  By calculating dynamical structure factor of two-dimensional doped attractive
Fermi-Hubbard model with Rashba spin-orbit coupling, we not only investigate
collective modes and single-particle excitations of the system during the phase
transition between Bardeen-Cooper-Schrieffer superfluid and topological
superfluid, but also propose a universal method to measure pairing gap
measurement in an optical lattice system. Our numerical results show that the
area of the molecular excitation peak at the transferred momentum ${\bf
q}=\left[\pi,\pi\right]$ is proportional to the square of the pairing gap in
the system with Rashba SOC. In particular, this method is very sensitive to the
pairing gap. This goes on verifying that this method is universal to measure
the pairing gap in a doped optical lattice with Rashba SOC. These theoretical
results are important for experimentally measuring the pairing gap and studying
the topological superfluid in an optical lattice.
