external stimuli (light, electrons, chemicals, or mechanical forces) for controlling the structure and properties, thus being applicable for dynamic photochemical and mechanochemical control in constructing new forms of matter made to order. Then, an overview of current challenges, limitations, as well as future research directions and opportunities in this field, are discussed.     

Polyester-based chiral materials for microwave absorption applications

Chiral materials are prepared by randomly embedding metallic helices in the dielectric host matrix of polyester and are studied for microwave absorption applications. An increase in the effective relative permeability of polyester-based chiral materials is reported in this paper. Electromagnetic properties of the said chiral samples are measured, using a free-space measurement technique, in the frequency range of 8–18 GHz. The reflection properties of planar chiral samples backed by a perfect conductor are studied and compared with those of a non-chiral polyester matrix. The results show that the chiral inclusions have considerably increased the effective magnetic permeability and have improved the effective dielectric permittivity of the chiral samples. These increases in effective permeability and effective permittivity of the chiral samples have resulted in enhanced absorption of electromagnetic waves in the chiral material in comparison with its non-chiral counterpart.     

Nonlinear Terahertz Generation: Chiral and Achiral Meta-Atom Coupling

Generation and manipulation of terahertz (THz) waves are of vital importance for advancing THz technology. Nonlinear metasurfaces allow effective integration of both processes into a single compact device. However, such existing THz devices commonly rely on utilizing a single meta-atom, which has fixed THz generation properties and thus limits the range of achievable functionalities. Here, it is demonstrate how coupling between different meta-atoms within the unit-cell can be used as a degree of freedom for controlling nonlinear THz generation, where achiral coupling provides full control over the amplitude of the generated THz field, while chiral coupling makes the THz generation sensitive to the handedness of the pump polarization. In particular, chiral coupling allows the realization of multiplexed pump-handedness-selective nonlinear metasurfaces, which is illustrated experimentally by selectively generating THz beams with different orbital angular momentum with a single nonlinear metasurface. This approach provides opportunities for developing various integrated nonlinear THz devices.     

Selective Excitation of Polarization-Steered Chiral Photoluminescence in Single Plasmonic Nanohelicoids

The development of chiral photoluminescence (PL) has drawn extensive attention owing to its potential applications in optical data storage, biosensing, and displays. Due to the lack of effective synthesis methods, colloidal metal nanostructures with intrinsic chiral PL have rarely been reported. Herein, the chiral excitation and emission properties of single gold nanohelicoids (GNHs) are reported for the first time. By measuring their circular dichroism (CD) response and excitation/emission polarization-resolved PL spectra, it is revealed that the intrinsic chirality arising from the geometric handedness of the GNHs induces the observed excitation-polarization-correlated chiral PL. Two models are developed to analyze the observed circular-polarization-steered effect: (1) a chiral PL phenomenological model quantitatively reproduces the PL dissymmetry features; (2) a chiral Purcell effect model reveals that the super-chiral near fields in the GNHs account for the far-field chiral responses such as the polarization-steered chiral PL. The findings not only provide an important understanding of the physical mechanism responsible for luminescent chiral plasmonic nanostructures, but also expand the research on chiral PL-active materials from achiral/chiral hybrid systems to metallic nanostructures with intrinsic structural chirality, thereby broadening the scope of applications in 3D chiral imaging and sensing as well as microstructure analysis.     

Trend of the development of metal-based heat dissipative materials

Fabrication of high-performance thermal management materials with ultra-high thermal conductivities and low CTEs is important in electronics fields. Recently we have initiated a series of investigations to fabricate such materials and found that metal-matrix composites (MMCs) uniquely fabricated are effective in improving their thermal and strength properties. In our study, to avoid the damage of filler particle surfaces, spark plasma sintering (SPS) processing was used as a processing technique. In this review article, various kinds of filler particle dispersed MMCs are introduced and their thermal properties of composites fabricated by using various methods, such as metal infiltration and vacuum hot pressing, are shown and well documented. This review article leads to strong indications regarding fabrication techniques for making better thermal management composites. Our SPS method is compared in terms of materials qualities to those fabricated by other fabrication techniques.     

A model for the static properties of DH lasers

The various subproblems for DH lasers such as field distribution, carrier profile, and temperature distribution are investigated. Solutions to these problems are obtained either analytically or by precise numerical methods. By combining the subproblems, a detailed model for the static properties is obtained. The model is applicable as well below as above threshold and properties of interest in the application of DH lasers such as threshold current, field distribution at a given current, and light current characteristics can be found. Nonlinear characteristics are found even for ideal symmetrical lasers. These "kinks" are associated with higher order modes and appear at relatively high values of the optical power.     

Approximate boundary conditions for thin chiral layers with curvilinear surfaces

One- and two-sided impedance approximate boundary conditions are obtained for a thin chiral layer with a coordinate surface described in generalized orthogonal curvilinear coordinates. The problem of diffraction of a plane electromagnetic wave by an infinite circular chiral-metallic cylinder is considered as an example illustrating the application of such conditions for calculation of reflecting structures with chiral layers. The main component of the field diffracted by the reflecting structure under study is calculated with the use of the approximate boundary conditions and by means of a rigorous approach, and the results obtained with the help of the two methods are compared.     

A graphical user interface (GUI) for plane-wave scattering from a conducting, dielectric, or chiral sphere

Various numerical techniques have been developed for modeling electromagnetic field propagation in various novel complex media. The validity of these techniques is usually verified by comparison to the exact solutions of canonical problems. Recently, research has focused on chiral media, a subclass of materials known as bianisotropic materials, and numerical techniques have been developed in order to calculate the interaction of electromagnetic fields with chiral objects. One canonical problem for these techniques is plane-wave scattering from a chiral sphere. This work presents a software package that displays and saves the calculated data for the scattering from a chiral, dielectric, or perfectly conducting sphere using a friendly graphical user interface (GUI).     

Controlling Chiral Spin States of a Triangular‐Lattice Magnet by Cooling in a Magnetic Field

Magnetic materials with a non‐collinear and non‐coplanar arrangement of magnetic moments hosting a nonzero scalar spin‐chirality exhibit unique magnetic and spin‐dependent electronic transport properties. The spin chirality often occurs in materials where competing exchange interactions lead to geometrical frustrations between magnetic moments and to a strong coupling between the crystal lattice and the magnetic structure. These characteristics are particularly strong in Mn‐based antiperovskites where the interactions and chirality can be tuned by substitutional modifications of the crystalline lattice. This study presents evidence for the formation of two unequal chiral spin states in magnetically ordered Mn_3.338Ni_0.651N antiperovskite based on density functional theory calculations and supported by magnetization measurements after cooling in a magnetic field. The existence of two scalar spin‐chiralities of opposite sign and different magnitude is demonstrated by a vertical shift of the magnetic‐field dependent magnetization and Hall effect at low fields and from an asymmetrical magnetoresistivity when the applied magnetic field is oriented parallel or antiparallel to the direction of the cooling field. This opens up the possibility of manipulating the spin chirality for potential use in the emerging field of chiral spintronics.     

Emerging Homochiral Porous Materials for Enantiomer Separation

Efficient resolution of racemates of chiral molecules is of great significance in the pharmaceutical, agrochemical, fragrances, and food additives industries. Emerging homochiral porous materials such as metal–organic frameworks, covalent-organic frameworks, porous-organic cages, and metal-organic cages with ultrahigh surface area, controllable pore chemistry and ample chiral recognition sites are promising for efficient chiral resolution, which display excellent properties for chiral separation applications. This review summarizes the design and synthesis strategies for the construction of homochiral porous materials, including direct synthesis, post-synthesis, and chiral induction synthesis. Following this, applications of emerging homochiral porous materials, including enantioselective adsorption, chiral chromatography, and membrane-based chiral separation are highlighted. Finally, the challenges in this area are discussed, with future perspectives provided.     

Nanoimprint Meta-Device for Chiral Imaging

The polarization of light is a valuable information channel that has been studied extensively in optical devices. There has been limited progress in developing low-refractive index contrast and large-scale chiral meta-devices that are easy to integrate and mass-produce. In this image, a chiral imaging meta-device with a large area and broadband chirality control is experimentally demonstrated. The centimeter-scale Moiré meta-device is achieved using nanoimprint technology. The Poynting vector and singularity features in the near field and chiral optical response in the far field are discussed. The proposed Moiré meta-devices can achieve circular dichroism (CD) of more than 10%. Further chiral imaging harnessing CD mechanisms are demonstrated, which may lead to significant potential in various fields, including encryption and security, materials science, biochemistry, and medicine.     

Measurement and Analysis of Propagation Mechanisms at 40 GHz: Viability of Site Shielding Forced by Obstacles

Mitigation of interference among adjacent radio systems is a topic of growing interest as the spectrum occupation increases. Site-shielding techniques appear as a method of improving millimeter-wave wireless communication system design, allowing frequency reuse and reducing cochannel interference. The viability of applying such techniques to systems operating in frequency bands around 40 GHz is the aim of this paper. Several propagation mechanisms are experimentally studied, including transmission across building obstacles, depolarization, reflection, and diffraction. The performance of some theoretical models of the different scattering mechanisms has been compared with measurement results. The measuring and processing procedures have also been improved. Values of the dielectric parameters of the materials in this frequency band have been obtained and are given in this paper. The attenuation results indicate that various materials, such as mortar, brick, and concrete walls, that present large values of attenuation in decibels per centimeter, can be used to shield base stations, reducing the frequency reuse distance in radio cellular networks. It can also be concluded that there is a significant diffracted field in the shadow region of brick corners.      

Non-van der Waals 2D Materials for Electrochemical Energy Storage

The development of advanced electrode materials for the next generation of electrochemical energy storage (EES) solutions has attracted profound research attention as a key enabling technology toward decarbonization and electrification of transportation. Since the discovery of graphene's remarkable properties, 2D nanomaterials, derivatives, and heterostructures thereof, have emerged as some of the most promising electrode components in batteries and supercapacitors owing to their unique and tunable physical, chemical, and electronic properties, commonly not observed in their 3D counterparts. This review particularly focuses on recent advances in EES technologies related to 2D crystals originating from non-layered 3D solids (non-van der Waals; nvdW) and their hallmark features pertaining to this field of application. Emphasis is given to the methods and challenges in top-down and bottom-up strategies toward nvdW 2D sheets and their influence on the materials’ features, such as charge transport properties, functionalization, or adsorption dynamics. The exciting advances in nvdW 2D-based electrode materials of different compositions and mechanisms of operation in EES are discussed. Finally, the opportunities and challenges of nvdW 2D systems are highlighted not only in electrochemical energy storage but also in other applications, including spintronics, magnetism, and catalysis.     

Waveguide characterization of chiral material: experiments

This paper is devoted to waveguide experiments on chiral materials: the constitutive parameters are extracted from the measured response of two chiral slabs of different length. Practical aspects of the implementation are discussed. This includes the injection molding technique used to manufacture the chiral material. Preliminary investigations are performed to qualitatively assess the material properties. These are quantitatively confirmed by the extracted constitutive parameters. The measurements are validated by experiments on a reference material     

Controlled Fabrication of 3D Chiral Microwrinkles via Asymmetrical and Biaxial Bucklings

Nano‐ and microsized chiral materials are receiving significant attention because of their unique characteristics, which include chiroptical activities and enantioselective interactions with living materials. However, the realization of chiral morphologies in such small‐scale materials has been an issue because of the complicated fabrication methods and limited material selection. In this study, a facile and reproducible method is developed for fabricating 3D chiral microwrinkles with twisted shapes by asymmetric and biaxial buckling. Soft polydimethylsiloxane (PDMS) substrates are asymmetrically stretched with angled biaxial strains and exposed to UV/ozone to prepare hard silica layers on top of the PDMS substrates to induce microwrinkles. The chiral shapes are controlled by changing the angle (θ) between the two strain axes and the UV/ozone exposure times (t₁, t₂) in each stage of buckling. The 3D chiral microwrinkles are shaped like “fusilli pasta” and occupy an area of 1.5 cm × 1.5 cm. The patterned area can be easily scaled, and no innate chiral biomaterial is necessary. This method could be widely extended to the fabrication of diverse types of chiral materials for advanced optical and bio‐applications.