High-Spectral-Contrast Symmetric Modes in Photonic Crystal Dual Nanobeam Resonators

We demonstrate accurate control of mode symmetry in suspended dual-nanobeam resonators on a silicon-on-insulator chip. Each nanobeam consists of a Fabry–Perot nanocavity bounded by tapered 1-D photonic crystals. Even and odd cavity-modes are formed due to lateral evanescent coupling between the two nanobeams. The odd cavity-mode can be excited by mode-symmetry-transforming Mach–Zehnder couplers. Modal contrasts over 27 dB are measured in fabricated structures. The influence of the optical field in the middle air slot on the background transmission and quality factors is discussed. The observed peak wavelength separations of the modes at various nanobeam spacings are in good agreement with simulation results. These nanobeam resonators are potentially useful in applications, such as ultrafast all-optical modulation, filtering, and switching.