Subwavelength Particle Spectroscopy by Measurements of Electromagnetic Scattering at Millimeter-Wave Frequency

A detailed study by theory, simulation, and measurement is presented for electromagnetic scattering from alumina ceramic spheres covering a frequency range from 50 to 220 GHz. The analysis is in context to sensing and characterization of low-to-moderate permittivity subwavelength-sized homogeneous dielectric spheres featuring small dielectric dissipation loss. This includes the description of novel techniques to characterize an unknown spherical particle to simultaneously determine both its electrical and geometrical attributes. For measurements, a broadband dielectric sensor on a millimeter-wave test bench is developed with the uncertainty in the experimental approach reduced by an over-the-air calibration. The complex electromagnetic scattering parameters and, thereby, extinction cross section are determined. In the nonresonant Rayleigh scattering domain, particle characterization is done through the exploitation of polarization-sensitive angular scattering measurements. In the resonant Mie scattering regime, the frequency-dependent forward and backward directive scattering is explored for single-particle spectroscopy. Finally, the dielectric dissipation factor of the tested alumina spheres is estimated from the magnitude of forward scattering.