Improving efficiency in photo diodes using photonic crystal structure

Abstract

In this thesis, high efficiency band edge lasers were created using various types of photonic structure. In chapter 2, cholesteric liquid crystal (CLC) cells were fabricated using three kinds of nematic liquid crystal (NLC) materials with different optical birefringent anisotropy Δn. Larger Δn of NLCs results in larger PBG width and higher density of state (DOS) at the PBG edge for each CLC cell. We were able to lower the threshold energy by a factor of more than two using CLC cells with broader photonic band gap width. In chapter 3, we inserted polystyrene colloidal crystal layer between two dielectric mirrors creating a defect layer inside the band gap of the mirrors. We were able to tune the defect position by changing the polystyrene bead size. Even though the full width half maximum value of the lasing emission was relatively large (7nm), the lasing emission follows the defect positions when changed. In chapter 4, we used all solid organic materials to create a low threshold, high efficiency lasing cell by simply inserting a PS colloidal crystal layer between two PCLC films. This is the first time to check how circular polarized light will act inside a PS crystal structure. As a result the PS layer will create defect modes and also enhance the reflectance. The enhancement of the reflectance and defect modes inside the cell will give rise to lower threshold amplified emission which may further be improved into lasing emission. In chapter 5, we sandwiched a grating layer between two PCLC films. The cell showed sharp peak, threshold behavior and a ring pattern, we were able to conclude that low threshold, high efficiency; all solid state laser can be fabricated by inserting a grating layer between two PCLC films. The grating layer will extract trapped light inside the plane into the normal direction, and the extracted light will go through a distributed feedback between two PCLC films, thus creating a low threshold, sharp lasing emission.