The theory of non-Markovian gain in semiconductor lasers

In this paper, non-Markovian optical gain of a semiconductor laser is derived from recently developed time convolutionless (TCL) quantum kinetic equations for electron-hole pairs, including the many body effects. Plasma screening and excitonic effects are taken into account using an effective Hamiltonian in the time-dependent Hartree-Fock approximation. To calculate the optical gain, equation of motion for the interband pair amplitude is integrated directly. It is shown that the line shape of optical gain spectra is Gaussian for the simplest, non-Markovian quantum kinetics, and the optical gain is enhanced by the excitonic effects caused by the attractive electron-hole Coulomb interaction and the interference effects (renormalized memory effects) between the external driving field and the stochastic reservoir of the system. Enhancement of optical gain by the memory effects suggests the violation of strict energy conservation on a very short time scale, as compared with the correlation time of the system governed by non-Markovian quantum kinetics     

1 THz Modulation in InGaAsP Multiple Quantum Wells for 40 Gb/s Applications

The terahertz (THz) rate modulation of quantum well (QW) electrooptic modulators necessitates a new way of thinking about how the modulation field modulates light; specifically, an incident narrow-band (with respect to the modulation frequency) signal once modulated acquires frequency components separated from the input signal center frequency by multiples of the modulation frequency. In this paper, we discuss the design of the QWs comprising the modulator to maximize the output at such THz sidebands of the incident optical frequency in InGaAsP QW based devices. We present a theoretical treatment of the case in which the THz modulation frequency is out of resonance between any exci- tonic levels near those exploited for the optical modulation, thus enabling an adiabatic treatment of the modulated optical susceptibility. We show that THz sideband conversion efficiencies of ~1% may be possible.     

Power Scaling on Efficient Generation of Ultrafast Terahertz Pulses

We have investigated power scaling for the efficient generation of the broadband terahertz (THz) pulses. These THz short pulses are converted from ultrafast laser pulses propagating in a class of semiconductor electrooptic materials. By measuring the dependence of the THz output on the pump beam in terms of incident angle, polarization, azimuthal angle, and pump intensity, we have precisely determined the contributions made by optical rectification, drift of carriers under a surface or external field, and photo-Dember effect. When a second-order nonlinear material is pumped below its bandgap, optical rectification is always the mechanism for the THz generation. Above the bandgap, however, the three mechanisms mentioned earlier often compete with one another, depending on the material characteristics and pump intensity. At a sufficiently high pump intensity, optical rectification usually becomes the dominant mechanism for a second-order nonlinear material. Our analysis indicates that second-order nonlinear coefficients can be resonantly enhanced when a material is pumped above its bandgap. In such a case, the THz output power and normalized conversion efficiency can be dramatically increased. We have also analyzed how the THz generation is affected by some competing processes such as two-photon absorption.     

太赫兹可控功能器件的研究进展

近年来太赫兹(THz)科学与技术的研究成为光学领域的研究热点,其在医学、通信和公共安全等领域具有深远的研究价值和重要的应用前景。但是THz波段功能器件尤其是可控功能器件匮乏,限制了太赫兹技术的进一步应用。表面等离激元光子学和奇异介质的出现有望改变这一局面,它能够在亚波长尺度对电磁波进行路由和操纵,有望成为下一代芯片技术的候选方案。首先介绍了基于表面等离体和奇异介质的可控太赫兹功能器件的物理机制,然后从电、磁、热、光四种主动控制手段来分别展示近年来太赫兹波段功能器件的研究进展,并从功能应用上对太赫兹功能器件进行了简单的分类总结,最后讨论了在应用中存在的问题和面临的挑战。     

Interferometric imaging with terahertz pulses

We demonstrate a new reflection imaging technique using single-cycle pulses, in which the sample to be imaged is placed at the focus of a lens in one arm of a Michelson interferometer. The detected signal is the superposition of the pulses from the two arms of the interferometer, one with a sample and one with a reference mirror. Because of the Gouy phase shift from the lens in the sample arm, these two pulses are nearly π out of phase, and can destructively interfere. This leads to a strong cancellation of the measured waveform and a nearly background-free imaging method. We demonstrate the technique using terahertz time-domain spectroscopy and show that the destructive interference provides enhanced sensitivity to features in the sample that are much thinner than the coherence length of the radiation. This technique could be valuable in any low-coherence tomographic measurement in which the reflected electric field can be measured directly     

太赫兹技术在军事和安全领域的应用

由于太赫兹波处于电磁波谱的特殊频段,属于电子学向光子学过渡区域,具有辐射小、透射性好、方向性强、频谱宽、通信传输容量大、对很多极性生物大分子有指纹谱等特点。因此太赫兹光谱技术、成像技术和雷达通信技术在军事和安全领域具有重要的应用价值。综述了近年来太赫兹技术在国内外军事、安全领域的应用成果,探讨了存在的问题,并对该技术在该领域的应用进行了展望。     

太赫兹科学技术在生物医学中的应用研究

介绍了近十年来国内外太赫兹科学技术在生物医学领域的研究与应用进展.重点总结了太赫兹辐射的生物效应、太赫兹光谱技术和太赫兹成像技术,在生物医学领域的研究与应用的3个大类的成果.有关太赫兹辐射的生物效应,分不同频率、不同功率和不同辐射时间下样品的生物学反应来介绍.又根据探测样品属性不同分为生物体、生物组织、生物细胞、生物细胞器和生物大分子等6个层次的内容.充分论证了太赫兹科学技术对生物医学的安全性和适用性.由于太赫兹光谱包含丰富的生物分子及分子间的组成信息和构象信息,在生物医学领域的研究与应用最多.对不同生物大分子的太赫兹特征频谱进行了解读,对太赫兹光谱技术的应用进行了梳理,目的为让更多的人了解太赫兹科学技术的优越性和应用前景.在太赫兹渡成像技术方面,总结了其在癌变组织诊断、皮肤烧伤探测等方面的研究应用.为使迷人的太赫兹波科学与技术更好的服务于人类,尝试较为全面地对太赫兹科学技术在生物医学领域取得的研究成果进行归纳与总结,同时也探讨了现阶段在生物医学领域所存在的不足,以及今后努力的方向.     

Study of molecular behavior in oxidation of insulating oil using terahertz spectroscopy

The stock of aged power equipment is increasing and it is important to clarify the characteristics of deteriorated insulating oil. This paper describes a basic study of molecular behavior in the oxidation of insulating oil by terahertz spectroscopy. The oxidation of insulating oil was modeled by using various compositions of dodecane and 2‐octanol. The terahertz measurements indicated that signi?cant absorption related to hydroxyl groups (‐OH) occurred between 6 and 7 THz. The absorption band was also observed in accelerated aging alkylbenzene, which is used in power equipment as insulating oil and is sensitive to hydrogen bonds. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 183(1): 9–15, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22335     

基于USB的主动太赫兹成像电路设计

简要介绍了一套针对双波段主动太赫兹无损检测系统研制的基于USB通信的电路系统.该电路系统可通过控制二维扫描机构实现太赫兹无损检测系统的二维扫描,利用双波段太赫兹探测器接收经目标反射的由双波段太赫兹源辐射的太赫兹信号,并对该信号利用过采样方式实现数据采集,通过USB实现数据的上位机传输,并最终利用频域滤波和小波变换等图像处理技术实现主动太赫兹成像.成像结果表明,电路系统符合实际需求,双波段太赫兹无损检测系统可实现分辨率3 mm的无损检测,能很好地满足泡沫等非极性材料的无损检测的应用需求.     

Body fitted grid generation with moving boundary and its application for dielectric waveguides

The stress birefringence and mode coupling effects in polarization-preserving fibres are most important problems which need to be improved. For a realization of some optical devices, the dielectric waveguide with sinusoidally varying circular cross-section has been investigated. It becomes very important to analyse the electromagnetic field distribution in a dielectric waveguide with a time-dependent moving boundary. This paper shows that numerical methods can simulate the effect of the external disturbance on the dielectric waveguide from time to time. The author has discussed body fitted grid generation with moving boundary for the Poisson and Laplace equations.^1,2 We have extended this technique for Maxwell's equation. The technique employs a kind of an expanded numerical grid generation. As the author adds the time component to grid generation, the time dependent co-ordinate system which coincides with a contour of moving boundary can be transformed into a fixed rectangular co-ordinate system. We could show the electric distribution in the waveguide time by time to verify the possibility of an application for an optical fibre. This technique makes it possible not only to analyse the effect of the external disturbance in a coherent optical communication system but also to fabricate optical devices.     

Development of a portable diagnostic apparatus for coil insulatorsin low-voltage induction motors

When most organic insulators are exposed to thermal stress, they turn darker in color. The increase of electronic transition absorption due to thermal oxidation of the resin is responsible for this darker coloration. A correlation between the degree of aging of these insulators and their reflective absorbances in the near IR (infrared) wavelength range has been established previously. In this paper, a novel non-destructive diagnostic apparatus using an optical fiber sensor developed to evaluate the aging of thermally deteriorated insulators is reported. The key point of this technique is measuring the change of reflective absorbance ratio at two different wavelengths. The sensor consists of two plastic optical fibers (for light transmitting and receiving), a plastic optical coupler, two kinds of near-IR light emitting diodes as light sources, and an optical power meter as a detector, and hence is very compact and lightweight (<2 kg). The aging of an insulator can be estimated using the diagnostic curve, which is obtained by an accelerated heating experiment using model pieces. We applied this measurement technique to predict the residual life of low-voltage induction motors used in elevators     

碳纤维复合材料不同深度缺陷的太赫兹检测研究

太赫兹(THz)无损检测技术具有非破坏性、非电离和非接触的优点,在航空航天领域纤维增强复合材料无损检测中得到了较快的发展和应用。在碳纤维复合材料层合板的4个不同深度(0.225、0.450、0.675、0.900 mm)插入聚四氟乙烯作为人工缺陷,采用太赫兹时域光谱和成像系统对其进行成像和光谱分析,探讨太赫兹波辐射下缺陷的成像效果和光谱特性。研究结果发现,在0.25~2.0 THz频率范围内,太赫兹反射成像可以成功检测出碳纤维复合材料中不同深度缺陷:随着缺陷深度的增加,太赫兹频域成像信号和光谱信号随缺陷深度线性增大,吸收系数成像信号和光谱信号随缺陷深度线性减小;随着频率的增加,缺陷的功率谱密度先增大后减小,吸收系数缓慢增大。该结果可以为碳纤维复合材料缺陷深度的可视化和定量化分析提供参考依据。     

Conduction mechanisms and high-field effects in syntheticinsulating polymers

The absorption and conduction currents in synthetic insulating polymers, viz., polyethylene, polypropylene, polyethylene-terephthalate and polyvinylidenefluoride, with dc fields may be electronic or ionic both and it may be difficult to determine their origins unambiguously. The charges which are present in the bulk of the polymer may be responsible for both the absorption and the conduction current. A limited displacement of charges together with the rotation of side chains may provide a decaying transient current in the presence of an externally impressed field whereas the conduction current may originate from a hopping of charge carriers in which the charges transfer between localized states. The complex dielectric response of these polymers with ac field may also be explained by intra- and inter cluster charge movement which would be consistent with the unified model, stated above for the case of dc fields     

Dynamics of dual-frequency solitons under the influence of frequency-sliding filters, third-order dispersion, and intrapulse Raman scattering

We analyze the structure of the optical field emerging from a superposition of two soliton-like pulses with different frequencies and arbitrary phase-shift between them, and show that the optical output contains either symmetric or antisymmetric two-soliton states. Furthermore, we study numerically the dynamics of these emerging two-soliton states under the influence of perturbative effects that are important for optical communications systems: frequency-sliding filters, third-order dispersion, and intrapulse Raman scattering.     

Scaling the modulation bandwidth and phase efficiency of a silicon optical modulator

We present an optimized design and detailed simulation of an all-silicon optical modulator based on a silicon waveguide phase shifter containing a metal-oxide-semiconductor (MOS) capacitor. Based on a fully vectorial Maxwell mode solver, we analyze the modal characteristics of the silicon waveguide. We show that shrinking the waveguide size and reducing gate oxide thickness significantly enhances the phase modulation efficiency because of the optical field enhancement in the voltage induced charge layers of the MOS capacitor, which, in turn, induce refractive index modulation in silicon due to free carrier dispersion effects. We also analyze the device speed by transient semiconductor device modeling. As both optical absorption and modulation bandwidth increase with increasing doping concentration, we show that, with a nonuniform doping profile in the waveguide, balance between the device operation speed and optical loss can be realized. Our simulation suggests that a TE-polarized optical phase modulator with a bandwidth of 10 GHz and an on-chip optical loss less than 2 dB is achievable in silicon.     

Homodyne In-Phase and Quadrature Detection of Weak Coherent States With Carrier Phase Tracking

We present a homodyne receiver structure for the detection of weak coherent states that uses sequential in-phase and quadrature measurements on the received optical signal. This receiver performs the optical carrier phase tracking requiring only a single balanced homodyne detector, by including a postdetection Costas-loop-type feedback, which additionally allows the use of suppressed carrier modulations in the received field, for efficient transmission. We report an experimental interferometric self-homodyne setup for the sequential detection of low photon number, binary phase-modulated optical signals that consist of strongly attenuated laser pulses by using a reference field as the local oscillator with an alternatively switched phase. A Costas loop postdetection subsystem is implemented in discrete time to perform fast real-time optical phase tracking. We also present the experimental results of the homodyne postdetection statistics for received BPSK signals with very low photon numbers, and compare them with the theoretical uncertainty limit. Finally, we conduct bit error rate measurements over a wide range of signal level, as well as a comparison with the standard quantum limit.      

Photonic Applications With the Organic Nonlinear Optical Crystal DAST

We review the recent progress in the development of photonic applications based on the organic crystal 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST). DAST is an organic salt with an extremely high nonlinear optical susceptibility chi⁽²⁾(-2omega,omega,omega)=580plusmn30 pm/V at 1.54 mum, a high electrooptic figure of merit n ³ r = 455plusmn80 pm/V at 1.54 mum, as well as a low dielectric constant epsiv = 5.2 . DAST is, therefore, very attractive for high-speed optical modulators and field detectors, as well as for frequency conversion and the generation of terahertz waves. Several techniques to microscopically structure this material have been developed recently; including modified photolithography, photobleaching, femtosecond laser ablation, graphoepitaxial growth, ion implantation, and direct electron-beam structuring, which open new perspectives of using this exceptional material for high-speed very-large-scale integrated photonics.     

Monochromatic-Tunable Terahertz-Wave Sources Based on Nonlinear Frequency Conversion Using Lithium Niobate Crystal

We review widely tunable terahertz (THz)-wave generation by optical parametric processes using lithium niobate crystal. Applying the parametric oscillation of or a MgO-doped crystal pumped by a nanosecond Q-switched Nd:YAG laser, we realized coherent THz-wave sources with a simple configuration widely tunable in the range of 0.7-3 THz. For efficient coupling of the THz wave, we used a monolithic grating coupler or a Si-prism array coupler. In addition, Fourier transform limited THz-wave spectrum narrowing was achieved by introducing the injection seeding method. A line width of about 110 MHz (0.003 cm^-1) was assured by measuring the absorption spectrum of low-pressure water vapor. Using the difference frequency generating method with a periodically poled crystal, we achieved higher conversion efficiency and realized continuous THz-wave generation. This room temperature operated, tabletop system promises to be a widely tunable THz-wave source suited to a variety of applications.     

New optical sampler based on TOAD and data postprocessing for subpicosecond pulse resolution

A low-complexity terahertz optical asymmetric demultiplexer-based optical sampler with simple data postprocessing for subpicosecond pulse shape characterization is proposed. Comparisons with commercial instruments demonstrate the accuracy of the implemented scheme reproducing optical pulses with a resolution of few hundreds of femtoseconds. Nevertheless, the proposed technique allows us to go down to femtosecond accuracy, slightly increasing the data postprocessing complexity. A further validation of the proposed optical sampler is numerically provided.     

Terahertz absorption characteristics of multilayer graphene/polymer

Abstract

In this article, we fabricated two different graphene structure samples, composed of single-layer graphene and multilayer graphene/PMMA structures. When w samples are pumped by the 980?nm laser with different powers, we measure the time-domain and frequency domain spectra of the terahertz transmission using the terahertz time domain spectroscopy system. The measured results show that the average terahertz transmission can be actively controlled by changing the pump power or the layer number of graphene, which exhibits potential applications in the terahertz absorption devices.