飞秒超强激光驱动太赫兹辐射特性的实验研究

强太赫兹源是太赫兹科学技术发展的关键,其中大能量强场太赫兹脉冲源在超快物态调控、新型电子加速器等领域具有重要的应用前景.超快超强激光与等离子体相互作用是近年来发展起来的一种新型的强场太赫兹辐射产生途径.本文报道了利用超强飞秒激光脉冲与金属薄膜靶作用产生太赫兹辐射的实验结果,研究了激光能量和离焦量对靶后太赫兹辐射能量的影响,并通过监测激光背向散射光谱,定性揭示了其变化规律与不同光强下的电子加热机制的相关性.实验表征了太赫兹辐射的频谱、偏振及聚焦光斑情况.测量结果表明,实验产生了脉冲能量达458μJ、聚焦场强高达GV/m量级的超宽带太赫兹辐射,为开展极端太赫兹脉冲与物质相互作用研究提供了一种新的强场太赫兹光源.     

宽带空芯THz单模单偏振光纤

传统的THz单模单偏振光纤大部分都是基于实芯光纤设计的,增加了THz波的传输损耗。最近几年,也有一些关于空芯THz单模单偏振光纤的报道。但是,他们的带宽太窄,往往只能在单一频率附近工作,影响了单偏振器件在THz范围内的使用价值和技术优势。为了解决这些问题,设计了一种新颖结构的空芯THz单模单偏振器件。它拥有超过0.1 THz的带宽,工作频率范围从1.63 THz到1.73 THz。这种器件对于THz通信网络的建设和应用具有重要意义。     

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     

1.54 THz激光成像实验与物质穿透特性

 用1.54 THz的波导型辉光放电激励的大功率连续波氰化氘（DCN）激光系统,进行了1.54THz激光成像实验研究和多种物质的穿透特性研究。利用简单的被测物体的2维扫描光学系统进行了连续波1.54 THz激光物体穿透成像,通过测量穿透光的不同强度清楚地识别了放入信封的塑料胸卡和金属硬币的位置和形状。实验中对不同物质进行了1.54 THz激光穿透特性测量,得到了多种不同物质的吸收系数。     

β-Zn3BPO7非线性光学晶体的光谱特性

测量了β-Zn3BPO7晶体(001)方向的红外－可见－紫外光波段的透射与反射光谱, 从而得到反射谱R透射谱T, 以及吸收系数, 并得出样品的禁带能隙Eg=3.198 eV。利用太赫兹时域光谱(THz—TDS)测量技术, 测量了THz辐射脉冲参考波形和透过样品后的信号波形, 进行傅立叶变换和简单的数学处理, 得到β-Zn3BPO7样品的介电系数的实部和虚部, 以及吸收系数的变化频谱α(ν)。在1.396, 2.002, 2.476和2.674 THz处, 分别存在明显的共振吸收峰。在THz波段的吸收较小, 特别是在0.5 T到1.5 T之间, 吸收几乎接近于零, 而样品在高频端有吸收增大的趋势。     

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。     

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.     

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。     

Broadband terahertz light source pumped by a 1 μm picosecond laser

We obtained a frequency tunable, low-coherence, picosecond, terahertz (THz) output with a high repetition rate from a picosecond Nd:YVO₄ bounce laser in combination with tandem periodically poled stoichiometric lithium tantalate and 4′-dimethylamino-N-methyl-4-stilbazolium tosylate crystals. The frequency of the THz output was tunable in the range 2.1–7.1 THz with a linewidth of ~3.5 THz at 2.2 THz. The THz output had a maximum peak power of ~180 mW and an average power of ~0.65 μW at 3.9 THz. This system has the potential to realize ultra-high speed, THz coherence tomography.     

Tunable few-cycle and multicycle coherent terahertz radiation from relativistic electrons

We report the generation of tunable, narrow-band, few-cycle and multicycle coherent terahertz (THz) pulses from a temporally modulated relativistic electron beam. We demonstrate that the frequency of the THz radiation and the number of the oscillation cycles of the THz electric field can be tuned by changing the modulation period of the electron beam through a temporally shaped photocathode drive laser. The central frequency of the THz spectrum is tunable from ～0.26 to 2.6 THz with a bandwidth of ～0.16 THz.     

Terahertz-wave generation with periodically inverted gallium arsenide

We overview methods of THz-wave generation using frequency down-conversion in GaAs with periodically-inverted crystalline orientation. First, we compare different nonlinear-optical materials suitable for THz generation, analyze THz generation process in quasi-phase-matched crystals and consider theoretical limits of optical-to-THz conversion. Then, we review single-pass optical rectification experiments with femtosecond pump pulses, performed in periodically-inverted GaAs, where monochromatic THz output tunable in the range 0.9–3.0 THz was produced. Finally, we describe a novel approach to create a compact highly efficient tunable (0.5–3.5 THz) room temperature monochromatic THz source, based on the concept of intracavity THz generation via resonantly-enhanced difference frequency mixing. This approach allowed generating of 1 mW of average THz power, potentially scalable to 10–100 mW.     

太赫兹数字全息术的研究进展

随着太赫兹成像技术的不断成熟,其空间分辨率和系统信噪比逐渐提高,成像速度逐渐加快,光学信息获取能力逐渐变强,人们对太赫兹成像在基础研究和工业应用的开发也逐渐深入。本文综述了近年来科研人员利用太赫兹数字全息成像系统进行的部分研究工作,包括对平板太赫兹元件的性能表征、对光控太赫兹元件的功能验证、对衍射太赫兹场中的纵向分量进行观测、以及对金属亚波长器件的太赫兹表面波进行分析。这些工作的完成对于太赫兹集成系统的研究和太赫兹成像技术的应用都具有积极的推动作用。     

Design of terahertz wave polarizing beam splitter

In this paper, we design a THz wave polarizer based on a periodic symmetrical thin film structure, which operates over a broadband THz frequency region (1.0–2.0 THz) and over an effective wide range of incident THz wave (72–84°). The spectral performance of this structure is characterized by transfer matrix method calculations. Results of simulations show that the polarizer is highly transmittance for TE polarized THz wave as well as highly reflecting for TM polarized THz wave.     

NOVEL TERAHERTZ PHOTOCONDUCTIVE ANTENNAS

The photoconductive (PC) antenna is a key device for the recent terahertz (THz) photonics based on laser-pumped generation and detection of THz radiation. In this paper we report on two new types of PC antennas: the Schottky PC antenna and the multi-contacts PC antenna. The former one is able to detect THz radiation intensity without the time-delay scan and useful for applications where spectroscopic information is not important, such as the THz intensity imaging. The latter one is useful for the polarization sensitive THz spectroscopy, such as the THz ellipsometry. The characteristic features of these new types of PC antennas are studied by using a THz time-domain spectroscopy system.     

Design and analysis of a metallic waveguide with a DAST cap for continuously phase-matched terahertz difference frequency generation

We present a novel source for continuous terahertz (THz) wave generation using an organic ionic salt, 4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST). THz waves are generated based on difference frequency generation (DFG) in the device. Phase matching condition and THz generation between 1.3 THz and 2.7 THz, for optical pump around 1.6 μm, are investigated. Our calculations predict that the device produces a relatively high THz output power of 11.07 μW from a 4 cm long waveguide at 2 THz.     

利用国内生长的有机晶体DAST作为THz源的研究

利用时域光谱技术分别测量了通过光整流法在国内新生长的有机晶体DAST和国际常用的高效能THz晶体ZnTe中产生的THz脉冲的时域光谱,也测量和分析了有机晶体DAST在THz波段的色散和吸收特性,并通过热辐射探测计(4.2 K Si-bolometer)测量了THz脉冲的绝对能量。结果表明,国内新生长的有机晶体DAST的确是一种高效率的THz产生晶体,优化生长条件后将可得到更高的产生效率。     

Terahertz polarization imaging with birefringent materials

We present a terahertz (THz) imaging method with a birefringent material which is sensitive to THz wave polarization. The subtle THz polarization modulations introduced by the target are extracted by the amplitudes of the two separated THz peaks passing through the birefringent material. The anisotropy of the industrial sprayed-on-foam-insulation (SOFI) is characterized by measuring its azimuthal angle dependent THz polarization response. This work demonstrated that this polarization sensitive THz imaging technique can be used for industrial inspection and biological related characterization.     

Terahertz (THz) Frequency Sources and Antennas - A Brief Review

In this paper we review THz radiation properties, generation methods, and antenna configurations. This paper suggests some new class of antennas that can be used at THz frequency, like optical antennas or Carbon nanotube antennas. THz technology has become attractive due to the low energy content and nonionizing nature of the signal. This property makes them suitable for imaging and sensing applications. But at the same time detection and generation of THz signals has been technologically challenging. This paper presents a comparative study of the generation techniques for THz frequency signals giving emphasis to the some new techniques like Quantum Cascade lasers which has created significant research interest. The main aim for this study is to find out the materials suitable for fabricating THz devices and antennas, a suitable method for generation of high power at THz frequency and an antenna that will make THz communication possible.     

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.     

ZnTe晶体中光学整流产生的THz辐射及其电光探测研究

借助抽运-探测技术研究了ZnTe晶体中光学整流产生的太赫兹(THz)辐射，利用ZnTe晶体的线性电光效应探测THz辐射场分布，观察到了较窄(约为0.2 ps)的THz场分布及相应较宽(响应超过4 THz，半峰宽约为2.4 THz)的THz频谱，并运用琼斯矩阵对实验结果进行了理论拟合. 研究了飞秒激光脉冲波长(750—850 nm)、脉冲宽度(56—225 fs)和晶体旋转与THz辐射产生的关系. 同时改变探测光偏振方向进行偏振调制，并从理论上分析了偏振调制对THz辐射探测的影响. 关键词： THz辐射 光学整流 电光探测 ZnTe