Design of multilayer high-dispersion mirrors using multi-swarm optimization method

Design of high-dispersion mirrors (HDMs) using a proposed multi-swarm optimization method is reported. We design HDMs for Yb:YAG disk oscillator at 1030 nm and ultrashort pulse Cr:YAG laser at 1550 nm. The results show that the optimum group delay dispersion and reflectance can be obtained with optimal number of layers. The proposed optimization method has a fast convergence rate and powerful global search ability and can be utilized effectively for the design of a variety of optical thin film filters.     

Transmission guided-mode resonance filters based on high reflection multilayer stacks

The transmission guided-mode resonance filters are implemented by integration of diffraction grating into classical thin films to produce high efficiency in the central wavelength and arbitrarily low sideband response over a quite large spectral range. Transmission guided-mode resonance filters require considerable fewer thin films to acquire narrow line width and high peak transmission in the central wavelength compared with classical multilayer high-reflectance coatings with a stack of quarter-wave thickness. The properties of transmission filters with single/double waveguide grating in the different layers of high refection stacks are compared each other. It is demonstrated that the narrow line width transmission filters can be abstained with only two different materials.     

Thin-film spatial filters

A thin-film optical filter used as a one-dimensional spatial filter is presented, and its design is briefly examined. The filter consists of a stack of quarter-wave dielectric layers upon a right-angle prism that selectively cancel a reflected or transmitted plane-wave front for various angles of incidence. Transmittance and reflectance are low-pass functions or high-pass functions of the angle of incidence with a high degree of steepness. In combination, these filters exhibit bandpass transmittance with a variable bandwidth. Applications to detection of extrasolar planets are briefly discussed.     

Modeling Thin Film Multilayer Broad-Band-Pass Filters in Visible Spectrum

Multilayer thin-film structures have been used to model and analyze the performance of broad-band-pass interference optical filters in visible spectrum. Starting from nine-layer configuration comprising quarter-wave layers, the number of layers is increased to make the pass band smooth with steep slopes and square band-pass characteristics. The layer materials are also varied to improve the performance of the filters. Further, matching layers are also introduced with incident medium and the substrate to reduce the magnitude of ripples in the pass band. Shift in the peak wavelength is also investigated as a function of angle of incidence.     

Design of Narrowband Optical Filters Using Binary Number Sequence Photonic Crystals

A theory to design narrow band optical filters by using a new photonic crystal structure is presented. This new photonic crystal structure is composed of low index layers and high index layers arranged in mod. 4 up and down binary number sequence. The new structure exhibits narrow transmission peaks in the forbidden frequency gap region with high optical transmission (greater than 99.98%) at C.W.D.M. (Coarse Wavelength Division Multiplexing) center wavelengths. The proposed filters use only 8 layers. These new binary number sequence photonic crystal narrowband optical filters are much smaller in size, lower in cost and easier to fabricate as compared to narrowband photonic crystal optical filters based on defect Fractal Cantor multilayers, suggested recently by a group of researchers.     

Simulation and fabrication study of porous silicon photonic crystal

The present work reports design and fabrication of porous silicon based one-dimensional (1D) photonic crystal. Distributed Bragg reflector (DBR) is a 1D photonic crystal composed of multilayer stack of high and low refractive index layers. Design of porous silicon DBR is a complex one and requires appropriate control in optical parameters of its constituent layers. In order to design DBR, two porous silicon single layer samples were fabricated using current density of 10 and 50 mA/cm². Optical characterization of single layer samples showed series of interference fringes. Reflective interferometric Fourier transform spectroscopy (RIFTS) method was employed to determine optical constants of porous silicon single layers. DBR simulation was carried out based on transfer matrix method. DBR was then fabricated using optical parameters obtained from RIFTS method. Reflection bandwidth of prepared DBR was found to be 216 nm, which is comparable to the simulated value of 203 nm.     

X波段叠层片式微带铁氧体环行器的设计

采用叠层片式化设计有望实现微波环形器与低温共烧陶瓷(LTCC)技术的结合。借助三维电磁仿真手段设计了一种X波段微带铁氧体环行器,基片采用具有不同饱和磁化强度的旋磁材料构成叠层片式结构。研究结果表明,叠片数目及其饱和磁化强度的搭配对环形器的回波损耗和隔离度特性影响显著,这与叠层结构引入的界面及叠片变化的饱和磁化强度影响了环形器的等效输入电路参数有关。通过优化设计,可以获得具有高回波损耗、高隔离度及低插入损耗等优良性能的器件,但带宽受叠片数目及其饱和磁化强度的搭配影响较小,难以通过优化进行拓展。     

Design of Multi-layer Dielectric Thin-film Polarizing Beam Splitters by Adaptive Simulated Annealing Method

Polarizing beam splitters (PBS’s) are essential components in optical systems such as liquid crystal displays, optical communication and optical recording. Adaptive simulated annealing (ASA) method is introduced here to design cubic PBS’s. A cubic PBS used in visual spectral is shown first, which has the characteristics of broadband, wide angular field. Then a cubic PBS used in communication C-band is illustrated, which has the advantage of large polarization extinction ratio. For the designed PBS’s, both the transmittance and reflectance can be utilized. It can be concluded that the ASA method can not only get significantly reasonable results but also show fast convergent speed and less memory occupation for designing PBS’s thin film filters.     

Optical stack filters using parallel fuzzy logic

Stack filters are more general and robust nonlinear filters than median filter, which possess threshold decomposition and stacking properties. Stack filters can be carried out one step using optical parallel fuzzy logic technique instead of three steps in threshold decomposition. All window, width 3 stack filters but two trivial stack filters are expressed in unified expression with coefficiencies to choose desired stack filter, so they are realized by an optical approach, window 3×3 separable stack filters can be realized by cascading the window width 3 stack filters. Window width 5 stack filters can be realized using the same method.     

Broadband dielectric film transflector for variable transmission optical filters

A broadband multilayer dielectric film transflector for applications in variable transmission optical filters is proposed. The dielectric film is composed of alternating TiO₂ and SiO₂ layers deposited on a substrate. The transmittance of the dielectric film transflector can be varied from 0 to 100% by adjusting the individual dielectric layer thickness and the number of layers. The conjugate gradient method is used to optimize the design of the transflectors. The obtained transflectors have good transmittance and reflectance within the whole visible range and the incident angle range 0-20°. The manufacturing tolerances on film thickness and refractive index are reasonably large.     

基于矢量衍射理论的相位型光瞳滤波器设计

根据矢量衍射理论,提出了一种设计纯相位型光瞳滤波器的新方法根据该方法所设计的滤波器主要应用于采用高数值孔径显微物镜的成像系统对点扩散函数中的指数因子进行二次近似处理后,可以得到超分辨参量与滤波器参量的近似关系式以三区相位型滤波器为例,讨论了该近似关系式的有效性给出一个设计实例,根据所要求的超分辨性能要求设计滤波器.模拟结果表明,该滤波器达到了设计要求,最后利用共焦系统点扩散函数的乘积特性,大大减弱了旁瓣的影响.     

Study on the feasibility of double stack high reflector coating at 355 nm

The feasibility of the idea of double stack HR coating was discussed in this paper both in theory and experiment. The theoretical simulation was made by employing optical coating design software. The analysis results showed that the design of double stack HR coating was feasible, which made the HR coating have ascendancy not only at reflectance but also at laser damage resistance. Then, the LaF₃/MgF₂ HR coating, the HfO₂/SiO₂ HR coating and the double stack HR coating were prepared for comparison, respectively. Transmittance spectra, surface morphologies and damage morphologies of these coatings were measured. Measurement of laser-induced damage threshold (LIDT) of S polarized light of the samples was performed at 355 nm, 45° incidence. The measurement results showed that the LIDT value of the LaF₃/MgF₂ HR coating with 30 layers was very high, but the reflectance was low. When the layer number was increased up to 36, lots of cracks appeared on the surface of the LaF₃/MgF₂ HR coating, with the LIDT badly declining. It was thought that the residual stress resulted in the cracks and the decline of the LIDT. The spectra measurement showed the double stack HR coatings could provide higher reflectance and wider reflection band than LaF₃/MgF₂ HR coating with less layer pairs. Any crack was also not found on the surface of the double stack coatings. Meanwhile, the double stack HR coatings possessed greater laser damage resistances than the HfO₂/SiO₂ HR coating. The damage morphologies showed that the damage of the double stack coating was even milder than that of the HfO₂/SiO₂ coating. Therefore, the double stack design was effective to gain high reflectance and great UV laser radiation resistance simultaneously.     

Narrowband reflection-and-transmission filter in an annular defective photonic crystal containing an ultrathin metallic film

The filtering properties for a narrowband reflection-and-transmission filter in an annular defective photonic crystal containing an ultrathin and strongly lossy metallic film are theoretically investigated based on the transfer matrix method for the cylindrical Bragg waves. At a certain design wavelength, simultaneous peaks in reflectance and transmittance can be found. The peak wavelength is shown to be dependent on the azimuthal mode number of the cylindrical waves. The peak heights in reflectance and transmittance can be directly varied by the stack numbers. In addition, the influence of the starting radius in reflectance and transmittance is also illustrated.     

The ballistic dimer resonance in the one-dimensional disordered photonic crystals

The propagation of electromagnetic waves in one-dimensional disordered dielectric layer stack is studied theoretically using the transfer matrix formalism. The presence of the dimer unit cells inside a host photonic crystal, as the intentionally short range disorder correlation, provides predicted dimer resonances, leading to the break down of the Anderson localization. However while suitably adjusting the intrinsic defect unit cell parameters (i.e. the defect dielectric constants), the light can be transmitted on larger localization length through a ballistic canal, opening up possibilities for performing better tailored ballistic optical filters. Moreover, by increasing the rate of disorder (i.e. the defects concentration and/or the length of the system) the quality of the transmission around the ballistic resonance can be improved with the smoother corresponding allowed mini bands.     

Design of Tunable Devices at Terahertz Frequencies Based on Quasi-Photonic Crystals Incorporated with Graphene

In this study, we present a new theoretical model including Thue-Morse and double-period sequences as quasi-photonic crystals are incorporation with graphene and investigate the transmission properties of the THz waves in both structures using a straightforward computational method. We also consider properties of nonlinear conductivity in addition to surface linear conductivity for graphene. We observe the sharp peaks and proper forbidden bands are created in the range of $0.3$ THz to $30$ THz. In addition, we find that by considering the nonlinear term of graphene and engineering the structural parameters such as the chemical potential of graphene, number of layers and the incidence wave angle, transmission levels of peaks enhance scientifically and quality factor improve considerably. These results show that it would be possible to design of high-Q tunable filters with multi-stop bands in the THz regime which can reduce the noise associated with THz frequency peaks and increase the number of sharp frequency peaks.     

Large range of omni-directional reflection in 1D photonic crystal heterostructures

A simple design of one-dimensional omni-directional reflector based on photonic crystal heterostructures structure has been proposed. The proposed structure consists of a periodic array of alternate layers of SiO₂ and Te as the materials of low and high refractive indices, respectively. The structure considered here has three stacks of periodic structures having five layers each. The lattice period of successive stack is increased by a certain multiple (say gradual constant, δ) of the lattice period of the just preceding stack. For numerical computation, the method of transfer matrix method (TMM) has been employed. It is found that such a structure has wider reflection bands in comparison to a conventional dielectric PC structure and the width of the omni-directional reflection (ODR) bands can be enlarged by increasing the value of the gradual constant δ.     

Spin-dependent resonant tunneling in ZnSe/ZnMnSe heterostructures

Using the transfer matrix method and the effective-mass approximation, the effect of resonant states on spin transport is studied in ZnSe/ZnMnSe/ZnSe/ZnMnSe/ZnSe structures under the influence of both electric and magnetic fields. The numerical results show that the ZnMnSe layers, which act as spin filters, polarize the electric currents. Variation of thickness of the central ZnSe layer shifts the resonant levels and exhibits an oscillatory behavior in spin current densities. It is also shown that the spin polarization of the tunneling current in geometrical asymmetry of the heterostructure where two ZnMnSe layers have different Mn concentrations, depends strongly on the thickness and the applied bias.     

一种新颖结构的高温超导信道滤波器设计

文章提出了一种新颖结构的高温超导信道滤波器设计方法,采用Sonnet软件进行微带电路的全波电磁场仿真.设计了一个一分二的高温超导信道滤波器,信道滤波器工作在S波段,每个信道的相对带宽约为0.4‰,采用的材料为多源热共蒸发法制备的DyBCO双面超导薄膜,衬底为0.5mm厚度的氧化镁.本文给出了本信道滤波器的等效电路、理论曲线、耦合参数和模拟仿真结果.     

Design of anisotropic reflector with birefringent thin films

A novel design for dielectric anisotropic mirrors with birefringent thin films for normal incidence is presented. This mirror consists of a stack of quarter-wave biaxial layers. The biaxial anisotropic layers can be fabricated by oblique deposition. The reflectance is different for two linear polarizations of light incidence on the mirrors. As a numerical example, the design is carried out on glass with TiO2 and ZrO2. These thin films could be applied to anisotropic reflective devices for lasers.     

Construction of high-quality narrow-band holographic filters

We have studied the factors limiting the quality of holographic notch filters. We have employed a novel recording scheme in which the sample is translated during exposure, eliminating flare due to reflections at the boundaries, and simultaneously reducing the scatter of the final hologram. We have also refined a gravity-settling dichromated gelatine (DCG) deposition technique and incorporated hardening of the DCG layers during the drying stage. With this, we have produced high optical quality, uniform, flare-free DCG filters with an optical density, OD > 5, bandwidth < 20 nm and scatter almost as low as that for state of the art dielectric notch filters.