  By placing a quantum emitter in the mouths of nanogaps consisting of two
metal nanoparticles nearly into contact, significant increases in emission rate
are obtained. This mechanism is central in the design of modern plasmonic
nanoantennas. However, due to the lack of general knowledge on the balance
between the different decay rates in nanogaps (emission, quenching, and metal
absorption), the design of light-emitting devices based on nanogaps is
performed in a rather hazardous fashion; general intuitive recipes do not
presently exist. With accurate and simple closed-form expressions for the
quenching rate and the decay rate into gap plasmons, we provide a comprehensive
analysis of nanogap light emitting devices in the limit of small gap thickness.
We disclose that the total decay rate in gap plasmons can largely overcome
quenching for specifically selected metallic and insulator materials,
regardless of the gap size. To confront these theoretical predictions, we
provide a comprehensive numerical analysis of nanocube-type antennas in the
limit of small gap thickness and further provide upper bounds for the
photon-radiation efficiency.
