How to simulate a spherical Target at the interface air/dielectric (epsilon dielectric is 2)

Summary:

I want to if it is possible to simulate a Target placed at the interface air/slab withing DDSCAT.

How to simulate a spherical Target at the interface air/dielectric (epsilon dielectric is 2)

Maximin (guest) 03 Aug 2017 19:10

I Sm a new user of DDSCAT and I want to if it is possible to simulate a Target placed at the interface air/slab withing DDSCAT. If possible please can anyone guide?

Thank you

draine (guest) 04 Aug 2017 16:13

Unfortunately, in its present form DDSCAT is not set up to treat the problem of a finite target near a semi-infinite slab.

For DDSCAT, the closest approximation to the posed problem would be to calculate the finite target placed at the surface of a rectangular block with the desired dielectric function representing the semi-infinite substrate. Use a block that is as large as you can afford, and repeat the calculation for different block sizes to see whether the results seem to be sensitive to the size of the block. This method is inelegant and unfortunately not very efficient; memory limitations will limit the size of the slab, given that the interdipole spacing d needs to be small enough to resolve the target geometry.

If your target happens to be a disk, you can use target option DSKRCTNGL. If your target happens to be a rectangular block, you can use target option RCTGLBLK2. If your target has another geometry, you can use target option FROM_FILE, and use your own software to prepare a file with the list of dipoles locations and compositions.

ozan (guest) 14 Sep 2017 14:04

Hello, is there a way to obtain scattering and absorption efficiencies of two materials separately when your example is concerned?

More specifically, suppose that I am trying to model a surface as a rectangular disk (as you describe above) and there is a spherical particle of different material on top of it. Can I have absorption efficiency of the particle on top separately? When I do the calculations what I see is the absorption efficiency of the whole system.

If yes, I will try to model core-shell particle on top of a surface (there will be at least 3 different materials! surface, core and shell). Can I calculate absorption efficiencies of each material? Is it possible?

Thank you for your help.

Have a nice day.

pcirrus 16 Sep 2017 02:38

If you look at equation (9) of Draine and Flatau, 1994 (J. Opt. Soc. Am. vol 11, no 4, April 1994) absorption cross section C_abs is given by summation over all dipoles:

C_abs= 4pik/E0^2 SUM_j=1^N [Im(P_j*(alpja_j) conj(Pj) - 2/3 k^2 P_j^2] (eq. 9)

and efficiency Q_abs is normalized C_abs (see DDSCAT User Guide https://arxiv.org/abs/1305.6497, section Absorption and Scattering By Finite targets). DDSCAT is calculating polarizations P_j in EVERY point of the shape/object. Thus, you can split the sum given in eq. 9 into contributions from subset of shape locations - for example upper part of disk and lower part of a disk. So if this is what you are interested in, the answer is yes. I think that the practical way of doing it is to store P_j and recalculate C_abs yourself. For that you can use nearfield option in DDSCAT. But, you can also get to the DDSCAT code and change the summation there (you will need to know FORTRAN and recompile the code).