Diagnostics of heterostructures of resonant-tunneling diodes during epitaxial growth. I. Estimation of the required accuracy of heterostructure growth

Based on the solution to the one-electron Schr?dinger equation, current-voltage characteristics of the resonant-tunneling diode based on GaAs/AlAs are numerically simulated. It is shown that the layer thickness accuracy during epitaxial growth of the diode active region should be no worse than one monolayer.     

What mesoscopic structures really “remember”: insufficiency of the open boundary approximation

We analyze the influence of contact electrons on quantum transport in a resonant-tunneling diode (RTD), using a many-body density matrix formalism for open systems. We explicitly relate the net current in the RTD to the memory-containing effective interaction between the RTD active region and the contacts. This effect can only be captured if the RTD active region is treated fully as a dynamically open system.     

Anisotropy of nonlinear optical transmission at the edge of the photonic band gap of an apodized layered medium

The effect of Kerr nonlinearity on the transmission of laser radiation in a one-dimensional photonic crystal has been investigated by the transfer-matrix method, modified to describe nonlinear effects. The crystal under study is a thin-film multilayer structure with a spatial distribution of the refractive index, which makes it possible to eliminate side bands in the transmission spectrum at each side of the photonic band gap and significantly increase the slope of the transmission curve. The transmission spectrum of such a photonic crystal structure has been studied for two opposite directions of laser radiation propagation. The anisotropy of nonlinear transmission is most pronounced near the edge of the photonic crystal band gap, which lies in the near-IR region. The proposed structure, having strong transmission anisotropy and sufficiently low reflection in the forward direction, can operate as an optical diode (analog of an electron diode).     

On the effect of the electron distribution in the near-contact region and asymmetry of the resonant-tunneling diode structure on the high-frequency response and the possibility of detecting the quantum amplification mode in an external high-frequency electric field

The conditions of the implementation of the quantum mode of microwave generation in semiconductor resonant-tunneling diode (RTD) structures were theoretically analyzed. The high-frequency response in structures with symmetric and asymmetric barriers was analyzed by numerical simulation methods. The effect of the Fermi distribution of carriers in the near-contact region was studied. It was shown that asymmetric structures with decreased impurity concentrations (10¹⁷ cm^?3) in the emitter region are optimum from the viewpoint of experimental observation of the quantum amplification mode.     

Anisotropy of interchain interaction in polymeric (SN)x crystals

The technique of polarized reflection microspectroscopy was used to study anisotropy of polymeric sulfur nitride (SN)_x in plasma edge region. The spectra taken from different faces of single crystals indicate the presence of strong anisotropy of interchain interaction due to orientation of the π-electron orbitals and suggest two-dimensional metallic behavior of the π-electron system in (SN)_x.     

Resonant-tunneling devices for millimeter-wave generation

Oscillations from resonant-tunneling diodes have been observed up to 200 GHz, and theoretical estimates predict that device performance should extend into the THz range. This paper addresses the issue of the ultimate frequency response and power generation capability of these devices. Techniques recently developed to solve the time-dependent Schrödinger equation are used to predict the rf power vs. frequency obtainable from resonant-tunneling diode oscillators, based on the calculated small-signal response. Factors limiting the rf power output from these devices are presented. Also, recently obtained dc experimental results for the In_.53Ga_.47As-In_xAl_1-xAs heterostructure material system grown on InP are presented. Using a quasi-static approximation, the rf power available from these devices under large-signal conditions is estimated.     

High-frequency response and the possibility of detecting the quantum amplification mode in resonant-tunneling diode structures

The conditions of the implementation of the quantum mode of microwave generation in semiconductor resonant-tunneling diode (RTD) structures were theoretically analyzed. Based on the constructed analytical model of the steady-state current and high-frequency response in symmetric RTD structures with finite barrier widths, high-frequency properties of RTDs in an external ac electric field were analyzed by numerical simulation methods. It was shown that the quantum amplification mode can appear not only in the high-frequency region of the terahertz range, but also at relatively low frequencies due to deformation of frequency dependences in a dc electric field.     

Point defects and amplification in active layers of InGaAs/AlGaAs heterostructures

The energy levels of gallium and arsenic vacancies as well as silicon impurities in the band gap of In _x Ga_{1 ? x} As have been found as functions of the indium content. The effect of defects on the lasing output power and on the optimal reflection coefficient of output mirrors of laser diode arrays (LDAs) based on the In_0.11Ga_0.89As/AlGaAs heterostructures has been estimated. It has been shown that the lasing output power of LDAs whose active level contains defects with deep energy levels in the band gap is substantially lower (with other conditions being the same) than that of LDAs with shallow level defects in their active layers.     

Line shapes of near-infrared DFB and VCSEL diode lasers under the influence of system back reflections

Laser diode line widths and line shapes are experimentally investigated in dependence on the diode current and on back reflections from an optical system. Four distributed-feedback (DFB)-type diode lasers and two vertical-cavity surface-emitting lasers (VCSELs) have been tested within the same optical setup and using the same fitting methods. System back reflection ratios of light reflected back to the laser have been varied between ?1?dB and ?45?dB and were below ?60?dB when all reflections were blocked. The background of this investigation is the evaluation of different laser types with respect to their suitability for sensor applications in which optical back reflections may occur, for example tunable diode-laser spectroscopy (TDLS). While DFB-type lasers showed almost pure Lorentzian line shapes and line widths of a few MHz, the tested VCSELs had a strong Gaussian contribution to the line shape, indicating stronger 1/f noise, which was also observed in the relative intensity noise of these particular lasers. System reflection ratios above ?25?dB had strong effects on the line width in both DFB diode lasers and VCSELs, while some influences have been observed at even lower reflection ratios for DFB diode lasers. As much smaller reflection ratios are typically required in TDLS systems to avoid etalon-like fringes and self-mixing interference effects, we conclude that the influence on the line width is not the most important reason to minimize back reflections in practical TDLS systems or to choose one type of diode laser over the other.     

Magnetotunneling spectroscopy of a biased triple-barrier semiconductor heterostructure

In this work we describe a magnetotunneling spectroscopy technique for probing the localization degree of two-dimensional states and mapping the subbands in the active region of a resonant-tunneling semiconductor heterostructure. The reported experimental data consist of the low-temperature tunneling current traces of an asymmetric triple-barrier structure measured by sweeping an in-plane magnetic field up to 10 T. According to our interpretation model, the main features observed in the tunnel current traces are due to the field-induced resonant transitions between two-dimensional states at the crossing region between dispersion curves. The data reveals the highly localized nature of the quantum states in an asymmetric double-quantum-well structure even for those with very narrow middle barriers.     

Single-well resonant-tunneling diode heterostructures based on In0.53Ga0.47As/AlAs/InP with the peak-to-valley current ratio of 22:1 at room temperature

Current-voltage (I–V) characteristics of resonant-tunneling diode In_0.53Ga_0.47As/AlAs/InP structures are studied at 300 and 77 K. The peak-to-valley current ratios were determined as 22:1 and 44:1 at temperatures of 300 and 77 K, respectively, which correspond to the maximum values for InGaAs/AlAs/InP heterostructures without an additional InAs layer of a quantum subwell in their configuration.     

基于可调谐激光吸收光谱的大气甲烷监测仪

可调谐二极管激光吸收光谱(TDLAS)技术利用二极管激光器波长调谐特性,获得被测气体的特征吸收光谱范围内的吸收光谱,从而对污染气体进行定性或者定量分析,这种方法不仅精度较高,选择性强而且响应速度快,已经被用于大气痕量气体监测以及工业控制。在对空气中的痕量气体进行检测中,由于气体浓度较低,需要和长吸收光程技术相结合。将可调谐二极管激光吸收光谱与经过108次反射后达到27 m光程的多次反射池相结合研制了用于地面环境空气中甲烷含量监测的便携式吸收光谱仪,并结合了用于微弱信号检测的二次谐波检测技术,从而达到了体积分数低于1×10-7的检测限,并利用不同体积分数的甲烷气体对系统进行了测试,得到了很好的测试结果。     

Universal transport properties of a quantum dot system with a laterally-coupled Majorana zero mode

We present a comprehensive analysis about the transport properties of a quantum dot (QD) system with a side-coupled Majorana zero mode. Our calculation result shows that when the coupling manners between the two leads and QDs are identical, the local Andreev reflection and the interlead normal tunneling have the same magnitude at the zero-bias limit. Accordingly, the zero-bias conductance value is always equal to e²/2h, which is exactly one half of the resonant-tunneling conductance. This result is independent of the level number and the level distribution in the single-QD case, and in the coupled-QD case it is irrelevant to the geometry of the QD molecule. The universal transport property is a powerful evidence for the feasibility to detect the MBSs based on a QD circuit. This result also means that the QD condition is not a key factor to achieve the detection. On the other hand, if the decoupling phenomenon appears, the Majorana zero mode may play a trivial role in contributing to the conductance property.     

Mathematical analysis of the lasing eigenvalue problem for the optical modes in a layered dielectric cavity with a quantum well and distributed Bragg reflectors

Optical modes and associated linear threshold values of material gain bringing them to lasing are investigated for a VCSEL-type cavity with a quantum well, sandwiched between two distributed Bragg reflectors. They are found as solutions to a specific novel eigenvalue problem with the “active” imaginary part of the quantum well refractive index. For the calculation of the Bragg mirror reflection coefficients, well-established method of the transfer matrices is used. The presented results accurately quantify intuitively predictable lowering of the modal thresholds for the modes whose lasing frequencies lay inside the reflectors rejection bands. Besides, they demonstrate that this approach automatically incorporates the account of overlapping between the active region and the modal E-field patterns and its effect on the thresholds.     

Giant magnetocurrent in triple-barrier ferromagnetic resonant-tunneling diode with different magnetization configurations

The authors theoretically investigate a triple-barrier ferromagnetic resonant-tunneling diode composing nonmagnetic couple wells and three GaMnN magnetic barriers, in which two kinds of antiparallel configurations are formed by changing the relative orientation of magnetization in the barriers. Based on a two-band model, the achievement of large magnetocurrents in the resonant bias regime is proposed under an optimal magnetization configuration. The magnetocurrents in both antiparallel configurations of triple-barrier system are much higher than that in double-barrier structure.     

Scattering-parameter measurements of laser diodes

An accurate and simple technique for measuring the input reflection coefficient and the frequency response of semiconductor laser diode chips is proposed and demonstrated. All the packaging parasitics could be obtained accurately using a calibrated probe, and the impedance of the intrinsic diode chip is deduced from the directly measured reflection coefficient. The directly measured impedance of a laser diode is affected strongly by the short bond wire. In the frequency response (S ₂₁) measurements of semiconductor laser diode chips, the test fixture consists of a microwave probe, a submount, and a bond wire. The S-parameters of the probe could be determined using the short-open-match (SOM) method. Both the attenuation and the reflection of the test fixture have a strong influence on the directly measured frequency response, and in our proposed technique, the effect of test fixture is completely removed.     

Compact liquid-refractive index measuring equipment

Based on total reflection principle, a compact liquid-refractive index measuring equipment was designed and fabricated, in which a diode laser was used as light source and a charge-coupled device (CCD) as photodetector. The influence on measurement accuracy of the wavelength shift and intensity fluctuation of the diode laser were surmounted by an effective feedback method. It is crucial whether the diode laser could be used in such a system.     

激光器型全光波长转换器的小信号分析

当外部光子注入到激光器有源腔中时 ,载流子把被注入光子消耗的一部分放大 ,进而激光器自身的输出功率将降低。基于此 ,可以实现全光波长转换。理论上基于载流子消耗机制 ,对激光器型波长转换器进行了小信号分析 ,给出了频率响应函数。理论分析表明 ,激光器型全光波长转换器的转换速度取决于激光器光子寿命以及激光器腔内的光子密度。