Strain-tunable Photonic Band Gap Microcavity Waveguides in Silicon at 1.55 μm

The majority of photonic crystals developed till-date are not dynamically tunable, especially in silicon-based structures. Dynamic tunability is required not only for reconfiguration of the optical characteristics based on user-demand, but also for compensation against external disturbances and relaxation of tight device fabrication tolerances. Recent developments in photonic crystals have suggested interesting possibilities for static small-strain modulations to affect the optical characteristics, including a proposal for dynamic strain-tunability. Here we report the theoretical analysis, device fabrication, and experimental measurements of tunable silicon photonic band gap microcavities in optical waveguides, through direct application of dynamic strain to the periodic structures. The device concept consists of embedding the microcavity waveguide on a deformable SiO2 membrane. The membrane is strained through integrated thin-film piezoelectric microactuators. We show a 1.54 nm shift in cavity resonances at 1.56 μm wavelengths for an applied piezoelectric strain of 0.04%. This is in excellent agreement with our modeling, predicted through first-order semi-analytical perturbation theory and finite-difference time-domain calculations. The measured microcavity transmission shows resonances between 1.55 to 1.57 μm, with Q factors ranging from 159 to 280. For operation at infrared wavelengths, we integrate X-ray and electron-beam lithography (for critical 100 nm feature sizes) with thin-film piezoelectric surface micromachining. This level of integration permits realizable silicon-based photonic chip devices, such as high-density optical filters and spontaneous-emission enhancement devices with tunable configurations.