Theory of the Two-Particle Emission from Superfluid Fermi Gases in the BCS-BEC Crossover

We present a theory of the emission of fermion pairs from a superfluid Fermi gas induced by a photon absorption. In the solid state physics, this type of process is called double photo-emission (DPE). The spectrum of the induced two-particle current (or DPE current) provides a direct insight into the pair-correlation of condensate fermion pairs. We develop a general formalism for two-particle current induced by DPE by treating the coupling of two Fermi gases with the time-dependent perturbation theory. This formalism is used to calculate energy distributions of DPE current from the superfluid Fermi gas in the BCS-BEC crossover at T=0. We show that the DPE current has distinct contributions of the condensed pair components and uncorrelated pair states. We also calculate the angular dependence of DPE current in the BCS-BEC crossover. The DPE current of the tightly-bound molecules in the BEC regime is found to be quite deferent from that of the weakly-bound Cooper pairs.