  Low propagation loss in high confinement waveguides is critical for
chip-based nonlinear photonics applications. Sophisticated fabrication
processes which yield sub-nm roughness are generally needed to reduce
scattering points at the waveguide interfaces in order to achieve ultralow
propagation loss. Here, we show ultralow propagation loss by shaping the mode
using a highly multimode structure to reduce its overlap with the waveguide
interfaces, thus relaxing the fabrication processing requirements.
Microresonators with intrinsic quality factors (Q) of 31.8 $\pm$ 4.4 million
are experimentally demonstrated. Although the microresonators support 10
transverse modes only the fundamental mode is excited and no higher order modes
are observed when using nonlinear adiabatic bends. A record-low threshold pump
power of 73 $\mu$W for parametric oscillation is measured and a broadband,
almost octave spanning single-soliton frequency comb without any signatures of
higher order modes in the spectrum spanning from 1097 nm to 2040 nm (126 THz)
is generated in the multimode microresonator. This work provides a design
method that could be applied to different material platforms to achieve and use
ultrahigh-Q multimode microresonators.
