  Strong light interaction with a subwavelength object has been a long pursuing
goal with difficulties mainly arising from the diffraction limit. We propose a
high finesse cavity with one mirror made of a subwavelength resonant particle
as a platform to enhance this interaction. High-quality eigenmode solutions are
obtained for such a highly non-paraxial cavity with a very high field
concentration at the particle. The eigenmode solutions interact with the small
particle in a more general way than by the electric dipole approximation. With
the help of the anapole excitation in the dipole term, the particle is designed
to scatter like a pure magnetic quadrupole, and in this way, it has anear-unity
reflectivity when used as a mirror for the strongly focused field of the
eigenmode. Light-matter interactions at the subwavelength scale can be greatly
enhanced due to the small size of the particle and the high finesse of the
cavity, which can be potentially interesting for applications in nano optics,
quantum optomechanics, nonlinear optics, and subwavelength metrology beyond the
electric dipole approximation.
