  Polaritons in metals, semimetals, semiconductors, and polar insulators, with
their extreme confinement of electromagnetic energy, provide many promising
opportunities for enhancing typically weak light-matter interactions such as
multipolar radiation, multiphoton spontaneous emission, Raman scattering, and
material nonlinearities. These highly confined polaritons are
quasi-electrostatic in nature, with most of their energy residing in the
electric field. As a result, these "electric" polaritons are far from optimized
for enhancing emission of a magnetic nature, such as spin relaxation, which is
typically many orders of magnitude slower than corresponding electric decays.
Here, we propose using surface magnon polaritons in negative magnetic
permeability materials such as MnF$_2$ and FeF$_2$ to strongly enhance
spin-relaxation in nearby emitters in the THz spectral range. We find that
these magnetic polaritons in 100 nm thin-films can be confined to lengths over
10,000 times smaller than the wavelength of a photon at the same frequency,
allowing for a surprising twelve orders of magnitude enhancement in magnetic
dipole transitions. This takes THz spin-flip transitions, which normally occur
at timescales on the order of a year, and forces them to occur at sub-ms
timescales. Our results suggest an interesting platform for polaritonics at THz
frequencies, and more broadly, a new way to use polaritons to control
light-matter interactions.
