微型二維二硫化鉬雷射之環形光柵共振腔設計

Abstract

二維材料是一種僅由單層原子或分子組合而成的材料，它具有許多不同於一般塊材下的物理特性：例如一般而言，碳構造物如石墨、鑽石等為絕緣體，並不導電；然而，由單層碳原子組合而成的石墨烯 (graphene) 卻具有令人驚嘆的高電子傳導率。除了導電率，部份的二維材料亦具有能隙 (band-gap) 可做為發光源。然而由於單原子層厚度小於 1 奈米，發光強度不佳；除此之外，些微的厚度變動會導致其性質劇烈改變，甚至消失，因此很容易受外在製程條件影響。要改善發光強度，並且在製程過程中同時保有原本二維材料特性，其中一種方式為使用可將 cavity mode 集中於一處之結構，利用空間疊合 (overlap) 使 cavity mode 的位置與二維材料所在之處重疊，在不變動二維材料本身的情況下增強其輸入能量。 本篇論文針對鋪設二維材料二硫化鉬 (MoS2) 於中心為環的圓形光柵結構 (circular grating cavity with ring center)之上探討，此結構的mode主要分布在空氣中，因此只需最後在結構上方鋪上MoS2，mode就容易受折射率影響而往MoS2移動，增加overlap機會。經由模擬計算發現，改變圓形光柵的中心半徑大小能夠有效控制結構的quality factor (Q)，而控制結構的duty cycle則能得到良好的overlap且同時保留較佳的Q。此外亦比較當鋪設MoS2後，光子晶體結構 (photonic crystal cavity) 和圓形光柵結構的overlap，證明圓形光柵結構更適合當作雷射的共振腔，並期許未來能夠在實驗上實現圓形光柵結構之二維材料雷射。

Two-dimensional materials (2D materials), which have been reported that have many unusual physical properties that are quite different with the bulk materials, have been widely used in recent years. One way is to be the light source. However, the light intensity of 2D material is too weak to illumine. Moreover, the properties are easily changed with a few nanometers varying of the thickness. In order to enhance the light intensity and keep the thickness below one nanometer (single layer) simultaneously, the overlapping issue about materials and cavity modes is important. One of the possible solution is to put 2D materials on the top of the structure which can enhance the light intensity from 2D materials by concentrating the cavity mode on 2D materials (that is, the overlap). Herein, we propose putting the 2D material of MoS2 on the top of the circular grating structure with ring center. Since the most part of the cavity mode of the structure distribute in the air, the MoS2 can easily overlap with the cavity mode because of the high refractive index. Moreover, we find one of the parameters of circular grating which can enhance the overlapping without dropping of the quality factor through calculation. In addition, comparing with the photonic crystal cavity, the circular grating cavity has larger overlap if putting MoS2 on the top of the structure. In sum, the circular grating cavity is a good selection to be the laser cavity for MoS2. We hope it will actually lasing in experiment and further, operate close to room temperature in the future.