  Lithium niobate on insulator emerges as a promising platform for integrated
microwave photonics because of its capability of ultralow-loss guidance and
high-efficiency modulation of light. Chip-level integration of microwave
filters is important for signal processing in the 5G/6G wireless communication.
Here, we employed the principle of bound states in the continuum for low-loss
waveguiding on an etchless lithium niobate integrated platform, and realized
high-performance microwave photonic filters thereon. These microwave photonic
filters consist of a high-quality photonic microcavity modulated by
piezoelectrically excited surface acoustic waves. Acoustic time delays from 21
to 106 ns and passbands with bandwidth as narrow as 0.89 MHz were achieved in
the fabricated filters operating at gigahertz frequencies. Our demonstration
may open up new applications on the lithium niobate integrated platform, such
as optical communication, signal processing, and beam steering.
