太赫茲波段材料量測與超穎表面設計

Abstract

本論文旨在研究太赫茲時域光譜技術，該原理是透過飛秒近紅外脈衝照射在光導天線上形成電子電洞對，再利用外加偏壓來加速電子電洞對的移動，形成瞬態電流，並產生太赫茲波。為了進行檢測，太赫茲脈衝與另一個近紅外脈衝重疊，並通過掃描近一端光路的延遲，記錄下太赫茲脈衝時間分辨的電場，最後透過傅立葉轉換得到樣品的光譜。利用模型比較穿過樣品與未穿過樣品的電場，可以測量太赫茲頻率範圍內材料的折射率與吸收係數，進而推倒介電常數與介電損耗。在架設太赫茲時域光譜系統後，為了驗證系統性能，吾人量測水線去比對標準的水線吸收頻，觀察系統的靈敏度，以及對照其他文獻量測材料的折射率與介電常數，來驗證吾人建立的系統。 本文第一部分主要介紹太赫茲時域光譜技術，分析各種參數的設定，來提升系統性能，提高訊雜比與動態範圍，其中訊雜比為60 dB，可用頻寬約0.1至1 THz。第二部分透過折射率回推材料的介電常數與介電損耗，並彙整許多材料的介電特性。由廠商提供的低頻板材介電參數透過Debye模型推出高頻的介電常數與損耗(其滿足因果性)，並與量測結果做比對。第三部分透過雷射雕刻機雕刻偏振片，可客製約10至50 dB的消光比與穿透率75%以上的太赫茲偏振片。為了再提升消光比，提出雙層偏振片的結構，透過模擬觀察其消光比相較單層偏振片提高20 dB。此外，也透過雷射雕刻出線寬50 μm的簡單圖形超表面材料，包含方形分裂環共振器與十字型開槽結構作為濾波器，最後使用太赫茲時域光譜量測穿透光譜來驗證特性。在分裂環共振器的量測結果中，可以看到多個共振頻，從軟體模擬電場和表面電流可以看到隨著共振數目的增加，兩個振幅中都會出現一個額外的節點，並且產生180度的相位變，皆與理論相符。

The purpose of this thesis is to study terahertz time-domain spectroscopy (THz-TDS). The principle involves using femtosecond near-infrared pulses to illuminate a photoconductive antenna, generating electron-hole pairs. These pairs are then accelerated by an applied bias, forming a transient current that produces terahertz waves. For detection, the terahertz pulses overlap with another near-infrared pulse, and by scanning the delay of the optical path on one end, the time-resolved electric field of the terahertz pulses is recorded. Finally, the sample’s spectrum is obtained through Fourier transform. By comparing the electric fields passing through the sample and without the sample, the refractive index and absorption coefficient of materials in the terahertz frequency range can be measured, leading to the derivation of dielectric constants and dielectric losses. After setting up the terahertz time-domain spectroscopy system, to verify its performance, we measured the water line to compare it with standard water absorption frequencies, observing the system''s sensitivity. Additionally, we compared the measured refractive indices and dielectric constants of materials with those reported in other literature to validate our established system. The first part of this paper mainly introduces the terahertz time-domain spectroscopy technique, analyzing various parameter settings to improve system performance, enhance signal-to-noise ratio (SNR), and increase dynamic range. The SNR is 60 dB, and the usable bandwidth is approximately 0.1 to 1 THz. The second part involves deriving the dielectric constants and loss tangents of materials through their refractive indices and compiling the dielectric properties of various materials. Using the dielectric parameters in low-frequency provided by manufacturers, we extrapolated the high-frequency dielectric constants and losses through the Debye model (satisfying causality) and compared them with our measurement results. The third part involves using a laser engraving machine to fabricate polarizers, achieving customized terahertz polarizers with extinction ratios of approximately 10 to 50 dB and transmittance above 75%. To further enhance the extinction ratio, a bilayer polarizer structure is proposed, which, through simulations, shows an improvement of 20 dB in extinction ratio compared to single-layer polarizers. Additionally, simple metasurface materials with 50 μm linewidths, including square split-ring resonators and cross-shaped slot structures, were engraved using the laser and used as filters. The transmission spectrum was measured using terahertz time-domain spectroscopy to verify their characteristics. In the measurement results of the split-ring resonators, multiple resonant frequencies were observed. From the software-simulated electric field and surface current, it was evident that with an increase in the number of resonances, an additional node appeared in both amplitude curves, producing a 180-degree phase change, consistent with theoretical predictions.