Silicon-based single-mode on-chip ultracompact microdisk resonators with standard silicon photonics foundry process

A silicon-based on-chip ultracompact microdisk resonator (MDR) with a super-high Q-factor to support single-mode operation is designed, fabricated, and tested. In our design, a compact MDR has an ultrasmall radius of 3.7 μm with an additional slab waveguide incorporated to wrap the disk and the bus waveguide with an aim to weaken the disk sidewall roughness, to increase the confinement of the optical field and to strengthen the optical coupling between the bus waveguide and the disk. By using the three-level etching capability offered by a standard silicon photonics foundry, two ultracompact MDRs with two different heights of 220 and 150 nm are fabricated, in which the incorporated slab waveguide is kept to have an identical height of 60 nm. Optical performance of the MDRs is evaluated. The measured transmission spectrums show that both the MDRs are operating in single mode with no resonance splitting observed. The MDR with a height of 220 nm has a significantly improved Q-factor of 75,000, which is useful for narrowband filtering. The MDR with a height of 150 nm has a strong energy distribution around the top surface of the disk, which is useful for refractive index sensing. In addition, for the MDRwith a height of 220 nm, owing to its ultrahigh light-confining capacity and ultracompact mode volume in the MDR, nonlinear optical response in the cavity is considerably enhanced and an optical bistability is observed experimentally when the input optical power is as small as -17 dBm.