THE NONLINEAR RESPONSE OF A QUANTUM-DOT IN A TIME-DEPENDENT FIELD

We calculate the rectification coefficient a(rect) and the second-harm onic generation coefficient a(2 Omega) for a quantum dot biased with a n ac field. The coefficients are calculated as functions of the gate v oltage (i.e. the positions of the resonant levels) and the frequency o f the applied field. The rectification coefficient has a peaked depend ence on the gate voltage with one central (negative) and two side (pos itive) peaks around the voltage at which the corresponding energy leve l for interacting electrons is equal to the collector electrode chemic al potential. The peak heights are related to the average number of el ectrons in the dot-a feature which is a direct consequence of the elec tron interactions. The second-harmonic generation coefficient has simi lar behaviour for low frequencies (a(rect) = a(2 Omega) at zero freque ncy). The frequency dependence of the rectification coefficient has fe atures resulting from the photon-assisted tunnelling through the dot. The resonant-like enhancement of the rectification coefficient cannot be attributed to the photon-assisted transitions between different res onant levels in the dot.