rough polyhedra on substrate

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Maxim Yurkin

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Jul 2, 2025, 9:24:42 AMJul 2

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Dear colleagues,

Let me bring your attention to the recent paper:
Virkki A.K. and Yurkin M.A. Microwave scattering by rough polyhedral particles on a surface, J. Quant. Spectrosc. Radiat. Transfer 345, 109547 (2025).

First, it enhances our understanding of optical properties of many atmosphereless bodies (Moon, asteroids, etc.), which have a fine regolith with sparse large boulders on top of it. When sensed by microwaves, these boulders (which we modelled by rough polyhedra) are important for the overall response, since their sizes are comparable to the wavelength. Surface mode of ADDA perfectly fits for such simulations. Systematic simulations allowed us to study the sensitivity of polarimetric signals to system parameters.

Second, we realized that the interpretation of such simulations is conceptually harder than in the free-space case. Thus, the paper provides discussion of methodological issues, relevant for any particle-on-substrate configuration. In particular:

* Ambiguities in defining measurable quantities for particles on substrate, such as Mueller matrix (and corresponding scattering plane) and scattering cross section.

* Classification of substrate effects. Specifically, we distinguished far-field interaction of 4 scattering pathways and near-field interaction (between particle and substrate).

* Using azimuthal instead of full orientation averaging, and quasi-equivalence between averaging over ensemble and orientations.

* Symmetries of Mueller matrix elements. Naturally, the presence of substrate breaks some symmetries. Still, some of them are satisfied at back scattering, some - only additionally at normal incidence. Moreover, some symmetries are approximately satisfied in a wider range - this may serve as indicator of weak near-field particle-substrate interaction.

* Setting the particle refractive index very close to 1 (as a test case), we got into the validity domain of the Born approximation. This turns off some of the interaction, causing more symmetries to be satisfied.

Third, the paper is accompanied by openly available data and scripts. Most relevant to ADDA, is the examples at https://github.com/adda-team/adda/tree/master/examples/papers/2025_surface , which allows one to compute the scattering properties of a spherical particle on a surface and to reproduce two figures of this paper.

I hope all of this will facilitate similar studies with ADDA for any particles on substrate.