基于光导微探针的近场/远场可扫描太赫兹光谱技术

太赫兹技术已经成为涉及公共安全、军事国防和国民经济等国家核心利益的前沿研究领域.以往太赫兹测量技术中通常以远场测量为主,如常用的太赫兹时域光谱仪.近年来太赫兹近场技术得到了迅猛的发展,特别是基于光导天线的探针技术的发展,为可扫描的太赫兹近场测量提供了可能.本文详细报道了我们近期在可扫描太赫兹近场光谱仪研究中的进展.采用光纤耦合的光导微探针实现了方便灵活的太赫兹近场/远场三维扫描,并同时获得振幅和相位信息.该系统将有可能广泛应用于人工微结构、石墨烯、表面等离子激元、波导传输、近场成像、生物样品检测、芯片检测等研究领域.     

Recent advances in terahertz imaging

We review recent progress in the field of terahertz “T-ray” imaging. This relatively new imaging technique, based on terahertz time-domain spectroscopy, has the potential to be the first portable far-infrared imaging spectrometer. We give several examples which illustrate the possible applications of this technology, using both the amplitude and phase information contained in the THz waveforms. We describe the latest results in tomographic imaging, in which waveforms reflected from an object can be used to form a three-dimensional representation. Advanced signal processing tools are exploited for the purposes of extracting tomographic results, including spectroscopic information about each reflecting layer of a sample. We also describe the application of optical near-field techniques to the THz imaging system. Substantial improvements in the spatial resolution are demonstrated. Received: 29 January 1999 / Published online: 7 April 1999     

Features of the scattering of focused terahertz radiation from the probe of a terahertz near-field microscope

The scattering of pulsed terahertz radiation from metallic probes in the form of thin cylinders and cones with a small opening angle, which are used in apertureless terahertz near-field microscopes, has been investigated. The extrema of the waveform of pulsed terahertz radiation scattered from a free probe are linearly shifted with a change in the vertical position of the probe, and the spectral distribution is characterized by an inversely proportional frequency dependence. In the presence of a reflecting surface under the probe, when new excitation and detection directions appear, the spectrum of scattered terahertz radiation does not differ from the spectrum of the incident radiation. The experimental data are in mutual agreement with the theoretical results obtained within the model of the generation of diffraction edge waves at the interface of inhomogeneous excitation between the excitation region and shadow region.     

Observation of magnetoconductivity with terahertz probes for ferromagnetic Fe films

Ultrafast characterization of spin-dependent transport behavior in ferromagnetic systems without contact probes has been of strong demand, recently. We have experimentally investigated spin-dependent complex conductivity for ferromagnetic Fe films of various thickness by means of terahertz time-domain transmission spectroscopy. Comparison of the transmitted terahertz wave amplitude and the spin-dependent conductivity reveals that the magnetization state of films effectively determines the complex conductivity. Non-invasive observation of spin-dependent conductivity by contact-free terahertz probe method is proven to be promising in further investigating spintronic materials, particularly on an ultrafast timescale.     

Optical terahertz bullets

Optical generation of broadband terahertz radiation in a system of resonant anisotropic molecules with a permanent dipole moment has been investigated taking into account the transverse dynamics of pulses. It has been shown that, if the carrier frequency of a femtosecond optical signal is higher than the eigenfrequency of a selected molecular transition by such a value that the spectrum of the pulse, owing to its finite length, covers the resonance transition, the generation occurs in the filamentation regime followed by transformation of the filaments into stable optical terahertz bullets localized in all directions. In this case, the energy generation efficiency can be as high as ～10^?3.     

How to realize an ultrafast electron diffraction experiment with a terahertz pump: A theoretical study

To integrate a terahertz pump into an ultrafast electron diffraction (UED) experiment has attracted much attention due to its potential to initiate and detect the structural dynamics both directly. However, the deflection of the electron probe by the electromagnetic field of the terahertz pump alters the incident angle of the electron probe on the sample, impeding it from recording structural information afterwards. In this article, we studied this issue by a theoretical simulation of the terahertz-induced deflection effect on the electron probe, and came up with several possible schemes to reduce such effect. As a result, a terahertz-pump-electron-probe UED experiment with a temporal resolution comparable to the terahertz period is realized. We also found that MeV UED was more suitable for such terahertz pump experiment.     

Coherent population trapping in quantum cascade photodetector as a new method for terahertz detection

In this article a novel terahertz photodetector based on coherent population trapping (CPT) and in quantum well cascade format is introduced and supplementary simulations and discussions are presented to fulfill the proposed idea. In this scheme, the interference of a short-wavelength probe signal and the terahertz (THz) signals modifies the absorption coefficient of the active region and thus, detection of the short wavelength probe is translated as detection of the THz signal. A ladder structure coupled to the active region extracts the probe-excited carriers and transports them to consequent period via phonon-mediated transport just like conventional quantum cascade structures.     

Comparative study of coherent terahertz emission from Fe/Pt ferromagnetic heterostructures

The ultrafast spin dynamic of in-plane magnetized Fe/Pt films was investigated by terahertz emission spectroscopy. The amplitude of the emitted terahertz wave is proportional to the intensity of the exciting laser beams.Both the amplitude and polarity of the terahertz wave can be adjusted by modifying the external magnetic field.The dependency of the amplitude on external magnetic fields is coincident to the hysteresis loops of the sample.Also, the polarity of the terahertz wave is reversed, as the magnetization orientation is reversed. The superdiffusive transient spin current with an inverse spin Hall effect is attributed to the main mechanism of the terahertz emission.     

Optical injection and terahertz detection of the macroscopic Berry curvature

We propose an experimental scheme to probe the Berry curvature of solids. Our method is sensitive to arbitrary regions of the Brillouin zone and employs only basic optical and terahertz techniques to yield a background-free signal. Using semiconductor quantum wells as a prototypical system, we discuss how to inject Berry curvature macroscopically and probe it in a way that provides information about the underlying microscopic Berry curvature.     

Reflective terahertz imaging of porcine skin burns

A reflective pulsed terahertz imaging system based on direct detection was developed and used to obtain high-resolution images of a porcine skin specimen with superficial partial-thickness (second-degree) burns. Images were also obtained of the sample through ten layers of dry medical (cotton) gauze with minimal image degradation. The burned and unburned regions of skin had large differences in terahertz reflectivity, displaying clear delineation [20 dB signal-to-noise ratio (SNR) difference signal] between both regions in the images. The terahertz images also exhibited a "halo" surrounding the burn areas that may correlate to the extent of burn injury. The system operated at a center frequency of 500 GHz with 125 GHz of 3 dB bandwidth and used whiskbroom scanning to generate images with a spatial resolution of 1.5 mm. Each pixel was acquired with a 16 ms integration time, resulting in a 40 dB postdetection SNR. The simplicity and high SNR of the reflective terahertz system are promising steps toward real-time terahertz medical imaging.     

On the generation of terahertz radiation by a phase-modulated optical pulse

The problem of terahertz radiation generation in a quadratic nonlinear medium by a nanosecond phase-modulated optical pulse possessing the properties of supercontinuum has been investigated. The possibility of generating both broadband and quasi-monochromatic terahertz signals has been demonstrated. The conditions have been revealed under which the generation efficiency of a phase-modulated nanosecond optical pulse is greater than that of a spectrally confined femtosecond signal.     

Development of a rapid-scan fiber-integrated terahertz spectrometer

Scientists in terahertz (THz) wave technologies have benefited from the recent developments in ultrafast laser technologies and RF technologies and applied these new gained techniques into characterizing a wide variety of phenomena. Undoubtedly, the most successful of these applications has been in the development of time-domain terahertz spectroscopic and imaging systems which has been utilized in the characterization of dielectrics and semiconductors. This pulsed technique has allowed users to characterize dynamical behavior inside materials under illumination with picosecond resolution. Typically pump/probe or similar dynamical measurements require the use of amplified pulses derived from free-space solid state lasers in the $\upmu $ J–mJ range and since interferometric techniques are typically used in pulsed measurements the measurement time of a THz spectrum can last at least tens of minutes. Better systems can be realized based on fiber laser technologies. Here we discuss the advantages of a THz spectrometer driven by an ultrafast Ytterbium doped fiber laser whose repetition rate can be tuned rapidly allowing for rapid dynamical measurements. The efficient gain medium, robust operation and compact design of the system opens up the possibility of exploring rapid detection of various materials as well as studying dynamical behavior using the high brightness source.     

连续太赫兹波双物距叠层定量相衬成像

针对连续太赫兹波叠层成像重建算法收敛较为迟滞的问题,提出一种连续太赫兹波双物距叠层成像方法及相关重建算法,使用不同记录距离形成的差异化衍射图幅值作为重建算法记录平面的约束条件,增加了记录平面数据多样性和衍射信息冗余度.仿真结果表明,本方法可以加快算法收敛速率,有效减少迭代次数,提高连续太赫兹波定量相衬成像计算效率.随后构建了基于2.52 THz光泵连续太赫兹激光器的双物距叠层成像实验装置,应用双物距记录方法及改进算法重建获得了聚丙烯基字母图案样品的幅值和相位分布,结果表明改进方法可以减少算法迭代次数,提升计算效率,同时改善相位像重建结果保真度.     

Contrast and resolution enhancement in a confocal terahertz video system

The effect of image contrast enhancement without the loss of the spectral composition of the radiation under investigation has been experimentally demonstrated for a terahertz far-field video system. The result has been obtained by using a confocal aperture placed in the intermediate-focus region of the optical system. The result is a fundamental advance towards the achievement of subwavelength resolution and the contrast of microscopy video systems in the terahertz spectral range.     

小波变换在太赫兹三维成像探测内部缺陷中的应用

采用Syn View Head 300对内部有胶和空气孔的样件进行了太赫兹二维扫描(xy轴方向),系统通过线性调频连续波技术得到样件内部的三维信息.检测薄层时,由于太赫兹源的波长在亚毫米量级,薄层的上下表面反射峰相距太近而难以辨别.为了提高太赫兹探测的纵向分辨率,采用小波变换对探测信号进行处理,对小波系数进行三维重构,获得的三维小波系数图像比原始三维探测信号更加精确.该方法有效提高了太赫兹成像的纵向检测精度,纵向分辨率可达1 mm.     

超快太赫兹时域光谱系统

超快太赫兹时域光谱系统是基于高速异步光学采样原理进行工作的,该系统使用2个重复频率可在1 GHz附近变化的飞秒振荡器,并使用高带宽反馈电路控制其重复频率。2个飞秒振荡器的重复频率存在Δf的失谐,一个飞秒振荡器的重复频率是1 GHz+Δf Hz,为泵浦脉冲;另一个飞秒振荡器的重复频率是1 GHz,为探测脉冲,由此提供泵浦脉冲和探针脉冲的时间差,时间延迟呈周期性变化,其扫描周期可以由1/Δf给出。此系统摒弃了传统THz-TDS系统所必需的机械延迟线,采用双光子探测器来产生触发信号。当设定Δf=1 kHz时,1 ms就可以探测出1个THz谱, 用时10.3 s即可得到动态范围为21 dB、频谱分辨率为5 GHz的太赫兹信号。该系统具有检测速度快和频谱分辨率高的优点,在需要快速测量的应用环境中有着传统太赫兹时域光谱系统不可比拟的优势。     

Influence of cutting off position of plasma filament formed by two-color femtosecond laser on terahertz generation

Femtosecond laser filamentation is a method of generating terahertz, which has wide application in terahertz subwavelength resolution imaging. In this paper, the plasma filament formed by femtosecond laser focusing was terminated with an alumina ceramics at different positions and the influence of the cutting off position of the plasma filament on the terahertz wave was studied. The results showed that the terahertz amplitude increases as the position approaches the end of the filament gradually. The stability of amplitude and peak frequency of the terahertz generated by the filament formed by two-color femtosecond laser via a lens with a longer focal length is lower than that through a lens with a shorter focal length, especially the terahertz amplitude at the end of the filament. The study will be helpful for future researchers in the field of THz sub-wavelength imaging utilizing femtosecond laser filament.     

"All air-plasma" terahertz spectroscopy

We demonstrate terahertz wave generation and detection capabilities up to 6 THz without the need for solid state materials, biased electrodes, or forward propagating signal collection. An "all air-plasma" terahertz system is used to encode explosive material resonant signatures into the 357 nm nitrogen fluorescence line of a bichromatic field-induced laser plasma filament. These results show the practicability to extend these measurements to remote locations where terahertz pulse information is no longer limited by water vapor absorption, phonon resonance, or signal collection directionality.     

太赫兹极高分辨力雷达成像试验研究

给出了一种载频0.14 THz、带宽5 GHz的极高分辨力太赫兹雷达成像系统样机。系统采用Ka波段毫米波信号二倍频后作为样机收发链路的谐波混频本振,以线性调频连续波信号作为发射信号,接收时采用去斜接收。利用该太赫兹雷达进行了成像试验并得到了1维距离像与逆合成孔径雷达（ISAR）成像结果。结果表明,THz雷达样机实现了3 cm的高分辨力,其ISAR像清晰,反映了目标的细微特征。     

Two-color laser wavelength effect on intense terahertz generation in air

Near-IR femtosecond lasers have been proposed to produce high-field terahertz radiation in the air via the laser-plasma interaction, but the physical mechanism still needs to be further explored. In this work, we theoretically investigate the effect of the two-color laser wavelength on the terahertz generation in the air based on a transient photocurrent model.We show that the long wavelength laser excitation can greatly enhance the terahertz amplitude for a given total laser intensity. Furthermore, we utilize a local current model to illustrate the enhancement mechanism. Our analysis shows that the terahertz amplitude is determined by the superposition of contributions from individual ionization events, and for the long wavelength laser excitation, the electron production concentrates in a few ionization events and acquires the larger drift velocities, which results in the stronger terahertz radiation generation. These results will be very helpful for understanding the terahertz generation process and optimizing the terahertz output.