folks keep contacting me about different things regarding this subject. I have advanced a bit, but still some issues remaining.
1) In excitation field definition you can forget about using complicated formulae with different expressions for z>0 and z<0. It looks like that if u use k_rfw, COMSOL will automatically consider refractive index of material into k vector in different media.
2) THERE is a big difference if u are solving for SCATTERD or TOTAL field. You should read Jiaming Jin's book to find out more. In short, if u are solving for SCATTERD field your excitation should be defined in the most precize analytical way, using Fresnel coefficients, and in that case you need complicated formulae with z>0 and z<0, since in SCATTERING MODULE you have to define field present everywhere except PML. In total field calculation, you have to insert incident wave on one boundary (port or scattering boundary condition with excitation), and propagation should be calculated... There are some advantages/disadvantages in both cases, depending what you whant to calculatet, how much memory you have, and some FEM issues that you can read in Jim's book...
3) About 2 step procedure: When u create new model name your dependents variables to scEx1,etc, and Application mode name to rfw1. Then you can make your geometry. After, you go to Multyphysics/Module navigator and ADD new scEx2,...,rfw2, and now you have two models on the same geometry. In both models you define separately Subdomain setting, Boundary settings, etc. Meshing is the same, since you have one geometry, and 2 models defioned on the same geometry. So, first you solve situation when your particle is made of air let's say, and later use that solution as excitation in your second model. Thus, in Physics/Scalar Variables you set Eoix... with Fresnel coefficients (or not if u don't want), and for E0ix2 you set Ex, E0iy2 set Ey, etc.... First step is to SOLVER MANAGER/Solve for you highlight rfw1, and for the output rfw1 (not necessary), and when it is done, you go to SOLVER MANAGER/Initial value, there you first Store solution, and check STORED SOLUTION option as initial values, SOLVE FOR you highlight rfw2, and the same for OUTPUT... That should work. Theoretical benefits of this procedure are not so clear to me. IF SOMEONE COMPARES IT; PLEASE PUT YOUR RESULTS AND CONCLUSION HERE....
4) Someone asked me "How did you separate the scattered field from the substrate and from the nanoparticle?" Maybe this 2 step procedure is the answer to that. Maybe you can normalized what you get for particle air and partice made of metal case, I don't know exactely.
5)Scattering cross-section: Read RF module reference guide. There is a Stratton-Chu formula implemented ion COMSOL for calculating far-fileld components. Onece you have them, you need spherical surface (it doesn't have to be the same as one for Stratton-Chu, can be smaller to save you integratin time) and one should do boundary integration of normEfar*normEfar, and that is proportional to scattering cross section. If you want to be more precise calculate this expresion: (normEfar*normEfar)/(Surface of particle*Ein*Ein*R*R) where R is the radius of your integrating spherical surface.
6) IF ANYONE HAVE GOOD RECIPE FOR PML THAT WORKS FINE, please POST IT!