The application of terahertz spectroscopy to liquid petrochemicals detection: A review

Petrochemicals, one of the most important energy sources, contribute to the remarkable development of human civilization. Therefore, the development of a kind of fast, safe, reliable and nondestructive detection technology is essential. Terahertz (THz) spectroscopy, containing abundant physical, chemical, and structural information of materials, shows significant applications in the fields of physics, chemistry, materials science, medicine, pharmacy and biology. As a promising detection technology, THz technology provides a new reliable analytic method in liquid petrochemicals detection due to the fact that low-frequency vibrational and rotational motions of hydrocarbon molecules lie in the terahertz region. In this article, we review the applications of the liquid petrochemicals detection based on the terahertz time-domain spectroscopy (THz-TDS) system, mainly containing the analysis of molecular properties, qualitative identification, quantitative analysis and the terahertz metamaterials sensing. In addition, we propose the further exploration of terahertz technology in the field of petrochemical industry.     

电光聚合物在全光技术实现THz波产生与检测中的研究进展

研究和开发基于超短波和低能量激光系统的高振幅、宽频带的太赫兹系统一直是太赫兹应用领域的一个热点。与传统电光晶体材料相比,以电光聚合物薄膜作为太赫兹波产生和检测的最大的优势在于： 一方面电光聚合物不具有晶格结构相应的不存在声子的吸收和太赫兹折射率的散射问题,避免光谱间隙的产生;另一方面电光聚合物薄膜与已用于宽频太赫兹系统中的超薄电光晶体材料相比,更易于加工和处理。另外,可以自行设计得到低相位失配和高电光系数的电光聚合物材料,得到高效率和宽频带的太赫兹辐射源。可以预见,电光聚合物在这一领域的应用必将有宽阔的发展空间和学术研究意义。介绍电光聚合物的电光效应及二阶非线性生色团合成等相关理论问题基础上,综述了近二十多年来,电光聚合物在基于全光技术实现太赫兹波产生与检测系统中的研究进展,主要包括: 掺钛蓝宝石飞秒激光激发下,共聚型和主客体型电光聚合物实现太赫兹波产生与检测;光纤飞秒激光激发下,主客体型电光聚合物实现太赫兹波的产生与检测。     

Efficient photoconductive terahertz source using line excitation

We report significant improvement in terahertz (THz) power and efficiency using photoconductive sources by use of a spatially extended line source excitation and the trap enhanced field effect that occurs in sources made on semi-insulating GaAs. The combination of high electric fields and reduced screening effects allows 10 microW of THz power to be generated with 14 mW of absorbed optical power, demonstrating nearly 0.1% optical-to-THz conversion efficiency.     

基于BS结构的太赫兹光谱数据库的设计与实现

随着太赫兹技术中关键科学与技术问题的解决和有关仪器发展,其在各个领域的应用也备受人们的关注。由于太赫兹光谱技术具有一些独特优势,在快速无损检测技术的发展方面具有重要的应用前景,该技术与其他方法互补,可以解决许多原来难以解决的检测问题。进一步开发太赫兹光谱技术实际检测方法的关键之一就是需要具有一套较为完整和可靠的太赫兹光谱数据库。介绍了近期开发的一个基于浏览器/服务器(Browser/Server)的太赫兹光谱数据库系统。根据实际需要设计了太赫兹光谱数据库的结构和主要功能。该数据库目前包括240多条太赫兹光谱信息,其数据来源包括三个部分：(1)收集了部分国外太赫兹光谱数据库的信息;(2)从文献中收集部分光谱数据;(3)国内部分实验室测量的太赫兹光谱数据。着重介绍了建立BS结构的太赫兹数据库的基本结构和项目,介绍了BS结构太赫兹光谱数据库的功能与检索方法。该数据库具有计算样品的光学参数功能,可以根据输入的太赫兹时域光谱数据计算出样品的吸收系数,折射率等其他光学参数,以便于样品特征研究和谱库检索。检索系统能方便的提供注册用户收集、查询、显示谱图及其实验数据和分子结构图、数据匹配等功能。该数据库可以登录http://www.teralibrary.com进行查询和使用。该数据库是根据目前的太赫兹光谱数据信息建立的,以后会逐渐完善。希望该太赫兹光谱数据库能够为用户提供强大、方便和高效的服务功能,进而推进太赫兹技术在更多领域的应用。     

A Terahertz Spectral Database Based on Browser/Server TechniqueSCIEI北大核心CSCD

With the solution of key scientific and technical problems and development of instrumentation, the application of terahertz technology in various fields has been paid more and more attention. Owing to the unique characteristic advantages, terahertz technology has been showing a broad future in the fields of fast, non-damaging detections, as well as many other fields. Terahertz technology combined with other complementary methods can be used to cope with many difficult practical problems which could not be solved before. One of the critical points for further development of practical terahertz detection methods depends on a good and reliable terahertz spectral database. We developed a BS (browser/server) -based terahertz spectral database recently. We designed the main structure and main functions to fulfill practical requirements. The terahertz spectral database now includes more than 240 items, and the spectral information was collected based on three sources: (1) collection and citation from some other abroad terahertz spectral databases; (2) collected from published literatures; and (3) spectral data measured in our laboratory. The present paper introduced the basic structure and fundament functions of the terahertz spectral database developed in our laboratory. One of the key functions of this THz database is calculation of optical parameters. Some optical parameters including absorption coefficient, refractive index, etc. can be calculated based on the input THz time domain spectra. The other main functions and searching methods of the browser/server-based terahertz spectral database have been discussed. The database search system can provide users convenient functions including user registration, inquiry, displaying spectral figures and molecular structures, spectral matching, etc. The THz database system provides an on-line searching function for registered users. Registered users can compare the input THz spectrum with the spectra of database, according to the obtained correlation coefficient one can perform the searching task very fast and conveniently. Our terahertz spectral database can be accessed at http://www.teralibrary.com. The proposed terahertz spectral database is based on spectral information so far, and will be improved in the future. We hope this terahertz spectral database can provide users powerful, convenient, and high efficient functions, and could promote the broader applications of terahertz technology.     

ZnO-based terahertz quantum cascade lasers

High-power terahertz sources operating at room-temperature are promising for many applications such as explosive materials detection, non-invasive medical imaging, and high speed telecommunication. Here we report the results of a simulation study, which shows the significantly improved performance of room-temperature terahertz quantum cascade lasers (THz QCLs) based on a ZnMgO/ZnO material system employing a 2-well design scheme with variable barrier heights and a delta-doped injector well. We found that by varying and optimizing constituent layer widths and doping level of the injector well, high power performance of THz QCLs can be achieved at room temperature: optical gain and radiation frequency is varied from 108 cm^?1 @ 2.18 THz to 300 cm^?1 @ 4.96 THz. These results show that among II–VI compounds the ZnMgO/ZnO material system is optimally suited for high-performance room-temperature THz QCLs.     

太赫兹技术在农产品检测中的应用研究进展

我国是农业大国,保障粮食安全是国家发展的战略需要。农产品检测技术的应用和发展对监控质量,预防由农产品品质问题引发的安全事故至关重要。太赫兹（Terahertz,THz）波位于电磁频谱空隙,频率高于微波而低于红外线,具备光子能量低、穿透性好、能表征分子结构等优点。基于太赫兹波的光谱检测技术受到研究人员广泛关注,在生物医学、安全检查等方面得到应用,被证明是一种可靠的检测手段。在农产品应用领域,太赫兹波特有的非接触、无标记检测能力为农产品成分分析、质量控制提供了技术手段,其良好的穿透性和无损害性,可以用来在不破坏农产品表面及外包装的前提下,检测内部成分变化。与其他光谱（超声、X射线、红外等）检测手段相比,太赫兹波频率范围宽、表征能力强,可实现对目标物质的快速无损检测。近几年,随着太赫兹发射源、探测器等设备以及光谱和成像技术的发展,其在农产品领域的应用有了新的进展。通过收集整理近期的文献资料,综述了太赫兹技术在农产品检测方面的应用拓展和研究成果,总结了目前存在的应用局限。在此基础上,对未来太赫兹光谱和图像检测的研究方向进行了展望,提出提高检测灵敏度和检测速度是农产品领域太赫兹技术产业化应用的研究重点。在检测系统中引入基于超材料的传感器是提升灵敏度的一种有效手段,可以突破原有的太赫兹光谱检测极限,对研究农药残留、真菌毒素等危害农产品安全的痕量污染物具有重要意义。在农产品快速成像检测方面,基于单像素成像和压缩感知理论的太赫兹计算成像技术是提高检测速度的可行方案。这些研究成果将为后续太赫兹技术的发展提供方向性指导,对农产品检测领域的应用推广具有重要参考价值。     

差频可调谐太赫兹技术的新进展

太赫兹技术在最近30年来得到快速发展, 并在医学、生物、农业、材料、安检、通信、天文等领域得到广泛应用. 从太赫兹源的频谱特性可以分为窄带(单频)太赫兹源和宽带太赫兹源. 从频谱技术方面来说, 相干的宽带和窄带太赫兹谱是一种互补性关系, 具有各自的技术特点和应用范围. 宽带太赫兹谱可以用于快速获取较宽频谱范围的分子振转谱, 实现混合特征谱的快速检测或成像. 窄带太赫兹源具有很好的光谱灵敏度和分辨率, 适用于太赫兹抽运-探测、分子振转能级谱精细结构分辨 以及太赫兹远程探测和成像. 因此研制具有可调谐的高峰值功率的窄带太赫兹源是适用于探测和识别分子振转能级指纹谱的应用需求, 而差频技术是获得高功率和宽调谐窄带太赫兹源最重要的技术之一. 为了突出该技术的最新进展, 本综述引证论文仅仅限于近5 年来基于差频技术产生太赫兹波的研究进展, 分为光学激光差频源和量子级联激光器差频源两大部分. 对于光学激光差频源, 分别对目前文献报道的各种双波长差频源和太赫兹产生用的非线性晶体进行分类介绍, 并给出所采用的技术和实验结果; 对于量子级联激光器差频源, 分别介绍了量子级联激光器中的差频产生技术和波长调谐技术的最新进展. 量子级联激光器差频太赫兹源是目前实现量子级联激光器在太赫兹波段室温运转的惟一技术, 是实现小型化、窄带宽调谐和室温运转太赫兹源的新发展领域, 值得关注.     

太赫兹片上系统和基于微纳结构的太赫兹超宽谱源的研究进展

目前太赫兹辐射信号的功率不高,辐射带宽也较窄,这些对于生物化学、含能材料的太赫兹检测应用领域来说是一大限制因素,因此如何获得宽谱高功率的太赫兹源对于太赫兹时域光谱系统的发展是非常重要的;另一方面,常规的太赫兹系统是在自由空间传输探测的,测量过程需要在氮气或者干燥空气环境中进行,以克服空气中水的吸收干扰,同时自由空间中的光场与物质相互作用的模式又降低了物质检测的灵敏度,这对于痕量物质检测来说构成了挑战.面对这一问题,太赫兹片上系统利用微纳结构中的局域场效应实现对物质的低浓度检测,此方案有助于解决这一应用难题.综上所述,本文分成以下两部分阐述:首先阐述了纳米金属薄膜作为新的太赫兹源,它可以同时产生非相干的和相干的太赫兹信号,其输出为超过100 THz的太赫兹-红外辐射,功率高达10 mW,这种超宽谱和高功率现象主要是由于非相干的热辐射效应引起的;第二,阐述了基于不同传输线结构、不同基底材料的太赫兹片上系统结构设计和光谱应用.基于共面带状线结构和聚合物材料基底的太赫兹片上系统有着较低的损耗,能够实现超过2 THz带宽的测量和生物化学应用.     

Terahertz sensing and imaging based on nanostructured semiconductors and carbon materials

The advantageous properties of terahertz (THz) waves, such as permeability through objects that are opaque for visible light and the energy spectrum in the microelectron‐volt range that are important in materials research, allow their potential use in various applications of sensing and imaging. However, since the THz region is located between the electronic and photonic bands, even the basic components such as detectors and sources have not been fully developed, unlike in other frequency regions. THz technology also has the problem of low imaging resolution, which results from a considerably longer wavelength than that of the visible light. However, the utilization of nanostructured electronic devices has recently opened up new horizons for THz sensing and imaging. This paper provides an overview of the THz detector and imaging techniques and tracks their recent progress. Specifically, two cutting‐edge techniques, namely, frequency‐selective THz‐photon detection and integrated near‐field THz imaging, are discussed in detail. Finally, the studies of superconductors and semiconductors with high‐resolution THz imaging are described.     

爆炸性物质太赫兹时间分辨光谱测量

利用自由空间电光取样方法,研究了四种炸药在太赫兹(THz)频段的光学特性。通过太赫兹时间分辨光谱测量,作者得到了四种炸药DNT(2,4-二硝基甲苯)、钝化的RDX(黑索今)、HMX(奥克托金)和TNT(2,4,6-三硝基甲苯)的透射光谱,进而计算得出它们在0.2～2.5 THz频段的吸收系数和折射率。作者发现,2,4-DNT在1.08 THz,HMX在1.82 THz存在显著的吸收尖峰,RDX在此频段存在多个吸收峰,TNT的吸收谱线相对其他三种样品比较平缓,这种共振吸收一般认为是由分子间相互作用或声子共振模式引起的。四种炸药对太赫兹波独特的吸收性质说明,太赫兹时间分辨光谱测量技术在炸药特征识别及安全检测领域具有潜在应用价值。作者对致癌物质偶氮苯进行了太赫兹光谱研究,发现了国产偶氮苯和进口偶氮苯在太赫兹波段均存在特征吸收峰,可用于物质鉴别。     

外差式相干探测时域光谱仪对磷化铟（InP）晶片的超宽频带太赫兹光谱的探测

磷化铟（InP）属于Ⅲ-Ⅴ族化合物半导体材料, 在毫米波的应用中展示出了高性能,在非线性太赫兹器件应用上具有很大的潜力。以前关于InP的研究主要集中于太赫兹频率在0.1～4 THz的频率范围内,在4～10 THz频率范围内InP的太赫兹光学数据还是空白。该研究利用空气等离子体相干探测太赫兹波的时域光谱系统研究了无掺杂的InP晶片在超宽THz频率范围（0.5～18 THz）内的光学特性, 实验中用电离的空气作为太赫兹的发射器和探测器, 利用可以调制的局部偏压诱导二次谐波产生,使在气体中太赫兹波的相干探测成为可能,明显提高了系统的动态范围和灵敏度。产生的太赫兹频谱宽度主要被激光脉冲持续时间所限制,太赫兹脉冲通过InP晶片后相对于参考脉冲会延迟,同时振幅会降低。另外,太赫兹信号的频谱振幅在6.7～12.1 THz范围内下降到本底噪声。同时还可以看出InP晶片在6.7～12.1 THz频率范围内不透光,在0.8～6.7 THz以及12.1～18 THz频率范围内InP的吸收系数相对较低,特别是在15～17.5 THz范围内吸收系数很低并且保持相对稳定,与此同时它的折射率单调增加。这些发现将有助于基于InP晶片的非线性太赫兹器件设计。     

Intense terahertz pulses by four-wave rectification in air

We describe a new four-wave rectification method for the generation of intense, ultrafast terahertz (THz) pulses from gases. The fundamental and second-harmonic output of an amplified Ti:sapphire laser is focused to a peak intensity of ~5x10(14)W/cm (2) . Under these conditions, peak THz fields estimated at 2 kV/cm have been observed; the measured power spectrum peaks near 2 THz. Phase-dependent measurements show that this is a coherent process and is sensitive to the relative phases of the fundamental and second-harmonic pulses. Comparable THz signals have been observed from nitrogen and argon as well as from air.     

单发太赫兹时域光谱技术

太赫兹时域光谱技术(THz-TDS)广泛应用于材料、生物医学、化学、药学、安检等诸多领域。传统扫描式THz-TDS技术需要通过改变探测光延时逐点扫描并重构时域信号,仅适合于具有较高重复频率且稳定的太赫兹辐射源情形下的样品探测。在低重复频率或涨落较大的太赫兹辐射源情形下和不可逆过程中样品的探测,扫描式THz-TDS不再适用,需要使用单发THz-TDS技术,单发THz-TDS技术原则上仅需要一个激光脉冲就可以获取一个完整的太赫兹时域脉冲波形。介绍几种主要的单发THz-TDS探测技术,这些技术都利用了电光晶体的泡克尔斯效应,通过测量探测光的某个物理量的变化来提取太赫兹信号。根据探测方法不同可分为光谱编码、空间编码和互相关等技术。在光谱编码技术中,探测光不同频率成分在时间上发生分离,不同时间成分分别被太赫兹脉冲不同时刻电场调制,通过测量探测光各个频率被太赫兹脉冲调制前后的光谱的变化提取太赫兹脉冲波形。该方法光路简单,测量结果直观,有较高的信噪比,但其时间分辨率较低,且被测太赫兹信号容易产生失真。为提高被测信号的时间分辨率,有人提出了空间编码技术,即不同位置探测光分别被太赫兹脉冲不同时刻电场调制,通过测量探测光各个位置太赫兹脉冲调制前后的光强变化提取太赫兹脉冲波形。根据不同空间展开方法可分为一维空间编码技术和二维空间编码技术。空间编码技术中虽然有较高的时间分辨率,但由于探测光在空间展开能量分散使得其信噪比相对较低。此外,还有一种较高时间分辨率的技术即互相关技术,可分为共线互相关和非共线互相关技术。在非共线互相关技术中,被太赫兹脉冲调制的激光啁啾脉冲与短脉冲互相关作用产生二次谐波,通过太赫兹脉冲调制前后二次谐波空间分布变化来提取太赫兹信号;在共线互相关技术中被太赫兹脉冲调制的啁啾脉冲与短脉冲共线入射到光谱仪,通过干涉条纹提取太赫兹信号,该技术提高了时间分辨率和信噪比,但光路布置复杂,不能进行实时监测。回顾了这几种单发THz-TDS探测技术的发展历程,综述探测技术的原理、实验方案和测量结果,并讨论了这些探测技术的优势和不足。     

生物基底材料的超宽频带太赫兹光谱研究

作为一种新兴的方式,太赫兹时域光谱和成像已经被广泛应用到研究不同生物组织的光学特性。在空气等离子体处施加偏置电场对太赫兹波脉冲进行外差式相干检测(air-biased-coherent-detection,ABCD)的太赫兹系统具有超宽频带和可以在较远距离进行成像的优点,十分适用于对生物组织进行超宽谱研究,而对生物组织进行光谱测量通常需要基底材料。利用太赫兹ABCD系统对四种典型的基底材料(石英,高密度聚乙烯,聚四氟乙烯和石蜡)的光学参数进行测定,并计算其在1~15THz频率范围内的吸收系数和折射率。结果表明,高密度聚乙烯和石蜡可以很好的被用作生物组织超宽频带太赫兹光谱测量的基底材料。同时,虽然石英和聚四氟乙烯都是窄带(0.1~3THz)太赫兹系统中常用的基底材料,但是由于它们在高于5THz的频率范围内对太赫兹波具有较强的吸收,所以不能用作超宽频带太赫兹光谱测量的基底材料。     

TERA-MIR radiation: materials,generation, detection and applications

This special edition is dedicated to original papers covering topics with the areas of interest of COST ACTION MP1204 whose main objective is to advance novel materials, concepts and device designs for generating and detecting THz (0.3–10 THz) and Mid Infrared (10–100 THz) radiation using semiconductor, superconductor, metamaterials and lasers and to beneficially exploit their common aspects within a synergetic approach. The results achieved benefit from the unique networking and capacity-building capabilities provided by the COST framework to unify these two spectral domains from their common aspects of sources, detectors, materials and applications. We are building a platform to investigate interdisciplinary topics in Physics, Electrical Engineering and Technology, Applied Chemistry, Materials Sciences and Biology and Radio Astronomy. In this sense THz and MIR are considered jointly, the driving force for both regimes being applications. The main emphasis of the research presented here is on new fundamental material properties, concepts and device designs that are likely to open the way to new products or to the exploitation of new technologies in the fields of sensing, healthcare, biology, and industrial applications. End users are: research centres, academic, well-established and start-up companies and hospitals. The strong coupling of THz radiation and material excitations has potential to improve the quantum efficiency of THz devices.     

Terahertz spectroscopy and imaging – Modern techniques and applications

Over the past three decades a new spectroscopic technique with unique possibilities has emerged. Based on coherent and time‐resolved detection of the electric field of ultrashort radiation bursts in the far‐infrared, this technique has become known as terahertz time‐domain spectroscopy (THz‐TDS). In this review article the authors describe the technique in its various implementations for static and time‐resolved spectroscopy, and illustrate the performance of the technique with recent examples from solid‐state physics and physical chemistry as well as aqueous chemistry. Examples from other fields of research, where THz spectroscopic techniques have proven to be useful research tools, and the potential for industrial applications of THz spectroscopic and imaging techniques are discussed.     

Single-shot spatiotemporal measurements of high-field terahertz pulses

The electric field profiles of broad-bandwidth coherent terahertz (THz) pulses, emitted by laser-wakefield-accelerated electron bunches, are studied. The near-single-cycle THz pulses are measured with two single-shot techniques in the temporal and spatial domains. Spectra of 0-6 THz and peak fields up to approximately or = 0.4 MV cm(-1) are observed. The measured field substructure demonstrates the manifestation of spatiotemporal coupling at focus, which affects the interpretation of THz radiation as a bunch diagnostic and in high-field pump-probe experiments.     

0.8 THz再生反馈振荡器的仿真模拟研究

随着太赫兹技术的发展,高频率、大功率的太赫兹辐射源一直是国内外研究的热点。再生反馈振荡器作为一种新型太赫兹源器件,具有可行性高、功率大的优点。基于0.8 THz太赫兹波成像系统的需求,采用折叠波导慢波结构,对再生反馈振荡器进行设计与研究。首先对0.8 THz折叠波导慢波结构进行设计并使用CST微波工作室中的本征模求解器进行参数优化,再通过CST粒子工作室中的PIC仿真模块对整管进行热特性仿真,验证了方案的可行性,仿真结果显示,最终可产生60 mW的稳定输出信号。