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.     

金属光子晶体薄膜可见光和微波波段传输性能

一维金属光子晶体薄膜是由金属-介质多层结构组成的等效均匀的各向异性超构材料。相比单层金属膜层,该结构在色散调控方面具有更多的自由度。在该结构中由于表面等离子体激元(SPP)的存在,可实现倏逝波的定向传输。在本文中,等效介质理论、时域有限元差分法(FDTD)的计算结果和实验结果都表明,传输倏逝波的波长、频宽和强度可通过金属光子晶体结构调整实现主动设计。金属膜厚比例越小,传输波长的中心和截止波长越长,频带越宽。当金属膜层厚度小于SPP穿透深度时,可获得宽频段的倏逝波的传输。同时,对金属光子晶体在微波波段的传输性能也进行了研究,发现其在微波波段等效介电常数为负,具有良好的反射性能。该结构的屏蔽效能远大于厚度相近的ITO薄膜的电磁屏蔽效能。在厚度只有几百纳米时,该结构即可实现良好的电磁屏蔽效能。通过金属光子晶体薄膜可实现电磁屏蔽材料的薄膜化、轻质化和可视化。     

Enlarged Photonic Band Gap in Heterostructure of Metallic Photonic and Superconducting Photonic Crystals

We show theoretically that the frequency range of photonic band gap of a hetero-structure which is made of a metallic photonic and superconducting photonic crystal can be enlarged due to the combination of the reflection band properties of the superconductor–dielectric (PC1) and metallic–dielectric (PC2) periodic structures. The transmittance and band structure of the considered structures are calculated using simple transfer matrix method and the Bloch theorem. Beside this, we have also calculated the transmittance of the superconducting photonic structure (PC1), metallic photonic structure (PC2) and heterostructure of metallic photonic and superconductor photonic crystals (PC1/PC2) for TE and TM-mode at the different angles of incidence.     

Investigation of TE-polarized photonic band gap properties in matallodielectric photonic crystals composed of metal coated dielectric cylinders

Using the Dirichlet-to-Neumann map method and generalization of this method, we have been able to calculate the photonic band structure of two-dimensional (2D) metallodielectric photonic crystals composed of metal-coated circular dielectric rods. The rods are embedded in an air background with a square array. We are interested in considering transverse electric (TE) mode of electromagnetic waves. The resulting band structures show the existence of photonic band gaps as well as some flat band regions. We theoretically study the effect of the dielectric constant and radius of the dielectric core on the photonic band structures. There are some interesting results compared to the case of solid metallic rods (without dielectric core) such as appearing the new photonic band gaps and a flat band region with the characteristic of cavity modes.     

Maximization of Photonic Bandgaps in Two-Dimensional Superconductor Photonic Crystals

In this paper, we investigate the properties of photonic band structures in two-dimensional superconductor photonic crystals (2D-SCPCs) using the frequency dependent plane wave expansion method. We consider two types of 2D-SCPCs, which are composed of superconductor (dielectric) rods embedded into a dielectric (superconductor) background, named type I (type II) SCPCs. We target maximization of the gap-to-mid-gap ratio by varying many parameters, namely, shape of the rods, the operating temperature, the permittivity of the dielectric material, and the threshold frequency of the superconductor. We show that the type II SCPCs have a higher gap-to-mid-gap ratio than the type I SCPCs. In addition, the PBGs can be tuned efficiently by the operating temperature. Moreover, the photonic band structures can be tailored by changing the dielectric constant of the background (rods) in the type I (type II) SCPCs.     

Dispersion requirements of low-loss negative refractive index materials and their realisability

A necessary dispersion requirement is given for the effective relative permeability μ_r(ω) and permittivity ϵ_r(ω) as a function of frequency for low-loss homogeneous dispersive materials and negative refractive index materials, in particular, if they exist at some dimensional scale D. The requirements are valid both for isotropic and uniaxial materials, the latter is defined by material properties that are invariant under a rotation about a principal axis. Using asymptotic requirements at zero frequency and Foster?s reactance theorem, the general bandgap structure is given for low-loss materials. It is shown that such materials can only be achieved if the material exhibits finite bandgaps separating regions of positive and/or negative refractive index regions. In degenerate cases, the bandgaps shrink to zero but low-loss propagation remains impossible in the neighbourhood of the critical frequencies. The effect of small losses are considered and compared with Stockman?s criterion. Losses may be arbitrarily small within a band provided there are no bounds to the size of the real parts of the relative permittivity and permeability at the band edges. A numerical example is given showing the effect of small losses.     

Optical properties of graphene based annular photonic crystals

The optical properties of a graphene based annular photonic crystal (APC) are theoretically investigated. The proposed structure is a hollow core cylindrical shell consists of the alternate dielectric layer and graphene monolayer immersed in free space. In order to study the photonic band structures of the APC, we obtained the optical spectra of the graphene based APC by employing the transfer matrix method in the cylindrical waves for both TE and TM polarizations. In this work we study the effect of different geometrical and optical parameters of the structure on the low loss high reflectance graphene induced band gap. It is found that the graphene induced band gap which appeared in the frequency below 10 THz is polarization independent and remains almost invariant with the change in the period number, the radius of the inner core region and the refractive indices of the inner core region and the surrounding medium. However, its width increases by increasing the azimuthal mode number and the chemical potential of the graphene monolayers and decreases by increasing the refractive index and the thickness of the dielectric layers.     

基于1550nm的全固态PBG—PCF的设计

设计了一种三角晶格结构的全内反射型光子晶体光纤,并在其包层孔内分别填充折射率为n=1．55～3．35（△n=0．3）的介电材料,使其等效为全固态光子带隙型光子晶体光纤,利用全矢量平面波展开法对其带隙特性进行分析,发现随着折射率的增加,光子带隙的位置逐渐向长波方向移动,导模也越来越少。设计一种工作波长为1550nm的全固态光子带隙型光子晶体光纤,计算得到其对应的归一化传播常数β=8．2时,导模的宽度大约为100nm。该光纤在光电转换或者电光转换等方面具有潜在的应用价值。     

One‐Dimensional Dielectric/Metallic Hybrid Materials for Photonic Applications

Explorations of 1D nanostructures have led to great progress in the area of nanophotonics in the past decades. Based on either dielectric or metallic materials, a variety of 1D photonic devices have been developed, such as nanolasers, waveguides, optical switches, and routers. What's interesting is that these dielectric systems enjoy low propagation losses and usually possess active optical performance, but they have a diffraction‐limited field confinement. Alternatively, metallic systems can guide light on deep subwavelength scales, but they suffer from high metallic absorption and can work as passive devices only. Thus, the idea to construct a hybrid system that combines the merits of both dielectric and metallic materials was proposed. To date, unprecedented optical properties have been achieved in various 1D hybrid systems, which manifest great potential for functional nanophotonic devices. Here, the focus is on recent advances in 1D dielectric/metallic hybrid systems, with a special emphasis on novel structure design, rational fabrication techniques, unique performance, as well as their wide application in photonic components. Gaining a better understanding of hybrid systems would benefit the design of nanophotonic components aimed at optical information processing.     

Limits on the performance of dispersive thin-film stacks

Dispersive thin-film stacks are interesting as compact, cost-effective devices for temporal dispersion compensation and wavelength multiplexing. Their performance depends on the total group delay or spatial shift that can be achieved. For general multilayer stacks, no analytic model exists relating the performance to the stack parameters such as the refractive indices and the number of layers. We develop an empirical model by designing and analyzing 623 thin-film stacks with constant dispersion. From this analysis we conclude that, for given stack parameters, the maximum constant dispersion value is inversely proportional to the wavelength range over which the dispersion is achieved. This is equivalent to saying that, for constant dispersion, there is a maximum possible spatial shift (or group delay) that can be achieved for a given material system and number of layers. This empirical model is useful to judge the feasibility of dispersive photonic nanostructures and photonic crystal superprism devices and serves as a first step in the search for an analytic performance model. We predict that an 8-channel wavelength multiplexer can be realized with a single 21-microm-thick SiO2-Ta2O5 thin-film stack.     

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.     

The properties of cutoff frequency in two-dimensional superconductor photonic crystals

In this paper, by means of frequency-dependent plane wave expansion method we investigate the properties of photonic band structures in two-dimensional superconductor photonic crystals. Effects of cut-off frequency are investigated by various parameters such as filling factor, the lattice constant alteration, threshold frequency of the superconductor, and shape of the rods as well. We show that the cut-off frequency can be efficiently tuned by the operating temperature. Moreover, it can be tailored by changing the dielectric constant of the background and the threshold frequency of the superconductor material.     

Fano Resonance by Means of the One-Dimensional Superconductor Photonic Crystals

In the present work, we introduce a study about the reflectance properties of a one-dimensional superconductor photonic crystal in a different manner. Our structure is designed from alternately layers of high-temperature superconducting material (BSCCO) and a conventional superconducting material (Nb) terminated by the dielectric cap layer. The investigated numerical results are essentially based on the basics of the transfer matrix method and the two-fluid model. The numerical results showed the appearance of two sharp peaks, which are referred to Fano and electromagnetic-induced reflectance resonances. Also, we demonstrate the effects of many parameters on the reflectance properties of Fano resonance and the electromagnetic-induced reflectance. The effects of the constituent layer thicknesses, the number of periods, the operating temperature, the angle of incidence, and the refractive index of the dielectric cap layer are all considered. Finally, the effects of hydrostatic pressure on the Fano and EIR resonances are studied and calculated. This structure can play an important role in optical switching devices.     

Plasmonic/metallic passive waveguides and waveguide components for photonic dense integrated circuits: a feasibility study based on microwave engineering

To investigate the potential for dense integration of photonic components, we analyse passive plasmonic/ metallic waveguides and waveguide components at optical frequencies by using mostly microwave engineering approaches. Four figures of performance are formulated that are utilised to compare the characteristics of four different slab waveguides with zero frequency cut-off modes. Three of these are metallic based whereas the fourth one, which also serves as a reference, is dielectric based with high index-contrast. It is found that all figures of performance cannot be optimised independently; in particular there is a trade-off between the waveguide Q-value and the transversal field confinement. Microwave methods are used to design several photonic transmission line components. The small Q-value of the metallic waveguides is the main disadvantage when using materials and telecom frequencies of today. Hence plasmonic waveguides do not offer full functionality for some important integrated components, being severe for frequency-selective applications. To achieve a dense integration, it is concluded that new materials are needed that offer Q-values several orders of magnitude higher than metals.     

Tuned switching of surface waves by a liquid crystal cap layer in one-dimensional photonic crystals

In this paper, we theoretically study the electromagnetic surface waves localized at the interface between a homogeneous dielectric medium and a semi-infinite, one-dimensional photonic crystal (1D PC). The semi-infinite 1D PC is made of alternative layers of right-handed (RH) and dispersive left-handed (LH) materials in the presence of a liquid crystal (LC) cap layer. In this structure, we derive the surface waves dispersion relation with tunable switching and localization by using an analytical direct matching procedure within the Kronig-Penny model. It is shown that, for both of layer arrangements, the variation of molecules orientation of the LC cap layer acts as an effective tool to tune the type (tuned switching) and localization of the surface waves and it also can create a surface mode with maximum localization in the first frequency bandgap.     

Planar photonic crystal structure with inherently single-mode waveguides

A planar photonic crystal that allows inherently gap-guided single-mode waveguides is proposed and discussed. This novel structure consists of a two-dimensional lattice of silicon rods embedded on a thin silica slab sandwiched between two silica claddings whose refractive indices are slightly lower than the index of the silica core. The physical parameters of the structure, i.e., rod radius and core thickness, are optimized to maximize the bandgap width for odd modes. Lossless guided modes inside the bandgap and below the claddings' light cone are obtained by reducing the radius of a row of rods. The waveguide bandwidth can be increased by inserting a thin silicon dielectric waveguide instead of the row of rods. The proposed approach may overcome many of the common drawbacks in conventional holes-on-dielectric planar photonic crystal waveguides.     

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.     

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.     

Angle- and Thickness-Dependent Photonic Band Structure in?a?Superconducting Photonic Crystal

The angle- and thickness-dependent photonic band structures in a one-dimensional superconducting photonic crystal are theoretically investigated based on the transfer matrix method. The band structure is studied near and below the threshold frequency at which the superconducting material has a zero permittivity. The gap structure is analyzed as a function of the thicknesses of the two constituent superconducting and dielectric materials. In the angular dependence of the band structure, it is found that in the TM-polarization there exists a strongly localized superpolariton gap in the vicinity of the threshold frequency. This gap is shown to be enhanced as the angle increases.