Simple asymptotic forms for Sommerfeld and Brillouin precursors

TL;DR

This paper derives simple analytical asymptotic expressions for Sommerfeld and Brillouin precursors in dense Lorentz media, clarifying their shapes, dependencies, and effects of attenuation and dispersion on pulse propagation.

Contribution

It provides explicit, simple asymptotic formulas for precursors, improving understanding of their formation and dependence on medium properties and initial pulse shape.

Findings

Sommerfeld precursor shape depends on initial discontinuity order

Brillouin precursor is determined by medium attenuation in asymptotic limit

Optimized pulse parameters enhance Brillouin precursor detection

Abstract

This article mainly deals with the propagation of step-modulated light pulses in a dense Lorentz-medium at distances such that the medium is opaque in a broad spectral region including the carrier frequency. The transmitted field is then reduced to the celebrated precursors of Sommerfeld and Brillouin, far apart from each other. We obtain simple analytical expressions of the first (Sommerfeld) precursor whose shape only depends on the order of the initial discontinuity of the incident field and whose amplitude rapidly decreases with this order (rise-time effects). We show that, in a strictly speaking asymptotic limit, the second (Brillouin) precursor is entirely determined by the frequency-dependence of the medium attenuation and has a Gaussian or Gaussian-derivative shape. We point out that this result applies to the precursor directly observed in a Debye medium at decimetric wavelengths. When attenuation and group-delay dispersion both contribute to its formation, we establish a more general expression of the Brillouin precursor, containing the previous one (dominant-attenuation limit) and that obtained by Brillouin (dominant-dispersion limit) as particular cases. We finally study the propagation of square or Gaussian pulses and we determine the pulse parameters optimizing the Brillouin precursor. Obtained by standard Laplace-Fourier procedures, our results are explicit and contrast by their simplicity from those derived by the uniform saddle point methods, from which it is very difficult to retrieve our asymptotic forms.