We design and fabricate a high Q silica nanobeam cavity that supports both transverse electric (TE) and transverse magnetic modes in the 1.55 μm wavelength range. The Q values obtained for both modes exceed 104 and are the highest reported values for photonic crystal (PhC) nanocavities made of silica. We also investigate the optimum conditions for coupling with the cavity in a side-coupled configuration. We achieve a coupling efficiency of 87% with the TE mode while maintaining a loaded Q of more than 104. We also found that the presence of a coupled waveguide reduces the intrinsic Q of the cavity, depending on the gap distance. This provides useful quantitative information for establishing an efficient scheme for coupling with low-index PhC nanocavities.
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