  Strong coupling is a phenomenon which occurs when the interaction between two
resonance systems is so strong that the oscillatory energy exchange between
them exceeds all dissipative loss channels. Each resonance can then no longer
be described individually but only as a part of the coupled, hybrid system.
Here, we show that strong coupling can occur in a deep subwavelength nanofilm
supporting an epsilon near zero mode which is integrated into a
metal-insulator-metal gap plasmon structure. To generate an epsilon near zero
mode resonance in the short-wave infrared region, an indium tin oxide nanofilm
of ~lambda/100 thickness is used. A polariton splitting value of 27%,
corresponding to a normalized coupling rate of 0.135, is experimentally
demonstrated. Simulations indicate that much larger coupling rates, well within
the ultra-strong regime where the energy exchange rate is comparable with the
frequency of light, are possible.
