  Waves that are perfectly confined in the continuous spectrum of radiating
waves without interaction with them are known as bound states in the continuum
(BICs). Despite recent discoveries of BICs in nanophotonics, full routing and
control of BICs are yet to be explored. Here, we experimentally demonstrate
BICs in a fundamentally new photonic architecture by patterning a
low-refractive-index material on a high-refractive-index substrate, where
dissipation to the substrate continuum is eliminated by engineering the
geometric parameters. Pivotal BIC-based photonic components are demonstrated,
including waveguides, microcavities, directional couplers, and modulators.
Therefore, this work presents the critical step of photonic integrated circuits
in the continuum, and enables the exploration of new single-crystal materials
on an integrated photonic platform without the fabrication challenges of
patterning the single-crystal materials. The demonstrated lithium niobate
platform will facilitate development of functional photonic integrated circuits
for optical communications, nonlinear optics at the single photon level as well
as scalable photonic quantum information processors.
