  We present a theoretical analysis for photon-assisted electron tunneling
through a hybrid superconducting-normal quantum point contact (QPC) consisting
of a superconducting lead (S), a normal two-level quantum dot (N), and a normal
lead (N). Using single particle non-equilibrium Green's function formalism, we
incorporate Floquet basis in the Nambu space and solve the Green's function
with finite matrix truncation to obtain the transport properties numerically.
Based on the proposed method, we studied the effects of photon-induced
single-level oscillations and quantum transition between two levels on the
current-voltage (I-V) characteristics of a superconducting QPC. For the single
level case, the main dc resonance in the I-V curve remains unchanged regardless
of the frequency and amplitude of the radiation, and a series of secondary
resonances due to multi-photon processes are present. When the transition
between two levels ($\epsilon_{1}$, $\epsilon_{2}$) is taken into account and
level oscillations are neglected, photons only have significant effects at Rabi
resonance when $\hbar\omega=(\epsilon_{2}-\epsilon_{1})$. At Rabi resonance,
the main dc resonance splits into two, and the separation between them is
determined by the coupling strength of the two levels.
