Terahertz time-domain spectrometer with module heads coupled to photonic crystal fiber

We constructed optical-fiber-based THz time-domain spectrometers (THz TDSs) with standard single-mode fibers (SSMFs) and large-mode-area photonic crystal fibers (LMA PCFs) and compared those to THz waves and spectra. The optical fibers are used for guiding optical pulses from ultra-fast lasers to a THz emitter and detector. The LMA-PCF-based THz TDS exhibits increased bandwidth from 1 to 2 THz and increased field amplitude by a factor of four compared with the SSMF-based THz TDS under the relatively higher excitation power of ultra-fast lasers. This improvement results from LMA PCFs that are suitable for high-power transmission without introducing nonlinear effects. We also fabricated compact THz emitter and detector module heads, which are connected with LMA PCFs. The LMA-PCF-based THz TDS had THz radiation power and bandwidth comparable with those obtained by a conventional THz TDS with a free-space optical arrangement. PACS 42.72.Ai; 42.65.-k; 42.81.DP     

Surface Emitting THz Wave Parametrical Oscillator Using MgO：LiNbO3

Room-temperature operation terahertz （THz） wave source is demonstrated using three MgO：LiNbO3 crystals which have a noncollinear arrangement. The experimental results show that the THz wave can be tunable from 0.8 THz to 3.0 THz, and the peak energy output is 103 pJ/pulse at 1.5 THz. The noncoilinear cavity configuration makes the THz beam have Gaussian-like spatial distribution, small divergence angle, perpendicularly eradiated from the crystal surface. The beam quality factor M2 is measured to be Mx^2 = 1.15, Mx^2 = 1.25 for characterizing the THz wave beam. Experiments also show that the THz beam can be focused by using a polyethylene lens, and the focal spot size is close to the diffraction limit.     

Development of variable‐magnification X‐ray Bragg optics

A novel X‐ray Bragg optics is proposed for variable‐magnification of an X‐ray beam. This X‐ray Bragg optics is composed of two magnifiers in a crossed arrangement, and the magnification factor, M, is controlled through the azimuth angle of each magnifier. The basic properties of the X‐ray optics such as the magnification factor, image transformation matrix and intrinsic acceptance angle are described based on the dynamical theory of X‐ray diffraction. The feasibility of the variable‐magnification X‐ray Bragg optics was verified at the vertical‐wiggler beamline BL‐14B of the Photon Factory. For X‐ray Bragg magnifiers, Si(220) crystals with an asymmetric angle of 14° were used. The magnification factor was calculated to be tunable between 0.1 and 10.0 at a wavelength of 0.112 nm. At various magnification factors (M≥ 1.0), X‐ray images of a nylon mesh were observed with an air‐cooled X‐ray CCD camera. Image deformation caused by the optics could be corrected by using a 2 × 2 transformation matrix and bilinear interpolation method. Not only absorption‐contrast but also edge‐contrast due to Fresnel diffraction was observed in the magnified images.     

Radial vibration of a slit-type annular ultrasonic concentrator

The radial vibration characteristic of an annular ultrasonic concentrator with multislits is studied.Based on the electro-mechanical analogy and by introducing an area ratio coefficient,the radial vibrational equivalent circuit and the frequency equation of the concentrator are derived.The radial displacement amplitude magnification factor of it is obtained.By numerical calculating,the relationship between the radial displacement amplitude magnification factor,the resonance frequency of the concentrator at the first and second resonance and its radius ratio are investigated.Also,the relationship between the radial displacement amplitude magnification factor,the resonance frequency of the concentrator and the length,the angle and the number of the silts are analyzed.It is illustrated that a maximum of displacement amplitude magnification factor of the concentrator appears with the increase of the radius ratio,and the amplitude magnification factor increase as the length,the angle and the number of the silts increase,while the radial resonance frequency of the concentrator decreases as the length,the angle and the number of the silts increase.The theoretical resonance frequencies are in good agreement with that of the FEM simulations and measurement.     

基于光放大的长脉冲抽运太赫兹激光

为了获得较窄增益带宽、较大光强的太赫兹辐射,尝试了利用TEA CO₂激光器对6 W可调谐连续波CO₂种子激光实行双程光放大实验,以期望获得长脉冲、高峰值功率的抽运光.通过实验,获得了大约30倍的放大系数,放大光输出功率随种子光注入功率的增加而增加,其中10 P （20）支线表现更加突出,并且被放大的激光支线半峰全宽在14 μs左右.基于这种长脉冲抽运源,建立了一种产生脉冲太赫兹的动力学模型,给出了产生太赫兹辐射的饱和抽运光强表达式,确定了抽运光的光强范围.另外, 关键词： 激光物理 长脉冲 动力学模型 太赫兹激光     

马来酰肼多形态的太赫兹光谱研究

分子的多形态(多晶型)是指化学组成相同但存在不止一种晶体形式的物质.这些多形态广泛存在于自然界中,其中药物的多形态尤其普遍.这些药物多形态虽然具有相同的化学分子组成,但其理化性质却存在差异,最终会导致药物作用功能的不同.近年来,随着太赫兹(THz)辐射源的产生方式成为一种常规技术后,太赫兹时域光谱技术(THz-TDS)...     

Nonlinear spoof surface plasmon polariton phenomena based on conductor metamaterials

We present nonlinear phenomena produced from spoof surface plasmon polariton (SSPP) modes. Below the THz spectrum, artificially textured conducting metastructures on a subwavelength scale generate surface-bound modes and are called SSPP modes, similar to surface plasmon polariton (SPP) modes in the visible spectrum. Even though nonlinear effects in the THz domain are negligible, subwavelength metallic gap structures are ideal candidates to realize nonlinear behavior in the THz domain because of slow light propagation, strong electromagnetic confinement, and a high quality factor Q. In particular, when SSPP structures are combined with Kerr nonlinear materials, nonlinear-bistable curves can be observed below the THz spectrum.     

Characteristics of terahertz coherent transition radiation generated from picosecond ultrashort electron bunches

This paper presents a method of generating terahertz (THz) coherent transition radiation (CTR) from picosecond ultrashort electron bunches including single and train bunches, which are produced by a photocathode radio frequency gun. The radiation characteristics of THz CTR including formation factor and energy spectrum are analysed in detail. With the help of a 2-dimensional particle-in-cell simulation, the radiation characteristics including power, energy and magnetic field are analysed. The results show that the radiation frequency can be adjusted by tuning the repetition frequency of the train bunch and the energy can be enhanced with the train bunches.     

几种糖衍生物分子的THz光谱研究

分子间氢键等振动可以出现在THz波段。糖类是重要的生物分子,是研究氢键的典型体系。糖的衍生物分子具有重要的功能,同时由于它们的特殊的分子结构,通常有大量氢键存在,使得它们在THz波段表现出一定的光谱特征。文章测定了几种糖衍生物分子的THz光谱,结果表明不同分子具有不同的光谱,异丙基-β-D-硫代葡萄糖具有1.17,1.35,1.93,2.23 THz等多个吸收峰;异丙基-β-D-硫代半乳糖的吸收峰位于1.93 THz处;甲基-(四-氧-乙酰基-β-D-半乳糖)的吸收峰位于1.87 THz处;氧-(2, 3, 4, 6-四-氧-乙酰基-β-D-葡萄糖)-氮-羟基琥珀酰亚胺具有1.23,1.70,1.84,2.23 THz等多个吸收峰。对结构相似的分子,尤其对于异构体来说,它们具有不同的峰位,这些低频振动为包含氢键在内的整个分子的振动,说明THz方法能够鉴别异构体,是对结构变化、空间构型等很敏感的技术,是红外光谱的有益补充。这也为大分子的光谱研究提供了基础。     

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.     

太赫兹液晶材料与器件研究进展

液晶是一种性能优异的可调控光电功能材料,基于液晶的太赫兹器件有着广泛的应用前景,但高性能太赫兹功能器件的研发仍处于初级阶段.本文综述了太赫兹领域液晶材料与器件的研究现状,探讨了液晶技术与太赫兹技术相结合的发展趋势.     

Simultaneous all-solid-state multi-wavelength lasers --- a promising pump source for generating highly coherent terahertz waves

A diode-end-pumped Nd:YAG dual-wavelength laser operating at 1319 and 1338 nm is demonstrated. The maximum average output power of the quasi-continuous wave linearly polarized dual-wavelength laser is obtained to be 2.1 W at a repetition rate of 50 kHz with an output power instability of less than 0.38% and beam quality factor M² of 1.45. Using the two lines, the highly coherent and narrow linewidth terahertz radiation of 3.23 THz can be generated in an organic 4-N, N-dimethylamino--methyl-stilbazolium tosylate (DAST) crystal. Meanwhile, the multi-wavelength red laser at 659.5, 664 and 669 nm is generated by frequency doubling and sum frequency processes in a lithium triborate (LBO) crystal. The average red laser output power is enhanced up to 1.625 W at a repetition rate of 15 kHz with an output power instability of better than 0.53% and beam quality factor M² of 6.05. Using the three lines, it is possible to generate the multi-wavelength THz radiation of 3.3, 3.43 and 6.73 THz in an appropriate difference frequency crystal.     

THz Radiation Generation via the Interaction of Ultra‐short Laser Pulses with the Molecular Hydrogen Plasma

In this paper, a two dimensional Particle In Cell‐Monte Carlo Collision simulation scheme is used to examine the THz generation via the interaction of high intensity ultra‐short laser pulses with an underdense molecular hydrogen plasma slab. The influences of plasma density, laser pulse duration and its intensity on the induced plasma current density and the subsequent effects on the generated THz signal characteristics are studied. It is observed that the induced current density in the plasma medium and THz spectral intensity are increased at the higher laser pulse intensities, laser pulse durations and plasma densities. Moreover, the generated THz electric field amplitude is reduced at the higher laser pulse durations. A wider frequency range for the generated THz signal is shown at the lower laser pulse durations and higher plasma densities. Additionally, it is found that the induced current density in hydrogen plasma medium is the dominant factor influencing the generation of THz pulse radiation. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

基于太赫兹光谱技术与CS-SVM的转基因产品鉴别

提出了一种基于太赫兹(THz)光谱技术以及布谷鸟搜索(CS)算法优化支持向量机(SVM)的有效的转基因产品鉴别方法(CS-SVM)。实验采用太赫兹时域光谱(THz-TDS)系统测量了三种转基因大豆种子及其亲本样品在0.2~1.2 THz波段的THz光谱,并采用SVM方法对转基因和非转基因大豆种子进行了分类鉴别研究,其中SVM的两个重要参数(惩罚因子和核参数)采用CS算法进行优化。实验结果表明,应用THz光谱技术结合CS-SVM方法为转基因和非转基因生物的检测和识别提供了一种快速、无损和可靠的分析方法。     

Ultra-narrow-linewidth continuous-wave THz sources based on multiplier chains

We demonstrate two different sources at 1.3 THz based on multiplier chains (72nd harmonic generation), which exhibit linewidths at the level of 2×10⁻¹² in relative units. The multiplication processes are shown not to contribute significantly to this linewidth. The phase noise of one of the sources and the fractional power in the carrier (76%) were determined. The application of these sources as references for quantum cascade THz lasers and for spectroscopy of ultracold molecules is suggested. Thus, rotational spectroscopy with few Hz resolution at 1.3 THz is possible with the present easy-to-use sources. An approach for reducing the linewidth by a factor on the order of 10³ to the 1×10⁻¹⁵ level using optical technology is proposed.     

Long Lifetime State of Shallow Donor Centres for Silicon Based THz Sources

Impurity centres in high purity silicon with ionization energies 38.32 and 40.09meV are observed by photothermal ionization spectroscopy measurements. Their typical shallow donor characteristics are approved by good agreement with theoretic evaluation under varying magnetic fields. The 2p0 state lifetime of one donor centres are more than 3 times longer than that of phosphorus in the same silicon sample. Compared to phosphorus which has been already successfully exploited to produce silicon-based THz radiations, this shallow donor centre has a pronounced enhancement on lifetime and quality factor of population inversion level. Hence, silicon with this donor centre would be a potential excellent candidate to develop improved silicon-based THz sources.     

Design of Tunable Devices at Terahertz Frequencies Based on Quasi-Photonic Crystals Incorporated with Graphene

In this study, we present a new theoretical model including Thue-Morse and double-period sequences as quasi-photonic crystals are incorporation with graphene and investigate the transmission properties of the THz waves in both structures using a straightforward computational method. We also consider properties of nonlinear conductivity in addition to surface linear conductivity for graphene. We observe the sharp peaks and proper forbidden bands are created in the range of $0.3$ THz to $30$ THz. In addition, we find that by considering the nonlinear term of graphene and engineering the structural parameters such as the chemical potential of graphene, number of layers and the incidence wave angle, transmission levels of peaks enhance scientifically and quality factor improve considerably. These results show that it would be possible to design of high-Q tunable filters with multi-stop bands in the THz regime which can reduce the noise associated with THz frequency peaks and increase the number of sharp frequency peaks.     

太赫兹波导滤波器的分析与设计

分析了不同宽边情况下对滤波器加工精度的影响,分析结果表明对于不同频段的滤波器,需要选择合适的谐振腔的宽边才能达到较好的性能,同时分析了不同谐振模式的滤波器对加工精度的影响,分析表明,对于太赫兹频段滤波器,选用TE101谐振模式时存在腔体长度会比波导的宽边小很多的情况,而选用高阶谐振模式不但可以提高滤波器的品质因数Q值,减少损耗,同时也能在一定程度上降低滤波器对加工精度的要求。最后以0.34 THz 4阶带通滤波器为例验证此方法的正确性,测试表明该滤波器最低损耗为-0.73 dB,在0.335~0.349 THz范围内损耗在-2 dB以内。     

不同种类岩石的太赫兹光谱特征研究

准确认识不同种类的岩石特征对地质构造、结构与年代的判断至关重要。常规光学显微手段虽然可以对岩石的颜色与表面形貌进行直接观测,但是对岩石种类的判断往往凭靠经验,一些颜色相近的矿物容易混淆,所得结果并不准确。光谱方法能够在不同频段给出样品的多个光学参数信息,通过建立光学参数与样品自身物性的联系,可以从多个维度确定样品的性质,从而有望实现对不同矿物成分与含量定性、定量评价。太赫兹光谱具备一定的穿透能力,能够透过一定厚度的岩石,是研究岩石物理性质的合理手段。基于太赫兹光谱技术,对取自不同地区的花岗岩、灰岩、砂岩和油页岩样品进行测试,分别计算得到每个样品的等效折射率n、衰减系数a,并以a为横坐标、n为纵坐标作图,结果表明,相同岩性岩石的n与a基本呈线性相关关系,而对于不同类型的岩石,其n随a的线性变化趋势存在明显区别。进而研究了岩石中的组成、结构等信息与其太赫兹光谱响应的联系,分析了不同岩性岩石的光谱响应机制,结果表明：花岗岩与灰岩的结构较为致密,其矿物组成是影响太赫兹光谱响应的主要因素,利用太赫兹参数能够估算岩石中铁、镁元素的相对含量;砂岩的成分较为单一,太赫兹光谱响应受孔隙度的影响;而对于油页岩来说,由于有机质具有强吸收、低折射率的特点,其有机质含量越高,折射率越低,对太赫兹吸收越强。结果表明,太赫兹光谱技术有望成为岩石物理性质现有研究手段的合理补充,为其评价、分析提供新技术、新指标,有着极其光明的应用前景。     

Optimization of wide band mesa-type enhanced terahertz photoconductive antenna at 1550 nm

A mesa-type enhanced InGaAs/InAlAs multilayer heterostructure(MLHS) terahertz photoconductive antenna(PCA)at 1550 nm is demonstrated on an InP substrate. The InGaAs/InAlAs superlattice multilayer heterostructures are grown and studied with different temperatures and thickness ratios of InGaAs/InAlAs. The PCAs with different gap sizes and pad sizes are fabricated and characterized. The PCAs are evaluated as THz emitters in a THz time domain spectrometer and we measure the optimized THz bandwidth in excess of 2 THz.