多散射介质中光子密度波扩散方程的求解

将光源作为一个复数点光源,根据实验研究模型的要求,求解了多散射介质中光子密度波扩散方程,找出了与入射面相对应面的光子通量和介质中吸收系数改变量之间的关系。所得结论能直接用于多散射介质中成像问题的实验研究之中,即通过对光子通量的检测,反演出吸收系数改变量的分布,从而达到多散射介质中的成像问题的解决。     

多散射介质中光子密度波扩散方程的二阶求解

对于生物医学光子学领域中的新技术-扩散光子密度波技术进行了理论上的研究与扩展,给出了长为a,宽为b,高为无限大的矩形边界条件下光子密度波扩散方程的二阶微扰解析解.由于实际测到的物理量多为光通量,演算出了与入射面相对应面处的光子通量和介质中的吸收系数改变量之间的关系.并利用人体前臂肌肉组织的典型光学参数,给出了二阶微扰解的模拟计算结果,并与一阶微扰计算结果进行了比较.结果表明,二阶微扰解比原有的一阶微扰解在计算精度上有一定的提高,尤其是在异常体附近精度更是有较大的提高.所得二阶结果不仅为利用扩散方程进行生物组织成像问题提供了更加精确的理论基础,而且对于图像质量尤其是分辨异物的能力的提高具有促进作用.     

沉没在高散射介质中的吸收体和散射体的 扩散光子密度波取象

我们使用扩散光子密度波法研究了沉没在高散射介质中的吸收体和散射体的取象,并对含有吸收体和加入不同颜色染料的散射体的取象实验结果进行了比较和分析。从实验结果可以得出用扩散光子密度波法可以探测强散射介质中吸收体或散射体的存在、大小和形状;以及在强散射介质中的散射体内加入不同颜色的颜料时,散射信号会发生不同的变化。     

计算光学成像在散射中的应用

自然界中普遍存在光散射现象。如何通过散射介质实现高分辨率成像是光学成像领域亟待解决的重要问题。在早期研究中,多重光散射被认为是雾霾、云层、生物组织等复杂介质成像中的障碍。然而,最近研究表明,散射并不是成像的基本限制:光子在经过多次散射后仍然包含了大量信息。为了深入了解新兴的计算光学成像是如何解决多重光散射问题的,文中主要介绍了波前整形、散斑相关及深度学习等方法在散射成像领域中的研究进展。最新的研究成果表明:波前整形可以实现动态散射介质内部的高分辨率快速聚焦;散斑相关能够利用单帧散斑实现非侵入式成像;基于深度学习的成像技术能恢复出隐藏在光学厚度为13.4的白色聚苯乙烯平板背后的物体。     

长方体内多量散射介质光子密度波扩散方程研究

进一步改进了已有物理模型,从理论上解决了三维有限体积内光子密度波扩散方程的求解问题,得到了长方体边界条件下的格林函数的解析解,给出了实验可测量光通量与待测物吸收系数改变量之间可进行数值计算的表达式。本物理模型更加接近实际情况,解决了已有模型与实际情况偏差较大的矛盾,为进行反演计算提高成像质量提供了理论保证。     

沉没在高散射介质中的吸收体和散射体的扩散光学密度波取象

我们使用扩散光子密度法研究了沉没在高散射介质中的吸收体和散射体的取象,并对含有吸收体和加入不同颜色染料的散射体的取象实验结果进行了比较和分析。从实验结果可以得出：用扩散光子密度法可以探测强散射介质中吸收体或散射体的存在、大小和形状;以及在强散射介质中的散射体内加放不同颜色的颜料时,散射信号会发生不同的变化。     

强散射介质取象波长的研究

本文报导了利用扩散光子密度波取象方法研究了强散射介质取象时应该选择的最佳波长，通过在强散射介质中加入少量弱吸收物质我们得到了更加清晰的象，因此我们得出，在研究强散射介质取象时，应该选择该介质有弱吸收的波长作为取象波长。     

强散射介质的共焦取像方法研究

近年来，随着激光器和探测器技术的飞速发展，推动了非破坏性的红光和近红外光生物医学成像的发展，这些研究最多的集中在采用时间域或频率域的测量方法，探测生物组织和生物组织模型的吸收和散射系数的变化．由于频率域测量有超过时间域测量的优点；迅速的数据采集时间，可以监控组织的动力学过程，特别是轻便的和紧凑的小型仪器设备，适于在床边使用，以及成本低廉，更适于在我国推广使用，本文研究了强散射介质（生物组织是典型的强散射介质）的强度调制（频率域）扩散光子密度波取像方法．一束强度调制光射人密度无序（强散射）介质，伴…     

康普顿散射对非磁化等离子体光子晶体禁带密度温度特性的影响

应用多光子非线性康普顿(Compton)散射模型和有限时域差分法,研究了多光子非线性Compton散射对非磁化等离子体光子晶体光子禁带密度和温度特性的影响,提出了将入射光、Compton散射光、等离子体的温度和密度作为调控光子禁带的新机制。研究结果表明,改变温度和等离子体密度分布可实现对等离子体光子晶体的光子禁带拓展,而Compton散射减小了这种拓展效应。Compton散射可使光子禁带向高频方向比散射前有较大的移动,能更好地实现等离子体光子晶体的高通滤波特性。     

利用飞秒时间选通技术对埋藏在高散射介质中的物体进行成像

利用５０ｆｓ互相关时间选通门和锁相放大相结合，成功地实现了对埋藏在不同散射介质中的物体进行透射式成像。互相关时间选门有效地选出了携带物体信息的弹道光子和蛇行光子，排除了给成像带来背景噪声的漫射光子。锁相放大器的采用进一步滤掉噪声，提高了对较厚散射介质的成像能力。     

Optical properties of wounds: diabetic versus healthy tissue

Diffuse photon density wave (DPDW) methodology at Near Infrared frequencies has been used to calculate absorption and scattering from wounds of healthy and diabetic rats. The diffusion equation for semi-infinite media is being used for calculating the absorption and scattering coefficients based on measurements of phase and amplitude with a frequency domain device. Differences observed during the course of healing in the two populations can be correlated to the delayed healing observed in diabetics. These results are encouraging and further work will focus on the implementation of this device to the clinical setting as a monitoring tool in chronic diabetic wounds.     

Initial theoretical study of solar cells with an intermediate band of non‐zero width and a thermalized electron population

A model based on detailed balance principles is developed to study how the thermalized nature of the electrons in the intermediate band (IB) affects the efficiency of intermediate band solar cells. Published work on intermediate band solar cells with finite IB width has focused on the fundamental case when the absorptivity is assumed to be high for all photon energies above the smallest band gap. In this work, an attempt is made to incorporate variations in the absorptivity due to the thermal distribution of the IB electrons. In a wide IB with a thermalized electron population, there will be a low density of electrons close to the upper band edge. The density of unoccupied electron states close to the lower band edge will also be low. As a consequence, the absorption coefficients for photon energies where the only energetically allowed transitions involve exciting electrons from or to, respectively, such states can be expected to be low. The presented model incorporates the effect of the thermalized electron population in an idealized way. In some cases, the calculated efficiency is well above the limit for single band gap cells, whereas in other cases it is not. It is concluded that absorption coefficients rising rapidly from very low values to higher values are advantageous, that overlap between the absorption coefficients can be beneficial when the IB becomes sufficiently wide, and finally, that a case‐by‐case study probably is required to evaluate whether a particular IB material can give cells with high efficiency. Copyright © 2011 John Wiley & Sons, Ltd.     

Determination of the Photon Lifetime for DFB Lasers

The photon lifetime of the distributed feedback (DFB) laser is obtained through the relations between cavity photon flux and power associated with the electric fields. The solution of the coupled wave equations provides the propagation parameters and the amplitude gain coefficients for which numerical and approximate solutions are obtained. Using the photon lifetime the threshold condition of the DFB laser within a fixed mirror cavity is determined. The evaluation of both Fabry-Peacuterot and DFB modes is shown by the calculation, using two different photon lifetimes, of all modal concentrations, with the Fermi energy as the independent variable. The current density is also evaluated using the Fermi energy and threshold current is identified as the value when the Fermi energy clamps     

Monte Carlo simulation for millimeter wave propagation and scattering in rain medium

As the operating frequencies of communications systems more higher into the millimeter wave range, the effects of multiple scattering in precipitation media become more significant. This paper treats the problems of electromagnetic multiple scattering in rain medium by the Monte Carlo method. The em wave is regarded as a Markov chain of photon collisions in a medium in which it is scattered and absorbed. For the sake of simplicity, the polarization is not taken into account, the above mentioned problems are described by the scale integro-diffierential equation of transfer. When the plane wave through a random medium with particle size distribution, the technique of weighted average is used to characterize the radiation intensity, including average scattering, absorption coefficients and phase function. The Monte Carlo simulation algorithms are done for the rain attenuation and reflectance at millimeter wavelength region. Our computational results are in good agreement with experimental data of rain attenuation.     

Computation of electromagnetic waves diffraction by spectralmoments method

We solve, for the first time, electromagnetic wave propagation equations in heterogeneous media using the spectral moments method. This numerical method, first developed in condensed matter physics, was successfully applied to acoustic waves propagation simulation in geophysics. The method requires the introduction of an auxiliary density function, which can be calculated by the moments technique. This allows computation of the Green's function of the whole system as a continued fraction in time Fourier domain. The coefficients of the continued fraction are computed directly from the dynamics matrix obtained by discretization of wave propagation equations and from the sources and receivers. We illustrate this method through the study of a plane wave diffraction by a slit in two-dimensional (2-D) media and by a rectangular aperture in three-dimensional (3-D) media. Comparison with analytical results obtained with the Kirchhoff theory shows that this method is a very powerful tool to solve propagation equations in heterogeneous media. Last, we present a comparison with other computing methods     

3-羟基黄酮的双光子吸收特性研究

3-羟基黄酮（3-HF）是一类具有激发态质子转移（ESPT）效应的有机分子，通过双光子诱导荧光研究了3-HF的ESPT的过程，并利用开孔Z-扫描的方法，在波长为532nm的皮秒脉冲激光激发下测量3-HF在环己烷溶液中的ESPT的双光子吸收系数。实验测得了在不同光强条件下的双光子吸收系数，结果表明，3-HF的双光子吸收系数随入射光强的增加而减小。这是由于在皮秒脉冲光的作用下，大量的3-HF分子被激发到激发态，而不能迅速返回，造成基态的粒子数不断减小，因此当光强增强时，双光子吸收系数减小。     

Time-dependent study of the photoionization of an electron out of a quantum well in an electric field

We have calculated the time-dependent probability for the photoionization of an electron out of a quantum well in the presence of an electric field. We employ the first-order time-dependent perturbation theory and describe the wave function of the electron outside the well in the form of a wave packet combining Airy functions with energies around the fundamental plus the excitation energy. No scattering is taken into account. The calculation is performed numerically and an asymptotic expression valid for long time periods is derived. The time evolution of the wave packet gives interesting clues to the way the wave function extends in space. It is shown that soon after excitation the wave packet does not extend infinitely in space; thus the electron is in a transiently localized state which rapidly evolves towards a combination of extended states leading to a propagating wave. In the case of photon energies which completely ionize the electron out of the well, oscillations are found for very short times with a period corresponding to the energy separation of resonances in the continuum. For transitions to a bound state, the asymptotic probability density is much higher than in the bound-to-free case, but the time delay before this density is reached is much longer, so that although the absorption is strong, the photocurrent will be much reduced by a highly probable relaxation to the ground state during the delay.     

A new representation for the Green's function of multilayer media based on plane wave expansion

In this paper, a new representation for the space domain Green's function of general multilayer media is presented. This approach is based on an efficient plane wave expansion of the source incident field. Using the transmission line model for the multilayer medium, the effect of the layered medium on the incident plane waves is determined by a transmission coefficient, and the amplitude and phase of each plane wave at the field point are obtained. The total field is evaluated by summing the resultant plane waves at the field point. The main advantage of the proposed method is that the spatial domain Green's function of multilayer media can be obtained easily as a summation of simple exponential functions, without the need for Sommerfeld integration or complex image approximation. The plane wave approximation is independent of the parameters of the medium and is valid over a wide frequency range. Furthermore, the exponential form of the plane wave solution makes it possible to compute the method of moments matrix elements analytically for most important types of basis functions in multilayer problems.