Studying the Scattering of Electromagnetic Wave by a Composite 3D Model at Terahertz Frequencies
The frequency range from 0.1 THz – 10 THz of the electromagnetic spectrum is designated as the Terahertz (THz) band; having unique properties such as high absorption in polar materials, characteristic spectral fingerprints and non-ionizing characteristics because of its weak photon energy (0.41 meV – 41.36 meV). These make it attractive for applications involving biological samples. However, structural inhomogeneities, microscopic variations in material density and compositional fluctuations inherent in biological samples, result in strong scattering of the incident radiation. Most calculations using scattering models based on Rayleigh, Mie theories however, are limited by many assumptions, such as spherical particle shape, independent scattering centers; and neglect the effects of multiple scattering even where each particle is also exposed to radiation scattered by other particles.
To counteract such limitations of these rigorous theoretical models, this work focuses on the study of scattering of electromagnetic waves by a composite 3D model, representing real life biological samples such as a leaf, using COMSOL Multiphysics®. This present simulation is performed with the help of Electromagnetic Waves, Frequency Domain of the Wave Optics module. The proposed model, with 5 small hemispheres (radius 30μm) upon a single cylindrical disk (radius 120μm, thickness 100μm) is designed based on a leaf structure with trichomes (thin hair-like formations) on its epidermal surface layer. To understand the nature of the interaction, three material combinations having different refractive indices (RI) are considered: Combination 1 (disk and hemispheres constituted of the same material), Combination 2 (lower RI for hemispheres) and Combination 3 (higher RI for hemispheres). With a sphere of a perfectly matched layer (PML) of radius six times that of the cylindrical disk, surrounding the physical structure, the model is parameterized over the frequency range [0.2 THz (~1498.962μm) to 1.8 THz (~166.55μm)] of our interest using a physics-controlled fine mesh. The scattering cross section of the composite structure (Fig.1(a)) varies with different combinations; while the 3D scattered electric field patterns for combination 2 at three different characteristic frequencies (@ 0.2 THz, 1 THz, 1.6THz) reveal that, (in Fig.1(b),Fig.1(c) and Fig.1(d)) the dominating scattering mechanisms at those frequencies are different. As expected, at lower frequencies, it is principally the Rayleigh scattering, showing the symmetric nature of the radiation pattern along the direction of propagation; at slightly higher frequency, the pattern shows a composite effect of both Rayleigh and Mie; while at a still higher frequency of 1.6 THz, the pattern shows a clear indication of Mie scattering with forward radiation lobe significantly larger than the backward lobe.
From the analyses of these patterns, frequency ranges are identified based on the proportion of backward scattering with respect to the forward scattering. This simulation classifies the mode of configuration (reflection/transmission) required for optimal data acquisition based on a specific frequency range for the actual experimental setup. Besides this immediate use of this analysis, the above model is also very much relevant for typical biological samples (leaves, petals, skin, etc.), common chemicals, food samples, patterned semiconductor heterostructures etc.
