Broadband terahertz ultrasonic transducer based on a laser-driven piezoelectric semiconductor superlattice

Spectral characteristics of laser-generated acoustic waves in an InGaN/GaN superlattice structure are studied at room temperature. Acoustic vibrations in the structure are excited with a femtosecond laser pulse and detected via transmission of a delayed probe pulse. Seven acoustic modes of the superlattice are detected, with frequencies spanning a range from 0.36 to 2.5 THz. Acoustic waves up to ∼2 THz in frequency are not significantly attenuated within the transducer which indicates excellent interface quality of the superlattice. The findings hold promise for broadband THz acoustic spectroscopy.     

高性能85mm短腔光学参变振荡器的THz电磁波输出特性分析

报道了以MgO∶LiNbO3为非线性光学介质,采用85 mm长的法布里珀罗单谐振腔结构形式的光学参变振荡器,产生THz电磁波的实验结果。使用波长为1064 nm的Nd∶YAGQ开关脉冲激光器作为抽运光源,通过改变入射角度使参变振荡器的相位匹配条件发生变化。采用Si真空量热器,并利用THz波干涉测量仪;或通过测量闲频光的频率对产生的THz波频率进行了测量。实验表明该参变振荡器输出频率调谐范围为0.9~3.0 THz。在抽运光能量为20 mJ/pulse,脉冲宽度16 ns,重复频率50 Hz条件下得到输出峰值位于1.2 THz,能量为102.5 PJ/pulse的THz波输出。通过引入Si棱镜阵列减小了THz波在晶体中的全反射,从而提高THz波的能量输出。使用金属缝隙探测器,对辐射的THz波的波束水平方向空间分布进行了测量,分析了Si棱镜阵列的衍射效应对THz波束空间分布的影响。     

Approach to broadband terahertz pulse field detection and recovery with electro-optic sampling measurement

Electro-optic sampling (EOS) measurement of terahertz (THz) pulses inherently leads to distortions, especially for relatively broadband THz pulses. In this paper, we proposed an approach to broadband THz pulse field detection and recovery with EOS measurement. In our approach, we use an EOS measurement utilizing a thick GaP crystal, which has a high transverse optical phonon frequency, to reduce the distortions caused by dispersive propagation, interface reflection and Fabry-Perot effect (multiple reflections within an EO crystal). And a retrieval algorithm is designed to eliminate the distortion of broadband THz pluses after passing through the thick GaP crystal (The distortion in a thick GaP crystal is mainly attributed to phase mismatching between the THz pulse and the sampling laser pulse.), in which preconditioned conjugated gradient method is used.     

Characterizing the polarity of a few-cycle infrared laser pulse

We demonstrate the characterization of polarity and carrier-envelope (CE) phase of a linearly polarized few-cycle infrared (IR) laser pulse (12?fs, 1.8???m) using Terahertz (THz) emission spectroscopy. Crossed polarizers electro-optic sampling geometry is applied to record temporal waveforms of far field THz emission from air filament driven by the few-cycle IR pulse. The THz waveforms are dependent on the carrier-envelope (CE) phase periodically. We confirm that the THz waveform and the intensity can be used jointly to determine the phase of a few-cycle laser field in laboratory frame.     

Broadband Terahertz Wave Generation from Monolayer Graphene Driven by Few-Cycle Laser Pulse

We theoretically investigate the characteristics of terahertz(THz) radiation from monolayer graphene exposed to normal incident few-cycle laser pulses, by numerically solving the extended semiconductor Bloch equations. Our simulations show that the THz spectra in low frequency regions are highly dependent on the carrier envelope phase(CEP) of driving laser pulses. Using an optimal CEP of few-cycle laser pulses, we can obtain broadband strong THz waves, due to the symmetry breaking of the laser-graphene system. Our results also show that the strength of the THz spectra depend on both the intensity and central wavelength of the laser pulses. The intensity dependence of the THz wave can be described by the excitation rate of graphene, while wavelength dependence can be traced back to the band velocity and the population of graphene. We find that a near single-cycle THz pulse can be obtained from graphene driven by a mid-infrared laser pulse.     

Coherent control of THz wave generation in ambient air

Our study of THz wave generation in the pulsed laser induced air plasma with individually controlled phase, polarization, and amplitude of the optical fundamental wave (omega) and its second harmonic (2omega) indicates that the third-order nonlinear optical process mixing the omega and 2omega beams in the ionized plasma is the main mechanism of the efficient THz wave generation. The polarity and the strength of the emitted THz field are completely controlled by the relative phase between the omega and 2omega waves. The measured THz field amplitude is proportional to the pulse energy of the fundamental beam and to the square root of the pulse energy of the second-harmonic beam once the total optical pulse energy exceeds the plasma formation threshold. The optimal THz field is achieved when all waves (omega, 2omega, and THz waves) are at the same polarization in the four-wave-mixing process.     

Terahertz spectroscopy studies on epitaxial vanadium dioxide thin films across the metal-insulator transition

We present results on terahertz (THz) spectroscopy on epitaxial vanadium dioxide (VO(2)) films grown on sapphire across the metal-insulator transition. X-ray diffraction indicates the VO(2) film is highly oriented with the crystallographic relationship: (002)(film)//(0006)(sub) and [010](film)//[2 ?1 ?10](sub). THz studies measuring the change in transmission as a function of temperature demonstrate an 85% reduction in transmission as the thin film completes its phase transition to the conducting phase, which is much greater than the previous observation on polycrystalline films. This indicates the crucial role of microstructure and phase homogeneity in influencing THz properties.     

Time-domain transillumination of biological tissues with terahertz pulses

We present time-domain transmission imaging of an opaque structure in pork-fat tissue obtained with a terahertz (THz) field sampling technique. Compared with imaging with near-infrared pulses, the terahertz sampling technique shows significantly enhanced contrast, as a result of low scattering. For enhanced spatial resolution, we show mid-infrared THz imaging of onion cells. Water absorption of THz pulse in muscle tissues is discussed.     

少周期激光脉冲与气体作用产生的离化电流和THz波辐射

本文通过理论和数值模拟,研究少周期激光脉冲电离气体原子产生的离化电流 以及相应的THz波辐射.研究表明,少周期激光脉冲离化气体后能产生较大的离化电流, 因而可以产生较强的THz辐射.不同的少周期激光脉冲相位导致电离出的 电子初始速度和电离起始时刻不同,从而产生的离化电流有所不同, 辐射的THz波随激光脉冲的相位成周期性变化.该理论得到一维PIC数值模拟的验证. 对于给定的激光脉冲相位,离化电流和THz辐射振幅并没有随入射激光振幅的增加而单调增加, 而是存在一些极值点.与均匀分布气体相比,当气体分布具有一定梯度时, 辐射表现相似的规律,但频谱会发生一定的变化.     

基于IrMn/Fe/Pt交换偏置结构的无场自旋太赫兹源

与目前商用的太赫兹源相比,自旋太赫兹源具有超宽频谱、固态稳定以及成本低廉等优点,这使其成为下一代太赫兹源的主要研究焦点.但使用自旋太赫兹源时,通常需要外加磁场使铁磁层的磁化强度饱和,才能产生太赫兹波,这制约了其应用前景.基于此,本文制备了一种基于Ir Mn/Fe/Pt交换偏置结构的自旋太赫兹波发生器,通过Ir Mn/Fe中的交换偏置场和Fe/Pt中的超快自旋流注入与逆自旋霍尔效应相结合,在无外加磁场下产生了强度可观的太赫兹波.在Ir Mn和Fe的界面中插入超薄的Cu,可以使Fe在厚度很薄时零场下实现饱和磁化,并且其正向饱和场最高可达–10 m T,从而进一步提升无场下的太赫兹发射效率.零场下出射的太赫兹波的动态范围超过60 d B,达到可实用化的水平.通过旋转样品,发现产生的太赫兹波的偏振方向也会随之旋转,并且始终沿着面内垂直于交换偏置场的方向.此外,在此交换偏置结构的基础上,引入了一层自由的铁磁金属层Fe,设计了一种以Ir Mn/Fe/Pt/Fe为核心结构的自旋阀太赫兹源,发现产生的太赫兹强度在两层铁磁层反平行排列时比平行排列以及不引入自由铁磁金属层时均大约提升了40%.结果表明,基...     

Integrated injection seeded terahertz source and amplifier for time-domain spectroscopy

We used a terahertz (THz) quantum cascade laser (QCL) as an integrated injection seeded source and amplifier for THz time-domain spectroscopy. A THz input pulse is generated inside a QCL by illuminating the laser facet with a near-IR pulse from a femtosecond laser and amplified using gain switching. The THz output from the QCL is found to saturate upon increasing the amplitude of the THz input power, which indicates that the QCL is operating in an injection seeded regime.     

THz generation via optical rectification with ultrashort laser pulse focused to a line

We report on efficient THz pulse generation via optical rectification with femtosecond laser pulses focused to a line by a cylindrical lens. This configuration provides phase-matched conditions in the superluminal regime. 35 pJ THz pulses have been generated with this technique in a stoichiometric LiNbO₃ crystal pumped by 2 μJ femtosecond laser pulses at room temperature. An unusual superquadratic rise of the THz pulse energy with the laser pulse energy has been observed at high laser energies. This extraordinary energy dependence of the THz generation efficiency is explained by self-focusing of the laser beam in the crystal. Z-scan measurements and comparison of the THz pulse spectra created with laser pulses having different energies confirm this interpretation.     

Studies on the mechanisms of powerful terahertz radiations from laser plasmas

A survey on the mechanisms of powerful terahertz (THz) radiation from laser plasmas is presented.Firstly,an analytical model is described,showing that a transverse net current formed in a plasma can be converted into THz radiations at the plasma oscillation frequency.This theory is applied to explain THz generation in a gas driven by two-color laser pulses.It is also applied to THz generation in a tenuous plasma driven by a chirped laser pulse,a few-cycle laser pulse,a DC/AC bias electric field.These are well verified by particle-in-cell simulations,demonstrating that THz radiations produced in these approaches are nearly single-cycles and linear polarized.In the chirped laser scheme and the few-cycle laser scheme,THz radiations with the peak field strength of tens of MV/cm and the peak power of gigawatt can be achieved with the incident laser intensity less than 10 17 W/cm 2.     

Terahertz-wave generation in a conventional optical fiber

We theoretically propose surface-emitted and collinear phase-matched terahertz (THz)-wave generation in a conventional optical fiber. The third-order nonlinear effect, four-wave mixing (FWM), is used to generate THz waves in an optical fiber. Surface-emitted THz-wave generation via FWM is realized using a single-mode fiber. Perfect phase matching is obtained at ~800 nm and 1.5 microm pumping, and it follows that third-order polarization in an optical fiber has the same phase at any point. In this situation, the optical fiber acts like a phased array antenna of the THz wave. Collinear phase-matching THz waves are obtained under the same conditions as for surface-emitted THz waves, and the THz wave is propagated in the silica cladding of the optical fiber. This is a promising method for realizing a reasonable THz-wave source.     

Investigation of Ag2O Thermal Decomposition by Terahertz Time-Domain Spectroscopy

Application of terahertz time-domain spectroscopy is demonstrated to study the process of Ag2O thermal decomposition. In the process of decomposition, the time-resolved signals are characterized by broad oscillations and decreased intensity, and Tttz pulse essentially contains two broad spectral components： one centered at around 0.35 THz and a band with a maximum at around 0.81 THz shift to 0.71 THz. Optical absorption spectra of different specimens are studied in the frequency range 0.3-1.4 THz and the data are analyzed by the relevant theory of the effective medium approach combined with the Drude-Lorentz model. The analysis suggests that optical properties stem from the Drude term for the metallic phase and the Lorentz term for the insulator phase in the complex system.     

Broadband THz emission from gas plasmas induced by femtosecond optical pulses: From fundamentals to applications

We review the generation of broadband THz radiation from femtosecond photo‐induced gas plasmas, with an emphasis on the highly efficient “AC‐bias” case where the plasma is generated and driven by a superposition of fundamental and second‐harmonic optical fields. The dependence on experimental parameters such as pulse energy, air pressure, polarization and focusing are presented, and compared to the predictions from semi‐quantitative models for the THz generation process, namely (i) a microscopic photocurrent model and (ii) a four‐wave mixing model. We also employ these models to the case of few‐cycle pulses, where the observed THz emission is related directly to the carrier‐envelope phase of the pulses, and hence provides a mechanism with which to measure this phase.}     

基于凸组合核函数的化合物太赫兹透射光谱分类

物质的太赫兹光谱包含着非常丰富的物理和化学信息。它对化合物晶体具有高的灵敏度、 单光子能量低等特点。但受到检测人员知识背景、 背景噪声、 识别算法精度等因素的影响,光谱样本识别准确率和效率较低。为了提高对太赫兹光谱的检测能力,提出应用基于凸组合核函数的support vector machines(SVM)对化合物的THz脉冲透射谱进行分类。在使用小波变换对数据进行滤波预处理之后,提取了传统波峰、 波谷位置特征和term frequency-inverse document frequency (TF-IDF) 最大间隔特征。TF-IDF方法使用信息论的原理确定每个采样点的权重,选择权重较大的点作为特征。针对太赫兹透射谱特征相似、 维数较低带来的分类困难问题,构建基于凸组合核函数的SVM分类模型。并利用核评价的方法,通过高维非线性规划方程求解最优凸组合参数。当最优凸组合参数被确定时,构建分类模型进行分类和预测。相比较于单一核函数,凸组合核函数将透射谱特征与分类模型融合起来。对于不同的检测样本,数据经过凸组合核函数映射到高维空间后,特征具有更显著的区分度。使用不同的太赫兹透射谱样本进行分类实验,结果表明,分类准确率得到极大提高。     

Optical Absorption Spectra and Intraband Dynamics in Terahertz-Driven Semiconductor Superlattice

We have theoretically investigated the optical absorption spectrum and intraband dynamics by subjecting a superlattice to both a terahertz (THz)-frequency driving field and an optical pulse by using an excitonic basis.In the presence of a THz dc field, the satellite structures in the absorption spectra are presented. The satellite structure is a result from the THz nonlinear dynamics of Wannier-Stark ladder excitons. On the other hand, the coherent intraband polarization is investigated. We find that the excitonic Bloch oscillation is driven by the THz field and yields an intraband polarization that continues to oscillate at times much longer than the intraband dephasing time. The temporal evolution of the slowly varying components of the intraband polarization is dependent on the THz frequency.     

Ultrashort-pulse modulation in adiabatically prepared Raman media

We demonstrate the efficient modulation of an approximately 100-femtosecond pulse in a Raman medium coherently prepared by nanosecond pulses. Raman sidebands of the ultrashort pulse spanning 360 THz are generated with an efficiency of >5%. We show that the mechanism permitting the sidebands to be generated is the preparation of a significant vibrational coherence in the medium that is robust to disturbance by an intense short pulse. If the observed sidebands were phase compensated, they would form a short train of approximately ten 3-fs pulses. Focusing would permit the realization of a peak intensity of >10(13) W cm(-2).