二維與三維砷化鎵光子晶體缺陷模態的量測與分析

Abstract

光子晶體是一個介電常數週期性變化的物質，在一些特定波段因破壞性干涉的影響，產生光子能隙(photonic bandgap)，在這能隙內的光對應不到實數的傳播常數而無法傳播。相對於完美的光子晶體，有缺陷結構的光子晶體更引起人的關注，原因在於缺陷產生的模態(defect mode)相對於一般模態在光子晶體區域內有著空間的侷限性，也就是說我們在光子晶體內可藉由些微結構的改變來控制缺陷模態的行為，這是一個非常有趣的現象，最簡單的例子為光子晶體波導，藉由適當的缺陷設計可以使光波照我們設計的路線行進。 利用發展出的自動化量測技術在空間上提供更精確及穩定的資訊，在二維的平板結構(slab structure)中，我們利用此技術把焦點放在光子晶體共振腔中，希望能觀察到其共振模態和其場型的分布，目前藉由電動平移台的協助，可以穩定的掃出空間上的強度分布，並進一步運用到此以技巧，去觀察到缺陷結構的散射頻譜和光場強度分布，藉由光場及頻譜的資訊更了解此共振腔的效應。在本文設計的結構中，藉由穿透頻譜和散射頻譜的對應，我們可以看到952nm與960nm的兩個共振模態。三維棋盤狀結構(checkerboard structure)上，因為自我複製法(auto-cloning method)在設計點缺陷的結構上較為複雜，我們採取線缺陷的結構，觀察缺陷結構產生的缺陷模態，並觀測缺陷模態的空間分佈，然而在空間的量測上，設計的兩種不同缺陷大小的場型空間分佈並無明顯差異，這可能是受限於偵測上解析度與穩定性問題。

Photonic crystals are materials with periodical dielectric constants. The band gap occurs according to the destructive interference, where certain wavelengths are not allowed to pass through. The light at those frequencies can not find a real propagation constant and thus will be scattered. Besides perfect photonic crystals, we are more interested in those photonic crystals with defects because those modes resulted from the defects are spatially confined in comparison with those non-defect modes. In other words, we can control the behavior of the defect modes by adjusting the fine structures in the photonic crystal. This is a very interesting phenomenon. An example of this is the photonic crystal waveguide, where lights will propagate along the direction we design. By developing automatic measurement techniques, we can acquire correct and stable information efficiently. For two-dimensional slab structures, we focus on the resonant cavities formed by the photonic crystals and expect to observe the resonant modes and the field distribution. With the assistance of the motorized stage, we could stably scan the spatial light intensity distribution and then observe the spectrum and the field intensity distribution of the defect modes. With the spectrum and the field distribution of the defect modes, we could understand more about the effects of the resonator. In the structure we designed in this work, we successfully find two resonant modes around 952nm and 960nm on the spectrum by observing the transmitted and the scattered spectrum. On the other hand, owing to the difficulty in constructing the point-defect by the auto-cloning method in three-dimensional checkerboard structures, we designed line-defects instead and then observed the effect from them. We observed the spectra and the spatial distribution of the defect modes. But we can’t find the obvious difference in spatial field distribution between two defect structures which have different sizes. It’s probably limited by the detection resolution and stability.