Coupled photonic crystal nanobeam cavities

We describe the design, fabrication, and spectroscopy of coupled, high quality $(Q)$ factor silicon nanobeam photonic crystal cavities. We show that the single nanobeam cavity modes are coupled into even and odd superposition modes, and we simulate the frequency and $Q$ factor as a function of nanobeam spacing, demonstrating that a differential wavelength shift of 70 nm between the two modes is possible while maintaining $Q$ factors greater than $106$⁠. For both on substrate and freestanding nanobeams, we experimentally monitor the response of the even mode as the gap is varied, and measure $Q$ factors as high as $2×105$⁠.