Single photon detection by cavity-assisted all-optical gain

We consider the free-carrier dispersion effect in a semiconductor nanocavity in the limit of discrete photoexcited electron-hole pairs. This analysis reveals the possibility of ultrafast, incoherent transduction and gain from a single photon signal to a strong coherent probe field. Homodyne detection of the displaced probe field enables an all-optical method for room-temperature, photon-number-resolving single photon detection. In particular, we estimate that a single photon absorbed within a silicon nanocavity can, within tens of picoseconds, be detected with ∼99% efficiency and a dark count rate on the order of kilohertz assuming a mode volume V_eff∼10⁻² (λ/n_Si)³ for a 4.5-μm probe wavelength and a loaded quality factor Q on the order of 104.