Simulation on Radiation Characteristics of an Electric Dipole near a Metallic Nanograting

我們利用陣列掃描法和有限元素法來模擬一個三維電偶極鄰近一維反射式金屬奈米光柵的輻射特性。金屬光柵包括純銀與銀-二氧化矽二種型式，其緊鄰之介質為氮化鎵。我們探討了輻射特性與電偶極的方向及位置的關係。吾人分別討論電偶極沿光柵變化方向、平行光柵方向與呈45度的方向下的輻射特性，亦改變電偶極的位置，從光柵突出正下方到凹槽正下方，以及均勻分布於這兩者之間的個別與綜合平均的效果。數值模擬結果顯示: 若光源屬層狀式結構，例如氮化鎵量子井，此種一維反射式金屬奈米光柵並沒有提升光偏振比的效果; 若光源屬侷限式結構，例如半導體奈米線、量子線和量子點，並置於光柵突出下方，此種結構才可能提升光偏振比。此外，銀-二氧化矽光柵系統的光總輻射量會比單純銀的光柵系統還多。The combination of the array scanning method (ASM) and the finite element method (FEM) is utilized to simulate a one-dimensional (1-D) reflection-type Ag or Ag-SiO2 grating structure contacting a GaN half space embedded with a three-dimensional dipole source. We find that the radiation characteristics of the system depend on the orientation and the location of the dipole. We first investigate the effect for different orientations of the dipole: in the grating-varied-, groove-, or 45-degree-direction. Then we examine the individual and average effects of the dipole located at the positions uniformly distributed between the protruding and groove parts of the grating. Our numerical results manifest that this kind of 1-D gratings may not be suitable for enhancing the polarization ratio if a light source is a layer-type structure, like a GaN quantum well. However, if a light source has some kind of localization, like a semiconductor nanowire, a quantum wire, or a quantum dot, and is located beneath the protruding part of the grating, this structure still has a chance to enhance the polarization ratio. Besides, the total radiation power for a Ag-SiO2 grating system is higher than that for a Ag grating system