- 學位論文
光子晶體發光二極體之探究及其高速調變特性
Investigation of Photonic Crystal Light-emitting Diodes and its High-speed Modulation
摘要
在光電元件中，週期性介電質奈米結構具有著傑出的電磁場及光-物質反應的控制能力，而許多研究已深入探討如何利用光子晶體結構，調控發光二極體和雷射元件的光傳播方向以及管理光子模態的時間上的響應。在本論文中，我們將闡述光子晶體發光二極體的操作特性及其應用於可見光通訊的可能性。 在過去，人們藉由理論上的計算及實驗驗證，負折射現象可藉由光注入的被動元件中呈現出來，在此我們利用電致發光元件展示出可見光波段的負折射現象，並在不同的極化方向上造成不同的自準直行為，我們更由平面波展開法模擬，計算出等頻率平面上的群速度等相關資訊，與實驗成果相互驗證。而光子晶體發光二極體中的自發性發光和角度相依的行為，我們利用模態的萃取距離理論及光子能帶的品質因素進行討論，在實驗中，具有500奈米晶格周期的光子晶體元件，展現出低階模態的高粹取效率，以及在垂直方向的自準直輻射場型。 由於近日科技的進步，氮化鎵發光二極體已變成可見光通訊的物理傳輸層中，頗具發展潛力的光傳輸器，但其調變頻寬通常受限於系統前端發光二極體中的多層量子井，其較長的自發性輻射復合生命週期。在我們的光子晶體發光二極體中，由於可以輻射出更同調的光，被侷限的光子模態可以被萃取出並偶合進高速傳輸的光訊號，並且伴隨著較短的光子生命週期。在此我們由時域上的模態萃取研究，比較其光致發光和高速電致發光的操作成果，在 8.56 KA/cm2 的偏壓電流密度下，光子晶體發光二極體可達到234 MHz電轉光的 -3 dB 的調變頻寬，此為相對於普通發光二極體1.1倍的頻寬增加，而在更大的元件中，更可達到2.1倍的增加比例。由萃取出來的元件本質小訊號參數，我們解釋了由於光子晶體奈米結構萃取出元件中的侷限模態可產生較快的輻射性載子復合，進而達成更快的調變速度。 我們也進一步比較不同光子晶體結構對其小訊號、大訊號動態行為的影響，在室溫下的時間解析光致螢光光譜及拉曼散射頻譜中，光子晶體可造成更快速的暫態響應及高效率的偶合出光，在最佳的設計下，可達到最高的347 MHz 的調變速度，相當於55%的普通發光二極體的頻寬增加量。在進一步的誤碼傳輸測試中，光子晶體發光二極體可成功通過偽隨機序列200 Mbps 的零錯誤碼傳送，其誤碼率小於 10^-10。而在數位調變的眼圖量測中，我們也可獲得較好的訊號傳遞完整性，測試的傳遞速度可達到800 Mbps。我們的研究顯示，由於不同的光子晶體結構會對元件的高頻行為及應力相關因素上產生多種的影響，因此在傳輸訊號測試中，會造成不同程度的訊號損耗改善，在最佳的測試下，光子晶體中有著較大的晶格周期及較大的空氣孔比例，可以表現出最好的頻寬及最佳的訊號傳輸品質。
並列摘要
Periodic dielectric nanostructures have shown its outstanding control over the electromagnetic (EM) field and the light-matter interaction in photonics. Much research has investigated the deployment of photonic crystals (PhCs) in light-emitting diodes (LEDs) and lasers in order to manipulate the propagation of light and manage the temporal response of the photonic mode. In this dissertation, we will elaborate the concept of the PhC-embedded LED (PhCLED), its characteristics, and the potential for visible light communication (VLC). In the past, the unique phenomena of negative refraction were demonstrated theoretically and experimentally by pumping optical power into a passive device. Here, we demonstrate an electroluminescent device with negative refraction in the visible wavelength range. Different self-collimation in far-field patterns of polarized light are observed. The experiment was further verified from theoretical calculations of the group velocity on the equifrequency contours (EFCs) based on the plane-wave expansion (PWE) simulation. The angle-dependence in the enhanced spontaneous emission of PhCLEDs was elucidated by. the modal extraction lengths and quality factors obtained from the photonic bands. In our cases, the PhC device with a larger PhC period of 500 nm exhibits the best lower-order mode extraction and self-collimated radiation toward the surface-normal direction. Recent progress has shown that GaN-based LED is one of promising optical transmitters in the physical layer (PHY) of VLC. However, the modulation bandwidth of the front-end LED is usually limited by the spontaneous radiative recombination lifetime of the multiple quantum wells (MQWs). For PhCLEDs, by emitting light more coherently, the guided photonic modes can be extracted and coupled into the high-speed optical signal with a shorter radiative recombination lifetime. The demonstrated devices were investigated based on the temporal modal extraction with the corresponding photoluminescence (PL) measurement and the high-speed electroluminescence (EL) modulation. At 8.56 KA/cm2 of the biasing current density, the electrical-to-optical (E-O) -3-dB bandwidth (f-3dB) up to 234 MHz (a 1.1-fold increment compared to that of a conventional LED) in PhCLED is achieved. A 2.1-fold increment in the bandwidth can be obtained for a larger size of PhCLED. The extracted intrinsic small-signal parameters explain the higher operation speed is attributed to the faster radiative carrier recombination of extracted guided modes from the PhC nanostructure. Finally, we compare various PhC structures with corresponding dynamic behaviors in both small- and large-signal modulation. Faster transient responses and higher efficiency of the out-coupled modes were obtained in the room-temperature time-resolved photoluminescence (TRPL) and Raman scattering measurement. With an optimal design of PhCs, the highest f-3dB of 347 MHz in the PhCLED is achieved, which corresponds a 55% enhancement compared to a convention LED. Moreover, an error-free transmission was tested under a pseudo-random bit sequence (PRBS) at 200 Mbps with a bit-error rate (BER) < 10^-10. Better signal integrity is also shown in digitally modulated eye patterns up to 800 Mbps. Our study reveals that the differences in the high frequency behaviors and strain-related effects of various PhCLEDs result in different improvements in the signal impairment during the data transmission. For the best performance, the PhC with a larger period and a larger air-hole portion characterizes PhCLEDs with the highest f-3dB bandwidth and the best transmitted signal quality.
參考文獻
延伸閱讀
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