We describe photon assisted tunneling (PAT) in single and coupled 
quantum dot systems which have  a relatively large zero-dimensional 
(0D) level  separation.  A microwave electric  field applied across 
the  tunneling  barrier  leads  to  the  formation  of  a  sideband 
structure. For a  single quantum  dot system, a PAT current peak is 
clearly observed in addition to the Coulomb oscillation peak. For a 
weakly-coupled  double-dot  system, we  observe resonant  0D-0D PAT. 
The resonant  PAT current  appears  only when two discrete  quantum 
levels  in  the  neighboring  dots are  exactly  separated  by  the 
microwave photon energy. The PAT current shows a narrower peak than 
the main  resonant  tunneling  current  in  accordance  with  time-
dependent   tunneling   theory.   The photon  stimulated   emission 
associated  with PAT from a higher-lying occupied level to a lower-
lying empty level  is obtained at  a drain voltage  sufficient that 
population inversion takes place. 

Fig 1. Schematic diagram of the double gate single electron transistor. A single and a coupled quantum dot are formed by applying relevant voltages to the Schottky gates.

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Fig. 2. Energy diagram of the PAT through a dot in the classical Coulomb blockade regime (a), and in the quantum dot regime (b).

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Fig. 3. A Coulomb oscillation peak and PAT current peaks through a quantum dot. Each curve is offset by 0.2 pA for clarity.

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Fig. 4. (a) Schematic charging diagram of the coupled dot system. (n, m) corresponds to the number of electrons in the left dot and in the right dot, respectively. The resonant 0D-0D tunneling peaks are observed at the vertices, V and V'. The thick lines, PLR, PRL, etc., indicate the conditions for the resonant 0D-0D PAT. (b) Energy diagram of the PAT for the condition on the line PLR, at the point V, and (d) on the line PRL.

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Fig. 5. (a) The drain current profile near the condition V' with and without applying microwave radiation. (b) The contour plot of the current profile. A part of the charging diagram is also attached on the top of the plot. The arrows indicate that E = (E_R + E_L)/2 and e = E_R - E_L are changed.

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Fig. 6. (a)The drain current profile with a main resonance peak at V' and satellite PAT peaks on P'LR and P'RL. (b) Schematic diagram for the photon absorption and stimulated emission processes