  We demonstrate direct probing of strong magnon-photon coupling using
Brillouin light scattering spectroscopy in a planar geometry. The magnonic
hybrid system comprises a split-ring resonator loaded with epitaxial yttrium
iron garnet thin films of 200 nm and 2.46 $\mu$m thickness. The Brillouin light
scattering measurements are combined with microwave spectroscopy measurements
where both biasing magnetic field and microwave excitation frequency are
varied. The cooperativity for the 200 nm-thick YIG films is 4.5, and larger
cooperativity of 137.4 is found for the 2.46 $\mu$m-thick YIG film. We show
that Brillouin light scattering is advantageous for probing the magnonic
character of magnon-photon polaritons, while microwave absorption is more
sensitive to the photonic character of the hybrid excitation. A miniaturized,
planar device design is imperative for the potential integration of magnonic
hybrid systems in future coherent information technologies, and our results are
a first stepping stone in this regard. Furthermore, successfully detecting the
magnonic hybrid excitation by Brillouin light scattering is an essential step
for the up-conversion of quantum signals from the optical to the microwave
regime in hybrid quantum systems.
