Terahertz electromagnetic waves emitted from semiconductor investigated using terahertz time domain spectroscopy

Ultrafast electromagnetic waves radiated from semiconductor material under high electric fields and photoexcited by femtosecond laser pulses have been recorded by using terahertz time domain spectroscopy (THz-TDS).The waveforms of these electromagnetic waves reflect the dynamics of the photoexcited carriers in the semiconductor material,thus,THz-TDS provides a unique opportunity to observe directly the temporal and spatial evolutions of non-equilibrium transport of carriers within sub-picosecond time scale....     

Precisely optical material parameter determination by time domain waveform rebuilding with THz time-domain spectroscopy

We offer an innovative optical material parameter extraction algorithm for Terahertz time-domain spectroscopy (THz-TDS). Unlike previously reported extraction algorithms in frequency domain, which have to be compromised between amplitude and phase, or the real part and imaginary part of the transfer function, we calculate material parameters by minimizing the difference between the measured sample waveform and the rebuilt one in time domain. The new method is effectively verified by different samples, and can be used either in transmission mode or in reflection mode for THz-TDS.     

基于Mueller矩阵椭偏仪的纳米压印模板与光刻胶光栅结构准确测量

在纳米压印工艺中,对模板和压印结构的几何参数进行快速、低成本、非破坏性地准确测量具有非常重要的意义.与传统光谱椭偏仪只能改变波长和入射角2个测量条件并且在每一组测量条件下只能获得振幅比和相位差2个测量参数相比,Mueller矩阵椭偏仪可以改变波长、入射角和方位角3个测量条件,而且在每一组测量条件下都可以获得一个4×4阶Mueller矩阵共16个参数,因此可以获得更为丰富的测量信息.通过选择合适的测量条件配置,充分利用Mueller矩阵中的测量信息,有望实现更为准确的纳米结构测量.基于此,本文利用自主研制的Mueller矩阵椭偏仪对硅基光栅模板和纳米压印光刻胶光栅结构进行了测量.实验结果表明,通过对Mueller矩阵椭偏仪进行测量条件优化配置,并且在光学特性建模时考虑测量过程中出现的退偏效应,可以实现压印工艺中纳米结构线宽、线高、侧壁角以及残胶厚度等几何参数更为准确的测量,同时对于纳米压印光刻胶光栅结构还可以直接得到光斑照射区域内残胶厚度的不均匀性参数.     

太赫兹吸收光谱的频率误差修正研究

在用太赫兹时域光谱(THz-TDS)系统测量样品THz吸收光谱的过程中,电光采样系统的采样误差往往导致测得的THz吸收频率与真实值间存在偏差。针对此问题,利用一氧化碳(CO)分子THz吸收峰的分布特性研究了对THz-TDS系统测量数据进行修正、从而提高THz吸收频率测量精度的方法。首先通过实验测得了2.0×105 Pa 压强下CO气体的一系列等间隔分布的THz吸收峰,然后把测得的吸收峰峰位与JPL标准数据库中CO分子的吸收频率进行对比,得到了实验数据的误差值。通过分析误差值随吸收频率的分布规律,发现二者间呈正线性相关,在此基础上拟合得到了实验数据的误差修正模型。用所建立的模型对实验数据进行修正后,最大误差值为3.36 GHz,较修正前降低了两个数量级,表明根据CO分子的THz吸收峰可有效修正THz-TDS系统的测量误差,从而提高THz吸收光谱的精度。本研究对材料分析识别以及分子光谱标准数据库的建立具有重要意义。     

Studies on 2,4-DNT Mixtures Using Reflection Terahertz Time Domain Spectroscopy for Explosives Detection

Absorption spectra （0.2-1.8 THz） of the mixtures of explosive 2,4-DNT and polyethylene（PE） at different ratios are obtained using reflection terahertz time domain spectroscopy （THz-TDS）. The pronounced absorption peak of 2,4-DNT at 1.08 THz is always observed for the mixtures with 2,4-DNT ratios above 20%. Experimental results demonstrate that more applicable and realistic THz-TDS in reflection geometry can be used to distinguish explosive mixed with other material having no THz fingerprints, and has a high potential in the detection of explosives.     

厚度误差对THz-TDS的测量不确定度分析

阐述了基于菲涅尔公式的透射式太赫兹时域光谱系统提取样品光学常数的方法和原理,分析了样品厚度误差对THz-TDS测量不确定度的影响,并建立了相应的不确定度模型。进行太赫兹时域光谱测量实验,提取硅片在太赫兹波段的折射率,并计算了误差对提取样品折射率的影响。结果表明,随着厚度误差的增大,系统测量偏差也随之增大。对于较厚样品,相同厚度误差对其测量结果影响较小。样品厚度为994μm时,在厚度存在1μm的测量误差情况下,系统测量折射率的偏差为0.001 2,接近模型的仿真值。实验结果验证了厚度误差对测量不确定度模型的有效性,了解了厚度误差对系统测量结果的影响情况,对测量过程及结果分析具有一定的指导意义。     

红薯淀粉中添加剂明矾的定性和定量太赫兹时域光谱技术检测

应用太赫兹时域光谱(THz-TDS)技术对红薯淀粉中的添加剂明矾进行了实验研究,获得了红薯淀粉和明矾的太赫兹时域光谱和频域光谱,通过计算得到二者的吸收系数谱和折射率谱,吸收系数谱显示明矾在太赫兹波段有明显的特征吸收峰,可以用THz-TDS技术对淀粉中的明矾进行特征识别。实验获得了红薯淀粉中掺杂不同百分比(质量分数)明矾的太赫兹时域光谱,计算得到吸收系数谱和折射率谱,发现随着明矾含量的增加吸收峰的幅度下降,折射率逐渐下降,说明THz-TDS技术可以用于淀粉中明矾的定性识别和定量检测。     

太赫兹波大气传输衰减模型

基于修正的Van-Vleck Weisskopf线型、辐射传输色散理论和水汽连续体吸收模型,结合HITRAN数据库,建立了太赫兹波大气传输衰减模型VVWH,形成了对宽频太赫兹波在真实大气中水平传输衰减的数值模拟能力。同时对太赫兹时域光谱技术（THz-TDS）获取的透射光谱实测数据进行了对比分析。计算结果与实验结果总体变化趋势一致,在吸收谱线处两者吻合良好,但在低频的大气窗口区,实验结果相比计算呈现出更强的传输衰减。考察了相对湿度对太赫兹波大气传输衰减特性的影响变化。     

基于Mueller矩阵成像椭偏仪的纳米结构几何参数大面积测量

为了实现有效的工艺监控, 在批量化纳米制造中对纳米结构的关键尺寸等几何参数进行快速、低成本、非破坏性的精确测量具有十分重要的意义. 光学散射仪目前已经发展成为批量化纳米制造中纳米结构几何参数在线测量的一种重要手段. 传统光学散射测量技术只能获得光斑照射区内待测参数的平均值, 而对小于光斑照射区内样品的微小变化难以准确分析. 此外, 由于其只能进行单点测试, 必须要移动样品台进行扫描才能获得大面积区域内待测参数的分布信息, 从而严重影响测试效率. 为此, 本文将传统光学散射测量技术与显微成像技术相结合, 提出利用Mueller矩阵成像椭偏仪实现纳米结构几何参数的大面积快速准确测量. Mueller矩阵成像椭偏仪具有传统Mueller矩阵椭偏仪测量信息全、光谱灵敏度高的优势, 同时又有显微成像技术高空间分辨率的优点, 有望为批量化纳米制造中纳米结构几何参数提供一种大面积、快速、低成本、非破坏性的精确测量新途径.     

Low noise spinning wheel technique for THz material parameter extraction

Double-modulated terahertz differential time-domain spectroscopy (double-modulated THz-DTDS), is a technique that is based on dithering the sample under test. In this paper, we report a measurement technique based on mounting the sample on a spinning wheel, in order to overcome fundamental limitations imposed by linear dithering. We demonstrate a proof-of-principle showing that noise decreases as a function of the spinning wheel modulation frequency. This technique does not suffer the mechanical noise limitation of traditional linear dithering and thus opens up future scope for further noise reduction via hardware advances in the modulation frequency of the wheel. The spinning wheel technique enables a rapid succession of measurements between the reference and sample signals with a single mechanical delay scan. As a result, an improvement in measurement time by at least a factor of two, as compared to the conventional THz-TDS measurement technique is observed. The spinning wheel technique is experimentally verified by measuring the dielectric properties of a thick polymer material.     

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。     

时空混合调制型偏振干涉成像光谱仪的全视场偏振信息探测研究

基于Savart板的新型时空混合调制型偏振干涉成像光谱仪(TSMPIIS)是一种静态、小型化、高稳定性的遥感偏振探测器件, 可以用来实现对目标的光谱信息以及偏振信息的探测. 目前, 已经确立了通过旋转TSMPIIS偏振片法来实现测量的方案, 然而该方案破坏了TSMPIIS系统原有的稳定结构, 降低了仪器的测量精度和可靠性. 为克服旋转偏振片法的不足, 本文从TSMPIIS的探测原理出发, 通过分析和计算TSMPIIS的Mueller矩阵, 推导出了全视场角度的TSMPIIS旋转偏振探测法的基本公式, 论证了TSMPIIS全视场偏振信息探测的可行性和准确性, 为TSMPIIS的遥感探测以及Stokes参数的反演提供了理论依据, 进一步拓展了TSMPIIS遥感探测的优越性.     

Terahertz Time-Domain Spectroscopy of Solids: A Review

Recent development of the terahertz time domain spectroscopy (THz-TDS) and its application to solids have been reviewed. This spectroscopy is unique in that the time-domain wave forms are measured at first and the complex optical constants are deduced directly by the Fourier transformation of them without resort to the Kramers-Kronig analysis. Various types of the THz-TDS systems are briefly described. Applications of the THz-TDS to various solids, i.e., semiconductors, superconductors, polymers, photonic crystals, and so on are also presented to demonstrate how widely this spectroscopy is applicable to characterization of solids.     

Differentiating characteristic microstructural features of cancerous tissues using Mueller matrix microscope

Polarized light imaging can provide rich microstructural information of samples, and has been applied to the detections of various abnormal tissues. In this paper, we report a polarized light microscope based on Mueller matrix imaging by adding the polarization state generator and analyzer (PSG and PSA) to a commercial transmission optical microscope. The maximum errors for the absolute values of Mueller matrix elements are reduced to 0.01 after calibration. This Mueller matrix microscope has been used to examine human cervical and liver cancerous tissues with fibrosis. Images of the transformed Mueller matrix parameters provide quantitative assessment on the characteristic features of the pathological tissues. Contrast mechanism of the experimental results are backed up by Monte Carlo simulations based on the sphere–cylinder birefringence model, which reveal the relationship between the pathological features in the cancerous tissues at the cellular level and the polarization parameters. Both the experimental and simulated data indicate that the microscopic transformed Mueller matrix parameters can distinguish the breaking down of birefringent normal tissues for cervical cancer, or the formation of birefringent surrounding structures accompanying the inflammatory reaction for liver cancer. With its simple structure, fast measurement and high precision, polarized light microscope based on Mueller matrix shows a good diagnosis application prospect.     

Error in determining Mueller matrix elements and its effect on solution of the inverse problem of polarimetry

We present an analysis of the errors in measurement of the Mueller matrix elements in polarimeters that are a combination of different types of Stokes polarimeters in the detection channel and controllable polarization converters in the probing channel. As polarization converters for the probing radiation, we consider a phase plate having four different angular positions in measuring the complete Mueller matrix and a linear polarizer having different angular positions in measuring the structural parts of an incomplete 4 × 3 Mueller matrix. We have shown that the error in determining the Mueller matrix elements is distributed nonuniformly over the matrix. The nature of the error distribution over the elements and its values are different for different combinations of detection and probing channels of the polarimeter, and depend on the anisotropy of the test object. The latter dictates the choice of the optimal layout for a Mueller polarimeter for studying media with the same or different types of anisotropy and the choice of Mueller matrix elements used to solve the inverse problem of determining the anisotropy parameters.     

超宽带太赫兹时域光谱探测技术研究进展

太赫兹时域光谱(THz time-domain spectroscopy, THz-TDS)技术是一种非常有效的相干探测技术,具有信噪比高,探测带宽,可在室温下工作,可进行时间分辨测量等特点,广泛应用于材料、化学、生物、安检等领域。较早时期的THz-TDS系统受限于太赫兹辐射源的带宽和光谱探测手段,测量范围有限(<5 THz),较高频段的光谱信息无法得到。为了进一步扩大太赫兹时域光谱探测技术的应用范围,迫切需要发展超宽带(≥10 THz)的太赫兹时域光谱探测技术。本文回顾了太赫兹时域光谱探测技术的发展进程,综述了实现超宽带太赫兹时域光谱探测的主要技术方法,展示了不同测量方法的典型实验方案,同时总结了不同探测方法的优缺点,并追踪了主要研究小组的前沿成果以及最新的应用进展。     

提高太赫兹时域光谱仪测量速度研究

太赫兹时域光谱仪的测量速度是一个非常重要的指标,为了提高光谱仪的测量速度需要提高延迟线的扫描速度。实验研究了扫描速度、时间常数、采样率对太赫兹时域光谱仪信号品质的影响。研究结果表明,当时间常数为10 ms时,提高扫描速度不会对信号幅度造成较大改变;而当时间常数为30,100和300 ms时,随着扫描速度的提高,信号幅度会迅速降低,且时间常数越大信号幅度衰减越快,因此,为了避免在提高测量速度时导致信号品质的劣化,应尽量设置较小的时间常数。另一方面,当采集一定时域长度的信号时,扫描速度越高,采集到的采样点数越少,时域波形失真越严重,因此在提高扫描速度时,不仅需要减小锁相放大器时间常数,同时还需要相应的提高采样率以避免因采集点数减少而造成信号波形的失真。该结果可为提高太赫兹时域光谱仪的测量速度提供参考。     

亮氨酸与异亮氨酸的低频集体振动的太赫兹光谱研究

太赫兹(THz)波是指频率在0.1～10 THz频段的电磁波。太赫兹光谱技术不同于以往的检测手段,可以用于检测氨基酸同分异构体,反映物质的分子结构和构型,对食品安全和药品药性控制有着重要的意义。亮氨酸与异亮氨酸属于同系的同分异构体,它们具有近似的分子结构,但物理化学性质有很大的差别。生物大分子的太赫兹吸收与其分子间氢键和分子内氢键的振动和转动能级相关的偶极跃迁有关,可以利用分子偶极跃迁进行生物分子的指纹识别。采用太赫兹时域光谱(THz-TDS)和傅里叶红外光谱(FTIR),对亮氨酸和异亮氨酸进行了测量。在中红外波段亮氨酸与异亮氨酸的吸收光谱几乎完全重叠,而在太赫兹频波段可以观察到它们的光谱存在明显差异,因此太赫兹光谱能够作为快速准确鉴别这两种物质的方法。采用密度泛函理论(DFT)对亮氨酸和异亮氨酸的低频集体振动模式进行理论模拟,并对其太赫兹光谱进行研究和讨论。通过比较实验和理论结果,计算得到的峰位与实验结果可以互相印证。     

基于声场重建的材料吸声系数测量方法

针对现有方法对材料吸声系数进行现场测量时存在低频测量误差大的问题,本文提出了一种利用扬声器线阵列对材料吸声系数进行现场测量的新方法。该方法使用基于能量比值约束的最小二乘法在待测材料表面进行平面波声场重建并结合双传声器传递函数法对材料的吸声系数进行测量。数值仿真表明在100～1600 Hz频率范围内,新方法在未加约束时能够对材料的吸声系数进行准确测量。在半消声室中利用新方法测量了三聚氰胺泡沫的吸声系数,分析了能量比值约束值对测量结果的影响,并和阻抗管以及其它两种现场测量方法的测量结果进行了对比。结果表明该方法能够对吸声材料在160～1600 Hz频段内的吸声系数进行准确测量,并且相较于现存的现场测量方法,新方法具有更低的测量频率下限。     

Spectroscopic measurements and terahertz imaging of the cornea using a rapid scanning terahertz time domain spectrometer

Spectroscopic measurements and terahertz imaging of the cornea are carried out by using a rapid scanning terahertz time domain spectroscopy(THz-TDS) system.A voice coil motor stage based optical delay line(VCM-ODL) is developed to provide a rather simple and robust structure with both the high scanning speed and the large delay length.The developed system is used for THz spectroscopic measurements and imaging of the corneal tissue with different amounts of water content,and the measurement results show the consistence with the reported results,in which the measurement time using VCM-ODL is a factor of 360 shorter than the traditional motorized optical delay line(MDL).With reducing the water content a monotonic decrease of the complex permittivity of the cornea is observed.The two-term Debye relaxation model is employed to explain our experimental results,revealing that the fast relaxation time of a dehydrated cornea is much larger than that of a hydrated cornea and its dielectric behavior can be affected by the presence of the biological macromolecules.These results demonstrate that our THz spectrometer may be a promising candidate for tissue hydration sensing and practical application of THz technology.