  Using the nonequilibrium Green function, we show that microwave irradiation
can reverse the supercurrent flowing through a superconductor / quantum-dot /
superconductor structure. In contrast with the conventional sideband effect in
normal-metal / quantum-dot / normal-metal junctions, the photon-assisted
structures appear near $E_{0}=\frac{n}{2}\hbar \omega (n=\pm 1,\pm 2...)$,
where $E_{0}$ is the resonant energy level of the quantum dot and $\omega $ is
the frequency of microwave field. Each photon-assisted structure is composed of
a negative and a positive peak, with an abrupt jump from the negative peak to
the positive peak around $E_{0}=\frac{n}{2}\hbar \omega $. The
microwave-induced $\pi $-junction transition is interpreted in the picture of
photon-assisted Andreev bound states, which are formed due to multiple
photon-assisted Andreev reflection between the two superconductors. Moreover,
the main resonance located at $E_{0}=0$ can also be reversed with proper
microwave strength and frequency.
