Accumulation of dwell times in light diffusion

We discuss various speeds of propagation that are relevant for the dynamics of classical waves in random media: phase velocity, group velocity and transport velocity. The transport velocity can be much smaller than the phase velocity due to the long dwelling of classical waves inside resonant scatterers. We show that the transport velocity of light can be obtained experimentally by study both transient and steady-state diffusion of light.     

Nonlinear polarizability and plasmon resonances in terahertz photoconductivity

The nonlinear polarization of an electron plasma in an incident terahertz (THz) field is examined in an iterated shielded potential approximation. It is found to exhibit resonant behavior when the incident terahertz frequency matches the plasma frequency, with concomitant resonant increase of the dielectric function. Such resonant behavior substantially reduces the effectiveness of the screened impurity scattering potentials, admitting resonant increase of conduction when the THz frequency matches the plasma frequency.     

Scattering and propagation of terahertz pulses in random soot aggregate systems

Scattering and propagation of terahertz pulses in random soot aggregate systems are studied by using the generalized multi-particle Mie-solution(GMM) and the pulse propagation theory. Soot aggregates are obtained by the diffusion-limited aggregation(DLA) model. For a soot aggregate in soot aggregate systems, scattering characteristics are analyzed by using the GMM. Scattering intensities versus scattering angles are given. The effects of different positions of the aggregate on the scattering intensities, scattering cross sections, extinction cross sections, and absorption cross sections are computed and compared. Based on pulse propagation in random media, the transmission of terahertz pulses in random soot aggregate systems is determined by the two-frequency mutual coherence function. Numerical simulations and analysis are given for terahertz pulses(0.7956 THz).     

Nanofocusing of terahertz wave on conical metal wire waveguides

The nanofocusing of the terahertz (THz) radiation energy is studied. By using a conical metal nanowire waveguide, we focus the energy of the terahertz surface plasmon polaritons (THz SPPs) to several nanometers’ scale. Another interesting property of the THz SPPs propagation on the waveguide is that the peak electric field at the waveguide tip enhances many times. What is more, both the phase velocity and the attenuation coefficient versus the wire radius are obtained. The terahertz energy nanofocusing opens the way to observe terahertz propagating and imaging on the nanoscale.     

THz-field-induced electronic transmission step-structure for a quantum wire

We study theoretically the low-temperature electronic transport property of a straight quantum wire under the irradiation of a finite-range transversely polarized external terahertz (THz) electromagnetic (EM) field. Using the freeelectron model and the scattering matrix approach, we show an unusual behaviour of the electronic transmission of this system. A sharp step-structure appears in the electronic transmission probability as the EM field strength increases to a threshold value when a coherent EM field is applied. We demonstrate that this effect physically comes from the inelastic scattering of electrons with lateral photons through intersubband transitions.     

Decorrelation and phase-shift of coda waves induced by local changes: multiple scattering approach and numerical validation

We report on theoretical predictions of the decorrelation and phase-shift of coda waves induced by local changes in multiple scattering media. Using the multiple scattering formalism, we show that both expressions (decorrelation and phase-shift) involve a same sensitivity kernel based on the intensity transport in the medium. We compare the kernels based on the diffusion approximation with the ones based on the radiative transfer approximation, showing that the latter is more accurate at short times or for changes located close to the source or the receiver. We also perform a series of numerical simulations of wave propagation (finite differences) to validate our models in different configurations.     

Transport properties of random media composed of core-shell spheres

With the energy-density coherent potential approximation method, a series of calculations concerning the contribution from the morphology and dispersion of random media composed of core-shell spheres on the transport properties of random media are conducted in terms of the scattering-cross-section efficiency factor, mean free path, velocity of electromagnetic energy, and diffusion coefficient. It is found that the core layer introduces more complicated resonant modes which lead to diverse possibilities to sharply decrease the transport of light within random media.     

太赫兹波在雾中的多重散射特性

太赫兹(THz)波作为微波和毫米波的延伸,它所提供的通信带宽和容量远大于毫米波。在随机介质中传播时,不但会发生时域和空域的形变,介质中的粒子还会对入射波发生散射,这些都会使得脉冲信号发生衰减。根据Mie理论与随机离散分布粒子的波传播与散射理论,计算了THz波信号入射下雾滴粒子的消光系数。结合雾滴粒子谱分布,得到了雾媒质的平均体系散射特性,采用蒙特卡罗法得到了平流雾对THz信号的多重散射特性,计算了THz波段信号对平流雾的透过率与反射率,分析了THz波段信号的前向、后向散射特性随散射角的分布。结果表明,低能见度大气环境中,雾对THz波产生的吸收和衰减不容忽视。相关研究结果对THz在大气传输、通信等方面的应用具有重要意义。     

Effect of spherical aberration and surface waves on propagation of lens-coupled terahertz pulses

Spatiotemporal measurements of a near-single-cycle terahertz pulse emitted from a photoconductive switch terahertz (THz) source show the effects of spherical aberration and surface waves on the pulse shape. The measured phase front has a swallow-tail shape described by catastrophe theory that contributes to the concentric ring structure of THz beam profiles. A time-of-flight model shows that the pulse shape is due to propagation along a cusp caustic and enhancement of the wings of the swallow-tail pulse is caused by surface waves.     

Transmission of terahertz radiation using a microstructured polymer optical fiber

A hollow-core microstructured polymer optical fiber was analyzed in the terahertz (THz) region. Spectral analysis of time domain data shows propagation of THz waves in both the hollow-core and the microstructured cladding with a time delay of approximately 20 ps. The frequency range and shift of the transmission bands between different sized waveguides suggested photonic bandgap or resonant guidance. Finite-difference time domain calculations agree relatively well to the experimental transmission results. Propagation losses were estimated to be as low as 0.9 dB/cm.     

多频带太赫兹滤波器的设计

基于不同大小形状的谐振环对电磁场不同响应的原理,设计了一种由6个嵌套式的方形封闭谐振环结构（CRR）组成的多频带太赫兹滤波器。通过利用时域有限差分法(FDTD)对该滤波器的透射特性进行研究,结果表明:当电磁波正入射或斜入射到谐振环所在平面时,能够出现六频带的滤波性,并且其透射系数对入射角度的变化并不敏感。该设计增加了滤波频带的个数,提高了滤波器对频率的敏感度,为多频带传感器、太赫兹通信技术等领域提供了理论方法。     

Performance optimization for terahertz quantum cascade laser at higher temperature using genetic algorithm

The aim of this work is to establish an approach for obtaining improved design parameters for high temperature operation of terahertz quantum cascade lasers using a multi-objective evolutionary algorithm. For studying the lasing conditions of a quantum cascade laser, a self-consistent model is adopted. This model uses standard wave function approximation and effective mass approximation with relevant scattering mechanisms to solve Schrodinger’s equation for the cascaded quantum wells. Fermi’s Golden Rule is then used to calculate the corresponding lifetime of each eigen states. To describe the coherent evolution of wave functions and phase breaking, density matrix formalism is employed. Subsequently, laser rate equations are used for calculating the parameters related to electronic transport in the device. These parameters are then utilized for investigating the temperature dependence of existing terahertz quantum cascade lasers. Finally, using an optimization technique based on Genetic algorithm, design parameters for resonant-phonon quantum cascade laser are obtained within the terahertz frequency range. The results illustrate that this optimization process can offer improvement in the performance of quantum cascade lasers in terahertz region at an elevated temperature. Moreover, the results also reveal that significant increase in operating temperature of a resonant phonon terahertz QCL is unlikely and hence novel design approaches should be considered for operating THz QCLs at room temperature.     

Generation of electromagnetic waves in the terahertz frequency range by optical rectification of a Gaussian laser pulse in a plasma in presence of an externally applied static electric field

We propose a theoretical model for the generation of electromagnetic waves in the terahertz (THz) frequency range by the optical rectification of a Gaussian laser pulse in a plasma with an applied static electric field transverse to the direction of propagation. A Gaussian laser pulse can exert a transverse component of the quasi‐static ponderomotive force on the electrons at a frequency in the THz range by a suitable choice of the laser pulse width. This nonlinear force is responsible for the density oscillation. The coupling of this oscillation with the drift velocity acquired by electrons due to the applied static electric field leads to the generation of a nonlinear current density. A spatial Gaussian intensity profile of the laser beam enhances the generated THz yield by many folds as compared to a uniform spatial intensity profile.     

太赫兹波段电磁超材料吸波器折射率传感特性

太赫兹超材料吸波器作为一类重要的超材料功能器件,除了可以实现对入射太赫兹波的完美吸收外,还可以作为折射率传感器实现对周围环境信息变化的捕捉与监测.通常从优化表面金属谐振单元结构和改变介质层材料和形态两个方面出发,改善太赫兹超材料吸波器的传感特性.为深入研究中间介质层对太赫兹超材料吸波器传感特性的影响,本文基于金属开口谐振环阵列设计实现了具有连续介质层、非连续介质层和微腔结构的3款太赫兹超材料吸波器,并对其传感特性与传感机理进行了深入研究.结果表明,为了提高太赫兹超材料吸波器的折射率灵敏度、最大探测范围等传感特性,除了可以选用相对介电常数较小的材料作为中间介质层外,还可以改变中间介质层的形态,进而减小中间介质层对谐振场的束缚,增强谐振场与被测分析物之间的耦合.与传统的具有连续介质层的太赫兹超材料吸波器相比,具有非连续介质层和微腔结构的超材料吸波器具有更优越的传感特性,可应用于对待测分析物的高灵敏度、快速检测,在未来的传感领域具有更加广阔的应用前景.     

Simulation of the propagation of a light pulse in a randomly inhomogeneous medium

The propagation of a Gaussian pulse in an inhomogeneous medium is considered. The detected pulse is represented as a series in the scattering multiplicities. The developed approach enables one to describe the passage of a pulse through a randomly inhomogeneous medium in the general case, beyond the framework of the diffusion approximation. The known results of the diffusion approximation are shown to represent limiting cases of the expansion constructed in the scattering multiplicities. The calculations are performed by the Monte Carlo method for cases of singly and multiply scattering media. The agreement between the results of the theoretical analysis and the numerical simulation allows one to use the approach developed in both optical problems and investigation of propagation of waves of other nature, in particular, seismic waves.     

Wave propagation in heterogeneous bistable and excitable media

Two examples for the propagation of traveling waves in spatially non-uniform media are studied: (a) bistable media with periodically varying excitation threshold and (b) bistable and excitable media with randomly distributed diffusion coefficient and excitation properties. In case (a), we have applied two different singular perturbation techniques, namely averaging (first and second order) and a projection method, to calculate the averaged front velocity as a function of the spatial period L of the heterogeneity for the Schlögl model. Our analysis reveals a velocity overshoot for small values of L and propagation failure for large values of L. The analytical predictions are in good agreement with results of direct numerical simulations. For case (b), effective medium properties are derived by a self-consistent homogenization approach. In particular, the resulting velocities found by direct numerical simulations of the random medium are reproduced well as long as the diffusion lengths in the medium are larger than the heterogeneity scale. Simulations reveal also that complex irregular dynamics can be triggered by heterogeneities.     

Propagation of p-polarized electromagnetic waves obliquely incident on stratified random media: Random phase approximation

We consider the propagation of p-polarized electromagnetic waves obliquely incident on stratified random dielectric media. Using the invariant imbedding method generalized to random media and applying the random phase approximation, we derive a simple analytical expression of the localization length and calculate the disorder-averaged reflectance and transmittance and the fluctuations of the localization length and the reflectance as functions of the incident angle. We also calculate the disorder-averaged intensity profile of the magnetic field inside the random medium. We find that within the random phase approximation, the p wave can be delocalized and transmitted completely at a certain critical incident angle, which is bigger than the Brewster angle in the uniform case.     

Particle Transport through Scattering Regions with Clear Layers and Inclusions

This paper introduces generalized diffusion models for the transport of particles in scattering media with nonscattering inclusions. Classical diffusion is known as a good approximation of transport only in scattering media. Based on asymptotic expansions and the coupling of transport and diffusion models, generalized diffusion equations with nonlocal interface conditions are proposed which offer a computationally cheap, yet accurate, alternative to solving the full phase-space transport equations. The paper shows which computational model should be used depending on the size and shape of the nonscattering inclusions in the simplified setting of two space dimensions. An important application is the treatment of clear layers in near-infrared (NIR) spectroscopy, an imaging technique based on the propagation of NIR photons in human tissues.