Metastable excited states of a closed quantum dot with high sensitivity to infrared photons

A closed quantum dot (QD) is studied via a conductance through a constricted conducting channel placed about ~1500.3em0exnm below the QD. As bias voltages of metal gates are scanned, remarkable hysteretic behavior shows up in the conductance, indicating different metastable charge states of the closed QD far away from the equilibrium. The sensitivity of the constricted conducting channel is high enough to detect single charge in the QD at 4.20.3em0exK, where the charging energy of the QD (EC≈0.190.3em0exmeV) is less than the thermal energy (kBT=0.360.3em0exmeV). Weak infrared illumination (wavelength of ~140.3em0exμm) causes intersubband excitation in the QD, leading to substantial reduction of the electrons in the QD. This largely modifies the charge state of the QD and entirely eliminates the hysteretic behavior. Single infrared photon is thereby detected.