1,721,108 research outputs found
An introduction to terahertz time-domain spectroscopic ellipsometry
Get PDFTerahertz spectroscopy has mainly been performed based on terahertz time-domain spectroscopy systems in a transmission or a window/prism-supported reflection configuration. These conventional approaches have limitations in characterizing opaque solids, conductive thin-films, multiple-layer structures and anisotropic materials. Ellipsometry is a self-reference characterization technique with a wide adaptibility that can be applied for nearly all sample types. However, terahertz ellipsometry has not yet been widely applied, mainly due to the critical requirement on the optical setting, the large discrepancy to traditional terahertz spectroscopy and conventional optical ellipsometry. In this paper we introduce terahertz time-domain spectroscopic ellipsometry from the basic concept, theory, optical configuration, error calibration to characterization methods. Experimental results on silicon wafers of different resistivities are presented as examples. This paper is serving as a tutorial to provide key technical guidance and skills for accurate terahertz time-domain spectroscopic ellipsometry
Stationary-wavelet Regularized Inverse Filtering: A Robust Deconvolution approach for Terahertz Reflection imaging
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Stationary-Wavelet Regularized Inverse Filtering for Terahertz Reflection Spectroscopy of Liquid Samples
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THz modulation and spectroscopy: device development and material characterization
No full textPh.D.Terahertz (THz) radiation consists of electromagnetic waves within the frequency range 0.1 x 10^12 Hz to 10 x 10^12 Hz, it is also referred to as T-rays. T-rays have many advantages over the microwave frequencies currently used in wireless communications. For example, shorter wavelength of T-rays leads to a smaller component size ; additionally, a higher frequency provides more bandwidth resources and also increases data transmission speed. In order to realize the potential in THz communications, high-speed and efficient THz modulators are required.THz frequencies match with the rotational transition frequencies of small molecules as well as the frequencies of vibrational mode transitions in many functional groupings of biological molecules. Thereby, the promising applications are THz material characterization, biomedical diagnosis and imaging. Current THz imaging systems are mostly based on raster scan designs, which are extremely slow; THz detector arrays have both a high cost and are challenging to fabricate. Compressive sensing theory introduces a viable single-pixel imaging system using a spatial modulator. Therefore, a THz spatial modulator will enable a single-detector THz imaging system. In this way, a high-speed and efficient THz modulator is crucial for THz communications and THz imaging.This thesis presents a new design of liquid crystal (LC)-based THz modulator first and then introduces novel THz modulators based on total internal reflection (TIR) geometry. The LC THz modulator utilizes a new design of electrodes which accelerates the switching speed by 10 times. By controlling an optically thin conductive layer at the interface of TIR geometry, the incident THz signal can be modulated. Equations to describe the conductive interface TIR (CI-TIR) model are derived and experiments to verify the equations are conducted both with electrically controlled graphene and optically pumped silicon. In TIR geometry, modulation depths over 99% are achieved by both graphene and photodoped Si. Based on the TIR geometry, a THz polarization converter design is presented by placing a subwavelength metal grating structure at the interface. Three typical polarization conversions are experimentally demonstrated with the same device. Based on metal grating integrated vanadium oxide thin-film structure in TIR geometry, an electrically active THz polarization converter is fabricated and measured. The electric field enhancement effect of the metal grating is further developed in the TIR regime to design a broadband, high modulation depth modulator with solid-state gated graphene. Finally, the potential of applying the theory to achieve thin-film TIR spectroscopy is discussed.太赫茲波指的是頻率在0.1x10^12 赫茲到10x10^12 赫茲之間的電磁波，通常也叫作T-ray。太赫茲波和現在使用的微波相比在無線通信領域有很多的優勢。例如，太赫茲波的波長更短，因此在太赫茲通信系統中可以使用尺寸更小的器件；另外，太赫茲的頻率更高，可以使用的帶寬資源也更多，同時數據的傳輸速率也更快。為了充分利用太赫茲通信的潛力，需要高速、高效的太赫茲調製器。太赫茲的頻率範圍和生物小分子的轉動頻率以及生物大分子功能團的振動頻率重合。太赫茲在材料表徵，生物醫學診斷和成像領域也同樣具有潛力。目前的太赫茲成像系統多採用逐點掃描方式，這種方式速度非常慢；太赫茲探測器陣列又非常難以製造且價格昂貴。壓縮感知技術(Compressive sensing) 可以實現單像素點成像，但需要結合一個空間光調製器。因此，一個太赫茲空間光調製器可以實現單一探測器的太赫茲成像。所以，一個高速、高效的太赫茲調製器對於太赫茲通信和太赫茲成像至關重要。本論文首先展示了一種基於液晶的新型太赫茲相位調製器，然後介紹了基於全反射系統的太赫茲調製器。液晶太赫茲調製器利用了一種新型的電極設計，可以使得液晶器件的開關速度提高10 倍以上。通過控制在全反射界面處的導電率，入射的太赫茲波可以得到調製。描述導電界面全反射(CI-TIR) 的公式在本文中被提出並展示了其推導過程。通過電控石墨烯和光泵浦矽半導體的實驗，上述公式得以驗證。利用全反射結構，在電控石墨烯和光泵浦矽半導體的實驗中都獲得了大於99% 的調製深度。基於全反射結構，一種太赫茲偏振調製器被提出，其結構是將一亞波長的光柵結構置於全反射的界面處。基於這種設計，三種典型的偏振旋轉結果在實驗中實現。基於光柵結合二氧化釩(Vanadium Oxide) 的結構，一種電控的太赫茲偏振調製器得以製造出並測試。亞波長光柵對於光的近場增強理論在本文中被發展到全反射結構中，基於這一新理論，一種寬帶、高效的太赫茲調製器被提出。在本文的最後，討論了全反射系統中太赫茲薄膜光譜學的應用前景。Liu, Xudong."July 2017."Thesis Ph.D. Chinese University of Hong Kong 2020.Includes bibliographical references (leaves 137-152).Abstracts also in Chinese.Title from PDF title page (viewed on 05, February, 2020)
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