共查询到20条相似文献,搜索用时 15 毫秒
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Anna A. Ezerskaya Dmitry V. Ivanov Sergey A. Kozlov Yuri S. Kivshar 《Journal of Infrared, Millimeter and Terahertz Waves》2012,33(9):926-942
We derive the spectral analogues of the Maxwell equations for describing the propagation of electromagnetic waves in linear and weakly nonlinear dielectric media, which can be useful for the THz spectroscopy of short pulses. We discuss the solutions of those equations for TM and TE polarized nonlinear waves. We obtain analytical solutions of these equations for the case of linear homogeneous isotropic and weakly nonlinear media, and also analyze the patterns of the Fresnel and Fraunhofer diffraction of single-cycle Gaussian THz pulses. 相似文献
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Anika Brahm Maryna Tymoshchuk Felix Wichmann Sebastian Merx Gunther Notni Andreas Tünnermann 《Journal of Infrared, Millimeter and Terahertz Waves》2014,35(11):974-986
In comparison to the X-ray computed tomography Terahertz technique significantly enhances the amount of the information acquired during the sample measurement. Not only amplitude, but also phase, time and spectral characteristics can be determined in THz time-domain spectroscopy. Thus, Terahertz tomography allows localization and identification of substances within the objects due to the characteristic fingerprints in this frequency range. Certainly, an appropriate data processing and comparison algorithms are crucial for the accurate identification of the substances in the measured sample. Therefore, we present a new wavelet-based identification method which is suitable even for the substances with broad absorption curves and small or no absorption peaks. The performance of this algorithm was evaluated with the help of a tomographic sample filled with four substances, which were previously characterized for the external database. The continuous wavelet transform was applied to every data cell of the tomographic measurement and compared to the database. Received sinograms were reconstructed into images which depict estimated similarity between the measured and database substances. Furthermore, we suggest a method for the reduction of spectral data after the continuous wavelet transform. This method is based on the extraction of the distinctive features in the form of ridge lines. 相似文献
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为了消除多级像和零级光的干扰,提出了一种单透镜结构的系统,获得了单一无扰的再现像.使用Gerchberg-Saxton(GS)算法获得菲涅尔全息图,预先叠加倾斜平面波相位至菲涅尔全息图上,使再现像移至中央位置.用成像透镜将空间光调制器(SLM)后方的虚像成像至前方,同时在成像透镜后方放置空间滤波器,滤去多级像和零级光.调整透镜与SLM的位置可方便调整再现像的大小和位置,缩短了光学系统的长度.利用虚像的方法为基于液晶SLM的计算全息显示提供了新的思路. 相似文献
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R. Emadi N. Barani R. Safian A. Zeidaabadi Nezhad 《Journal of Infrared, Millimeter and Terahertz Waves》2016,37(11):1069-1085
A photoconductive antenna (PCA) has been numerically investigated in the terahertz (THz) frequency band based on a hybrid simulation method. This hybrid method utilizes an optoelectronic solver, Silvaco TCAD, and a full-wave electromagnetic solver, CST. The optoelectronic solver is used to find the accurate THz photocurrent by considering realistic material parameters. Performance of photoconductive antennas and temporal behavior of the excited photocurrent for various active region geometries such as bare-gap electrode, interdigitated electrodes, and tip-to-tip rectangular electrodes are investigated. Moreover, investigations have been done on the center of the laser illumination on the substrate, substrate carrier lifetime, and diffusion photocurrent associated with the carriers temperature, to achieve efficient and accurate photocurrent. Finally, using the full-wave electromagnetic solver and the calculated photocurrent obtained from the optoelectronic solver, electromagnetic radiation of the antenna and its associated detected THz signal are calculated and compared with a measurement reference for verification. 相似文献
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OCT图像散斑的形成机理和清除方法 总被引:3,自引:1,他引:3
根据光相干层析成像(OCT)的光学和光电检测系统特点,结合现有的理论模型,分析了OCT成像和散斑形成机理。指出失去相干性的多次散射光部分会被滤波电路排除,散斑是由高散射组织相干长度之内的不同散射截面上的若干具有nπ光程差的相干散射光彼此叠加形成的。最后讨论了消除散斑的方法。 相似文献
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Peter Uhd Jepsen Hannes Merbold 《Journal of Infrared, Millimeter and Terahertz Waves》2010,31(4):430-440
We present spectroscopic measurements of the full dielectric function of aqueous solutions of sodium chloride and lithium
chloride at concentrations approaching their solubility limits at room temperature. We find that the dielectric properties
of the two salts are rather different at THz frequencies. Whereas both the real and imaginary part of the permittivity of
NaCl increases with concentration, we see that the imaginary part of the permittivity of LiCl (related to the absorption)
decreases with increasing salt concentration. We relate these changes to the behavior of slow and fast Debye relaxation processes
in the solutions. 相似文献
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Detection Sensitivity of Brillouin Scattering Near Fresnel Reflection in BOTDR Measurement 总被引:1,自引:0,他引:1
We discuss the effect of Fresnel reflection on BOTDR measurement. We show that the theoretical detection sensitivity with respect to Brillouin scattering is limited by the pulse modulation effect and phase noise of the light source that is incorporated into the Brillouin spectrum by way of the Fresnel reflection, when the Brillouin sensor is located near the Fresnel reflection point. We describe the detection sensitivity in the presence of Fresnel reflection in terms of the minimum detectable sensor length. In regard to the pulse modulation effect, we find that when the pulsewidth is short, the noise in the Brillouin spectrum is large and the detection sensitivity deteriorates. We derive quantitative equations and confirm the equations experimentally. When the pulsewidth is 10 ns with a 1-MHz linewidth laser, the boundary Fresnel reflection determining whether or not we can measure a 1-m sensor fiber near the Fresnel reflection is about 30 dB lower than that for a 100 ns pulsewidth. A 1-m sensor fiber near the Fresnel reflection could be measured with a 10-ns pulse only when the Fresnel reflection was very low at about . Moreover, the effect of the laser linewidth is also discussed analytically. The analysis suggests that a laser with a narrower linewidth will provide better sensitivity. 相似文献
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Abufadda Mahmoud H. Mbithi Nelson M. Polónyi Gyula Nugraha Priyo S. Buzády Andrea Hebling János Molnár László Fülöp József A. 《Journal of Infrared, Millimeter and Terahertz Waves》2021,42(11-12):1065-1077
Journal of Infrared, Millimeter, and Terahertz Waves - The transmission of THz, near-infrared (1030 nm), and green (515 nm) pulses through Eisenia andrei body wall is studied,... 相似文献
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Transmission and Reflection Terahertz Spectroscopy of Insensitive Melt-Cast High-Explosive Materials
Norbert Palka Mateusz Szala 《Journal of Infrared, Millimeter and Terahertz Waves》2016,37(10):977-992
Currently, artillery shells and grenades that are introduced into the market are based on melt-castable insensitive high explosives (IHEs), which do not explode while they run a risk of impact, heat or shrapnel. Particles of explosives (such as hexogen, nitroguanidine and nitrotriazolone) are suspended in different proportions in a matrix of 2.4-dinitroanisole. In this paper, we investigated samples of commonly used IHEs: PAX-41, IMX-104 and IMX-101, whose internal structures were determined by a scanning electron microscope. Terahertz time domain spectroscopy was applied in both transmission and reflection configurations. At first, the complex refraction indices of four pure constituents creating IHEs were determined and became the basis of further calculations. Next, the experimentally determined transmission and reflection spectra of IHEs and pure constituents were compared with theoretical considerations. The influence of the grain size of constituent material and scattering on the reflection spectra was analysed, and good agreement between the experimental and theoretical data was achieved. 相似文献
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Masahiko Tani Kohji Yamamoto Elmer S. Estacio Christopher T. Que Hidekazu Nakajima Masakazu Hibi Fumiaki Miyamaru Seizi Nishizawa Masanori Hangyo 《Journal of Infrared, Millimeter and Terahertz Waves》2012,33(4):393-404
Recent studies on the techniques and development of photoconductive (PC) semiconductor devices for efficient generation and detection of terahertz (THz) pulsed radiation are reported. Firstly, the optimization of PC antenna design is discussed. The PC detection of THz pulsed radiation using low-temperature grown GaAs with 1.55-μm wavelength probe is then described. Finally, the enhancement of THz radiation from InSb by using a coupling lens and magnetic field is investigated. These results reveal valuable insights on the design of an efficient, compact, and cost-effective THz time-domain spectroscopy system based on 1.55-μm fs laser sources. 相似文献
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Soble Paul Rangayyan Rangaraj M. Gordon Richard 《IEEE transactions on bio-medical engineering》1985,(5):330-335
Images reconstructed using a set of a few projections spanning a narrow angular range suffer from a systematic geometric distortion due to the point spread function of the reconstruction process. This distortion can be removed by deconvolving a complementary set of projections calculated from the initial reconstruction. Homomorphic deconvolution and inverse filtering techniques were used for this purpose. A second reconstruction is computed from the union of both sets of projections. Although the distortion is removed, the results are noisy due to problems associated with inverse filtering. Two-dimensional (2-D) filtering of the second reconstruction is performed to reduce the noise, while preserving the reduction in geometric distortion. Results obtained using several 2-D filters were compared visually and by point noise content, distortion, and projection error measures. Quantitative measures of geometric distortion removal for test images are also described. 相似文献
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FENGDi LUSi TANQiao-feng 《半导体光子学与技术》2003,9(2):107-111
In the Fresnel transform domain,an effective improvement to the conventional interative algorthm for designing the diffractive optical elements (DOEs)used for spatial beam shaping has been proposed.The algorithm can successfully achieve to design DOEs for beam shaping.Compared with conventinal algorithm,this algorithm can provide faster convergence,more powerful ability to overcome local minimum problem and better shaping quality.By computer simulation,the result has shown that the DOEs designed by this algorithm has snch advantages as high uniformity at the main lobe,low profile error and steep edge. 相似文献
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为克服超宽带太赫兹通信中的波束色散,当前已设计了多种基于实时延(TTD)的天线结构,但其功耗大、复杂度高。为解决这一问题,该文提出一种低功耗、低复杂度的基于串行等间距延时器的稀疏射频链天线结构。通过联合优化延时器时延和移相器相位,可以改变子载波波束方向,实现多载波波束扩展与波束聚拢,从而为不同分布场景的用户服务。具体而言,在所提天线结构下,提出一种优化延时器时延和移相器相位的方案将多载波波束扩散到不同方向,实现波束扩展,以服务分布在不同方向上的用户。然后,通过调节时延和相位使所有载波波束对准同一方向,实现波束聚拢,以服务分布在同一方向上的用户。仿真结果表明了所设计天线结构和所提优化方案的有效性。 相似文献
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Badano A. Kyprianou I.S. Freed M. Jennings R.J. Sempau J. 《IEEE transactions on medical imaging》2009,28(5):696-702
We quantify the variation in resolution due to anisotropy caused by oblique X-ray incidence in indirect flat-panel detectors for computed tomography breast imaging systems. We consider a geometry and detector type utilized in breast computed tomography (CT) systems currently being developed. Our methods rely on MANTIS, a combined X-ray, electron, and optical Monte Carlo transport open source code. The physics models are the most accurate available in general-purpose Monte Carlo packages in the diagnostic energy range. We consider maximum-obliquity angles of 10deg and 13deg at the centers of the 30 and 40 cm detector edges, respectively, and 16deg at the corner of the detector. Our results indicate that blur is asymmetric and that the resolution properties vary significantly with the angle (or location) of incidence. Our results suggest that the asymmetry can be as high as a factor of 2.6 between orthogonal directions. Anisotropy maps predicted by MANTIS provide an understanding of the effect that such variations have on the imaging system and allow more accurate modeling and optimization of breast CT systems. These maps of anisotropy across the detector could lead to improved reconstruction and help motivate physics-based strategies for computer detection of breast lesions. 相似文献