  Cavity optomagnonics has emerged as a promising platform for studying
coherent photon-spin interactions as well as tunable microwave-to-optical
conversion. However, current implementation of cavity optomagnonics in
ferrimagnetic crystals remains orders of magnitude larger in volume than
state-of-the-art cavity optomechanical devices, resulting in very limited
magneto-optical interaction strength. Here, we demonstrate a cavity
optomagnonic device based on integrated waveguides and its application for
microwave-to-optical conversion. By designing a ferrimagnetic rib waveguide to
support multiple magnon modes with maximal mode overlap to the optical field,
we realize a high magneto-optical cooperativity which is three orders of
magnitude higher compared to previous records obtained on polished YIG spheres.
Furthermore, we achieve tunable conversion of microwave photons at around 8.45
GHz to 1550 nm light with a broad conversion bandwidth as large as 16.1 MHz.
The unique features of the system point to novel applications at the crossroad
between quantum optics and magnonics.
