Double-slit diffraction of terahertz wave generated by tilted-pulse-front pumping

We propose a spatial diffraction diagnostic method via inserting a millimeter-gap double slit into the collimated terahertz beam to monitor the minute variation of the terahertz beam in strong-field terahertz sources, which is difficult to be resolved in conventional terahertz imaging systems. To verify the method, we intentionally fabricate tiny variations of the terahertz beam through tuning the iris for the infrared pumping beam before the tilted-pulse-front pumping setups.The phenomena can be well explained by the theory based on the tilted-pulse-front technique and terahertz diffraction.     

太赫兹科学与技术研究回顾

太赫兹光谱系统利用远红外辐射获得分子的某些光谱信息，这些信息在电磁谱的其他波段中是很难得到的．材料的研究是现代太赫兹系统中的重要的组成部分，一方面因为新的、大功率的太赫兹光源极大地依赖于如量子级联结构一类的新材料；另一方面，太赫兹的光谱和成像技术也为扩展诸如半导体和生物有机分子等材料的应用提供了一个有力的工具．     

Level spacing statistics for two-dimensional massless Dirac billiards

Classical-quantum correspondence has been an intriguing issue ever since quantum theory was proposed. The searching for signatures of classically nonintegrable dynamics in quantum systems comprises the interesting field of quantum chaos. In this short review, we shall go over recent efforts of extending the understanding of quantum chaos to relativistic cases. We shall focus on the level spacing statistics for two-dimensional massless Dirac billiards, i.e., particles confined in a closed region. We shall discuss the works for both the particle described by the massless Dirac equation(or Weyl equation)and the quasiparticle from graphene. Although the equations are the same, the boundary conditions are typically different,rendering distinct level spacing statistics.     

基于Rydberg原子天线的太赫兹测量

Rydberg原子在微波和太赫兹频段具有极大的电偶极矩,利用量子干涉效应可实现对该频段电磁波场强的高灵敏探测,理论上灵敏度可达到远高于现有探测技术的水平.基于Rydberg原子量子效应的电磁场探测及精密测量技术在太赫兹的场强和功率计量、太赫兹通信和太赫兹成像等方面有着巨大的应用前景.本文回顾了基于Rydberg原子量子干涉效应实现电磁波电场自校准和可溯源测量的基本理论和实验技术,详细介绍了基于Rydberg原子的高灵敏太赫兹场强测量、太赫兹近场高速成像和太赫兹数字通信的基本原理和技术方案.最后简单介绍了本研究团队正在开展的基于Rydberg原子的太赫兹探测工作.     

Questions and answers

The aim of this paper is twofold. First, we shall derive the Fermi-Dirac (FD) and Bose-Einstein (BE) distributions from the classical Maxwell-Boltzmann (MB) distribution by introducing into the classical system the consequences of quantum mechanical indistinguishability in a direct and simple manner. Next, we go through a brief introduction to feedback systems and see how the FD and BE systems may be viewed as classical systems with appropriate feedback. We shall see that the resemblance to feedback systems is more than formal and that a feedback mechanism does exist in systems obeying quantum statistical mechanics.     

Quantum dot materials for terahertz generation applications

Compact and tunable semiconductor terahertz sources providing direct electrical control, efficient operation at room temperatures and device integration opportunities are of great interest at the present time. One of the most well‐established techniques for terahertz generation utilises photoconductive antennas driven by ultrafast pulsed or dual‐wavelength continuous wave laser systems, though some limitations, such as confined optical wavelength pumping range and thermal breakdown, still exist. The use of quantum dot‐based semiconductor materials, having unique carrier dynamics and material properties, can help to overcome limitations and enable efficient optical‐to‐terahertz signal conversion at room temperatures. Here we discuss the construction of novel and versatile terahertz transceiver systems based on quantum dot semiconductor devices. Configurable, energy‐dependent optical and electronic characteristics of quantum‐dot‐based semiconductors are described, and the resonant response to optical pump wavelength is revealed. Terahertz signal generation and detection at energies that resonantly excite only the implanted quantum dots opens the potential for using compact quantum dot‐based semiconductor lasers as pump sources. Proof‐of‐concept experiments are demonstrated here that show quantum dot‐based samples to have higher optical pump damage thresholds and reduced carrier lifetime with increasing pump power.

     

Terahertz emission with a time-dependent amplitude in asymmetric triple quantum wells

In this work, we have numerically integrated in space and time the effective-mass Schrödinger equation for an exciton in a GaAs/GaAlAs triple quantum well heterostructure. Considering a Coulomb interaction between the electron-hole pair in a triple quantum well, we have obtained a time-varying dipole moment in the system whose amplitude is also oscillating with time. In this way, we have shown the possibility of having a new kind of terahertz electromagnetic radiation emerging from a semiconductor device after an optical excitation of the system.     

Terahertz spectroscopy of quantum phase transitions and the temperature-frequency scaling

In the last few decades, significant progress has been achieved in the development of generators and detectors of terahertz radiation (at frequencies in the range from ≈300 GHz to ≈3 THz). Different terahertz spectroscopic techniques have been widely used now in investigating semiconductors, superconductors, molecular magnets, multiferroics, metamaterials, and other promising objects. It has been demonstrated that terahertz spectroscopy offers wide but not completely realized possibilities for studying quantum phase transitions in electron-correlated systems.     

Nonlinear terahertz emission in semiconductor microcavities

We consider the nonlinear terahertz emission by the system of cavity polaritons in the regime of polariton lasing. To account for the quantum nature of terahertz-polariton coupling, we use the Lindblad master equation approach and demonstrate that quantum microcavities reveal a rich variety of nonlinear phenomena in the terahertz range, including bistability, short terahertz pulse generation, and terahertz switching.     

On the Initial Conditions and Solutions of the Semiclassical Einstein Equations in a Cosmological Scenario

In this paper we shall discuss the backreaction of a massive quantum scalar field on the curvature, the latter treated as a classical field. Furthermore, we shall deal with this problem in the realm of cosmological spacetimes by analyzing the Einstein equations in a semiclassical fashion. More precisely, we shall show that, at least on small intervals of time, solutions for this interacting system exist. This result will be achieved providing an iteration scheme and showing that the series, obtained starting from the massless solution, converges in the appropriate Banach space. The quantum states with good ultraviolet behavior (Hadamard property), used in order to obtain the backreaction, will be completely determined by their form on the initial surface if chosen to be lightlike. Furthermore, on small intervals of time, they do not influence the behavior of the exact solution. On large intervals of time the situation is more complicated but, if the spacetime is expanding, we shall show that the end-point of the evolution does not depend strongly on the quantum state, because, in this limit, the expectation values of the matter fields responsible for the backreaction do not depend on the particular homogeneous Hadamard state at all. Finally, we shall comment on the interpretation of the semiclassical Einstein equations for this kind of problems. Although the fluctuations of the expectation values of pointlike fields diverge, if the spacetime and the quantum state have a large spatial symmetry and if we consider the smeared fields on regions of large spatial volume, they tend to vanish. Assuming this point of view the semiclassical Einstein equations become more reliable.     

High-temperature all-optical intersubband quantum well Terahertz switch

In this article an asymmetric intersubband quantum well structure as a high temperature terahertz (THz) optical switch is proposed. In our proposed structure the incoming low power energy photon (THz control signal) causes an optical switching. In this structure we introduce an optical terahertz switch based on coherent population trapping (CPT) phenomena. In the presence of electromagnetic THz field, quantum interference between the terahertz control field and short-wavelength probe field under appropriate condition, the medium becomes transparent (zero absorption) for the probe field. So the absorption and refraction characteristic of optical probe field can be modified with THz radiation. Therefore this idea is suitable for all – optical terahertz switching.     

Spectral measurement of weak THz waves with quantum Hall detectors

A terahertz (THz) microspectroscope is developed, in which the frequency of extremely weak THz radiation is resolved by scanning the magnetic field for a quantum Hall detector. The electron density of the detectors is controlled by the back-gate biasing, so that the detector sensitivity is calibrated over a spectral range studied. Reliable spectral measurements with a spectral resolution of 1.2 cm⁻¹ has been made with a sensitivity better than 10 femtowatt level over 1 s integration time.     

Photomagnetic effect in bilayer two-dimensional electron–hole systems

In tilted magnetic fields a bilayer electron–hole system is found to generate a photocurrent under terahertz radiation as the system is tuned to electron cyclotron resonance conditions. The photoinduced current amplitude oscillates with the magnetic field in correlation with Shubnikov–de Haas oscillations for electrons. The phenomenon is accounted for by a photomagnetic effect in electron–hole systems in the quantum Hall regime and has potentialities for terahertz detection and spectroscopy.     

掺杂石墨烯系统电场调控的非线性太赫兹光学特性研究

石墨烯是单原子厚的二维狄拉克相对论费米子系统,其优秀的光电学性质得到了广泛的关注和研究. 本论文利用量子理论研究掺杂石墨烯系统外电场和光场共同作用下的非平衡载流子的非线性太赫兹光学性质. 研究发现,掺杂石墨烯带内光吸收表现出强的非线性太赫兹光学特性. 随着外加偏压电场的增大,石墨烯非线性光学响应增强;随着外界太赫兹光频率的减小,非线特性增强. 研究表明通过改变电场强度,可以有效调节石墨烯系统太赫兹非线性光学特性. 研究结果为探索和发展以石墨烯为基础的新型纳米太赫兹光电器件的研究和实际应用提供了理论依据. 关键词： 石墨烯 太赫兹 非线性 光电流     

Terahertz and infrared photodetectors based on multiple graphene layer and nanoribbon structures

We consider new concepts of terahertz and infrared photodetectors based on multiple graphene layer and multiple graphene nanoribbon structures and we evaluate their responsivity and detectivity. The performance of the detectors under consideration is compared with that of photodetectors made of the traditional structures. We show that due to high values of the quantum efficiency and relatively low rates of thermogeneration, the graphene-based detectors can exhibit high responsivity and detectivity at elevated temperatures in a wide radiation spectrum and can substantially surpass other detectors. The detector being discussed can be used in different wide-band and multi-colour terahertz and infrared systems.     

Solving the two-dimensional Schrödinger equation using basis truncation: A hands-on review and a controversial case

Solutions of the Schrödinger equation by spanning the wave function in a complete basis is a common practice in many-body interacting systems. We shall study the case of a two-dimensional quantum system composed of two interacting spinless electrons and see that the correctness of the matrix approach depends inexplicably on the type of interaction existing between particles. Also, we shall extend the present study to other systems of special interest in order to ilustrate the method.     

Theory of superionic conduction in hydrogen-bonded systems

Among superionic conduction phenomena in various ionic materials, the conduction phenomenon associated with the motion of protons in hydrogen-bonded systems has aroused considerable interests with regard to a problem of whether the proton motion should be treated quantum mechanically or classically. In this paper we first describe a quantum mechanical mechanism of proton-induced ionic conduction in the superionic phase in zero-dimensional hydrogen-bonded M₃H(XO₄)₂[M = K, Rb, Cs, X = S,Se] materials, by giving a brief review on the theory developed by Ito and Kamimura. Then we discuss the characteristic difference between quantum mechanical and clasiical mechanisms in the case of proton-induced superionic condcuction, in paticular, by comparing characteristic time scales in quantum mechanical and classical diffusions in hydrogen-bonded systems. Finally a prediction is made on an anomalous behavior of terahertz spectra for Rb₃H(SeO₄)₂.     

Single-particle distribution function of a quantum dot system at finite temperature

In the present paper,we shall rigorously re-establish the result of the single-particle function of a quantum dot system at finite temperature.Unlike the proof given in our previous work(Phys.Rev.B 74 195414(2006)),we take a different approach,which does not exploit the explicit expression of the Gibbs distribution function.Instead,we only assume that the statistical distribution function of the quantum dot system is thermodynamically stable.As a result,we are able to show clearly that the electronic structure in the quantum dot system is completely determined by its thermodynamic stability.Furthermore,the weaker requirements on the statistical distribution function also make it possible to apply the same method to the quantum dot systems in non-equilibrium states.