Properties of a Raman atom-laser output coupler

We have studied the characteristics of an atom laser created by removing at oms from a Bose-Einstein condensate (BEC) via a two-photon Raman process. I n contrast with outcoupling by RF transitions, Raman outcoupling offers spa tial selectivity within the condensate and delivers significant momentum tr ansfer to the outcoupled atoms. We develop a version of zero-temperature me an-field theory for BECs which includes the internal state of the atom and the effect of laser fields focused into the cloud. For a model condensate w e find that the outcoupled matter wave develops sharp peaks at spatial posi tions where the two-photon Raman process is resonant with the hyperfine spl itting of the atomic ground state. As time proceeds the width of the outcou pled wave expands and a steady stream of matter waves travels outside the c ondensate volume.