Combined analyser-based and propagation-based phase-contrast imaging of weak objects

A new theoretical method combining analyser-based and propagation-based hard X-ray phase-contrast imaging is investigated. Unlike the previous theoretical model of the combined imaging method constructed under the assumption of slow variation of the individual transfer functions (large Fresnel numbers), a new model proposed in this paper uses the assumption of a weak scatterer (analogous to the first Born approximation). Consequently, the results are not limited to the case of short propagation distances or low-resolution imaging. An explicit expression for the combined transfer function is derived and analytical and numerical examples solving related inverse imaging problems are presented.     

突发公共卫生事件对系统性金融风险的冲击及传染效应研究

首先基于面板向量自回归模型考察了突发公共卫生事件对系统性金融风险的冲击影响,接着综合考虑突发公共卫生事件的影响及其所导致的收益率的非对称性构建单指标非对称CoVaR模型,最后借助LASSO惩罚函数与局部估计法进行求解,以此构建有向网络分析金融机构间的传染效应.研究发现:(1)突发公共卫生事件冲击会使系统性金融风险水平短...     

A half-blind color image hiding and encryption method in fractional Fourier domains

We have proposed a new technique for digital image encryption and hiding based on fractional Fourier transforms with double random phases. An original hidden image is encrypted two times and the keys are increased to strengthen information protection. Color image hiding and encryption with wavelength multiplexing is proposed by embedding and encryption in R, G and B three channels. The robustness against occlusion attacks and noise attacks are analyzed. And computer simulations are presented with the corresponding results.     

Multiple-encoding retrieval for optical security

We propose a method for image encryption by multiple-step random phase encoding with an undercover multiplexing operation. The true image is stored in a multiple record we call encodegram; and then we can reconstruct it by the use of the appropriate random phase masks and a retrieval protocol. To increase the security of the true hidden image and confuse unauthorized receivers, we add to the encodegram an encoded fake image with different content. This fake image has only a small effect on the retrieval of the true hidden image, owing to the specific property of this protocol. In the decryption step, we can reveal the true image by applying the inverse protocol to two cyphertexts, one the encodegram containing the true image along with the fake image; and the other helping to get the random phase key to achieve the true image. Computer simulations verify the validity of this method for image encryption. Digital implementation of the method makes it particularly suitable for the remote transmission of information.     

Image transfer by modulation of short light pulses

This paper shows that the Fourier-processing based modulation of short laser pulses can be successfully used for all-optical image transfer. The system under consideration consists of a transmitter, where the transferred image modulates the frequency profile of a carrier pulse, and a receiver, where after the reverse processing an original image is restored. The main limitations are discussed and the information capacity of a single pulse is obtained. The results from preliminary experiments using a CPM-ring dye laser as a source of carrier pulses are presented.     

Finite element modelling of dielectric elastomer minimum energy structures

This paper presents an experimentally validated finite element model suitable for simulating the quasi-static behaviour of Dielectric Elastomer Minimum Energy Structure(s) (DEMES). A DEMES consists of a pre-stretched Dielectric Elastomer Actuator (DEA) adhered to a thin, flexible frame. The tension in the stretched membrane causes the frame to curl up, and when a voltage is applied, the frame returns to its initial planar state thus forming a useful bending actuator. The simulation method presented here incorporates a novel strain energy function suitable for simulating general DEA actuator elements. When compared against blocked force data from our previous work, the new model provides a good fit with an order of magnitude reduction in computational time. Furthermore, the model accurately matched experimental data on the free displacement of DEMES formed with non-equibiaxially pre-stretched VHB4905 membranes driven by 2500 V. Non-equibiaxially pre-stretching the membranes allowed control of effective frame stiffness and bending moment, this was exploited by using the model to optimise stroke at 2500 V in a hypothetical case study. Dielectric constant measurements for non-equibiaxially stretched VHB4905 are also presented.     

Novel Ghost Imaging Method for a Pure Phase Object

We propose a novel ghost imaging scheme which is especially served to a pure phase object. A spatially incoherent beam is mixed with a coherent beam of the same frequency field by a beamsplitter. Then we perform the ghost imaging scheme using the mixed beam. Our theoretical result shows that this approach is capable of reconstructing a pure phase object in joint-intensity measurement. The visibility of the images is also analysed for two pure phase objects, an optical wedge and a phase grating.     

Fullerene solubility–current density relationship in polymer solar cells

During the last decade polymer solar cells have undergone a steady increase in overall device efficiency. To date, essential efficiency improvements of polymer–fullerene solar cells require the development of new materials. Whilst most research efforts aim at an improved or spectrally extended absorption of the donor polymer, not so much attention has been paid to the fullerene properties themselves. We have investigated a number of structurally related fullerenes, in order to study the relationship between chemical structure and resulting polymer–fullerene bulk heterojunction photovoltaic properties. Our study reveals a clear connection between the fullerene solubility as material property on one hand and the solar cells short circuit photocurrent on the other hand. The tendency of the less soluble fullerene derivates to aggregate was accounted for smaller current densities in the respective solar cells. Once a minimum solubility of approx. 25 mg/ml in chlorobenzene was overcome by the fullerene derivative, the short circuit current density reached a plateau, of about 8–10 mA/cm². Thus the solubility of the fullerene derivative directly influences the blend morphology and displays an important parameter for efficient polymer–fullerene bulk heterojunction solar cell operation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical color image encryption with redefined fractional Hartley transform

We propose a new method for color image encryption by wavelength multiplexing on the basis of two-dimensional (2-D) generalization of 1-D fractional Hartley transform that has been redefined recently in search of its inverse transform. A color image can be considered as three monochromatic images and then divided into three components and each component is encrypted independently with different wavelength corresponding to red, green or blue light. The system parameters of fractional Hartley transform and random phase masks are keys in the color image encryption and decryption. Only when all of these keys are correct, can the image be well decrypted. The optical realization is then proposed and computer simulations are also performed to confirm the possibility of the proposed method.     

All-optical continuous-tunable image switch based on nonlinear photoinduced anisotropy in bacteriorhodopsin film

Using nonlinear photoinduced anisotropy in bacteriorhodopsin (bR) film, we presented and demonstrated an image switch in which the output can be tuned continuously by the intensity of a pumping beam. A laser with wavelength 532 nm was used as the pumping beam, and a He-Ne laser at wavelength 632.8 nm was used as the probe beam. Without pumping light, a little of polarized probe beam can transmit the crossed polarizers and the output is very low. With the presence of pumping light, owing to photoinduced anisotropy in the bR film, a portion of the probe beam transmits the crossed polarizers, depending on the intensity of the pumping beam. For the low-intensity probe beam (0.44 mW/cm²), the output is dependent on a wide range of pumping beam (2-30 mW/cm²). On the contrary, for the high-intensity probe beam (0.80 mW/cm²), the output is dependent on a narrow range of pumping beam (2-7 mW/cm²).