Transient surface photoconductivity of GaAs emitter studied by terahertz pump-emission spectroscopy

This paper investigates the ultrafast carrier dynamics and surface photoconductivity of unbiased semi-insulating GaAs in detail by using a terahertz pump-emission technique. Based on theoretical modelling, it finds that transient photoconductivity plays a very important role in the temporal waveform of terahertz radiation pulse. Anomalous enhancement in both terahertz radiation and transient photoconductivity is observed after the excitation of pump pulse and we attribute these phenomena to carrier capture in the EL2 centers. Moreover, the pump power- and temperature-dependent measurements are also performed to verify this trapping model.     

Tailoring terahertz near-field enhancement via two-dimensional plasmons

We suggest a novel possibility for electrically tunable terahertz near-field enhancement in flatland electronic materials supporting two-dimensional plasmons, including recently discovered graphene. We employ electric-field effect modulation of electron density in such materials and induce a periodic plasmonic lattice with a defect cavity. We demonstrate that the plasmons resonantly excited in such a periodic plasmonic lattice by an incident terahertz radiation can strongly pump the cavity plasmon modes leading to a deep subwavelength concentration of terahertz energy, beyond λ/1000, with giant electric-field enhancement factors up to 10(4), which is 2 orders of magnitude higher than achieved previously in metal-based terahertz field concentrators.     

0.14 THz高功率太赫兹脉冲的参数测量

给出了0.14 THz高功率单次脉冲信号的频率和功率的测量方法及实验结果。针对高功率太赫兹脉冲频率高、峰值功率高和脉宽短的特点,实验中采用了截止波导滤波法与谐波混频法相结合的方式准确测定了脉冲信号频率,利用辐射场功率密度积分法获取了辐射脉冲的远场功率分布,并给出了单次脉冲的辐射功率。某实验条件下的测量结果表明,0.14 THz高功率太赫兹脉冲的频率为0.146 3 THz,脉宽约为1.5 ns,功率不小于0.5 MW。     

A structure for enhanced terahertz emission from a photoexcited semiconductor surface

A structure that can provide enhancement of terahertz emission from a semiconductor surface excited with femtosecond laser pulses is proposed. The structure consists of a semiconductor layer on a Si substrate with metal coating on the upper surface of the layer and a Si lens attached to the bottom of the substrate. The semiconductor is excited through a hole in the coating and emits terahertz radiation through the substrate lens. We demonstrate theoretically that the proposed structure can increase the terahertz yield by orders of magnitude as compared to the previously used schemes of terahertz emission from a semiconductor surface.     

Determination of the temporal profile of a terahertz pulse by applying the nonlinear-optical cross-correlation method

The nonlinear-optical cross-correlation method is proposed to determine the temporal profile of the electric field of a pulse terahertz radiation modulating the phase of the basic femtosecond laser pulse in an electrooptical crystal. In this method the measurement of the temporal profile of the electric field of the terahertz radiation pulse is realized in one laser pulse.     

太赫兹计量研究与标准研制进展

太赫兹作为新的技术手段在物质成分识别、高速通信、生物医学、安检成像和军事国防等领域具有重要的作用。太赫兹技术的各种应用都建立在对太赫兹本身及其与物质相互作用测量的基础上,因此准确的太赫兹参数测量及相关量值溯源是太赫兹应用的技术支撑和保障。介绍中国计量科学研究院在太赫兹辐射参数计量标准研究中形成的测量技术和溯源方法、研制的测量仪器和测量装置、制定的计量校准法规和建立的计量标准装置,对太赫兹辐射时域、频域、空域和强度等参数给出了量值溯源传递图并进行了测量不确定度分析。提出的太赫兹计量方法和标准装置可保障太赫兹技术研究和应用中的量值可靠,也可为其他太赫兹参数的测量提供参考。     

Generation of terahertz radiation by optical rectification in presence of resonance impurities

We demonstrate the possibility of an efficient generation of terahertz radiation under optical rectification in a uniaxial crystal containing impurities with resonance two-level quantum transitions. The group velocity of an optical pulse entering the regime of self-induced transparency decreases to the value of phase velocity of the terahertz radiation, which results in phase matching of long- and short-wavelength waves. The terahertz generation is accompanied by strong self-modulation of the optical pulse. Two-component soliton-like structures can be formed at long distances accompanied by spectral red shift of the optical component.     

Terahertz radiation from armchair carbon nanotube dipole antenna

This paper investigates the electromagnetic radiation characteristics of a metallic, large aspect ratio single walled carbon nanotube antenna in the terahertz frequency region below 12.5 THz. The key features of terahertz pulse have been revealed on the carbon nanotube antenna in comparison with conventional photoconductive switching. The terahertz waveforms, radiation power and their field distributions have been evaluated and are analysed. The Fourier transformed spectra over the whole frequency range demonstrate that the carbon nanotube antenna can be used as radiation source for broadband terahertz applications.     

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.     

On the generation of terahertz radiation by a phase-modulated optical pulse

The problem of terahertz radiation generation in a quadratic nonlinear medium by a nanosecond phase-modulated optical pulse possessing the properties of supercontinuum has been investigated. The possibility of generating both broadband and quasi-monochromatic terahertz signals has been demonstrated. The conditions have been revealed under which the generation efficiency of a phase-modulated nanosecond optical pulse is greater than that of a spectrally confined femtosecond signal.     

Thermal radiation control in the terahertz region using the spoof surface plasmon mode

In this Letter, the enhancement of thermal radiation in the terahertz region is presented theoretically by using the spoof surface plasmon mode on the microcavity array. We found that the terahertz wave is radiated selectively in a vertical direction. As a result, a quasi-monochromatic terahertz source can be created.     

200mJ光泵浦氨气太赫兹激光器北大核心CSCD

利用光栅选支调谐的TEA CO2激光器作为泵浦源设计了高能量脉冲光泵浦太赫兹激光器。太赫兹激光谐振腔由2m长的石英玻璃管、GaAs泵浦光输入窗和SiO2太赫兹光输出窗组成,氨气充入谐振腔内作为增益介质。太赫兹激光的波长由自主设计的金属线栅F-P干涉仪测量。实验中在多个波长处都获得了强烈的太赫兹光输出,其中151.5μm太赫兹光的输出脉冲能量高达204mJ,对应的泵浦光能量为32J。最后,还给出了利用151.5μm太赫兹光进行透视成像的实验结果。     

Tailored terahertz pulses from a laser-modulated electron beam

We present a new method to generate steady and tunable, coherent, broadband terahertz radiation from a relativistic electron beam modulated by a femtosecond laser. We have demonstrated this in the electron storage ring at the Advanced Light Source. Interaction of an electron beam with a femtosecond laser pulse copropagating through a wiggler modulates the electron energies within a short slice of the electron bunch with about the same duration of the laser pulse. The bunch develops a longitudinal density perturbation due to the dispersion of electron trajectories, and the resulting hole emits short pulses of temporally and spatially coherent terahertz pulses synchronized to the laser. We present measurements of the intensity and spectra of these pulses. This technique allows tremendous flexibility in shaping the terahertz pulse by appropriate modulation of the laser pulse.     

Investigation of parameters of terahertz pulses generated in single-domain LiNbO<Subscript>3</Subscript> crystal by step-wise phase mask

A new scheme for the efficient generation of broadband terahertz radiation via optical rectification of femtosecond laser pulses in the single-domain lithium niobate crystal equipped with the step-wise phase mask (SPM) is investigated. It is shown that using the SPM one can provide the phase matching for all the spectral components of a terahertz pulse by providing the effective conversion of laser radiation in the terahertz region. The angular distribution of spectral components, as well as the temporal shape of terahertz pulses in the wave zone is studied. These results can be applied in the time-domain spectroscopy, the imaging of hidden objects, and etc.     

Ladder climbing on the anharmonic intermolecular potential in an amino acid microcrystal via an intense monocycle terahertz pulse

We experimentally demonstrated 20 ladder climbing steps on the anharmonic intermolecular potential in the amino-acid microcrystals with an intense monocycle terahertz pulse. Absorption spectra show the suppression of the peak and enhancement of the low-frequency absorption for the incident electric field amplitude. These results are reproduced by simulations based on coherent transition processes between quantum levels in the anharmonic potential. The appearance of such nonlinearity allows us to control macroscopic motion via a phase-controlled terahertz pulse.     

Two-color laser wavelength effect on intense terahertz generation in air

Near-IR femtosecond lasers have been proposed to produce high-field terahertz radiation in the air via the laser-plasma interaction, but the physical mechanism still needs to be further explored. In this work, we theoretically investigate the effect of the two-color laser wavelength on the terahertz generation in the air based on a transient photocurrent model.We show that the long wavelength laser excitation can greatly enhance the terahertz amplitude for a given total laser intensity. Furthermore, we utilize a local current model to illustrate the enhancement mechanism. Our analysis shows that the terahertz amplitude is determined by the superposition of contributions from individual ionization events, and for the long wavelength laser excitation, the electron production concentrates in a few ionization events and acquires the larger drift velocities, which results in the stronger terahertz radiation generation. These results will be very helpful for understanding the terahertz generation process and optimizing the terahertz output.     

Generation of terahertz radiation during reflection of femtosecond laser pulses from a metal surface

The features of terahertz radiation generated upon oblique incidence of a femtosecond laser pulse on a metal surface are investigated theoretically. We propose a Cherenkov-type generation mechanism associated with excitation of low-frequency surface currents by a p-polarized optical field. The nonlinear surface currents and corresponding electrodynamic characteristics of low-frequency radiation are calculated analytically and by numerical simulation using the hydrodynamic model. The features of Cherenkov generation of terahertz radiation are analyzed, and techniques are proposed for increasing efficiency.     

半绝缘GaAs光电导开关产生太赫兹波电场屏蔽效应的二维Monte Carlo模拟

利用Ensemble-Monte Carlo模拟方法，对不同实验条件下半绝缘GaAs(SI-GaAs)光电导开关作为偶极辐射天线在辐射太赫兹电磁波(太赫兹波)中体内电场的分布以及空间电荷屏蔽效应对太赫兹波辐射的影响进行了模拟.载流子的时域空间电场分布表明:用高能量激光脉冲触发低压偏置的GaAs开关，空间电荷屏蔽是限制太赫兹波辐射功率的一个重要因素，并且空间电荷屏蔽能够引起太赫兹波呈现双极性.当高能量飞秒激光脉冲以全电极间隙触发大孔径光电导天线时，空间电荷电场屏蔽效应对太赫兹波的影响不大. 关键词： 光电导开关 Ensemble-Monte Carlo模拟 辐射场屏蔽 空间电荷屏蔽     

Strange Quark Matter and the Astrophysical Nature of Anomalous Effects in 1–100 PeV Cosmic Rays

JETP Letters - Energy, pulse duration, and frequency of the spectral maximum are important characteristics of pulsed terahertz radiation. The unipolarity of radiation is another characteristic that...