We have investigated terahertz (THz) photon-assisted tunneling in single self-assembled InAs quantum dots (QDs). Two types of photon-assisted tunneling processes have been observed in the THz range; the ground state resonance and the photon-induced excited state resonance, depending on the relative magnitude between the orbital quantization energy of the QDs and the THz photon energy. Furthermore, we could realize a very high coupling efficiency between the THz waves and the QDs and observed multi-photon absorption up to the fourth-order during the tunneling process, resulting in almost complete lifting of the Coulomb blockade.
