  We propose a scheme for infrared single-photon detection based on two-photon
absorption at room-temperature in Si avalanche photodiodes, where the detected
photon's energy is lower than the bandgap and the energy difference is
complemented by a pump field. A quantum non-perturbative model is developed for
non-degenerate two-photon absorption in direct and indirect semiconductors
yielding proper non-divergent rates allowing device efficiency optimization.
The proposed monolithic detector is simple, miniature, integrable and does not
require phase matching, while not compromising the performance and exhibiting
even better efficiency than the competing up-conversion schemes (~1 order of
magnitude) for similar optical pump levels.
