  Ultrahigh quality factor (Q) microcavities have been emerging as an appealing
compact photonic platform for various applications. The Q factor plays a
critical role in determining the nonlinear optical performance of a
microcavity. However, a silica microcavity suffers from severe degradation of
its Q value over time during storage or use in air due to the accumulating
surface absorption loss, which would deteriorate their nonlinear photonic
performance. Here, we report a new type of ultrahigh Q silica microcavity that
effectively prevents the Q degradation over time. The Q values of the devices
remain unchanged over time under storage in air. Optical frequency combs are
generated with sustainable ultralow threshold performance in the course of time
from the devices in open air. This approach would greatly facilitate ultrahigh
Q silica-based photonic devices for next generation photonic applications.
