In this thesis we show several modelling tools which are used to study nonlinear photonic
band-gap structures and microcavities. First of all a nonlinear CMT and BPM were implemented
to test the propagation of spatial solitons in a periodic device, composed by an array
of parallel straight waveguides. In addition to noteworthy theoretical considerations, active
functionalities are possible by exploiting these nonlinear regimes. Another algorithm was developed
for the three-dimensional modelling of photonic cavities with cylindrical symmetry,
such as microdisks. This method is validated by comparison with FDTD. We also show the
opportunity to confine a field in a region of low refractive index lying in the centre of a silicon
microdisk. High Q-factor and small mode volumes are achieved. Finally the characterization
of microdisks in SOI with Q-factor larger than 50000 is presente