  Integrated photonic circuits have transformed data communication, biosensing,
and light detection and ranging, and hold wide-ranging potential for optical
computing, optical imaging and signal processing. These applications often
require tunable and reconfigurable photonic components, most commonly
accomplished through the thermo-optic effect. However, the resulting tuning
window is limited for standard optical materials such as silicon dioxide and
silicon nitride. Most importantly, bidirectional thermal tuning on a single
platform has not been realized. For the first time, we show that by tuning and
optimizing the deposition conditions in inductively-coupled plasma chemical
vapor deposition (ICPCVD) of silicon dioxide, this material can be used to
deterministically tune the thermo-optic properties of optical devices without
introducing significant losses. We demonstrate that we can deterministically
integrate positive and negative wavelength shifts on a single chip, validated
on amorphous silicon carbide (a-SiC), silicon nitride (SiN) and
silicon-on-insulator (SOI) platforms. We observe up to a 10-fold improvement of
the thermo-optic tunability and, in addition, demonstrate athermal ring
resonators with shifts as low as 1.5 pm/{\deg}C. This enables the fabrication
of a novel tunable coupled ring optical waveguide (CROW) requiring only a
single heater. In addition, the low-temperature deposition of our silicon
dioxide cladding can be combined with lift-off to isolate the optical devices
resulting in a decrease in thermal crosstalk by at least two orders of
magnitude. Our method paves the way for novel photonic architectures
incorporating bidirectional thermo-optic tunability.
