半導體奈米雷射之設計與實現

Abstract

在過去幾十年中，微縮同調光源的研究不曾停滯過，半導體雷射的共振腔尺寸在近幾年中已經從微米尺度縮小到奈米尺度，而奈米尺度的半導體雷射元件具有較小的佔地面積與低功率消耗的特性，所以在生物醫學，光纖通訊，積體光路和平行運算有極高的應用淺力。此外，在微型共振腔中，光與物質的物理作用也是一門有趣的研究，所以在過去幾年中，各式各樣的微型共振腔半導體雷射元件被成功開發，如光子晶體共振腔雷射、微型碟盤式雷射、布拉格反射鏡雷射、垂直共振腔面射型雷射與奈米線雷射等。雖然這些半導體雷射的共振腔尺度都接近奈米等級，但是這些同調光源元件需要在其共振腔外圍建立數個波長的週期性結構來維持高的品質因子(Q-factor)，所以整體元件面積還是在數個波長的尺度，並沒有真的實現奈米尺度的同調光源製作。2007年Hill等人成功製作出金屬披覆型雷射，此結構只需要幾奈米厚度金屬就可以把光場有效地侷限於共振腔裡面，有效的縮小元件面積。然而，此類型的金屬共振腔還是無法突破物理繞射極限。在2009年電漿子雷射成功開發，使得同調光源得尺寸可以進一步的突破物理繞射極限，使同調光源開始進入到奈米的世代。由於電漿子必須存在於金屬和介電材料的介面，導致電漿子雷射損耗很大，一般電漿子雷射的操作閾值極高，所以只能在極低溫的環境下操作。本篇論文主要在研究電漿子雷射的特性，電漿子雷射的操作原理主要在於降低金屬損耗、提高共振腔品質因子及降低電漿子移動的群速度使得增益可以被有效放大。我們首先研究銀的電漿子雷射，由於銀在波長320奈米附近有劇烈的能帶吸收損耗，造成色散曲線在此波段有劇烈的變化(彎折效應)。我們利用氧化鋅奈米線當做增益材料，使雷射操作波長接近銀的能帶吸收波段，來研究其物理特性。我們藉由調整氧化層厚度來調變電漿色散曲線，使得銀電漿子的群速度可以有效降低到光速的1/80，成功開發出模態體積最小的可室溫操作紫外光波段電漿子雷射。此外，我們也研究鋁電漿子雷射，藉由改善鋁金屬薄膜和氧化層的品質，我們成功開發全世界第一顆可室溫操作單晶鋁電漿子雷射，此雷射具有極低的操作閾值(0.28mJ/cm2)和極高的特徵溫度(178K)。

Quest of searching for small coherent light sources has never been stopped for these micro-to-nano scale light sources are not only practically essential for small footprint, low power consumption, high density, optical integrated circuit and parallel signal processing applications but also provide insightful way to investigate the interaction between light and matter. Several designs have been developed to scale down the optical cavity volume to contain only few photons modes, such as photonic crystal defect type lasers, microdisk lasers, and nanowire lasers. These lasers, however, require the cavity size in the order of few (/n)3 to sustain a proper mode profile with a reasonable cavity Q value. Recently, optical cavities surrounded with metal claddings have been developed to reduce the cavity volume because the optical field penetrating into the metal claddings decay so rapidly that the optical mode can be further shrunk at the cost of a lower cavity Q value because of the strong absorption of metal. However, metal can provide a method of drastically diminishing the cavity mode beyond the diffraction limit by forming the surface plasmon at the interface of metal and dielectric layers. The ultra-small electromagnetic field distribution of the surface plasmon mode can substantially facilitate the interaction between light and matter by enhancing the Purcell factor. Successful demonstration of a plasmon nanolaser typically relies on its enhanced Purcell factor, which is inversely proportional to the mode volume. In addition, the slower propagating speed of plasmons can raise the Purcell factor by increasing opportunities for interaction between the gain medium and surface plasmons. Slow group velocity can be achieved at the band edge provided by the distributed feedback mechanism in the two dimensional periodic structure. However, the distributed feedback mechanism requires a relatively large area, which violates the small dimension requirement of nanolasers. The refractive index of silver (Ag) has a high variation at the ultraviolet (UV) wavelength range because of the interband absorption. This index variation can directly influence the dispersion of the surface plasmon to achieve a large group index, resulting in a small mode volume and large Purcell factor. To demonstrate this effect, we used the zinc oxide (ZnO) nanowire as the gain medium to match the UV wavelength range. We adjust the dielectric spacer thickness to tune the surface plasmon dispersion curve so that the lasing wavelength can be located at a large dispersion region, thereby achieving a low group velocity, which is of 1/80 of the speed of light. Beside, we also presented a high-performance Al-based plasmonic nanolaser in the ultraviolet regime. The interfacial roughness and, in particular, metal film quality play a key role in the ZnO nanolasers. By using molecular beam epitaxy to grow a high-quality single-crystalline Al film, followed by ultra-smooth Al2O3 layer prepared by atomic layer deposition and ZnO nanowire placement, we have realized an ultraviolet plasmonic nanolaser with a very low threshold pumping energy density (0.28mJ/cm2) and a high characteristic temperature (178K).