Dielectric Metalens by Multilayer Nanoimprint Lithography and Solution Phase Epitaxy

Metasurfaces have ushered in a huge development for their superior ability in manipulating light properties including phase, amplitude, and polarization, which show great potential as alternatives for the refractive optical devices. Recently, many applications of metasurface including metalens have been proposed and investigated, aiming at substituting their refractive counterparts. However, the commonly used fabrication approaches employ electron-beam lithography (EBL) followed by dry etching or atomic layer deposition (ALD) of dielectric materials, which are expensive and inefficient. Besides, dry etching of dielectric materials at sub-100 nm scale with a high aspect ratio is challenging. Herein, a new approach for dielectric metalens fabrication is presented, which combines multilayer nanoimprint lithography and solution phase epitaxy. High aspect ratio ZnO nanopillars with a height-to-diameter ratio of over 7:1 are demonstrated. By using the multilayer nanoimprint lithography, increased aspect ratio nanostructures from shallow imprinting molds are obtained. The highly anisotropic growth characteristic enables nanopillars to grow at a height that exceeds the resist thickness. With this ability, ZnO metalenses are fabricated where the height of nanopillar reaches 1.1 μm, achieving a focusing efficiency of 50%. The process is cost-effective with a high throughput, which can be widely used for many optical applications.     

介质多层膜的热吸收

根据薄膜光学基本理论,利用一级近似数学方法,给出S,P偏振态平行光在两弱吸收介质边界反射率及透射率的简单表达式,并依此研究了弱吸收介质单层膜及多层膜的热吸收。结果表明,单一膜层的热吸收率,在膜层参数确定的条件下,不仅取决于此膜层材料的消光系数,而且取决于相邻膜层的消光系数。     

Multilayer dielectric structure for enhancement of evanescent waves

We describe a type of multilayer dielectric coating designed to enhance the intensity of evanescent waves. The coating consists of a stack of alternating high- and low-index quarter-wave layers followed by a final low-index layer whose thickness is chosen to optimize the evanescent-wave intensity. Indirect measurements of the film properties are in good agreement with theory.     

Multifiber ceramic capacitor

A new type of capacitor, multifiber capacitor (MFC) is proposed. It is made up of fiber capacitors (elements) connected in parallel and bonded together with a binder. Each element consists of fiber core (inner electrode), dielectric coating and outer electrode. Analysis indicates that MFC has the optimum capacitance in comparison with multilayer capacitor (MLC) when the diameter of fiber core is carefully matched by the thickness of dielectric coating. Since dielectric layer of a wide range of thickness can be produced more easily as fiber coating than as flat tape, MFC can cover a wider range of capacitance than MLC. Apart from as a possible substitute or supplement to MLC, MFC is potentially useful to bridge the gap between ceramic MLC and thin film capacitor used in integrated circuits. MFC also has better resistance to dielectric breakdown. The possible techniques for the preparation of MFC are also described.     

磁场调制直接重写磁光存储介质的记录特性研究

用直流共溅射方法制备了富稀土－过渡族金属非晶态合金薄膜，用数值计算的方法计算了激光照射下多层膜结构盘片的温度场分布，分析了盘片材料成分和结构对热磁记录的影响，对盘片结构进行了优化设计并选定了合适的磁光层和电介质层厚度。实验盘片可在低偏场和低激光功率下记录，盘片的读写性能基本符合２．５英雨可擦重写磁光盘（ＭｉｎｉＤｉｓｋ）的使用要求。     

Accurate and rapid optical characterization of an anisotropic guided structure based on a neural method

Optimal performances of integrated optical devices are obtained by the use of an accurate and reliable characterization method. The parameters of interest, i.e., optical indices and thickness of the waveguide structure, are calculated from effective indices by means of an inversion procedure. We demonstrate how an artificial neural network can achieve such a process. The artificial neural network used is a multilayer perceptron. The first result concerns a simulated anisotropic waveguide. The accuracy in the determination of optical indices and waveguide thickness is 5 x 10(-5) and 4 nm, respectively. Then an experimental application on a silica-titania thin film is performed. In addition, effective indices are measured by m-lines spectroscopy. Finally, a comparison with a classical optimization algorithm demonstrates the robustness of the neural method.     

Layer thickness fluctuations in optical coatings with non-quarter-wave design

Previous research on highly reflective multilayer optical coatings has shown that varying the thickness of layers from their traditional quarter-wave values gives the potential for greater damage thresholds without sacrificing the desired optical properties. We numerically investigate the influence of unintended layer thickness fluctuations in these nontraditional designs, concluding that such fluctuations should have minimal effect on the optical properties of the coating. This result makes the prospect of producing nontraditional designs more promising.     

Direct optical monitoring on the rotating substrate holder

Single wavelength optical monitoring is frequently applied in large box coaters. In state of the art optical coating systems a testglass changer with stationary test glasses is used for thickness monitoring while the substrates are located on a rotating substrate holder. The accuracy of this so called indirect monitoring technique is mainly limited by variations of the tooling factor. Direct monitoring on a substrate which is located on the rotating substrate holder avoids these errors and takes advantage of automatic thickness error compensation effects. The paper reports substantial progress which has been achieved for coating systems with large area substrate holders (up to Ø 1500 mm). Intermittent monitoring on a substrate or a witness was applied. This technique enables rapid prototyping with tight specifications and high yields in large area batch coaters. Application results of challenging optical multilayer systems are demonstrating clearly the potential of this powerful monitoring technique. The monitoring capability was investigated for a lot of different layer systems such as dielectric mirrors, anti‐reflection coatings, sophisticated edge filters, polarizer coatings, beam splitters multiple cavity band‐pass filters and notch filter coatings. Strong coincidence of theory with experiment was achieved with plasma ion assisted deposition (PIAD) and magnetron sputtering. Reproducibility experiments have clearly shown the benefits of this monitoring technique.     

R.F. sputtering system for the preparation of multilayer optical filters

Thin film multilayer optical filters are usually prepared by vacuum evaporation of successive layers. The deposition of thin films by sputtering has many advantages over the vacuum evaporation technique and hence it is employed for the fabrication of better quality optical filters. However, the filters may be more time consuming to prepare if a single-target sputtering system is used because two different materials are required for alternate layers. For the preparation of multilayer optical filters, a special r.f. sputtering jig arrangement with two target holders is fabricated. Using this the successive layers can be deposited in one pumpdown. A simple coupling arrangement used to match the r.f. generator output impedance to the load is explained. With all the sputtering parameters such as the pressure, the temperature and the power maintained constant except the time, control of the thickness of the multilayers is achieved. A calibration chart of sputtering time versus thickness of the materials to be deposited is prepared for TiO₂ and SiO₂ for the above purpose. Using this calibration chart, the deposition time of different layers of any optical filters based on these dielectric materials can be fixed to obtain optical filters of acceptable quality and reproducibility.     

Stepwise graded refractive-index profiles for design of a narrow-bandpass filter

A combination of stepwise graded refractive-index profiles and a cavity structure is used for designing narrow-bandpass filters of TiO(2)/SiO(2) multilayer films upon BK7 glass substrates. Symmetrical profiles of stepwise graded refractive indices result in high transmittance of passbands for the designed filters. The bandwidth of the narrow-bandpass filter is controlled by adjustment of parameters such as the thickness and the number of layers in the multilayer stack. This design is proposed as a new and simple method for coating synthesis of optical filters.     

新型多纤维陶瓷电容器的设计

提出了一种新型多纤维陶瓷电容器(MFC)。MFC由众多纤维电容器并联而成，而每根纤维电容器由内电极(导电纤维)、介电层和外电极构成。理论分析表明，当纤维直径与介电层厚度相匹配时，MFC的电容比多层电容器(MLC)的电容大，而且MFC也具有更优异的抗击穿性能。     

Thermally invariant dielectric coatings for micromirrors

Thermal expansion-induced curvature becomes a major effect in micromirrors as the mirror diameter exceeds 100 microm. Such mirrors are used for optical switching, scanning, and many other applications. By using multilayer coatings instead of a single metal reflector, one can use the mechanical properties of the multilayer to create mirrors with zero curvature across temperature. We demonstrate the fabrication of such thermally invariant mirrors using dielectric coatings. A semianalytic model based on free-plate elastic theory is developed that uses empirical parameters in place of the true thermal expansion coefficients of the coating materials. Micromirrors are demonstrated that maintain their design curvature to within lambda/60 for lambda = 633 nm across an operating range from 21 degrees C to 58 degrees C.     

Design of X-ray supermirrors

A new approach is proposed for the design of wide band-pass multilayer optical elements operating in the hard X-ray spectral region. The method, based on the combination of analytical and numerical methods, solves the inverse problem consisting of inferring the composition profile of a depth-graded multilayer coating. The key feature of our approach consists in using an analytical expression for the depth-distribution of the period as initial solution for direct computer calculations. This allows a global minimization of the merit function and a many-fold decrease of the computer run time. Simulations of two particular cases are presented: a constant reflectivity over a wide spectral range and a complicated reflectivity profile. The best choice of material pairs for composing a depth-graded multilayer structure is discussed from the viewpoint of maximum achievable reflectivity and least number of bi-layers. Features of depth-graded multilayer mirrors, which are distinctive from conventional periodic mirrors, are examined. The factors influencing the optical quality of broad-band multilayers are also considered.     

Radiation engineering of optical antennas for maximum field enhancement

Optical antennas have generated much interest in recent years due to their ability to focus optical energy beyond the diffraction limit, benefiting a broad range of applications such as sensitive photodetection, magnetic storage, and surface-enhanced Raman spectroscopy. To achieve the maximum field enhancement for an optical antenna, parameters such as the antenna dimensions, loading conditions, and coupling efficiency have been previously studied. Here, we present a framework, based on coupled-mode theory, to achieve maximum field enhancement in optical antennas through optimization of optical antennas' radiation characteristics. We demonstrate that the optimum condition is achieved when the radiation quality factor (Q(rad)) of optical antennas is matched to their absorption quality factor (Q(abs)). We achieve this condition experimentally by fabricating the optical antennas on a dielectric (SiO(2)) coated ground plane (metal substrate) and controlling the antenna radiation through optimizing the dielectric thickness. The dielectric thickness at which the matching condition occurs is approximately half of the quarter-wavelength thickness, typically used to achieve constructive interference, and leads to ～20% higher field enhancement relative to a quarter-wavelength thick dielectric layer.     

Development of thin broad band radar absorbing materials using nanostructured spinel ferrites

Stealth applications now emphasise on development of efficient Radar Absorbing Materials of light weight, less coating thickness, broad bandwidth of absorption along with cost effective raw materials and manufacturing techniques. Therefore, in this paper an attempt has been made to develop such an efficient cost effective radar absorbing material which possesses broad band absorption with less coating thickness. Unconventionality of acquiring impedance match for a double layer absorber of nanostructured nickel ferrite (NF) with reasonably good dielectric properties and its cation substituted counterpart, nickel zinc ferrite (NZF) with enhanced magnetic properties has been thoroughly investigated for radar wave absorption for very low coating thickness. Complex permittivity and permeability of NF and NZF with crystallite sizes of 10.0 and 16.0 nm, respectively, are measured and used as the data bases. The microwave absorption properties of the ferrites are correlated with their size, morphology, permittivity, permeability, thickness and bandwidth of absorption. Multilayering has been performed using the optimization through Genetic Algorithm in order to attain suitable impedance matching layer for minimum reflection loss (RL) at lower thickness. The results indicate an enhancement in the absorption with RL value of ?45.0 dB for a moderately low coating thickness of 1.72 mm. Single layers NF, NZF and multilayer NF–NZF are fabricated over the aluminium sheets and the results are experimentally verified.     

介质薄膜的透射光谱测量及其光学参数的分析

介绍介质薄膜透射光谱的测量以及基于分析薄膜透射光谱的计算薄膜光学参数的方法。对制备在玻璃基板上的二氧化钛、二氧化硅和氧化锌薄膜进行了可见光谱区的透射比测量，并用包络线方法和最优化方法对这些透明薄膜的光学参数进行了计算和分析。着重讨论了最优化方法在分析薄膜光学参数中的应用及其误差分析。此外，还对包络线方法和最优化方法进行了比较。     

空心微球上Al-W多层涂层的制备与表征

在惯性约束聚变研究中,高-低Z金属涂层对提高靶丸的性能具有重要的作用。本实验采用直流磁控溅射技术和驱动微球运动的倾斜旋转托盘与敲击装置,在GDP空心微球表面溅射了不同工作气压的Al-W多层涂层,利用扫描电镜(SEM)、原子力显微镜(AFM)、X射线衍射仪(XRD)研究气压变化对多层涂层质量的影响规律。结果表明:多层涂层的厚度均匀性好,达90%以上,涂层均呈致密的柱状晶生长,残余应力小,涂层质量受工作气压影响显著。涂层结构的致密度随气压增大而下降,涂层的形貌与颗粒尺寸发生相应的变化,致使多层涂层的表面粗糙度随气压增大呈现出先减小后增大的变化趋势。当工作气压为0.5 Pa时,Al-W涂层的综合性能最好,表面颗粒形貌为细小均匀的塔形,表面粗糙度值低至331.1 nm,残余应力小。     

Control of resonant wavelength from organic light-emitting materials by use of a Fabry-Perot microcavity structure

We report the fabrication of Fabry-Perot microcavity structures with the organic light-emitting material tris-(8-hydroxyquinoline) aluminum (Alq3) and derive their optical properties by measuring their photoluminescence (PL) and absorption. Silver and a TiO2-SiO2 multilayer were used as metal and dielectric reflectors, respectively, in a Fabry-Perot microcavity structure. Three types of microcavity were prepared: type A consisted of [air[Ag[Alq3]Ag]glass]; type B, of [air[dielectric[Alq3]dielectric]glass]; and type C, of [air[Ag[Alq2]dielectric]glass]. A bare Alq3 film of [air[Alq3]glass] had its PL peak near 514 nm, and its full width at half-maximum (FWHM) was 80 nm. The broad FWHM of a bare Alq3 film was reduced to 15-27.5, 7-10.5, and 16-16.6 nm for microcavity types A, B, and C, respectively. Also, we could control the PL peak of the microcavity structure by changing the spacer thickness, the amount of phase change on reflection, and the angle of incidence.     

Independent electromagnetic optimization of the two coating thicknesses of a dielectric layer on the facets of an echelle grating in Littrow mount

Abstract

Electromagnetic investigations using the integral equation system method with parametrization (IESMP) show that the two-coating thicknesses of a dielectric layer on the facets of an echelle grating in a Littrow mount have to be independently optimized. While the optimal coating thickness on the blaze facet is the same for maximal efficiency and minimal absorption in both polarizations, this is not the case for the anti-blaze facet. Therefore, it is only possible to optimize the two-coating thicknesses for one of the purposes. On the blaze facet, a simple formula based on thin-film optical considerations describes the optimal thickness very well. Additionally, we found that resonance anomalies can significantly reduce efficiency if the wrong coating thickness is used on the anti-blaze facet. The coating thickness creating the resonance anomaly can be deduced by investigating the poles of the reflection coefficient of a dielectric coated metallic mirror in grazing incidence. This value can be used to optimize the layer for maximal efficiency. Consequently, we are generally able to describe the optimal coating thicknesses for minimal absorption as well as for maximum efficiency in both, TE- and TM-polarization, using only thin-film optical considerations without any further rigorous calculation.     

Multilayer optical thin films for use at terahertz frequencies: method of fabrication

A new method of fabricating multilayer optical coatings used at terahertz frequencies has been developed. Using plasma-enhanced chemical-vapor deposition, a multilayer antireflection coating for germanium optics at terahertz frequencies was fabricated. The coating consists of amorphous silicon and silicon-oxide layers. The transmittance and structure of the coating were experimentally investigated. The transmittance spectrum of the coating on the Ge substrate shows a wideband antireflection behavior in the 40-120 cm(-1) region.