Multiple-Hole Defects: Optimizing Light-Matter Interaction in Photonic Crystal Cavities
Silicon photonic crystal (PhC) cavities have attracted great interest recently due to their ability to highly confine light both spatially and temporally. The small mode volumes and long photon lifetimes associated with PhC cavities are desirable for sensing applications, which utilize the sensitivity of the light-matter interaction (LMI) inside the cavity region. In pursuit of enhancing the LMI of PhC cavity-based sensors, this dissertation focuses on the design, simulation, fabrication, and testing of Multiple-Hole Defect (MHD) PhC cavities. MHD PhCs are 2D silicon slab PhCs with small, sub-wavelength sized “defect holes” placed directly into point defect PhC cavities. The insertion of MHDs increases the spatial overlap in the PhC cavity between the modal field and any surface perturbations, such as captured molecules, made within the cavity. Several designs of MHD PhC cavities were explored using Finite-Difference Time Domain (FDTD) simulations in order to understand the effect of MHD integration on the PhC cavity resonance frequency and quality factor. It was found that the LMI is maximized when defect holes are placed in regions of highest field localization within PhC cavities. The sensitivity of MHD PhCs to bulk refractive index changes was investigated by wetting the structures with different fluids. The bulk index sensitivity of MHD PhC cavities with 80 nm diameter defect holes was found to be 98 nm/RIU, which is larger than comparable PhC cavity sensors without defect holes. The sensitivity of MHD PhCs to small refractive perturbations on the sensor surfaces was explored by binding small-molecules to the surfaces of MHD PhCs treated with either a native oxide or an atomic layer deposition (ALD) silicon dioxide. It was found that the sensitivity of PhC sensors to a 0.8 nm thick, surface-bound aminosilane monolayer was increased by 160% when three 60 nm diameter defect holes were added to an L3 PhC cavity. These results represent the initial steps towards highly sensitive, compact, label free optical sensors, and with further improvements could result in improved handheld lab-on-chip type sensor devices.