NONLINEAR CURRENT RESPONSE OF A MANY-LEVEL TUNNELING SYSTEM - GENERATION OF HIGHER HARMONICS

The fully nonlinear response of a many-level noninteracting tunneling system to a strong alternating field of high frequency omega is studie d in terms of the Schwinger-Keldysh nonequilibrium Green's functions. The non-linear time dependent tunneling current I(t) is calculated exa ctly and its resonance structure is elucidated. In particular, it is s hown that under certain reasonable conditions on the physical paramete rs,the Fourier component I-n is sharply peaked at n=(Delta E/(h) over bar omega), where Delta E is the spacing between two levels. This freq uency multiplication results from the highly nonlinear process of n ph oton absorption (or emission) by the tunneling system. It is also conj ectured that this effect (which so far is studied mainly in the contex t of nonlinear optics) might be experimentally feasible.