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The fully nonlinear response of a many-level tunneling system to a strongalternating field of high frequency $\omega$ is studied in terms of theSchwinger-Keldysh nonequilibrium Green functions. The nonlinear time dependenttunneling current $I(t)$ is calculated exactly and its resonance structure iselucidated. In particular, it is shown that under certain reasonable conditionson the physical parameters, the Fourier component $I_{n}$ is sharply peaked at$n=\frac {\Delta E} {\hbar \omega}$, where $\Delta E$ is the spacing betweentwo levels. This frequency multiplication results from the highly nonlinearprocess of $n$ photon absorption (or emission) by the tunneling system. It isalso conjectured that this effect (which so far is studied mainly in thecontext of nonlinear optics) might be experimentally feasible.Comment: 28 pages, LaTex, 7 figures are available upon request from  goldin@bgumail.bgu.ac.il, submitted to Phys.Rev.

Nonlinear current response of a many-level tunneling system: Generation of higher harmonics