Imprinted diffractive optics for terahertz radiation

Terahertz diffractive optic elements have been fabricated in polypropylene by imprinting with a silicon master. A silicon master is created with eight phase levels for high diffraction efficiency and etched using inductively coupled plasma reactive ion etching. This technique enables the rapid replication of complex optical structures in a high transmission material. Excellent replication of multilevel high efficiency Fresnel lenses is shown. The resulting lenses were tested with a 2 THz quantum cascade laser. The signal strength at the focus was 70 times the base signal strength.     

Subwavelength resolved terahertz real-time imaging based on a compact and simplified system

A real-time imaging system based on a compact terahertz laser is constructed by employing one off-axis parabolic mirror and one silicon lens. Terahertz imaging of water, water stains, leaf veins, human hairs, and metal wire is demonstrated. An imaging resolution of 68 μm is achieved. The experiments show that this compact and simplified imaging system is suitable for penetration demonstration of terahertz light, water distribution measurement, and imaging analysis of thin samples.     

基于太赫兹时域光谱技术的伪色彩太赫兹成像的实验研究

提出了一种伪色彩太赫兹成像技术. 通过引入频域色彩区间积分, 建立了一套基于太赫兹时域光谱技术的伪色彩太赫兹成像系统, 实验分别研究了乳糖和对氨基苯甲酸两种不同白色化学粉末的伪色彩成像和灰度成像, 研究了不同颜色区间定义对伪色彩图像的影响, 讨论了利用不同频率信息成像系统所能达到的空间分辨率. 研究结果表明, 伪色彩成像技术可以将不同的物质信息同时成像在一张太赫兹图像中, 通过不同物质在太赫兹图像中呈现出的颜色差别来区分不同的物质及其分布. 克服了传统的太赫兹灰度成像技术中, 需要多张图像来区分不同的物质的问题, 提高了成像速度, 降低了筛选难度. 利用高频信息进行伪色彩成像, 可以将系统成像的空间分辨率提高到0.4 mm. 伪色彩成像方式可以更直观快捷地显示样品的基本属性, 对于实现太赫兹安检的初检和快速筛选具有重大的现实意义.     

Semi-quantitative time resolved LIBS measurements

Received: 2 February 1996/Accepted: 20 January 1997     

All-semiconductor room-temperature terahertz time domain spectrometer

We report what we believe to be the first demonstration of an all-semiconductor room-temperature terahertz time domain spectrometer. An optical Stark mode-locked vertical-external-cavity surface-emitting laser with 480 fs pulses at 1044 nm was used to illuminate low-temperature-grown photoconductive antennae with 5 mum-gap bow-tie-shaped electrodes. The coherently detected spectrum has a bandwidth close to 1 THz, in which water absorption lines at 0.555 and 0.751 THz can be resolved.     

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。     

Papyrus imaging with terahertz time domain spectroscopy

Terahertz time domain spectroscopic imaging (THz-TDSI) is a non-ionizing, non-contact and non-destructive measurement technique that has been recently utilized to study cultural heritage artifacts. We will present this technique and the results of non-contact measurements of papyrus texts, including images of hidden papyri. Inks for modern papyrus specimens were prepared using the historical binder, Arabic gum, and two common pigments used to write ancient texts, carbon black and red ochre. The samples were scanned in reflection at normal incidence with a pulse with a spectral range between 0.1 and 1.5 THz. Temporal analysis of the signals provides the depths of the layers, and their frequency spectra give information about the inks.     

Three-dimensional time resolved measurements from holographic records

Some work with scatter plate holographic systems is reviewed, proceeding from single exposure recordings of transilluminated amplitude distributions to time resolved recordings of three-dimensional phase distributions. The display of sequences of holographic records of an event developing in time may be facilitated by transforming the original holographic records into a single multiplex hologram. An effective form of this techniquw is described, it results in a single reflecting phase element located at a plane near the reconstructed images. Reconstructions belonging to separate instants of time are recovered by selection in the pupil plane.     

Picosecond time resolved luminescence change upon saturation

A new sensitive method is reported for the measurement of picosecond duration luminescence change upon saturation from photoexcited samples. Examples of 600 ps and 20 ps lifetimes are given.     

时间分辨电子显微镜的可行性研究

文章从理论上分析了飞秒激光与金属阴极的相互作用过程．将飞秒激光与金属阴极产生的超短电子脉冲作为电子源，提出了一种具有时间分辨能力的电子显微镜(TREM)的概念设计．文章作者详细地研究了这种电子显微镜的可行性，并讨论了超短电子脉冲的传输和控制问题．     

时间分辨电子显微镜的可行性研究

文章从理论上分析了飞秒激光与金属阴极的相互作用过程.将飞秒激光与金属阴极产生的超短电子脉冲作为电子源,提出了一种具有时间分辨能力的电子显微镜(TREM)的概念设计.文章作者详细地研究了这种电子显微镜的可行性,并讨论了超短电子脉冲的传输和控制问题.     

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.     

Characterizing the rock geological time by terahertz spectrum

<正>In the field of geology, it is necessary to get the information about the rock geological time, which includes absolute geologic age and relative geologic age [1]. Tracing the evolutionary history of the earth is one of the research tasks of earth science. Time, space, material and motivation are the basic elements for studying geological processes and environmental evolution, and time is the basis for studying     

On the response time of plasmonic terahertz detectors

The response time of a plasmonic terahertz detector is investigated using two independent experimental techniques relying on time-resolved and heterodyne measurements, respectively. Both methods demonstrate that the detector response time does not exceed 110 ps. The results obtained suggest that the designed terahertz detector can be used as a component of high-speed telecommunication systems of a new generation.     

Magneto optics of single photons emitted from single InAs/GaAs self-assembled quantum dots in a planar microcavity

We fully characterize the fine spectral structure of neutral and negatively charged single microcavity quantum dot excitons, using polarization-sensitive magneto-photoluminescence spectroscopy. We show that the microcavity allows the simultaneous detection of both the bright and dark excitons using Faraday configuration. Thus, we were able to fully determine the fine structure and the g-factors of the neutral and negatively charged single exciton states within the same single quantum dot. Our measurements are in excellent agreement with novel, many carrier model calculations, which take into account Coulomb and exchange interactions among all the confined e–h pair states.     

Advanced analysis concepts for terahertz time domain imaging

Imaging based on ultrashort terahertz (THz) pulses (100-3000 μm) is investigated. The measured pulses are analyzed and the resulting amplitude and time delay information are compared. An algorithm for discrimination of multiple pulses is presented, which can distinguish several layers inside an object. A new measurement concept is presented, which accelerates the measurement of samples with small optical path differences about two orders of magnitudes. Exemplarily different applications from the field of quality management are shown.     

Perturbation approach to time resolved stimulated Brillouin scattering

The laser beam intensity changes, due to time resolved stimulated Brillouin scattering, were studied analytically using perturbation approach. It was shown, that the derived expressions enables a qualitative understanding of the previously reported results of the experimental as well as numeric investigations.     

On the theory of ultra-short time resolved photo-detection

We discuss the theory of photo-detection with arbitrary time-resolution. In the limit of ultra-short response times, we show how in the quantal heterodyne detection process both difference and sum-frequencies occur in the detector output, contrary to what seems to be the general suggestion. The results compare with purely classical quadratic detection.     

时间分辨电子显微镜的数值模拟

对时间分辨电子显微镜进行了数值模拟。通过求解从样品透射出来的电子在静态磁场和动态电场的混合场中的运动,评价时间分辨电子显微镜的动态时空特性。根据该数值模拟,时间分辨电子显微镜能够在荧光屏上获得样品在不同时刻的6幅显微分幅图像。     

Picosecond time resolved reflectivity of direct gap semiconductors

The relaxation of optically excited hot carrier distributions in GaAs and CdSe has been measured by time resolved reflectivity measurements with sub-picosecond optical pulses from a mode-locked cw dye laser. A characteristic transient in the reflectivity has been interpreted by an analysis of the perturbed optical dielectric function. Estimated energy loss rates are 0.4 eV/ps for electrons and holes having mean energies in the range of 1 to 2 eV for GaAs at room temperature.