Discussion Forum

Dear All,
I am new to Comsol, and I am now dealing with the stimulation of the Scattering/Absorption/Extinction cross-section of a Gold Nanorod concave structure（Fig.1）. My work is based on the APPs: Scattering nanosphere & scatterer on substract.

The absorption cross section as a function of wavelength was calculated by integrating the resistive heating losses over the volume of the nanoparticles. The scattering cross section as a function of wavelength was calculated by integrating the dot product of the scattered intensity vector and the normal vector pointing outwards from the particle surface across the entire particle surface. The extinction cross section was calculated by summing the absorption and scattering cross sections.

Here are some of my questions:
Someone mentioned me that we should use a sign function:
ofact = if(nx*x+ny*y+nz*z>=0,1,-1) ; onx = nx*ofact; ony = ny*ofact; onz = nz*ofact;
to make sure the normal vector is outward. But my structure has a Concave inner surface, so the situation can be complicated.

1. Does the vector (nx,ny,nz) just mean the outward normal vector?

2. How to define the ofact sign function of the complicated structure?
nx*x+ny*y+nz*z>0 for some parts of the concave inner surface, while the others <0. （Fig.2）

3. While I define ofact=if(nx*x+ny*y+nz*z>=0,1,-1), the scattering value sometimes be negative, does a negative value of scattering corss-section meaningful? (Fig.3)

4. While I define ofact=1; The scattering value could be so low compared with the Absorption value, is it right?