Unidirectional Beam in FDTD

Hello everybody,

I want to simulate scattering of laser beam from nano-particles (something like air pollutions or dust).
How can I inject a unidirectional beam (like laser) in my simulation.
I use TF/SF technique, but it does not produce a narrow unidirectional beam.

I hear some thing that I can adjust the phase part of my source to get a unidirectional beam. Could anyone please help me

you applied the Gaussian shape beam on the input plane with the help of TFSF technique, only fix the EM fields on that plane.
however, how narrow the beam will be is dependent on the background material and the centeral wavelength of laser source.