"Units" for field, polarization, polarizability in DDSCAT and how to convert output to SI units?

"Units" for field, polarization, polarizability in DDSCAT and how to convert output to SI units?

Dmitriy (guest) 14 Jul 2016 20:20

Hello, I would like to leverage DDSCAT 7.3 to characterize the decay of electric field away from a plasmonic resonator relative to the dipole moment of that resonator for various specific conditions. Therefore, I would like to convert the electric field, polarization and polarizability values output by DDSCAT to physical SI units. All of the following outputs appear to be approximately on the order of 1.

(from DDPOSTPROCESS.f90)
! CXEINC(1-NXYZ,3) = complex incident macroscopic E field at all points
! CXESCA(1-NXYZ,3) = complex radiated macroscopic E field at all points
! CXPOL(1-NXYZ,3) = complex polarization/d^3 at all points
! CXADIA(1-NXYZ,3) = diagonal element of polarizability/d^3 at all pts

Can you please explain how each of these outputs are normalized? Specifically, can you please help me understand how to convert scattered electric field, incident electric field and polarization values extracted in DDSCAT to SI units?

draine (guest) 14 Jul 2016 22:07

Hi Dmitriy,

DDSCAT assumes the equations of Gaussian (aka as "cgs") electromagnetism. The user is allowed to set the incident electric field to a chosen value in whatever system of units the user prefers.

DDSCAT and DDPOSTPROCESS will report electric and magnetic fields in the same units that were used for CXEINC. If you set CXEINC to have magnitude unity, the scattered field CXESCA in the vicinity of the target will naturally have a magnitude similar to the value of CXEINC, unless you use a target material with refractive index close to unity (i.e., close to that of the ambient medium). What matters, of course, is the *ratio* of the scattered field to the incident field — take this from DDSCAT, and then multiply by whatever you think the incident field should be.

DDSCAT calculates the polarization/d^3 at all points, where d is the interdipole spacing — this is the polarization density P. In cgs electromagnetism, D = E + 4*pi*P .

One more important point: as explained in the UserGuide (section 29), the electric field reported by DDSCAT is the "macroscopic" electric field E_macro — this is the field such that D = epsilon*E_macro , where epsilon is the dielectric function.