Brillouin precursors from magnon and optical phonon polaritons from a random-walk media

Abstract

The dispersion of a broadband electromagnetic pulse in a dielectric was theorized by Sommerfeld and Brillouin to separate into parts, with two groups of frequencies traveling faster—or “running ahead”—of the main signal from a Lorentz-like media. The observation of one such group, the Brillouin precursor (BP), arising from magnon and phonon polaritons generated in a random-walk ultra-small particle ferrite media is reported for the first time. Upon input of a broadband low-frequency radio wave in the form of a very low frequency (VLF) square wave to the ferrite particle dielectric core of the Random Walk Antenna (RWA), BP was generated and detected by the VLF antenna. The BP observation propagating in free space and water is supported by two salient percussor features (1) temporal Bessel-like waveform, and (2) algebraic, rather than exponential, attenuation with distance. The Brillouin precursor observed here in the VLF region can lead to precursors in the THz and optical regime with respect to the resonance frequency of the quasi-particles of optical phonons and excitons, respectively