Highly engineered mesoporous structures for optical processing

Arranging periodic, or quasi-periodic, regions of differing refractive index in one, two, or three dimensions can form a unique class of mesoporous structures. These structures are generally known as photonic crystals, or photonic quasicrystals, and they are the optical analogue of semiconducting materials. Whereas a semiconductor's band structure arises from the interaction of electron or hole waves with an arrangement of ion cores, the photonic crystal band structure results from the interaction of light waves with an arrangement of regions of differing refractive index.What makes photonic crystals highly attractive to the optical engineer is that we can actually place the regions of differing refractive index in a pattern specifically tailored to produce a given optical function, such as an extremely high dispersion, for example. That is, we can define the geometrical arrangement of the dielectric foam to provide us with the form of band structure we require for our optical functionality.In this paper, the optical properties and applications of these highly engineered mesoporous dielectrics will be discussed.     

Bioinspired Metamaterials: Multibands Electromagnetic Wave Adaptability and Hydrophobic Characteristics

Although various photonic devices inspired by natural materials have been developed, there is no research focusing on multibands adaptability, which is not conducive to the advancement of materials science. Herein, inspired by the moth eye surface model, state‐of‐the‐art hierarchical metamaterials (MMs) used as tunable devices in multispectral electromagnetic‐waves (EMWs) frequency range, from microwave to ultraviolet (UV), are designed and prepared. Experimentally, the robust broad bandwidth of microwave absorption greater than 90% (reflection loss (RL) < ?10 dB) covering almost entire X and Ku bands (8.04–17.88 GHz) under a deep sub‐wavelength thickness (1 mm) is demonstrated. The infrared emissivity is reduced and does not affect the microwave absorption simultaneously, further realizing anti‐reflection and camouflage via the strong visible light scattering by the microstructure, and can prevent degradation by reducing the transmittance to less than 10% over the whole near UV band, as well as having hydrophobic abilities. The mechanism explored via simulation model is that topological effects are found in the bio‐structure. This discovery points to a pathway for using natural models to overcome physical limits of MMs and has promising prospect in novel photonic materials.     

Optical negative refraction by four-wave mixing in thin metallic nanostructures

The law of refraction first derived by Snellius and later introduced as the Huygens-Fermat principle, states that the incidence and refracted angles of a light wave at the interface of two different materials are related to the ratio of the refractive indices in each medium. Whereas all natural materials have a positive refractive index and therefore exhibit refraction in the positive direction, artificially engineered negative index metamaterials have been shown capable of bending light waves negatively. Such a negative refractive index is the key to achieving a perfect lens that is capable of imaging well below the diffraction limit. However, negative index metamaterials are typically lossy, narrow band, and require complicated fabrication processes. Recently, an alternative approach to obtain negative refraction from a very thin nonlinear film has been proposed and experimentally demonstrated in the microwave region. However, such approaches use phase conjugation, which makes optical implementations difficult. Here, we report a simple but different scheme to demonstrate experimentally nonlinear negative refraction at optical frequencies using four-wave mixing in nanostructured metal films. The refractive index can be designed at will by simply tuning the wavelengths of the interacting waves, which could have potential impact on many important applications, such as superlens imaging.     

Hot-wire photonics: materials, science, and technology

The prospect of an integrated photonic technology has fueled an effort to understand the optical properties and to gauge the photonic engineering potential of hydrogenated amorphous silicon-based materials. Of particular interest for photonic engineering is the tunable range of the refractive index in amorphous silicon and the fast and slow light induced optical changes. The advance of photonic-engineered amorphous silicon technology requires an investigation into the relationships among fabrication processes, material properties, and the interrelations among the various optically important parameters. Here, the experimental investigation into H-implant refractive engineered amorphous silicon materials is detailed. Interestingly, the H-implant can interact with the amorphous structure to produce compacting of the structure, which may indicate refractive index increase. In addition, the evolving prospects for an amorphous silicon-based photonic technology will be up-dated. Waveguide-based light valve structures for the further scientific investigation of light induced refractive index change in amorphous silicon and technological applications are described.     

可调谐手征超表面电磁特性研究进展

手征超表面是由具有特定电磁响应的平面手征单元结构构成的超薄超材料,由于其具有自由控制电磁波的奇异能力而引起了极大的关注.通过在超表面设计中加入可调谐材料,可以实现其功能受外部激发控制的可调谐或可重构的超器件,为动态调谐电磁波开辟了新的道路.本文介绍了可调/可重构手征超表面电磁特性的一些理论基础,当线偏振光进入可调谐手征...     

Shine Bright Like a Diamond: New Light on an Old Polymeric Semiconductor

Brilliance usually refers to the light reflected by the facets of a gemstone such as diamond due to its high refractive index. Nowadays, high-refractive-index materials find application in many optical and photonic devices and are mostly of inorganic nature. However, these materials are usually obtained by toxic or expensive production processes. Herein, the synthesis of a thin-film organic semiconductor, namely, polymeric carbon nitride, by thermal chemical vapor deposition is presented. Among polymers, this organic material combines the highest intrinsic refractive index reported so far with high transparency in the visible spectrum, even reaching the range of diamond. Eventually, the herein presented deposition of high quality thin films and their optical characteristics open the way for numerous new applications and devices in optics, photonics, and beyond based on organic materials.     

Normal-incidence reflection spectra of the (111) photonic opal surface in the regions of the first and second photonic band gaps

We have studied the reflection spectra of opal photonic crystals with air-or ethanol-filled pores at different diameters of the silica spheres. An experimental technique has been proposed which enables identification of both the first and second photonic band gaps in the reflection spectrum of opal. The ability to observe the second band gap allowed us to derive a dispersion relation for the refractive index of the infiltrated substance. The calculations were performed using a model for a one-dimensional periodic layered medium with two refractive indices. We obtained ω(k) dispersion curves for electromagnetic waves in a photonic crystal (at normal incidence). The ω(k) dispersion law was used to find a dispersion relation for the reflectance of the photonic crystal.     

Focusing concave lens using photonic crystals with magnetic materials

The guided modes lying in the upper gap-edge band in the photonic band structure of photonic crystals have negative values of refractive index. This feature generates many interesting optical phenomena, and some spectacular photonic devices such as focusing slabs have been developed. We report the design of a photonic-crystal, planoconcave lens for focusing incident parallel light, and theoretically analyze the chromatic aberrations for TM and TE modes. In addition to dielectric photonic crystals, the chromatic aberration of a magnetic photonic-crystal planoconcave lens was investigated because the magnetic permeability may also contribute to the periodic index contrast in photonic crystals, especially at long wavelengths. A significant difference was found in the chromatic aberration for a TM mode propagating in a dielectric than in a magnetic photonic-crystal planoconcave lens.     

Electromagnetic analysis of ultra-refraction and negative refraction

Abstract

From the notion of dispersion diagram of a photonic crystal, an analytic study of ultra-refraction and negative refraction phenomena is achieved. In these phenomena, a photonic crystal simulates a material having an optical index that is positive and less than unity (ultra-refraction) or negative (negative refraction). The study is made in both harmonic and time-domain regimes in conditions close to experimental ones: a beam or a light pulse illuminates a semi-infinite photonic crystal. The form of the transmitted light as well as its phase and group velocities are described analytically. Special attention is paid to frequencies very close to the edge of a gap, where an effective index independent of the incidence angle can be defined. The present analysis extends to the case of the so-called ‘left-handed materials’ recently reported in the literature.     

Negative refraction in semiconductor metamaterials

An optical metamaterial is a composite in which subwavelength features, rather than the constituent materials, control the macroscopic electromagnetic properties of the material. Recently, properly designed metamaterials have garnered much interest because of their unusual interaction with electromagnetic waves. Whereas nature seems to have limits on the type of materials that exist, newly invented metamaterials are not bound by such constraints. These newly accessible electromagnetic properties make these materials an excellent platform for demonstrating unusual optical phenomena and unique applications such as subwavelength imaging and planar lens design. 'Negative-index materials', as first proposed, required the permittivity, epsilon, and permeability, mu, to be simultaneously less than zero, but such materials face limitations. Here, we demonstrate a comparatively low-loss, three-dimensional, all-semiconductor metamaterial that exhibits negative refraction for all incidence angles in the long-wave infrared region and requires only an anisotropic dielectric function with a single resonance. Using reflection and transmission measurements and a comprehensive model of the material, we demonstrate that our material exhibits negative refraction. This is furthermore confirmed through a straightforward beam optics experiment. This work will influence future metamaterial designs and their incorporation into optical semiconductor devices.     

Chemically Responsive Elastomers Exhibiting Unity‐Order Refractive Index Modulation

Chameleons are masters of light, expertly changing their color, pattern, and reflectivity in response to their environment. Engineered materials that share this tunability can be transformative, enabling active camouflage, tunable holograms, and novel colorimetric medical sensors. While progress has been made in creating artificial chameleon skin, existing schemes often require external power, are not continuously tunable, and may prove too stiff or bulky for applications. Here, a chemically tunable, large‐area metamaterial is demonstrated that accesses a wide range of colors and refractive indices. An ordered monolayer of nanoresonators is fabricated, then its optical response is dynamically tuned by infiltrating its polymer substrate with solvents. The material shows a strong magnetic response with a dependence on resonator spacing that leads to a highly tunable effective permittivity, permeability, and refractive index spanning negative and positive values. The unity‐order index tuning exceeds that of traditional electro‐optic and photochromic materials and is robust to cycling, providing a path toward programmable optical elements and responsive light routing.     

基于超材料的无标记光学生物传感

超材料(metamaterials)因为能够在亚波长尺度范围内精细调控电磁波而受到人们广泛关注。超材料具有丰富的电磁模态,在表面支持高度局域场增强且对周围介电环境极其敏感,可应用于无标记光学生物传感领域。与传统光学生物传感器相比,超材料生物传感器具有小型化、集成化、高度灵敏、多功能可定制等突出优点。本文总结了近年来超材料生物传感器在可见光、近红外、中红外以及太赫兹波段的研究进展,包括折射率生物传感、表面增强拉曼散射、表面增强红外吸收和太赫兹生物传感等。     

Spectrometric determination of the refractive index of optical wave guiding materials used in lab-on-a-chip applications

The design and optimization of light-based analytical devices often require optical characterization of materials involved in their construction. With the aim of benefiting lab-on-a-chip applications, a transmission spectrometric method for determining refractive indices, n, of transparent solids is presented here. Angular dependence of the reflection coefficient between material-air interfaces constitutes the basis of the procedure. Firstly, the method is studied via simulation, using a theoretical algorithm that describes the light propagation through the sample slide, to assess the potentially attainable accuracy. Simulations also serve to specify the angles at which measurements should be taken. Secondly, a visible light source and an optical fiber spectrometer are used to perform measurements on three commonly used materials in optical lab-on-a-chip devices. A nonlinear regression subroutine fits experimental data to the proposed theoretical model and is used to obtain n. Because the attainable precision using this method of refractive index determination is dictated by the uncertainty in the transmission measurements, the precision (with 95% confidence) for mechanically rigid samples, namely glass and poly(methyl methacrylate) (PMMA), is higher than those estimated for the elastomer sample (in-house-molded poly(dimethylsiloxane) (PDMS)). At wavelengths with the highest signal-to-noise ratio for the spectrometer setup, the estimated refractive indices were 1.43+/-0.05 (580 nm) for PDMS, 1.54+/-0.02 (546 nm) for glass, and 1.485+/-0.005 (656 nm) for PMMA. Accurate refractive index estimations with an average precision equal to 0.01 refractive index units (RIU) were obtained for PMMA and glass samples, and an average precision of 0.09 RIU for the PDMS molded slide between 550 and 750 nm was obtained.     

A Lithography‐Free and Field‐Programmable Photonic Metacanvas

The unique correspondence between mathematical operators and photonic elements in wave optics enables quantitative analysis of light manipulation with individual optical devices. Phase‐transition materials are able to provide real‐time reconfigurability of these devices, which would create new optical functionalities via (re)compilation of photonic operators, as those achieved in other fields such as field‐programmable gate arrays (FPGA). Here, by exploiting the hysteretic phase transition of vanadium dioxide, an all‐solid, rewritable metacanvas on which nearly arbitrary photonic devices can be rapidly and repeatedly written and erased is presented. The writing is performed with a low‐power laser and the entire process stays below 90 °C. Using the metacanvas, dynamic manipulation of optical waves is demonstrated for light propagation, polarization, and reconstruction. The metacanvas supports physical (re)compilation of photonic operators akin to that of FPGA, opening up possibilities where photonic elements can be field programmed to deliver complex, system‐level functionalities.     

Dirac cones induced by accidental degeneracy in photonic crystals and zero-refractive-index materials

A zero-refractive-index metamaterial is one in which waves do not experience any spatial phase change, and such a peculiar material has many interesting wave-manipulating properties. These materials can in principle be realized using man-made composites comprising metallic resonators or chiral inclusions, but metallic components have losses that compromise functionality at high frequencies. It would be highly desirable if we could achieve a zero refractive index using dielectrics alone. Here, we show that by employing accidental degeneracy, dielectric photonic crystals can be designed and fabricated that exhibit Dirac cone dispersion at the centre of the Brillouin zone at a finite frequency. In addition to many interesting properties intrinsic to a Dirac cone dispersion, we can use effective medium theory to relate the photonic crystal to a material with effectively zero permittivity and permeability. We then numerically and experimentally demonstrate in the microwave regime that such dielectric photonic crystals with reasonable dielectric constants manipulate waves as if they had near-zero refractive indices at and near the Dirac point frequency.     

Silica nanorod-array films with very low refractive indices

The refractive-index contrast is an important figure of merit for dielectric multilayer structures, optical resonators, and photonic crystals. This represents a strong driving force for novel materials that have refractive indices lower than those of conventional optically transparent materials. Silica nanorod-array dielectric films with unprecedented low refractive indices of 1.08 are demonstrated and shown to have viable optical properties including enhanced reflectivity of a single-pair distributed Bragg reflector.     

Photonic Crystals

Abstract

In this paper, we review the early motivation for photonic crystal research which was derived from the need for a photonic bandgap in quantum optics. This led to a series of experimental and theoretical searches for the elusive photonic bandgap structures: those three-dimensionally periodic dielectric structures which are to photon waves, as semiconductor crystals are to electron waves. We shall describe how the photonic semiconductor can be ‘doped’, producing tiny electromagnetic cavities. Finally, we shall summarize some of the anticipated implications of photonic band structure for quantum electronics and the prospects for the creation of photonic crystals in the optical domain.     

Sequences of exact analytical solutions for plane waves in graded media

We present a new method for building sequences of solvable profiles of the electromagnetic (EM) admittance in lossless isotropic materials with 1D graded permittivity and permeability (in particular profiles of the optical refractive index). These solvable profiles lead to analytical closed-form expressions of the EM fields, for both TE and TM modes. The Property-and-Field Darboux Transformations method, initially developed for heat diffusion modelling, is here transposed to the Maxwell equations in the optical-depth space. Several examples are provided, all stemming from a constant seed potential, which makes them based on elementary functions only. Solvable profiles of increasingly complex shape can be obtained by iterating the process or by assembling highly flexible canonical profiles. Their implementation for modelling optical devices like matching layers, rugate filters, Bragg gratings, chirped mirrors or 1D photonic crystals, offers an exact and cost-effective alternative to the classical approaches.     

Waveguiding Fresnel Lenses: Modelling and Fabrication

Abstract

This paper examines the methods, problems and results of modelling Fresnel lenses, and the progress towards fabricating experimental devices that has resulted from these evaluations. The computer model used ray-tracing through phase-weighted two-dimensional Fresnel lenses overlaid onto optical waveguide structures. Several types of lenses were considered, digital phase shift, analogue and photolithographically optimized forms being selected for particular study. The evaluation was directed to determining the effect on performance of changes in dimensions, selected focal length and materials. Negative form lens structures were included, as were a range of fabrication methodologies, so that substrate anisotropy and overlayer refractive index were necessarily surveyed. The lens fabrication will ultimately employ three main technologies: dilute-melt proton-exchange in LiNbO₃, GaAlAs and Nb₂O₅ layers as the guiding films, with lens overlays in a range of deposited films with refractive indices up to 3·85. We report preliminary experimental results leading towards such lenses. For a typical lens, aberration and optical power transfer efficiency were measured and examined in relation to the expected coupling efficiency of the lens overlay to the optical waveguide (the ‘overlap’ integral).     

Dispersion law in photonic crystals in sinusoidal and quasi-relativistic approximations

ω(k) dispersion curves are calculated for electromagnetic waves in a photonic crystal (at normal incidence). The calculations rely on a model of a one-dimensional periodic layered medium with two refractive indices. The ω(k) dispersion law thus derived is used to find the frequency-dependent reflectance of the photonic crystal. Approximations are proposed that allow the dispersion law ω(k) to be obtained in explicit form.