  In this work we present a scheme to control the optical dipole trap potential
in an N-type four-level atomic system by using chirped femtosecond Gaussian
pulses. The spatial size of the trap can be well controlled by tuning the beam
waist of the Gaussian pulse and the detuning frequency. The trapping potential
splits with increasing Rabi frequency about the center of the trap, a behavior
analogous to the one observed experimentally in the context of trapping of
nanoparticles with femtosecond pulses. An attempt is made to explain the
physics behind this phenomenon by studying the spatial probability distribution
of the atomic populations.
