Wang Zhanshan

High Efficiency One-Way Transmission by One-Dimensional Photonic Crystals with Gratings on One Side

We propose a scheme of optical one-way transmission by using one-dimensional photonic crystals (PhCs) with diffraction gratings on one side. The one-way transmission is realized by making the PhC opaque to the zeroth diffraction order and transparent to another propagating (in air) diffraction order. For such a structure with 10-period PhC, 93% of the incident energy passes through when an electromagnetic wave impinges from one side, and the transmittance decreases to the order of 0.001% as the electromagnetic wave illuminates from the other side.

Broadening of Non-Transmission Band of Ultra-Narrow Band-Pass Filters Using Heterostructures

Transmission characteristics are studied for the hybrid structures combining defect and multiple heterostructures. It is shown that the non-transmission frequency range can be substantially enlarged and the phenomenon of narrow band-pass filter can be realized by adjusting the number, position and size of the defect. The theoretical and experimental results on heterostructures containing Ta2O5/SiO2 multilayer films are presented. With perfect non-transmission frequency range and high peak transmissivity, this structure opens a promising way to fabricate ultra-narrow band-pass filters with wide non-transmission frequency range.

Determination of Tungsten Layer Profiles in Bilayer Structures Using X-Ray Reflectivity Method

An effectual method is presented to determine the profiles of a tungsten (W) layer, such as the density, the thickness and the roughness in the multilayer structures, using the x-ray reflectivity technique. To avoid oxidation effects of tungsten, a B4C capping layer is deposited onto to the W layer. To observe the profiles of the tungsten layer with different thicknesses, three groups of W/B4C bilayers with different thicknesses are prepared by using ultra high vacuum dc magnetron sputtering and measured by an x-ray diffractometer. A type of genetic algorithm called the differential evolution is used to simulate the measurement data so as to obtain the parameters of bilayers. According to the simulation, it is shown that the W layer density varies from 95.26% to 97.51% compared to the bulk. In our experiment, the deposition rate is 0.044 nm/s, and the thickness is varied in the range of 9.8–19.4 nm.

HfO 2 /SiO 2 chirped mirrors manufactured by electron beam evaporation

A HfO2/SiO2 chirped mirror was manufactured by electron beam evaporation to increase the laser resistance. The hybrid monitoring strategy utilizing both monochromatic monitoring and quartz crystal monitoring was applied to the deposition compared to the single optical monitoring method. The coatings were characterized by transmission spectrophotometer and white light interferometry, and the experimental results showed that the chirped mirror monitored with the hybrid strategy possessed high reflectivity (>99.7%) and tolerable group delay dispersion oscillation (-50±20 fs2) in the spectra range of 740-860 nm.

Design of Grazing-Incidence Broad-Band Multilayers for Hard X-Ray Reflectors

A new method of designing x-ray supermirrors with broad angular or energy response for use as coatings in x-ray optics is presented. The design is based on an analytical method with oversimplified analytical and semi-empirical formulae, and an extensive numerical method is used in the optimization design. A better initial multilayer is obtained with the former method and optimized with the latter method. In the optimization, a good design is achieved with much less computing time. In addition, the saturation effect due to the interfacial roughness in multilayer also emerges in the design of x-ray supermirrors with definite performances. The reflectivity of C/W x-ray supermirrors as a function of photon energy at the fixed grazing incident angle 0.5 degrees is presented.

Fabrication and Characterization of Ni Thin Films Using Direct-Current Magnetron Sputtering

Ni films are deposited by using ultra high vacuum dc magnetron sputtering onto silicon substrates at room temperature, and the high-quality and high-density films are prepared. The parameters, such as thickness, density and surface roughness, are obtained by using small-angle x-ray diffraction (XRD) analyses with the Marquardt gradient-expansion algorithm. The deposition rate is calculated and the Ni single layer can be fabricated precisely. Based on the fitting results, we can find that the surface roughness of the Ni films is about 0.7 nm, the densities of Ni films are around 97% and the deposition rate is 0.26 nm/s. The roughness of the surface is also characterized by using an atomic force microscope (AFM). The changing trend of the surface roughness in the simulation of XRD is in good agreement with the AFM measurement.

Spectral Resolution Improvement of Mo/Si Multilayers

Theoretically, the spectral resolution of a multilayer can be improved through a combination of utilizing high reflectance orders and by decreasing the thickness of the scattering layer. We fabricate Mo/Si multilayers in the first, second, third, fourth and fifth reflectance orders with Mo layer thicknesses of 3.0 nm and 2.0 nm, respectively, using direct current magnetron sputtering. The structure of the multilayers is characterized with a grazing angle x-ray diffractometer (XRD). Then the reflectivity of the multilayers is measured in a synchrotron radiation facility. The results show that the spectral resolution increases with the increasing reflectance order and with the decreasing Mo layer thickness. The highest spectral resolution is improved to 117.5 in the 5th order for dMo = 2 nm, where the reflectivity is 18%.

Hard X-ray one dimensional nano-focusing at the SSRF using a WSi2/Si multilayer Laue lens

The multilayer Laue lens (MLL) is a novel diffraction optics which can realize nanometer focusing of hard X-rays with high efficiency. In this paper, a 7.9 mu m-thick MLL with the outmost layer thickness of 15 nm is designed based on dynamical diffraction theory. The MLL is fabricated by first depositing the depth-graded multilayer using direct current (DC) magnetron sputtering technology. Then, the multilayer sample is sliced, and both cross-sections are thinned and polished to a depth of 35-41 mu m. The focusing property of the MLL is measured at the Shanghai Synchrotron Facility (SSRF). One-dimensional (1D) focusing resolutions of 205 nm and 221 nm are obtained at E=14 keV and 18 keV, respectively. It demonstrates that the fabricated MLL can focus hard X-rays into nanometer scale.

Influence of working gas pressure on the performance of W/Si multilayers

The effect of Ar pressure on the performance of W/Si multilayers is investigated. W/Si multilayers were deposited by a high vacuum DC magnetron sputtering system. The Ar pressure was changed from 1.0 to 5.0 mTorr with an interval of 1.0 mTorr during the deposition process. Electron probe microanalysis and Rutherford backscattering are performed to determine the Ar content incorporated within these multilayers. The results demonstrate that less Ar is incorporated within the sample when more Ar is used in the plasma, which could be explained by the increase of the collision probability and the decrease in the kinetic energy of Ar ions arriving at the substrate when more Ar exists. The grazing incident X-ray reflectivity (GIXR) at 0.154 nm is used to determine the structural parameters of the layers. The results show that the structures of these multilayers prepared at different Ar pressure are very similar and that the interface roughness increases quickly when the Ar pressure is higher than 3.0 mTorr. The measurements of the extreme ultraviolet (EUV) reflectivity indicate that the reflectivity decreases when Ar pressure increases. The fitting results of GIXR and EUV reflectivity curves indicate that with an increase of Ar pressure, the density and decrement of the refractive index are increased for W and decreased for Si, which is mainly due to (1) the decrease in Ar content incorporated within these multilayers which affects their performance and (2) the increase of collision probability for sputtered W and Si, the decrease of their average kinetic energy arriving at the substrate, and thus the loosing of their layers.

Knife-edge technique for laser cooling

The transfer characteristics of an atomic beam and the effect of laser were investigated in this paper. In the application of knife-edge technique, the temperature of atoms through laser cooling was measured. Results indicate that, after atoms are emitted from an atomic oven, the longer the atoms move, the worse the distribution of the atomic beam shows, regardless the laser cooling is taken or not. Laser cooling can reduce the transverse velocity of the atomic beam to several orders of magnitude and also increase the uniformity of an atomic beam. Knife-edge technique can measure the temperature of an atomic beam through laser cooling. The measurement accuracy depends on the pixel size of the charge coupled device (CCD), which is used for the fluorescent imaging of the atomic beam. The results are very important for the future experiments of laser cooling.