兆赫/遠紅外波段共振吸收器與高效率熱輻射源之研究

Studies of Terahertz/Far-infrared Resonant Absorbers and High Efficiency Thermal Emitters

鐘佩鋼(Pei-Kang Chung)

指導教授 ： 顏順通

國立交通大學/電機學院/電子工程系所/博士(2020年)

摘要

本篇論文研究兩種形式的兆赫/遠紅外波段的光學元件:(1) 單頻與
雙波段可獨立調頻的共振吸收器與 (2) 高效率熱輻射源。
第一部分，我們先提出兩個簡單且可靠的方法，透過量測單張含有干涉條紋的反射率頻譜，萃取出介電平板的複數折射率。第一個方法仰賴模型化介電函數，適用於極性材料，而第二個方法移除上述限制，可應用於任何均勻且半透明的介電平板。上述方法成功地應用於有 (或無) 背金屬的砷化鎵、磷化銦與鍺半導體平板。接著我們研究兩種窄頻吸收器，包括半導體極性薄膜/金屬構成的雙層吸收器與金屬/絕緣體/金屬形成的微共振腔。針對雙層吸收器，我們透過理論分析發現在厚度 1.2 微米下，砷化鎵薄膜能於特徵頻率提供一完美吸收的共振。根據理論預測，我們實作出一個底層鍍金、厚度約為 1.4 微米的砷化鎵薄膜吸收器，於 7.9 THz 產生一個單頻吸收，其吸收率可達約 96% 而品質因子為 66。另一方面，我們研究表層金屬含有正方形與十字接頭形孔洞陣列的兩種微共振腔的共振特性。透過電場場形與色散分析，我們發現共振吸收來自於兩種相互競爭的機制: 傳播的表面波與侷域的法布立-培若 共振，其主導權隨孔洞參數的變化而改變。相較於正方形孔洞，十字接頭形孔洞有更多的結構調整自由度，使我們得以設計出一個雙波段可獨立調頻的吸收器。
第二部分，我們研究兩種電晶體 (高電子遷移率電晶體與異質接面雙載子電晶體) 的熱輻射特性。透過頻譜與功率分析，我們辨識出兩種熱輻射機制，並證實兩種電晶體可作為高效率熱輻源。適當偏壓下的電晶體，能於元件內部產生高能或熱電子，並透過各種散射機制釋放能量。高電子遷移率電晶體內部存在強烈的電子-聲子散射，有效地產生參與輻射的熱聲子，並於特徵頻率貢獻高效率熱輻射。在 20 THz以下的頻段，單一元件輻射功率可達 13 µW，功率轉換效率約 3×10−5，高出電阻 20%。由於結構上的差異，異質接面雙載子電晶體內部存在強烈的電子-離子散射，進而產生高效率的軔制輻射。相同頻段下，單一元件輻射功率可達約 20 µW。而功率轉換效率約為1×10−4，相較高電子遷移率電晶體有逾三倍的提升。

關鍵字

兆赫 ； 遠紅外 ； 共振吸收 ； 干涉 ； 熱輻射 ； 微共振腔 ； 電晶體

並列摘要

In the dissertation, we focus on two types of optical devices in the THz/far-IR region: (1) single-band and independently tunable dual-band absorbers and (2) high efficiency thermal emitters. In the first part, we begin with proposing two simple and reliable methods for
extracting complex refractive index of a dielectric slab from a single reflectance spectrum with interference fringes. The first method requires a modeled dielectric function in the extraction process and is suited for polar materials while the second one removes such constraint
and is applicable to any homogeneous and semi-transparent slabs. The methods have been successfully applied to GaAs, InP, and Ge semiconductor slabs with or without backside metal. Next, we investigate two narrowband absorbers, including a two-layer absorber consisting of a semiconductor polar film on metal and a metal-insulator-metal microcavity. In the first case, our theoretical analysis shows that a GaAs film of 1.2 µm in thickness on perfect metal provides a narrowband resonance with perfect absorption at characteristic frequency. Based on the prediction, we experimentally demonstrate quasi-monochromatic absorption at 7.9 THz with an absorbance of ∼ 96% and a quality factor of 66 in a ∼ 1.4 µm-thick GaAs film on gold. In the second case, we investigate the resonant properties of two microcavities, one with
surface metal perforated with a square hole array while the other with a cross-joint hole array. Through electric field and dispersion analysis, we show that the resonant absorption originates from two competing mechanisms, propagating surface wave and localized Fabry-Pérot resonance, and the dominance of which varies with hole parameters. Compare with the square hole, the cross-joint hole provides more degree of freedoms for structural manipulation, which allows us to design a dual-band absorber with independently tunable frequencies.
In the second part, we investigate thermal radiation properties of two kinds of transistors, a high electron mobility transistor and a heterojunction bipolar transistor. Through spectral and power analysis,
we identify different thermal radiation mechanisms for each transistor and verify their high efficiency performance. Under appropriate bias conditions, the transistors produce high-energy or hot electrons within
that relax their energy through various scattering mechanisms. For the high electron mobility transistor, strong electron-phonon scattering produces hot phonons that involve in the radiation process, leading to efficient thermal radiation at characteristic frequencies. For frequencies below 20 THz, a single transistor can produce radiation power up to 13 µW and a power conversion efficiency of ∼3 × 10−5, which is 20% higher than that of a resistor. For the heterojunction bipolar transistor, strong electron-ion scattering occurs within due to structural difference and produces intense bremsstrahlung radiation. Under the same frequency range, a single transistor can produce radiation power up to 20 µW with a conversion efficiency of ∼1 × 10−4, which is more than 3
times the efficiency of a high electron mobility transistor.

並列關鍵字

Terahertz ； Far infrared ； resonant absorption ； interference ； thermal emission ； microcavity ； transistor

參考文獻

[1]M. Tonouchi, “Cutting-edge terahertz technology,” Nature photonics, vol. 1, p. 97, 2007.

[2]W. L. Chan, J. Deibel, and D. M. Mittleman, “Imaging with terahertz radiation,” Reports on Progress in Physics, vol. 70, p. 1325, 2007.

[3]M. Walther, B. M. Fischer, A. Ortner, A. Bitzer, A. Thoman, and H. Helm, “Chemical sensing and imaging with pulsed terahertz radiation,” Analytical and Bioanalytical Chemistry, vol. 397, pp. 1009–1017, 2010.

[4]D. Saeedkia, Handbook of terahertz technology for imaging, sensing and communications. Elsevier, 2013.

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