Evanescent wave scattering by particles on a surface: Validation of the discrete dipole approximation with surface interaction against microwave analog experiments

- The DDA-SI is used for predicting evanescent wave scattering by particles.
- DDA-SI predictions are compared against scaled microwave experiments and the FEM.
- The DDA-SI and the FEM are in agreement with the experiments for lossless objects.
- The DDA-SI diverges from the FEM and experiments for a larger absorbing object.

The discrete dipole approximation with surface interaction (DDA-SI) is analyzed and expanded for the modeling of far-field scattering by objects on a surface illuminated by an evanescent wave generated by total internal reflection. More specifically, the electric field scattered in the far zone predicted via the DDA-SI is compared against scaled microwave experiments; additional comparisons are also performed using results from a Finite Element Method (FEM). Three cases are considered: a lossless cube with a side length of λ/1.79 (size parameter x=1.76), a lossless sphere with a diameter of λ/1.92 (x=1.63) and an absorbing sphere with a diameter of λ/0.87 (x=3.63), where λ is the wavelength. For lossless scatterers, a good agreement between the DDA-SI, the FEM and scaled microwave analog experiments is observed, especially when modified Fresnel reflection coefficients are used for computing the surface interaction in the DDA-SI. For an absorbing sphere, the experimental and FEM results are in reasonable agreement, while the DDA-SI exhibits a different trend. This behavior might be due to the fact that the accuracy of the DDA decreases as the permittivity and the size parameter increase. This work suggests that the DDA-SI could be used as a forward model in an evanescent wave-based characterization framework given that a thorough convergence and accuracy analysis is carried on in order to improve the performance when dealing with objects having large permittivity and/or size parameter.