光子晶體波導內的慢光現象及整合型光子晶體面射型雷射之研究

Abstract

在本論文中，我們報告兩種利用光子晶體所製造的光電元件。由於光子晶體本身的特殊能帶結構，使得這兩種光電元件具有特殊的性能，被認為會對未來多功能的光積體電路發展有所幫助。 首先，第一種元件是一種能讓光的傳輸速度減慢的光波導元件。我們設計了特殊的光子晶體波導，使得在波導內的傳輸光其群速度能大幅度的被減慢，這種現象稱為慢光。第二種元件是一種整合式的面射型雷射。我們將光子晶體共振腔整合在傳統的邊射型半導體雷射，使產生的雷射光耦合到光子晶體共振腔，並由其表面射出單一模態的雷射光。上述的這兩種光電元件，將會是未來發展多功能光積體電路中的重要關鍵元件。 在慢光的研究部分，我們首先設計了單一線缺陷的光子晶體波導 (亦稱為W1型)來瞭解光波在其內部傳輸的行為模式。我們所製作的W1型光子晶體波導具有非常低的傳輸損失率(2dB/mm)。我們在其所量測到的頻譜圖上，觀察到在頻譜截斷區域的Fabry-Perot振盪週期迅速變小。將此異常週期代入Fabry-Perot公式，可得到異常大的群折射率(200~300)。同時我們也利用時間領域的方法將3GHz的調變訊號導入W1型波導中，並量測其相位變化，間接推得光訊號再頻譜截斷區域有非常長的傳輸延遲時間。我們並將此測到的實驗數據與從能帶圖上計算得到的群速度理論值作了比較，發現這種的慢光現象是由於W1型光子晶體波導在1st Brillouin zone中，能帶邊緣的缺陷模態所造成。 根據上述由W1型光子晶體波導所得到的結果，我們更進一步研究一種具有特殊能帶結構的光子晶體耦合型波導。自此類型波導元件中，其光子能帶係內存在著一種S型的耦合缺陷能帶，此能帶有利於慢光現象的觀察。我們利用了Mach-Zenhder干涉方法所得到的光波群速度對頻率響應圖，證明了在那特殊S型耦合能帶上，的確存在一種異於能帶邊緣的慢光模態，我們稱之為轉折點型慢光模態。同時，我們也利用高速示波器來紀錄光波在此耦合型波導傳輸中，波形隨著時間變化的情形。藉由調整入射光的頻率，使其慢慢趨近耦合缺陷模態的頻率，我們觀察到逐漸增加的傳輸延遲時間，經由換算，我們得到了一個極小光波群速度，只有0.017c。從能帶圖上所推得的群速度理論值也符合了我們所量測的結果。 在光子晶體共振腔部份，我們展示了一種整合型的奈米共振腔-雷射結構。在這結構中，由注入電流產生的雷射光會直接耦合到光子晶體共振腔，並從共振腔表面射出單一模態雷射光。同時藉由頻譜圖的分析，我們可到此光子晶體共振腔具有高品質因子(Q-factor)。另外，我們也觀察到從此共振腔射出的雷射光對溫度具有高性能的穩定性，此特徵值是一般量子井半導體雷射的5倍。最後，我們設計了兩個並肩型的光子晶體共振腔雷射，展示多重波長同時射出可能性。

In this dissertation, two optoelectronic devices based on photonic crystals are presented. These two devices with their unique properties derived from the special band structure of the photonic crystals will be useful for future photonic integrated circuits with multiple functions. The first device is an optical delay line that slows down the speed of the propagation light. A special waveguide based on line-defects in photonic crystal was designed to support waveguide mode with a significantly reduced group velocity. The second device is an integrated surface emitting laser. A regular in-plane laser was integrated with an photonic crystal nanocavity to couple the laser emission into a single mode surface emitting light. Both devices will be the key components for multi-function photonic integrated circuits. In the part of slow light, single line defect (W1 type) photonic crystal waveguides are first studied in order to understand the behaviors of propagation light over them. Very low propagation loss (~2dB/mm) of the fabricated waveguides is obtained. The rapidly diminishing Fabry–Perot oscillation periods at the cutoff region of the measured transmission spectrum determine extremely large group indices of 200~300. The group delay time measurements by detecting phase shift of 3G Hz modulated signals through the waveguides also show a very large time-delay (>200psec) near the cutoff. In comparison with theoretical group velocities derived from the band structure, these experimental results are ascribed to the effect of the defect modes at the band edge of the first Brillouin zone. Based on successful results of single line defect waveguides, we further investigate a photonic crystal coupled waveguide, where the unique guided mode band structure has a flat band region within the photonic band gap allowing for slow light observation. The spectral dependence of group velocity, which is measured by Mach-Zehnder interference method, indicates the existence of slow light modes around the inflection point of the unique flat band, rather than at the band edge. Time-domain observation of optical pulses propagating along two-dimension slab photonic crystal coupled waveguides is also demonstrated by using a high speed oscilloscope. By adjusting the wavelength of the input pulses toward the flat band of the coupled defect modes, an increasing duration time between reference and output pulses are clearly observed. An extremely small group velocity of 0.017c is thus obtained. Calculated group velocities show good agreement with our measured results. In the part of photonic crystal nanocavity lasers, we demonstrate an integrated nanocavity laser structure, where the laser light is directly coupled to photonic crystal nanocavities (H1 and H2) and emits out from the surface with selected wavelengths of the resonant modes of the nanocavities. Single mode emission with high Q factors [Q(H1)=1890 and Q(H2)=3800] is obtained with electrical pumping. Excellent temperature stability (0.097nm/0C) of laser emission from the nanocavity is observed as well. The wavelength shift versus temperature is about five times better than that of regular quantum well lasers. Dual wavelength emission from two side-by-side photonic crystal nanocavities is also demonstrated.