Effect of Superficial Tissue on Spatial Sensitivity Profile of Optical Brain Activity Measurement Evaluated by Direct Hybrid Monte Carlo-Diffusion Method

The light propagation in the brain is highly affected by the superficial tissues such as skin, skull and cerebrospinal fluid (CSF). The development of an adequate model to calculate light propagation in the head is very important for optical brain activity measurement. We developed a novel approach, the direct hybrid Monte Carlo-diffusion method (DHMDM), which can calculate light propagation in the three-dimensional head models including low scattering regions in which the light propagation does not obey either the diffusion approximation or the radiosity theory. The effect of thickness of the superficial tissue upon the spatial sensitivity profiles for optical brain activity measurement is evaluated by the DHMDM. The sensitivity to the brain activity decreases with increasing thickness of the superficial layer whilst the spatial decay of sensitivity on the brain surface depends little upon the thickness of the superficial layer.     

Image recording on photothermoplastic media with different thicknesses of the thermoplastic layer

The quality of images recorded on photothermoplastic media (PTPMs) with various thicknesses of the thermoplastic layer is investigated. It is found that the potential of a corona-forming electrode initiating deformations on a PTPM surface and the diffraction efficiency, corresponding to the additional maxima in its dependence on the spatial frequency, increase with decreasing thickness of the thermoplastic layer, in contrast to the diffraction efficiency at the resonance frequency. It is shown that the frequencies corresponding to additional maxima in the dependence of the diffraction efficiency on the spatial frequency are preferable for application as the carrier frequency for raster-scan recording and as the center carrier frequency for holographic image recording on PTPMs in order to reach a higher diffraction efficiency, image contrast, and signal-to-noise ratio than those attainable at the resonance frequency. An anomalous relation between the resolution and sensitivity of PTPMs is found under the suggested recording conditions and carrier frequencies. It is shown that both the resolution and the sensitivity of PTPMs increase with decreasing thickness of the thermoplastic layer.     

Optical data transfer by switching waves in bistable interferometers with a specified optical thickness distribution

The evolution of switching wave fronts in thin-layer bistable Fabry-Perot interferometers with a specified topology of the transverse distribution of the optical thickness of an intermediate layer is studied. The conditions of passage of switching waves through the boundary between regions with different thicknesses and of tunneling of waves through a barrier of a variable thickness and width formed by two such boundaries are determined by numerical modeling. The possibilities of the formation of planar waveguide channels, regions of retardation and acceleration of the motion of switching waves, and points of the splitting and joining of their fronts are demonstrated, as well as the possibility of creating planar structures on this basis that can perform optical logic operations.     

Ellipsometric and reflectance studies of GaAs/AlAs superlattices

Multiple-angle-of-incidence ellipsometry at 632.8 nm and optical reflectance spectra are used to study GaAs/AlAs superlattices on GaAs substrates. The diagnostic potential of the two methods are compared to each other. Precise values of the total film thicknesses are obtained from the ellipsometric data. Significant disagreement between the measured and nominal thicknesses based on RHEED controlled MBE growth conditions is observed. Nonuniformity of a superlattice film is identified with the spatial resolution of about 1 mm². The thickness variations are accompanied by pronounced variations of the optical constants.     

Locating an acoustic point source scattered by a skull phantom via time reversal matched filtering

This paper examines the utilization of the time reversal matched filtering method to resolve the location of an acoustic point source beneath a skull phantom (variable thickness layer), without the removal of this layer. This acoustical process is examined experimentally in a water tank immersion system containing an acoustic source, a custom-made skull phantom, and a receiving transducer in a pitch-catch arrangement. The phantom is designed to approximately model the acoustic properties of an average human skull bone (minus the diploe layer), while the variable thickness of the phantom introduces a variable time delay to the acoustic wave, relative to its entry point on the phantom. This variable delay is measured and corrected for, and a matched filtering time reversed process is used to determine the location of the point source. The results of the experiment are examined for various positions of the acoustic source behind the phantom and compared to the reference cases with no phantom present. The average distance between these two cases is found to be 4.36 mm, and within the expected deviation in results due to not accounting for the effects of refraction.     

Influence of a skull cranial window on the measurement of haemoglobin concentration in cortical tissue by multi-spectral imaging analysis

The simplified cranial window, which is transparent thinned skull, has been used for the optical imaging of cortical tissue of small animals to measure the concentration change in haemoglobin as an intrinsic signal of brain activation. The multi-spectral images of the cortical tissue of guinea pigs through the skull cranial window were compared with those of the exposed cortex to evaluate the influence of the scattering and absorption properties of the skull on the measurement of the concentration change in haemoglobin. Although skull thickness affects the sensitivity of the optical signal due to a decrease in mean optical path length in the cortical tissue, the influence of the skull cranial window on the wavelength dependence of optical path length can be ignored when the skull thickness is less than approximately 100 mm. Accurate concentration changes in oxygenated and deoxygenated haemoglobins can be calculated from the optical signal measured through a skull cranial window and the wavelength dependence of optical path length for the exposed cortex.     

Optimization of a guided-mode resonance filter by varying the thickness of the buffer layer

In this Letter, we demonstrate that by adjusting the thickness of the buffer layer, the optical responses of a guided-mode resonance filter(GMRF) can be improved for sensor applications. The GMRF is fabricated using a replica molding with a plastic substrate and a UV-curable polymer. SiO_2 buffer layers of different thicknesses are deposited before the waveguide-layer deposition. The sensitivity of the GMRFs decreases slightly with increasing SiO_2 layer thickness. By contrast, the full width at half-maximum reduces substantially with increasing SiO_2 layer thickness, resulting in the improvement of the overall figure of merit.     

硅基有机太阳能电池光学性能分析

采用传输矩阵法的光学模型以及MATLAB软件模拟了硅基有机太阳能电池对入射光的吸收率和各层厚度的关系。模拟表明活性层对入射光的吸收率主要受其自身厚度影响,且由于微腔效应,这种结构的电池可以很大程度上优化活性层厚度;另外通过调节折射率匹配层厚度和传输层厚度也可以优化活性层对入射光的吸收率。在本文所讨论的厚度中,最佳传输层厚度为10 nm左右,最佳匹配层厚度为30 nm ZnS或者60 nm Alq₃。     

Optical optimization of organic solar cell with bulk heterojunction

The work is devoted to the optimization of layer thickness in an organic photovoltaic cell. It presents the applied calculation method which is based on the optical transfer matrix 2×2 formalism. We present the influence of thickness of a PEDOT:PSS layer and of an active layer on the normalized modulus squared of optical electric fields distribution inside devices and on the distributions of exciton generation rate. We present the relationship between optimal thicknesses of the PEDOT:PSS layer and the active layer. We also present the influence of antireflection coating on distributions of optical electrical fields, as well as the distributions of exciton generation rate. Perpendicular and oblique illumination of the photovoltaic structure is discussed.     

非晶Si/SiO2超晶格的制备及其光谱研究

用磁控溅射方法在玻璃基底上制备了非晶Si/SiO₂超晶格.利用透射电子显微镜 (TEM) 和X射线衍射技术对其结构进行了分析,结果表明,超晶格中Si层大部分区域为非晶相,局域微区呈现有序结构,其厚度由1.8—3.2nm变化,SiO₂层厚度为4.0nm.并采用多种光谱测量技术,如吸收光谱、光致发光光谱和Raman光谱技术,对该结构的光学性质进行了系统研究.结果表明,随纳米Si层厚度的减小,光学吸收边以及光致荧光峰发生明显蓝移,Raman峰发生展宽,即观测到明显的量 关键词：     

Image differentiation with the aid of a phase knife

A scheme for optical differentiation of images based on spatial filtering is proposed. A phase knife—a glass plate consisting of two halves, whose thicknesses are chosen so that the optical path difference between the light waves passed through them makes up half the wavelength—serves as a filter. The linearity of the transformation and the sensitivity of the system to noise are investigated theoretically. The scheme was experimentally implemented on the basis of a liquid-crystal light valve. The experimental results confirm that such a setup can be efficiently used for processing images with a narrow-band spatial spectrum and for visualization of phase objects.     

Effects of silver and gold catalytic activities on the structural and optical properties of silicon nanowires

The metal-assisted chemical etching of silicon in an aqueous solution of hydrofluoric acid and hydrogen peroxide is established for the fabrication of large area, uniform silicon nanowire (SiNW) arrays. In this study, silver (Ag) and gold (Au) are considered as catalysts and the effect of different catalysts with various thicknesses on the structural and optical properties of the fabricated SiNWs is investigated. The morphology of deposited catalysts on the silicon wafer is characterized by atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM). It is shown that the morphology of the fabricated silicon nanostructures remarkably depends upon the catalyst layer thickness, and the catalyst etching time directly affects the structural and optical properties of the synthesized SiNWs. FESEM images show a linear increment of the nanowire length versus time, whereas the etching rate for the Au-etched SiNWs was lower than the Ag-etched ones. Strong light scattering in SiNWs caused the total reflection to decrease in the range of visible light, and this decrement was higher for the Ag-etched SiNW sample, with a longer length than the Au-etched one. A broadband visible photoluminescence (PL) with different peak positions is observed for the Au- and Ag-etched samples. The synthesized optically active SiNWs can be considered as a promising candidate for a new generation of nano-scale opto-electronic devices.     

Influence of film thickness on laser ablation of hydrogenated amorphous carbon films

Single-pulse damage thresholds of hydrogenated amorphous carbon (a-C:H) films were measured for 8-ns laser pulses at 532-nm wavelength. Layer thicknesses from below the optical penetration depth to above the thermal diffusion length (60 nm–13 μm) were examined. After correction of the damage-threshold values for the fraction of energy effectively absorbed by the material, the damage threshold was found to increase linearly with the layer thickness, also for film thicknesses below the optical penetration depth of a-C:H. The threshold fluence reached the bulk value for a layer thickness equal to the thermal diffusion length. The thermal diffusion coefficient was obtained from fitting the experimental data. Several phenomena like graphitization, blistering, exfoliation, and ablation were observed for different fluence regimes and film thicknesses. PACS 79.20.Ds; 06.60.Jn; 78.66.Jg     

基于波前技术的人眼神经对比敏感度测量

搭建了基于波前像差的神经对比敏感度(NCSF)测试系统。该系统在测试人眼空间对比敏感度(CSF)的同时,利用Hartmann-Shack波前传感器测量人眼波前像差,通过计算进而得到人眼的NCSF。与通过两种设备分别测量全视觉CSF和波前像差获得NCSF相比,该方法避免了不同测试状态下像差波动的影响,简化了测试过程;和传统激光干涉方法测量NCSF相比,该方法避免了激光干涉产生的相干噪音和激光散斑等不利因素,并且通过改变不同亮度不同颜色视标,可以得到不同亮度,不同波长下的NCSF。选用绿光视标对四例正常人眼的NCSF进行了测量,结果表明:该系统可以同时获得人眼的全视觉CSF、屈光系统调制传递函数和NCSF;在同等亮度下,不同人眼的NCSF存在个体差异;对同一个体,NCSF曲线的最大值对应的空间频率比全眼空间CSF曲线的最大值对应的空间频率高一些。     

Spectral-domain optical coherence phase microscopy for quantitative phase-contrast imaging

We describe a novel microscopy technique for quantitative phase-contrast imaging of a transparent specimen. The technique is based on depth-resolved phase information provided by common path spectral-domain optical coherence tomography and can measure minute phase variations caused by changes in refractive index and thickness inside the specimen. We demonstrate subnanometer level path-length sensitivity and present images obtained on reflection from a known phase object and human epithelial cheek cells.     

Heat-insulation film with gold layer deposited on antireflection subwavelength-structured surface

The optical properties of gold layer deposited on antireflection subwavelength-structured surface are first exposed. The experimental results of the reflectances and transmittances for several different thicknesses of gold-deposited layers on the subwavelength structures are carried out. The nanostructured surface with spatial period and a diameter of about 230 nm and height of about 150 nm on polyethylene terphthalate (PET) film is fabricated by micro-replication process of UV imprinting. Comparing these with the bare gold-deposited layer and bare nanostructure, the results show that the optical films with the suitable gold layer deposited on antireflection subwavelength-structured surface has high transmittance and low glare in the visible spectral range and high reflectance in the infrared range. That is to say, when the antireflection subwavelength-structured surface is coated with a gold layer of several tens of nm thickness, it will have some unique optical characters.     

镀膜法改善有机薄膜太阳能电池光学性能

有机活性材料的低载流子迁移率使得有机光伏电池的电极收集到的电荷较少。增加活性层光吸收能够增加激子的产生数从而增加电极收集到的电荷,提升器件的性能。通过对器件模拟的方法,研究以P3HT：PCBM为活性层的薄膜太阳能电池的光学性能。 在此基础上,提出采用镀多层高反射膜的方法改善电池器件的光学性能。结果表明：活性层厚度对电池器件的光吸收起到主导作用;镀多层高反射膜在活性层厚度小于160 nm、Ag厚度小于20 nm时能大幅度改善电池器件的光学性能,光生激子总数随活性层厚度的增加而迅速增多,并且在活性层厚度约为150 nm时为一个最佳值。     

In situ sensitive optical monitoring with proper error compensation

We introduce a way to estimate the fluctuation of the refractive index during thin film deposition, through an optical monitor. The thicknesses and error-compensated thickness for each layer are analyzed. A novel monitoring method is thereby derived. The revised refractive index and the choice of highly sensitive monitoring wavelengths help us predict the termination points more accurately. The performance of a narrow-bandpass filter monitored by this method is demonstrated.     

基于传输矩阵法的聚合物太阳能电池光学性能分析

采用基于传输矩阵法的光学模型及Matlab软件,模拟了以聚合物P3HT:PCBM为活性层的倒装太阳能电池,并分析了模拟电池的吸光率及其内部光电场分布。探讨了厚度、入射角度以及新结构对电池光学性能的影响。模拟结果表明,电池的光吸收主要由活性层厚度决定,分别随着电子传输层和空穴传输层厚度的增加而下降。当光入射角度增大时,由于光程的增加电池的吸光率随之增加,在40°入射角时达到最大;由于其他光学作用,器件吸光率在40°入射角以后反而降低;证明了光斜入射时电场在整个器件中分布是不连续的。通过在基本结构的适当位置插入一层薄膜构成的微腔器件,由于光学共振效应能够有效提高电池的光吸收。     

Research on quantum efficiency of transmission-mode InGaAs photocathode

To investigate the quantum efficiency of high performance transmission-mode InGaAs photocathode, the quantum efficiency curve is fitted by using the multilayer optical thin film theory and the first principle calculation. A series of the performance parameters are obtained with the relative error less than 5%. It indicates that the thickness of the emission layer is 0.7–0.9 μm, the thickness of the window layer is 0.1–0.3 μm. Meanwhile the results from the first principle calculation are proved to be reliable as well. In addition, the long-wave response will increase and the highest integral sensitivity will be obtained when the thickness of the emission layer is the optimum value. For the InGaAs photocathode, the back interface recombination velocity mainly leads to the low integral sensitivity, which is caused by the material lattice mismatch.