  As a high-order quantum transition, two-photon emission has an extremely low
occurrence rate compared to one-photon emission, thus having been considered a
forbidden process. Here, we propose a scheme that allows ultrafast two-photon
emission, leveraging highly confined surface plasmon polariton modes in a
degenerately-doped, light-emitting semiconductor thin film. The surface plasmon
polariton modes are tailored to have simultaneous spectral and spatial overlap
with the two-photon emission in the semiconductor. Using degenerately-doped
InSb as the prototype material, we show that the two-photon emission can be
accelerated by 10 orders of magnitude: from tens of milliseconds to
picoseconds, surpassing the one-photon emission rate. Our result provides a
semiconductor platform for ultrafast single and entangled photon generation,
with a tunable emission wavelength in the mid-infrared.
