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

Abstract

A closed quantum dot (QD) is studied via a conductance through a constricted conducting channel placed about $\sim 150\mathrm{nm}$ 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 $4.2\mathrm{K}$, where the charging energy of the QD $(E_C\approx 0.19\mathrm{meV})$ is less than the thermal energy $(k_{B}T=0.36\mathrm{meV})$. Weak infrared illumination (wavelength of $\sim 14\mu \mathrm{m}$) 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.