Transmission properties of frequency selective structures with air gaps

The transmission properties of compound frequency selective structures with dielectric slab and air gaps were investigated by computation and experimentation.Mechanism analyses were also carried out.Results show that the air gaps have a distinct influence on the transmission properties.Resonant frequency of the structure would increase rapidly when the air gap appears.After the gap gets larger to a specific value,generally 1/5 wavelength corresponding to the resonant frequency,the transmission properties would change periodically with the gap thickness.The change of transmission properties in one period has a close relationship with the dielectric thickness.These results provide a new method for designing a bandpass radome of large incidence angle and low loss with the concept of stealth shield radome.     

一维光子晶体光学特性的复平面波展开法研究

应用复平面波展开法对一维光子晶体的光子带隙, 透射特性进行了分析. 通过对色散关系和透射系数的数值计算发现一维光子晶体周期结构个数以及折射率分布对光学晶体透射系数以及光子带隙的影响. 对于含有整数个周期结构的光子晶体有共振点出现在光子帯隙外的频率范围内, 共振点的个数比周期结构个数少1. 带隙倾斜斜率等于折射率的比值. 折射率比值越大, 带隙的范围越大.     

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在理想条件下,为了研究等离子体参数、填充率、入射角度和介质层相对介电常数对一维三元磁化等离子体光子晶体的禁带特性的影响,用由传输矩阵法计算得到的TE波任意角度入射时的左旋极化波(LCP)和右旋极化波(RCP)的透射系数来研究其禁带特性。结果表明,仅增加等离子体碰撞频率不能实现禁带宽度的拓展,改变等离子体频率、填充率和介质层的相对介电常数能实现对禁带宽度和数目的调谐。改变等离子体回旋频率能实现对右旋极化波的禁带的调谐,但对左旋极化波的禁带几乎无影响。入射角度的增大使得禁带低频区域带宽变大,而高频区域带宽则是将先减小再增大。     

Manipulation of resonant tunneling by substrate-induced inhomogeneous energy band gaps in graphene with square superlattice potentials

We investigate the resonant transmission of Dirac electrons through inhomogeneous band gap graphene with square superlattice potentials by transfer matrix method. The effects of the incident angle of the electrons, Fermi energy and substrate-induced Dirac gaps on the transmission are considered. It is found that the Dirac gap of graphene adds another degree of freedom with respect to the incident angle, the Fermi energy and the parameters of periodic superlattice potentials (i.e., the number, width and height of the barriers) for the transmission. In particular, the inhomogeneous Dirac gap induced by staggered substrates can be used to manipulate the transmission. The properties of the conductance and Fano factor at the resonant peaks are found to be affected by the gaps significantly. The results may be helpful for the practical application of graphene-based electronic devices.     

Transmission properties of two-dimensional photonic crystal channel waveguides

We study the transmission properties of straight channel waveguides designed in a two- dimensional (2D) photonic crystal patterned into an AlGaAs heterostructure. 2D dispersion calculations show the existence of small gaps occurring along the dispersion branch of the fundamental mode. We show that their location agree very well with mini-stop bands observed on the transmission spectra.     

TM模式下二维非磁化等离子体光子晶体的禁带调制特性分析

采用时域有限差分法(FDTD)中的分段线性电流密度卷积(PLCDRC)算法研究了TM波入射时二维非磁化等离子体光子晶体的禁带特性.从频域角度分析得到微分高斯脉冲的透射系数,并讨论该光子晶体的介质圆柱的介电常数、晶格常数、介质圆柱半径,周期常数和等离子体参数对其禁带特性的影响.结果表明,增加周期常数和等离子体碰撞频率不会改变禁带宽度,增加介质圆柱的相对介电常数和等离子体频率可以展宽禁带的宽度. 当填充率一定时,减小介质圆柱的半径和晶格常数可以实现禁带的拓展. 关键词： 等离子体 光子晶体 禁带 PLCDRC算法     

Isotropic photonic gaps in a circular photonic crystal

We investigated the optical properties of a circular photonic crystal (CPC) for which the distance between lattices was systematically distributed. The transmission spectra of CPC composed of alumina cylinders were examined in the frequency region from 0 to 20 GHz. We show that photonic gaps are obtained not only in CPCs but also in phase-shifted CPCs. The isotropic photonic gaps are evidenced by changes in the incident angle of a millimeter wave.     

Defect and dope selective of transmission properties of 2D photonic crystal

The effects of the defect and doping on the transmission properties are investigated in two-dimensional photonic crystal (PC) with triangular rods using the finite difference time domain (FDTD) method. The results show that the width and the central position of the photonic band gap (PBG) depend on the thickness of rods with defect or composite-defect, and the refractive index of doped dielectric rods. The transmission properties of composite-defect combined with doping are further investigated. The thinner the nested concentric triangular rod and the smaller the refractive index is, the wider the band gaps is.     

Multiband Terahertz Photonic Band Gaps of Subwavelength Planar Fractals

Optical transmission properties of subwavelength planar fractals in terahertz （THz） frequency regime are studied by means of time-domain spectroscopy. The transmission spectra with multiple pass bands and stop bands are observed. The tunable photonic band gaps are realized by changing the angle between the principle axis of planar fractal and the polarization of THz wave. The possible application of the subwavelength optical component is discussed. We attribute the detected transmittance from subwavelength fractals to localized resonances.     

Al nanogrid electrode for ultraviolet detectors

Optical properties of Al nanogrids of different pitches and gaps were investigated both theoretically and experimentally. Three-dimensional finite-difference time-domain simulation predicted that surface plasmons at the air/Al interface would enhance ultraviolet transmission through the subwavelength gaps of the nanogrid, making it an effective electrode on GaN-based photodetectors to compensate for the lack of transparent electrode and high p-type doping. The predicted transmission enhancement was verified by confocal scanning optical microscopy performed at 365 nm. The quality of the nanogrids fabricated by electron-beam lithography was verified by near-field scanning optical microscopy and scanning electron microscopy. Based on the results, the pitch and gap of the nanogrids can be optimized for the best trade-off between electrical conductivity and optical transmission at different wavelengths. Based on different cutoff wavelengths, the nanogrids can also double as a filter to render photodetectors solar-blind.     

2维光子晶体中的掺杂效应数值研究

 把平面波展开方法(PWM)用于2维光子晶体掺杂情况下透射特性的研究,计算得到了粗细不同的空气柱掺杂、相同半径不同介质柱掺杂、不同柱体形状掺杂情况下2维光子晶体的透射系数与入射光频率的关系曲线。结果表明2维光子晶体的禁带的宽度、位置、透过率与掺杂体半径、掺杂体介电常数、掺杂体柱体几何形状等因素有关,掺杂因素相差越大透射曲线变化越明显,特别是损耗介质的掺入更使得禁带可调要素增加。     

Photonic crystals built on contrast in attenuation in an immersion medium

Optical properties have been studied for a one-dimensional photonic crystal, where the contrast between the impedances of layers is achieved due to the difference not in the real parts but in the imaginary parts of their permittivities. The spectral dependences of the transmission, reflection, and absorption coefficients of a finite-thickness slab of the crystal have been calculated for the case where it is placed in an immersion medium with the same magnitude of the real part of the permittivity as in layers of the photonic crystal. It has been demonstrated that, although these structures do not form band gaps, there are indications (traces) of the gaps and the optical properties in these regions can differ strongly from those in the homogenized case.     

High throughput combinatorial screening of semiconductor materials

This article provides an overview of an advanced combinatorial material discovery platform developed recently for screening semiconductor materials with properties that may have applications ranging from radiation detectors to solar cells. Semiconductor thin-film libraries, each consisting of 256 materials of different composition arranged into a 16×16 matrix, were fabricated using laser-assisted evaporation process along with a combinatorial mechanism to achieve variations. The composition and microstructure of individual materials on each thin-film library were characterized with an integrated scanning micro-beam x-ray fluorescence and diffraction system, while the band gaps were determined by scanning optical reflection and transmission of the libraries. An ultrafast ultraviolet photon-induced charge probe was devised to measure the mobility and lifetime of individual thin-film materials on semiconductor libraries. Selected results on the discovery of semiconductors with desired band gaps and transport properties are illustrated.     

内插扩张室声子晶体管路带隙特性研究

声子晶体管路的带隙特性,可以实现管路系统在特定频率下的噪声控制.利用二维模态匹配法推导出单个内插扩张室元胞的传递矩阵,结合Bloch定理,得到声子晶体管路的能带结构计算方法;验证了二维方法在计算能带结构时的准确性.研究发现,内插扩张室声子晶体管路存在布拉格带隙和局域共振带隙.进一步研究了晶格常数以及内插管长度对能带结构的影响,结果表明,晶格常数主要控制布拉格带隙,而内插管长度对局域共振带隙有较大的影响,并研究了两种参数变化下的带隙耦合.研究结果可以为管路降噪设计提供新的思路.     

Waveguide modes of electromagnetic radiation in hollow-core microstructure and photonic-crystal fibers

The properties of waveguide modes in hollow-core microstructure fibers with two-dimensionally periodic and aperiodic claddings are studied. Hollow fibers with a two-dimensionally periodic cladding support air-guided modes of electromagnetic radiation due to the high reflectivity of the cladding within photonic band gaps. Transmission spectra measured for such modes display isolated maxima, visualizing photonic band gaps of the cladding. The spectrum of modes guided by the fibers of this type can be tuned by changing cladding parameters. The possibility of designing hollow photonic-crystal fibers providing maximum transmission for radiation with a desirable wavelength is demonstrated. Fibers designed to transmit 532-, 633-, and 800-nm radiation have been fabricated and tested. The effect of cladding aperiodicity on the properties of modes guided in the hollow core of a microstructure fiber is examined. Hollow fibers with disordered photonic-crystal claddings are shown to guide localized modes of electromagnetic radiation. Hollow-core photonic-crystal fibers created and investigated in this paper offer new solutions for the transmission of ultrashort pulses of high-power laser radiation, improving the efficiency of nonlinear-optical processes, and fiber-optic delivery of high-fluence laser pulses in technological laser systems.     

Photonic band gap in 1D Photonic crystals with gradient profile of pitch and amplitude of modulation

The optical properties of one-dimensional photonic crystals with graded profile of pitch and that of the amplitude of modulation are studied theoretically. The development method, elaborated by the authors earlier, is applied. We show that there exist photonic band gaps in their transmission spectra, and, moreover, they have much wider bandwidth.     

Effect of stacking faults on the optical properties of inverted opals

Stacking faults appear to be the most common type of defect in inverted opals which are good candidates for photonic crystals with absolute gaps in the visible range of light. In this Letter we present for the first time a systematic study of the effect of stacking faults on the optical properties of self-assembled photonic crystals, by means of large-scale transmittance calculations for macroscopic slabs of inverted opals with randomly distributed stacking faults. We show that frequency gaps, as seen in optical transmission experiments, will in general appear wider in the presence of stacking faults. We attribute the above to Anderson localization of light due to disorder.     

Uncoupled photonic band gaps

We theoretically investigated the symmetry properties of the modes in two-dimensional square lattice photonic crystals in order to study phenomena that would enable new frontiers in the applications of photonic crystals. Using group theory, symmetry analysis of the photonic crystals bands has been done. Particular attention was given to the search for the uncoupled B modes that cannot be excited by the external plane wave because they are symmetry forbidden. The existence of the uncoupled modes enabled to define new physics phenomena: uncoupled photonic band gaps. For the frequency ranges inside the uncoupled photonic band gaps, zero transmission is obtained. Therefore, there are two different types of photonic gaps in the photonic crystals: photonic band gaps and uncoupled photonic band gaps. The appearance of uncoupled photonic band gaps in photonic crystals could at least improve the application of the existing photonic materials and structures or even enable the usage of new ones for devices like waveguides, filters, and lasers.     

Linear periodic and quasiperiodic anisotropic layered media with arbitrary orientation of optic axis—A numerical study

We study numerically the linear properties of periodic and quasiperiodic anisotropic layered media. Each anisotropic slab can have arbitrary orientation of optic axis. We apply the general numerical code to recover the known results for solc filters. We propose novel periodic structures where the location and width of the gaps can be controlled easily. We also study the transmission properties of a Fibonacci sequence of anisotropic layers and show the interesting features like self-similarity and scaling.     

Optical properties of one-dimensional photonic crystals containing graded materials

The optical properties of an one-dimensional photonic crystal containing graded materials are studied theoretically. The graded layers have space dispersive permittivity and magnetic permeability which vary along the direction perpendicular to the surface of the layer. The gradation profiles of permittivity are studied in detail. We show that the structure possesses forbidden band gaps in its transmission spectra and the gradation profiles of permittivity affect the band gaps significantly. For the exponential gradation profile ε₁(x) = α e^βx, the number of the band gaps increases and the total frequency region corresponding to the gaps becomes large with increasing parameter β. On the other hand, the position of band gaps can be changed by the adjustment of the gradation profiles even if possessing same volume-average permittivity in the graded layers. Therefore, we can achieve suitable photonic band gaps by choosing gradation profiles of permittivity.