Laser-induced collective excitations in a two-component Fermi gas

We consider the linear density response of a two-component ~superfluid! Fermi gas of atoms when the perturbation is caused by laser light. We show that various types of laser excitation scheme can be transformed into linear density perturbations; however, a Bragg spectroscopy scheme is needed for transferring energy and momentum into a collective mode. This makes other types of laser probing schemes insensitive for collective excitations and therefore well suited for the detection of the superfluid order parameter. We show that, for the special case when laser light is coupled between the two components of the Fermi gas, density response is always absent in a homogeneous system.