高指向性及高效率兆赫波光子發射器之開發

Abstract

近年來由於兆赫波科技的快速發展，許多兆赫波發射器被廣泛地開發，其中包括共振穿隧二極體、光導天線、單傳輸載子光二極體、傳輸波光偵測式兆赫波光子發射器、兆赫波量子級聯雷射、磷化銦高電子傳輸電晶體及異質介面偶極電晶體等等。而作為一個兆赫波發射源，其效率及指向性在一些特殊應用裡尤為重要。例如，手持式兆赫波探測頭必須擁有較高的輻射指向性，以便在即時偵測時得到較高的訊雜比及解析度；在生物檢測晶片應用裡，激發能量必須加以限制以避免對待測樣本加熱造成影響。因此，一個高效率的兆赫波源（意指低功率消耗）在這類的應用裡也扮演著一個不可或缺的角色。在本論文中，我們提出並且實驗證明兩式依垛行開槽陣列天線為設計基礎的兆赫波光子偵測器。實驗過程中，我們不需使用任何高藉電係數的聚焦媒介便可輕易得到高指向性的兆赫波輻射。在0.9兆赫的輻射場型中，不論在電場或磁場平面，其3dB束寬都可以被有效地侷限在30度輻射角以內。此外，我們將光電路重新設計在一接地共平面波導結構上以便提升其輻射效率及高頻操作。此新設計的優點在於能夠保有厚基板，在適當的設計下，基板能夠收集儲存散逸在電路表面及基板內的兆赫波並且形成有效共振，對於提升輻射效率及高頻操作有其顯著的效果，亦有利於未來的大尺寸光電積體電路整合。除了單頻操作，我們亦利用超短脈衝光作為寬頻兆赫波的激發。利用相位偵測的技術，我們成功觀測兆赫波在光子發射器上的傳播、共振及輻射行為，其特性與原始電路設計有著相當高度的吻合，因此，我們相信所發展的高效率及高指向性的兆赫波發射器將非常適合可做為一顆低功率消耗�高敏感度生物檢測晶片，其中代測物之高頻電性可藉由精密兆赫波電路做更進一步的相位檢測。 除了上述高效率及高指向性兆赫波光子發射器之開發，在本論文中，我們也成功建立一套光纖化兆赫波同調控制系統作為兆赫波光子激發系統。我們知道環境對於兆赫波有強烈的吸收及散射影響，因此將兆赫波資訊載於光訊號上是未來發展兆赫波技術的一個重要方式之一。光纖化傳播除了有利於實現一套簡易可撓手持式或桌上型系統，對於未來的長程光纖通訊，所建立的光纖化兆赫波同調控制系統亦可提供其技術支援。本論文中，我們使用的是一根大面積模態光子晶體光纖。實驗結果顯示其頻率範圍可成功地由0.15兆赫線性調至約3.7兆赫。因此，我們希望藉由結合所發展之高效率高指向性兆赫波光子發射器以及簡易可撓式光纖化兆赫波同調控制系統，未來能夠提供更廣泛的兆赫波科技應用。

Many Terahertz (THz) radiation devices have been created for the reason of upcoming THz applications, e.g., resonant-tunneling diodes (RTDs), photoconductive (PC) switch antennas, uni-traveling-carrier photodiodes (UTC-PDs), traveling-wave photodetector-(TWPD) based photonic transmitters, THz quantum cascade lasers, InP-based high electron mobility transistors and heterojunction bipolar transistors etc. As a THz radiation source, its directivity and efficiency should be of great concern for specific purposes. For instance, a handheld THz probe head needs a highly concentrated THz beam to enhance the sensitivity of instant detections. Low excitation power is critical for on-chip biosensing due to the thermal effects. A highly efficient THz source for such an application is thus eagerly desired. In this dissertation, we have proposed to use the rampart slot array antenna design to achieve highly directional THz radiation patterns without any focusing medium. A 3dB beam width less than 30° in both E and H planes at ~0.9THz was demonstrated by exciting single radiation source. To enhance radiation efficiency and high-frequency performances, we modified the circuit design of the THz photonic transmitter where a grounded coplanar waveguide structure was utilized to collect more resonating waves in the substrate. Besides, a broadband THz excitation was also performed to study the on-chip THz wave propagation, resonance, and radiation phenomena. The good agreement of circuit design and THz wave characteristics shows a great potentiality for the present circuit to be utilized as an on-chip detection device, where phase-sensitive detection could be realized. We also proposed a fiber-based coherent control system for THz photonic generation. For handheld devices or tabletop systems, combination with an optical fiber is a promising way to save the THz information from environmental disturbances. Long-range communication relies on a fiber-optic communication system as well. Here we utilized a large-mode-area photonic crystal fiber to realize a neat tunable narrow-band THz excitation system with lower hardware cost and higher flexibility. The tunable range was demonstrated from 0.15 to ~3.7 THz. In the future applications, we are convinced that a high-efficiency/low power consuming THz system could benefit from our newly proposed device design and neat THz excitation system.