超常介质非线性光学研究进展

光与超常介质相互作用时,光的磁场也变得很重要。超常介质的色散磁导率导致光与超常介质的非线性相互作用呈现出许多新特性。总结了超常介质中二阶非线性光学现象的国内外研究进展,报道了在三阶非线性超常介质中光的传输特性方面的研究结果,包括传输模型的建立、调制不稳定性、孤子和自聚焦现象等。     

Calculation of linear and nonlinear intersubband optical absorptions in a quantum well model with an applied electric field

Analytic forms of the linear and the third-order nonlinear optical intersubband absorption coefficients are obtained for general asymmetric quantum well systems using the density matrix formalism, taking into account the intrasubband relaxation. Based on this model, we calculate the electric field dependence of the linear and the third-order nonlinear intersubband optical absorption coefficients of a semiconductor quantum well. The energy of the peak optical intersubband absorption is around 100 meV (wavelength is 12.4 μm). Thus, electrooptical modulators and photodetectors in the infrared regime can be built based on the physical mechanisms discussed here. The contributors to the nonlinear absorption coefficient due to the electric field include 1) the matrix element variation and 2) the energy shifts. Numerical results are illustrated.     

Anomalously Strong Second-Harmonic Generation in GaAs Nanowires via Crystal-Structure Engineering

GaAs-based semiconductors are highly attractive for diverse nonlinear photonic applications, owing to their non-centrosymmetric crystal structure and huge nonlinear optical coefficients. Nanostructured semiconductors, for example, nanowires (NWs), offer rich possibilities to tailor nonlinear optical properties and further enhance photonic device performance. In this study, it is demonstrated highly efficient second-harmonic generation in subwavelength wurtzite (WZ) GaAs NWs, reaching 2.5  ×  10⁻⁵ W⁻¹, which is about seven times higher than their zincblende counterpart. This enhancement is shown to be predominantly caused by an axial built-in electric field induced by spontaneous polarization in the WZ lattice via electric field-induced second-order nonlinear susceptibility and can be controlled optically and potentially electrically. The findings, therefore, provide an effective strategy for enhancing and manipulating the nonlinear optical response in subwavelength NWs by utilizing lattice engineering.     

半抛物量子阱中的线性与非线性光学吸收系数

采用密度矩阵与迭代处理的方法,给出了计算施加电场的半抛物量子阱(QWs)系统中的线性与非线性光学吸收系数的详细过程.并且推导了此系统中的简单解析的线性与非线性光学吸收系数.对GaAs材料构成的QWs进行了数值计算.结果显示,系统中的线性与非线性光学吸收系数敏感地依赖于外加的电场强度、QWs的受限势频率以及入射光的强度.     

Terahertz Photon Mixing Effect in Gapped Graphene

We theoretically calculate the terahertz waves mixing effect in doped graphene with a finite bandgap. The temperature dependence of the nonlinear intraband optical response at bandgap opening of few tens of meV are investigated. When the external electric field is weak, a moderate level of bandgap opening is found to slightly enhance the nonlinear optical response. The optical response is however significantly altered under strong-field condition. The strong-field nonlinear optical conductivity exhibits two distinct response ‘hot spot’: (i) low temperature with large bandgap and (ii) high temperature with small bandgap. The electric field required for the nonlinear response to dominate over the linear response is typically in the order of 10⁴ V/cm. This value increases rapidly by a factor of 10 in large bandgap and high temperature regimes. Our results suggest that photon mixing effect in gapped graphene is strongly gapped dependent and hence the bandgap opening has to be carefully engineered in order to optimize the photon mixing effect in gapped graphene.     

Electric-field enhancement and extinguishment of opticalsecond-harmonic generation in asymmetric coupled quantum wells

The second-harmonic susceptibilities of asymmetric coupled quantum wells (ACQW) and compositionally asymmetric coupled quantum wells (CACQW) under the influence of the applied electric field are investigated theoretically. Analytic forms of the second-harmonic susceptibility are derived by using the density matrix formalism. Coupled one-dimensional Schrodinger equation and Poisson's equation are solved self-consistently to find the subband eigenenergies and the envelope wavefunctions for the ACQW and CACQW structures. Dipole moment matrix elements for the ACQW and CACQW are evaluated from the resulting envelope wavefunctions and large nonlinear optical effects are predicted for these two structures. Based on the theoretical calculations, the second-harmonic susceptibilities of 50 nm/V and 35 nm/V can be achieved for the ACQW and the CACQW, respectively. This is more than two orders of magnitude enhancement as compared to that of the bulk GaAs. By a suitable choice of the composition of CACQW, a novel structure which the second-order nonlinear optical effect can be turned on or off by the applied electric field is proposed based on the results of the theoretical calculation. This phenomenon is attributed to the symmetry restoration of envelope wavefunctions of the CACQW under the quenching electric field F_off. A simple physical model to estimate the F_off has also been developed successfully     

Current-limiting thermistors for high-power applications

Applications of positive-temperature-coefficient polymer thermistors as current-limiting elements in high-power systems are complicated by the nonlinear scaling of device parameters. The spatial and temporal evolution of the internal quantities temperature and electric field, and the temporal evolution of the external device property resistance, are described here as calculated according to a one-dimensional electrothermal model. Under a steeply ramping current excitation, a longitudinal heating instability compresses the electric field into the center of the material, thereby inducing dielectric failure. Insight into the design of a useful device is provided by results of calculations under three different conditions     

准相位匹配（QPM）技术研究新进展

准相位匹配是一种人为地制备周期结构以实现非线性频率转换技术,它拓宽了非线性晶体应用范围,提高了频率转换效率。本文详细介绍了准相位匹配主要技术优势,并描述周期极化畴反转技术,这一当前普遍用来制备准相位匹配非线性晶体的实验方法,同时总结了这一领域国内外研究的最新进展。     

Optical-fiber communications for electric power companies in Japan

The Japanese electric power companies have long been interested in the optical fiber communication-system features, such as non inductivity, high-insulation property, large transmission capacity etc. Since 1974 they have been conducting field experiments utilizing optical-fiber communications for transmitting power-system protection, supervision, and control data. Based on these works, six Japanese electric companies have installed nine optical fiber communication systems since 1978, and their performance records are fully satisfactory.     

用ATR方法研究液晶表面光学性质

本文用衰减全反射方法研究液晶在基片表面上的性质，测试在外电场作用下液晶分子重新定向引起光学性质的变化。液晶薄层的光学常数和厚度用ＡＴＲ曲线拟合计算。     

Nonlinear thermo-optic model for the characterization of optical self-heating in electro-optic semiconductors

A nonlinear thermo-optic model for the characterization of electro-optic semiconductor probes used to simultaneously measure electric field and temperature is presented. Optical thermal self-heating of the semiconductor is shown to be the primary limiting mechanism for temperature dynamic range, electro-optic modulation power, temperature sensitivity, temperature contrast, and temperature invasiveness. Tradeoff considerations of these parameters as a function of optical wavelength and input optical power are discussed. Optical temporal defocusing is shown to minimize the effects of nonlinear absorption. Simulation results are compared with measurements.     

矢量光场激发三阶非线性光学效应的研究进展（特邀）

具有偏振结构分布的强激光与非线性光学材料相互作用导致了多种新颖的非线性光学效应,反映了材料的非线性光学特性,调制了光场的传播行为。笔者概述了矢量光场激发三阶非线性光学效应的研究进展。首先简要介绍了任意偏振光激发三阶非线性光学效应的理论,如非线性薛定谔方程、光束传播方程、各向同性和各向异性三阶非线性光学系数。也简要介绍了表征三阶非线性光学系数的 Z-扫描技术。在弱聚焦条件下,给出了诸如径向偏振光、杂化偏振光和柠檬型庞加莱光束这三种类型矢量光场的焦场表达式。其次,重点回顾了多种矢量光场激发的各向同性/各向异性三阶非线性光学效应,包括径向偏振光激发的各向异性非线性光学效应、杂化偏振光激发的各向同性和各向异性非线性光学效应、柠檬型庞加莱光束激发的各向同性/各向异性非线性光学效应。最后,简要讨论了矢量光场在非线性偏振旋转、光束整形、可控光场塌缩与成丝和光限幅方面的应用。     

Nonlinear polarization rotation in semiconductor optical amplifiers: theory and application to all-optical flip-flop memories

We present a model for polarization-dependent gain saturation in strained bulk semiconductor optical amplifiers. We assume that the polarized optical field can be decomposed into transverse electric and transverse magnetic components that have indirect interaction with each other via the gain saturation. The gain anisotropy due to tensile strain in the amplifier is accounted for by a population imbalance factor. The model is applied to a nonlinear polarization switch, for which results are obtained, that are in excellent agreement with experimental data. Finally, we describe an all-optical flip-flop memory that is based on two coupled nonlinear polarization switches.     

Optically nonlinear waves in thin films

A new treatment of the behavior of TE nonlinear waves in an optically nonlinear film is given. The new mathematical results are expressed in terms of the physical parameters of the system and represent a straightforward way to introduce the necessary Jacobian elliptic functions. The optical nonlinearity is of the Kerr type and the numerical calculations are performed for a self-focusing medium. Dispersion curves labeled with optical power density at the lower film boundary, detailed plots of the variation of electric field amplitude as the wave number changes, and details of the power distribution across the guide are given. Since two values of a wave number can exist for the same power level and power thresholds exist, the system is of device interest in the area of optical switching.     

Response of the Internal Electric Field in CdZnTe to Illumination at Multiple Optical Powers

Manipulation of CdZnTe (CZT) crystals using illumination is a useful tool for altering the internal electric field present under normal bias conditions. The interactions with carriers that are trapped at either terminal are visualized by the electric field distribution through polarization. In this report, we demonstrate an ability to selectively manipulate the internal electric field of CZT using multiple-wavelength light illumination at various optical powers. The internal electric field polarization can be controlled using changes in optical power. We also investigate the electric field distributions using multiple optical powers to examine the light response as a function of light penetration depth.     

A numerical solution to full-vector electromagnetic scattering bythree-dimensional nonlinear bounded dielectrics

This paper deals with electromagnetic scattering by nonlinear dielectric objects. In particular, a numerical approach is developed that is aimed at determining the distributions of the electromagnetic field vector inside a three-dimensional nonlinear, inhomogeneous, isotropic scatterer illuminated by a time-periodic incident electric field vector. An integral-equation formulation for the full-vector scattering problem is considered, and the nonlinear effect is taken into account by introducing equivalent sources and a Fourier-series representation. A system of integral equations (for each harmonic vector component and for the static term) is obtained that includes the internal electric field distribution as the unknown. After discretization, the solution is reduced to solving an algebraic system of nonlinear equations. Some preliminary numerical results are reported concerning scatterers that exhibit a specific (quadratic) dependence of the dielectric permittivity on the total electric field. The harmonic components of the scattered electric field outside the objects are also computed     

Donor impurity-related optical absorption coefficients and refractive index changes in a rectangular GaAs quantum dot in the presence of electric field

Within the quasi-one-dimensional effective potential model and effective mass approximation, we obtain the wavefunctions and energy eigenvalues of the ground (j=1) and first 2 excited states (j=2 and 3) of a donor impurity in a rectangular GaAs quantum dot in the presence of electric field. The donor impurity-related linear and nonlinear optical absorption as well as refractive index changes for the transitions j=1-2 and j=2-3 are investigated. The results show that the impurity position, incident optical intensity and electric field play important roles in the optical absorption coefficients and refractive index changes. We find that the impurity effect induces the blueshift for j=1-2 and redshift for j=3-2 in the absence of the electric field, but it leads to redshift for j=1-2 and blueshift for j=3-2 in the existence of the field. Also, the optical coefficient for the higher energy transitions j=2-3 is insensitive to variation of impurity positions, while that for the low energy transition j=1-2 depends significantly on the positions of impurity. In addition, the saturation and splitting phenomenon of the optical absorption are observed as the incident optical intensity increases.     

Monte Carlo simulation of THz maser based on optical phonon transittime resonance in GaN

The linear and nonlinear analysis of microwave power amplification and generation in the terahertz (THz) frequency range is carried out by the Monte Carlo method in bulk wurtzite GaN. It is shown that a dynamic negative differential mobility persists up to 80 K due to the transit-time resonance associated with optical phonons. The generation frequency depends almost linearly on the static electric field strength and can be tuned in the wide frequency range from 0.2 to 3 THz when the electric field varies from 1 up to 12 kV/cm. The optimum amplitude of the microwave electric field generated by bulk GaN is found to be of the order of the static electric field thus leading to a significant maximum efficiency of about 1 to 1.5% for microwave power generation in the THz frequency range     

Direct measurement of nonlinear coefficient of optical fibre using linear optical sampling

The nonlinear phase shift induced on a time-varying optical source propagating in an optical fibre is directly measured using linear optical sampling, a coherent sampling oscilloscope that can measure samples of the electric field of an optical source. Comparison of the measured amplitude and phase yields a direct measurement of the nonlinearity of the optical fibre.     

纳米光存储薄膜结构的光学性质

近场超分辨纳米薄膜结构可以突破衍射极限实现纳米尺寸信息存储,是下一代海量存储技术的重要方案之一,也是纳米光子学研究中的热点.纳米膜层结构基于激光作用下的非线性局域光学效应实现超分辨.分析了超分辨近场薄膜结构突破衍射极限的光学原理,对超分辨纳米薄膜结构的表面等离子体激发特性、非线性光学特性、近场光学特性和超透镜效应等重要光学性质的最新研究进展做了系统介绍.