Re: Optical forces
since you want to consider a volume with this laser fields, a boundary condition probably is not enough.
Which method are you using for the simulation of the particle?
If you are using a continuous field for the particle like in the eulerEuler bifurcation3d example the solution is probably to apply a different diffusion coefficient in the respective region.
If you are using a discrete representation like in the eulerLagrange bifurcation3d example you could introduce an additional force, describing the impact of the mentioned laser field.