Study of Scattering Distribution for Spherical Particles
Terahertz (THz) radiation lies in between the microwaves and the infrared region of electromagnetic wave spectrum corresponding to the wavelength 30 µm to 3,000 µm. This EM range has found widespread applications in several industrial fields such as, material characterization, medical diagnostics, food and agriculture, security screening etc. The interaction of THz radiation with material leads to material specific rotational and vibrational spectra, which can be used for the identification of unknown samples. Often, this spectral identification becomes complicated due to the presence of other simultaneous optical responses, such as scattering effects in the sample, which affect the spectral signature of the sample, especially in granular forms.
In this current study, using COMSOL Multiphysics®, we have analyzed the scattering effect of spherical particles of different sizes over a wide range of wavelengths (75 µm to 3,000 µm). COMSOL Multiphysics® is a widely used simulation tool providing interactive environment ideal to study this scattering physics in this wide range. The scattering response changes from Mie to Rayleigh through a transition phase when contributions from both types of scattering are present.
Spherical, non-absorbing particles of refractive index 1.54 with different radii of 20 µm, 80 µm and 160 µm were considered for 3D simulation using frequency domain study available in wave optics module. A perfectly matched layer (PML) was considered with radius ten times the radius of the particle and with thickness three times the particle radius. The incident wave was propagating in the positive z-direction with polarization along x-axis. The PML domain was discretized using triangular elements whereas fine mesh elements were used for particle discretization. Scattering pattern was obtained for different frequencies using parametric sweep in frequency ranging from 0.1 THz to 4 THz with step size of 0.2 THz.
The scattering electric field pattern obtained for 20 µm particle at three different frequencies (1.2 THz, 2.4 THz and 3.8 THz) is shown in fig. 1. We observed that when wavelength of the incident radiation is large, scattering distribution follows the Rayleigh scattering. The direction of the scattered field pattern is symmetrical along the polarization plane, as expected. As the frequency of the incident radiation increases (in other words, the wavelength decreases), the scattering interaction becomes stronger. The scattered field pattern becomes asymmetrical with a distinct bias towards the forward direction. This is indicative of the Mie scattering interaction. Same scattering pattern is equally observed using this simulation by other particles of larger radii; but the frequency at which the pattern of scattered field becomes asymmetrical is found to be lower for larger particles. As observed in figure 1 (c), when the particle size becomes comparable to the wavelength of the incident radiation, the scattered field pattern is highly asymmetrical with large forward direction amplitude. Prior knowledge of scattering effect is essential for effective experimental data analyses. Based on this frequency and particle size related study, experimental work will be performed using THz time domain spectroscopy for identification of different granular samples such as sugar, salt and flour.
