TiO₂ microring resonators with high Q and compact footprint fabricated by a bottom-up method

Titanium dioxide (TiO₂) microring resonators (MRRs) with high quality factors (Qs) are demonstrated by using a new, to the best of our knowledge, bottom-up fabrication method. Pattern platforms with a T-shaped cross section are first defined by etching a thin top layer of silicon nitride and a thick bottom layer of silica and partially undercutting the silica. Then, TiO₂ is deposited on the platforms to form the TiO₂ waveguides and devices. TiO₂ MRRs with different bending radii, waveguide widths, and gaps in the bus waveguide are fabricated and measured. The intrinsic Q(Q_int) is achieved to be ∼1.1 × 10⁵ at the telecommunication wavelengths, corresponding to a bend waveguide loss of 3.9 dB/cm while the compact MRR with a radius of 10 µm can still sustain a Q_int of ∼ 10⁵. These results not only unfold the feasibilities of the proposed bottom-up method for fabricating TiO₂ waveguides and MRRs with high Qs and compact footprints but also suggest a new approach for fabricating waveguides in other materials, of which direct etching is not easily accessible.