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Non-invasive near-field THz imaging using a single pixel detector

机译:使用单像素检测器的无创近场太赫兹成像

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摘要

The terehertz radiation potentially has many interesting applications. From air port security, non-destructive evaluations of electronics and space shuttle panels, to non-ionizing photon energies with the potential to detect cancer growths and quality control of pharmaceutical tables, the list of potential applications is vast as shown in chapter 1. However, there is a lack of cheap, robust and efficient THz sources, detectors and modulators. Further, the long wavelengths render micron sized details unseeable with far-field imaging techniques. This has rendered most imaging applications unusable in the real world. This thesis is based around demonstrating an imaging technique that uses a near-field THz modulator to obtain sub-wavelength images. There are five distinct experimental demonstrations that show the full capacity of the imaging technique developed here. Chapter 2 gives an outline of the background physics knowledge needed to understand the entirety of the thesis. An outline of the mathematics used for modellingis given in the latter part of the chapter as well. Chapter 3 gives a background on the THz generation and detection techniques used in our THz-TDS system, optical rectification and electro-optic sampling in ZnTe. Further more, our system is capable of photoexciting a sample in conjunction to it being probed with a THz pulse. For the most part, we photoexcite a silicon wafer in order to use its photoconductive properties to modulate our THz pulse. Our photoexcitation pulse is spatially modulated, via a digital micromirror device, which in turn spatially modulates our THz pulse. This patterned THz pulse can then be used with a single-element detector to perform imaging. How to do this and the type of patterns needed is described in the latter part of chapter 3. Chapter 4 is the first demonstration that photo-induced conductivity in silicon can be used to manipulate evanescent THz fields for sub-wavelength imaging. For this, we imaged a 1D sub-wavelength slit and were able to obtain the slit profile with 65μm (λ/6 at 0.75T Hz) resolution. Chapter 5 demonstrates what limits the resolution in our imaging system. Namely, the distance which the patterned THz pulse propagates to the object from where itwas spatially modulated. We demonstrate 9μm (λ/45 at 0.75T Hz) resolution using an ultra-thin (6μm) silicon wafer. At such sub-wavelength scales polarization becomes an important factor. We show how one can use polarization in order to detect 8μm breaks in a circuit board hidden by 115μm of silicon. Chapter 6 concerns itself with showing how noise affects our images. Further more, our imaging system is compatible with compressed sensing where one can obtain an image using fewer measurements than the number of pixels. We investigate how different under-sampling techniques perform in our system. Note under-sampling at sub-wavelength resolutions, as is done here, is rather unusual and is of yet to be demonstrated for other part of the electro-magnetic spectrum. Chapter 7 shows that one does not need to photoexcite silicon. One can in principle illuminate any material, hence we photoexcite graphene with our spatially modulated optical pulses. This allows us to obtain the THz photoconductive response of our graphene sample with sub-wavelength resolution (75μm ≈ λ/5 at 0.75T Hz). We compare our results with Raman spectra maps. We find a clear correlation between THz photoconductivity and carrier concentration (extracted from Raman). Chapter 8 exploits the full capacity of our imaging system by performing hyper-spectral near-field THz imaging on a biological sample. For this, in our imaged field of view, we measured the full temporal trace of our THz pulse at a sub-wavelength spatial resolution. This has allowed us to extract the frequency dependent permittivity of our biological sample, articular cartilage, over our spectral range (0.2-2T Hz). We find the permittivity to change on a sub-wavelength scale in correlation with changes in the structure of our sample. However, the permittivity extraction procedures that have been developed make a far-field approximation. We mathematically show the presence of the THz near-fields to render the long wavelength spectral parts of our extracted permittivity to be wrong. Chapter 9 is where we conclude and point out the main problem that needs to be addressed in order to make the measurements presented here more accessible to others. Namely, the cost of the laser system powering the THz-TDS and how to further reduce the acquisition time.
机译:太赫兹辐射可能具有许多有趣的应用。从航空港的安全性,对电子设备和航天飞机面板的无损评估,到具有检测癌症增长和药物桌质量控制潜力的非电离光子能量,潜在的应用范围非常广泛,如第1章所示。 ,缺乏廉价,坚固和有效的太赫兹源,检测器和调制器。此外,长波长使微米尺寸的细节无法用远场成像技术看到。这使得大多数成像应用在现实世界中无法使用。本文基于演示一种成像技术,该技术使用近场太赫兹调制器来获得亚波长图像。有五个不同的实验演示,显示了此处开发的成像技术的全部功能。第2章概述了理解整个论文所需的背景物理学知识。本章的后半部分还提供了用于建模的数学概述。第3章介绍了我们的THz-TDS系统中使用的THz产生和检测技术,ZnTe中的光整流和电光采样的背景知识。此外,我们的系统能够结合以THz脉冲探测的样品对样品进行光激发。在大多数情况下,我们光激发硅晶片,以便利用其光电导特性来调制我们的THz脉冲。我们的光激发脉冲是通过数字微镜设备在空间上调制的,而该设备又会在空间上调制我们的THz脉冲。然后,该图案化的THz脉冲可与单元素检测器一起使用以执行成像。第3章的后半部分介绍了如何执行此操作以及所需的图案类型。第4章是第一个论证,表明硅中的光感应电导率可用于操纵e逝的太赫兹场用于亚波长成像。为此,我们对一维亚波长狭缝成像,并能够获得分辨率为65μm(0.75T Hz时为λ/ 6)的狭缝轮廓。第5章演示了限制成像系统分辨率的因素。即,图案化的太赫兹脉冲传播到被空间调制的物体的距离。我们使用超薄(6μm)硅晶片演示了9μm(在0.75T Hz时为λ/ 45)分辨率。在这样的亚波长范围内,极化成为重要的因素。我们展示了如何利用极化来检测由115μm硅隐藏的电路板上的8μm断裂。第6章探讨了噪声如何影响我们的图像。更进一步,我们的成像系统与压缩传感兼容,在压缩传感中,可以使用比像素数更少的测量值来获取图像。我们研究了系统中不同的欠采样技术如何执行。注意,如此处所做的那样,在亚波长分辨率下进行欠采样是很不常见的,还有待论证电磁频谱的其他部分。第7章表明,不需要光激发硅。原则上可以照亮任何材料,因此我们用空间调制的光脉冲激发石墨烯。这使我们能够获得亚波长分辨率(0.75T Hz时为75μm≈λ/ 5)的石墨烯样品的THz光电导响应。我们将结果与拉曼光谱图进行比较。我们发现太赫兹光电导率和载流子浓度之间有明显的相关性(摘自拉曼)。第8章通过对生物样本进行高光谱近场THz成像来充分利用我们成像系统的全部功能。为此,在成像的视野中,我们以亚波长空间分辨率测量了THz脉冲的完整时间轨迹。这使我们能够在我们的光谱范围(0.2-2T Hz)范围内提取生物样品(关节软骨)的频率相关介电常数。我们发现介电常数随样品结构的变化而在亚波长范围内变化。但是,已经开发出的介电常数提取程序可以进行远场近似。我们用数学方法显示了太赫兹近场的存在,使提取的介电常数的长波谱部分错误。第9章总结并指出了需要解决的主要问题,以使此处介绍的度量更易于他人使用。即,为THz-TDS供电的激光系统的成本以及如何进一步减少采集时间。

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    Stantchev Rayko Ivanov;

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  • 年度 2017
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