You can decide by yourself if you want to reference F2P and its authors in any publication for which you used it. You are not legally required to do so.

The preferred citation for referencing on related topics:

               [1]M. Qiu, "Effective index method in heterostructure slab waveguide based two-dimensional photonic crystals", Appl. Phys. Lett., Vol. 81, No. 7, 1163-1165 (2002).

               [2]M. Qiu, B. Jaskorzynska, M. Swillo, and H. Benisty, "Time-domain 2D modeling of slab-waveguide based photonic-crystal devices in the presence of out-of-plane radiation losses", Microwave Opt. Techn. Lett., Vol. 34, No. 5, 387-393 (2002).

The preferred citation for the F2P software:

               [1]M. Qiu, F2P: Finite-difference time-domain 2D simulator for Photonic devices, http://www.imit.kth.se/info/FOFU/PC/F2P/