双层深度编码探测器的晶体表面优化

设计了双层LSO晶体和位置灵敏型光电倍增管耦合构成的用于小动物PET成像的 深度编码探测器. 众所周知, 晶体的不同的表面处理影响着光输出量, 进而影响着它们构建的PET探测器的性能. 为了优化设计的深度编码探测器的性能, 测试了两种不同表面处理的LSO闪烁晶体阵列探测器的晶体分辨能力及其能量、时间和空间分辨率, 结果表明, 光滑表面LSO晶体构建的深度编码探测器显示出良好的空间、能量及时间分辨特性.     

基于蛇形光路的PET探测器作用深度提取方法

为实现正电子发射断层成像(Positron Emission Tomography,PET)探测器的作用深度(Depth Of Interaction,DOI)信息获取,本文提出一种基于分光技术的探测器设计方案.探测器采用晶体单元与硅光电倍增管(Silicon Photomultiplier,SiPM)一对一耦合、蛇形光路的设计和单端Anger加权读出方法进行DOI解码.基于GATE软件进行蒙特卡罗模拟,建立8×1的LSO晶体阵列(单根晶体尺寸3.1×3.1×20mm3);模拟泛场照射获取位置查找表;并进行不同深度的模拟,获得各晶体在各深度的空间分辨率.结果显示所模拟的探测器模块DOI分辨率在1.0~6.7mm之间,平均值为3.2mm.本文提出的基于蛇形光路的PET探测器方案能在维持系统成本和复杂度的前提下实现DOI解码,提升PET系统的成像性能.     

PET用新型深度编码探测器设计

平行性问题或作用深度问题影响着正电子放射层析系统(PET)的成象分辨率。本文提出和评价了一种二阶深度编码探测器,这个探测器是由两层LSO晶体和位置灵敏型光电倍增管(PS-PMT)组成,上层的晶体相对于下层的晶体的位置在X和Y方向上都有半个晶体阵列单元的错位,因为每个晶体的输出光的重心位置分布在PS-PMT上不同的位置,所以通过计算其重心位置就可在象平面上区别开每个晶体。根据PET系统的应用要求,我们验证了探测器的性能。伽马射线垂直入射时,上层晶体阵列的固有空间分辨率为1.15mmFWHM,下层晶体列阵为1.34mm FWHM.钠伽马辐射源的511keV能量峰处的平均能量分辨率对于下层晶体为19.7%,上层为23.6%.通过类似的技术亦可实现3阶、4阶深度编码探测器。     

利用作用深度信息提高正电子断层成像仪分辨率一致性

获取作用深度信息是设计小动物正电子断层成像仪的关键技术之一.我们利用新设计的含有作用深度信息的探测器，从模拟与实验两个方面观测了作用深度信息对于分辨率一致性的影响.结果表明：在γ射线垂直入射时，深度编码探测器和一般的无深度检出机能的探测器，均获得了高分辨率，而γ射线斜入射时，深度编码探测器与一般的探测器相比能提供更好的空间分辨率. 关键词： 作用深度 空间分辨率 正电子断层成像仪     

Calculations of potential of mean force: application to ion-pairs and host–guest systems

ABSTRACT

We report the calculation of the potential of mean force (PMF) of different types of associations by various techniques: two no biased methods (thermodynamic integration and finite difference thermodynamic integration), a constraint biased technique (adaptive biasing force) and an umbrella biased method (umbrella sampling). We apply these methodologies to the association between two methane molecules in water, to the formation of an ion pair in water and to the formation of an insertion complex between a macrocycle and a cation. The different PMFs are compared to each other on the basis of the depth of the free energy minimum and on the location of different specific points.     

光纤激光泵浦周期极化晶体双波段激光器

针对激光定向干扰系统要求对抗1 m ~3 m和3 m ~5 m 2种类型探测器,需要输出相应2种波段激光,通过高重频调Q技术和种子注入光放大技术,获得高功率高光束质量1.06 m光纤激光输出,外置起偏器获得2束激光输出,分别为泵浦周期极化钽酸锂和周期极化铌酸锂晶体,实现高功率1 m ~3 m 和3 m ~5 m激光输出。在电源输入电流60 A,调Q驱动频率50 kHz的条件下,获得最高功率7.5 W的2 m激光和4.2 W的3.9 m激光,频率转换效率为39.5%。实验结果表明:通过光纤激光器泵浦光参量振荡器,可获得高功率1 m ~3 m和3 m ~5 m双波段激光输出。     

Nonlinear shift of spatial solitons at a graded dielectric interface

We investigate total internal reflection of optical spatial solitons at the interface between two regions of nematic liquid crystals with different optical densities. Due to nonlinear molecular reorientation, the solitons experience a penetration depth, hence, a lateral shift that depends on the excitation, with lateral shifts from 0.7 to 1.2 mm as input powers increased from 1.6 to 9.3 mW.     

Micro-Raman analysis on LiNbO3 substrates and surfaces: Compositional homogeneity and effects of etching and polishing processes on structural properties

In this work we report on micro-Raman analysis on lithium niobate (LN) substrates in order to study the compositional homogeneity of the crystals and to clear up the effects of etching and polishing processes on the surface of wafers and crystals.The fact that the linewidth of some Raman modes scale with the composition of LN crystals, together with the use of a confocal microscope, allowed a three-dimensional determination of the sample stoichiometry and of the crystalline quality. This local tool can supply additional information, which can be complementary to the electro-optic coefficients, carefully measured as well in order to check functional parameters.Raman spectra from buried regions were obtained on as-grown, etched and polished crystals and wafers. The depth profile of the peak energy and the linewidth of the Raman mode at 872 cm⁻¹ indicate that mechanical processing of surfaces causes, in some cases, structural modifications till a depth of 15 μm.     

A surface sensitive optical method for the evaluation of processed ZnO: exploitation of LO phonon interaction

We report on a surface property of bulk ZnO crystals and an optical method to evaluate it. Bulk ZnO crystals have a damaged surface layer due to chemomechanical polishing. We prepared the ZnO crystals by etching, and evaluated the improvement of the surface by high-resolution X-ray diffraction (XRD) and photoluminescence (PL). In PL measurements, the relative intensity of the first order longitudinal optical phonon replica of free exciton (FX-1LO) to second order process (FX-2LO) was compared. The relative intensity becomes weak with increasing etched depth and finally saturates at the etched depth of 5 μm. This result agrees well with XRD results.     

Self assembly of inorganic nanocrystals in 3D supra crystals: Intrinsic properties

Here we describe how arrangements of nanocrystals can self-organize in 3D arrays called supra crystals. The 3D arrays can fall into the familiar categories of face centered cubic (fcc), hexagonal compact packing (hcp) crystals, and body centered (bcc) crystals. Intrinsic collective properties of these 3D arrangements are different from the properties of individual nanoparticles and from particles in bulk.We demonstrate by two various processes and with two types of nanocrystals (silver and cobalt) that when nanocrystals are self ordered in 3D superlattices, they exhibit a coherent breathing mode vibration of the supra crystal, analogous to a breathing mode vibration of atoms in a nanocrystal.Comparison between the approaches to saturation of the magnetic curve for supra crystals and disordered aggregates produced from the same batch of nanocrystals is similar to that observed with films or nanoparticles either highly crystallized or amorphous.     

Variation of photon density distribution with system component within Intralipid emulsion

In order to recover distributions of optical parameters of underlining tissues utilizing backreflection optic tomography or diffuse reflectance spectroscopy for diagnostic proposes, trajectories of photons should be correctly estimated within tissue. Therefore, we have simulated the soft tissues with 1% Intralipid emulsion and have determined photon density distribution as a function of depth inside the Intralipid emulsions using laser light of wavelength 808 nm and two different detectors for different source-to-detector (S-D) separations. In this study we have shown difference in path distribution of the detected photons with two different detectors with different sensitivity for the same S-D separations in the Intralipid emulsions. This study has also shown that, the maximum penetration depth (MPD) of the detected photons is not an increasing function of the S-D separations. After reaching a maximum depth, the penetration depth decreases as S-D separation increases.     

Directional coupler design based on coupled cavity waveguides in photonic crystals

We propose a directional coupler design based on coupled cavity waveguide in photonic crystals. The plane wave expansion is used to give the dispersion of the coupled cavity waveguides and two parallel such waveguides. The couple length is got from the dispersion curves. Based on the research of the dispersion, we present a directional coupler and the transmission property is given. This structure is potentially important for highly efficient directional coupler in integrate optical circuit.     

向列相液晶中强非局域空间光孤子传输的理论研究

对向列相液晶中非局域空间孤子的传输进行了理论研究.基于非线性液晶孤子传输方程，采用Gauss形式的试探解，不仅得到了空间孤子的解析解,而且还在临界功率附近得到了呼吸子的解析解.通过数值模拟证明我们的结果比Conti和Assanto等人的结果更合理.同时，对液晶中的非局域孤子模型和Snyder等提出的强非局域孤子模型进行了全面的比较. 关键词： 向列相液晶 空间孤子 非局域非线性 呼吸子     

Dark–bright optical solitary waves and modulation instability analysis with (2 + 1)-dimensional cubic-quintic nonlinear Schrödinger equation

This paper addresses the (2+1)-dimensional cubic-quintic nonlinear Schrödinger equation (CQNLS) that serves as the model to study the light propagation through nonlinear optical media and non-Kerr crystals. A dark–bright optical solitary wave solution of this equation is retrieved by adopting the complex envelope function ansatz. This type of solitary wave describes the properties of bright and dark optical solitary waves in the same expression. The integration naturally lead to a constraint condition placed on the solitary wave parameters which must hold for the solitary waves to exist. Additionally, the modulation instability (MI) analysis of the model is studied based on the standard linear stability analysis and the MI gain spectrum is got. Numerical simulation and physical interpretations of the obtained results are demonstrated. It is hoped that the results reported in this paper can enrich the nonlinear dynamical behaviors of the CQNLS.     

Microscopic fluorescence imaging of bulk defect clusters in KH(2)PO(4) crystals

A microscopic fluorescence imaging system is used to detect optically active centers located inside a transparent dielectric crystal. Defect centers in the bulk of KH(2)PO(4) crystals are imaged based on their near-infrared emission following photoexcitation. The spatial resolution of the system is 1mum in the image plane and 25mum in depth. The experimental results indicate the presence of a large number of optically active defect clusters in different KH(2)PO(4) crystals, whereas the concentration of these clusters depends on the crystal sector and growth method.     

Investigation of a UV-laser generated planar polymeric waveguide directly and after five years of preparation using an interferometric method

Polymeric integrated-optical waveguides were prepared in a planar chip using UV-laser lithographic method. Five waveguide samples were irradiated by an excimer laser of wavelength 248 nm with different irradiation parameters (fluence and irradiation doses). Using Mach–Zehnder interferometer, the refractive index depth profiles of these samples are determined in two cases, directly and five years later of preparation. A crucial change of refractive indices profiles has been recorded after five years of preparation. In the first case we got double region waveguides fitted a Gaussian shape, while in the second one we observe a single region waveguide which has exponential shape. The photochemical reactions responsible for these changes in the two cases are demonstrated. This is quite important when such waveguides are used in the applications. Also it was interesting to investigate the aging impact on mode field distributions and the effective mode indices. The mode parameters are determined based on a theoretical model and the experimentally obtained data. The results show a notable change in the mode field distributions and the propagation coefficients as influence of aging.     

非常偏振光在单轴晶体表面的全反射分析

利用透射波函数和由菲涅尔公式求解反射相位差并对其求导的方法,分析了非常偏振光在单轴晶体表面发生的全反射现象,求解出晶体光轴在入射面内时,非常偏振光从各向同性介质入射到晶体和从晶体出射到各向同性介质两种情况的隐失波穿透深度和Goos-Hnchen位移的一般表达式。通过计算机模拟给出了单轴晶体为方解石和水晶情况时的穿透深度和Goos-Hnchen位移图像。结果表明,对于不同的晶体,光轴的取向对穿透深度和Goos-Hnchen位移的影响是明显不同的,若选取合适的晶体、光轴取向和入射角,可以得到较大的隐失波穿透深度和Goos-Hnchen位移。     

一维金属/介质光子晶体透射特性分析

阐述了一维金属/介质光子晶体的概念,采用时域有限差分法研究了不同金属等离子体频率和碰撞频率对一维金属/介质光子晶体透射特性的影响。计算结果表明,一维金属/介质光子晶体具有在可见光波段透明,在紫外波段以及红外至微波波段不透明的特性。对于结构相同的金属光子晶体,金属等离子体频率越低,金属光子晶体在可见光波段的透射允带就越宽,透射率越高,而金属的碰撞频率对透射允带没有显著的影响。     

Direct observation of a large London penetration depth in an organic superconductor

Careful observation of the diamagnetic shielding signal in small fields applied along the a-axis of deuterated (TMTSF)₂ClO₄ crystals reveal a systematic dependence of the diamagnetic susceptibility on the thickness (b- and c-dimensions) of the crystals, which we explain by a macroscopically large value of 0.004 cm for the London penetration depth.     

修正的频域有限差分法在二维金属光子晶体分析中的应用

与介质光子晶体相比,金属光子晶体的带隙特性在毫米波和亚毫米波波段有着重要的应用价值.基于Yee网格的频域有限差分法推导得出的本征模方程,求解后能方便而又可靠地得出介质光子晶体的带隙图和场分布.但由于金属与介质的本质差异,该方法不能直接应用于金属光子晶体.文中引入了金属表面边界条件,推导了二维金属周期结构的光子带隙本征模方程.通过数值计算,得出了不同晶格结构（正方/三角格子）下两种模式（TE/TM）的全禁带特性,并与介质周期结构的禁带特性进行对比,分析了金属周期结构在模式选择和器件集成方面的优点. 关键词： 金属光子晶体 频域有限差分法 全禁带