  Thermo-optic (TO) phase shifters are very fundamental units in large-scale
active silicon photonic integrated circuits (PICs). However, due to the
limitation of microheater materials with a trade-off between heating efficiency
and absorption loss, designs reported so far typically suffer from slow
response time, high power consumption, low yields, and so on. Here, we
demonstrate an energy-efficient, fast-response, and low-loss TO phase shifter
by introducing hydrogen-doped indium oxide (IHO) films as microheater, and the
optimized electron concentration with enhanced mobility endows the IHO high
conductivity as well as high near-infrared (NIR) transparency, which allow it
to directly contact the silicon waveguide without any insulating layer for
efficient tuning and fast response. The TO phase shifter achieves a
sub-microsecond response time (970 ns/980 ns) with a {\pi} phase shift power
consumption of 9.6 mW. And the insertion loss introduced by the IHO microheater
is ~ 0.5 dB. The proposed IHO-based microheaters with compatible processing
technology illustrate the great potential of such material in the application
of large-scale silicon PICs.
