Chirped femtosecond pulse scattering by spherical particles

Generalized Lorentz-Mie formulas are used to study the scattering characteristics when a chirped femtosecond pulse illuminates a spherical particle. For a linear chirped Gaussian pulse with the envelope function g(τ) = exp[-π(1 + ib)τ(2)], dimensionless parameter b is defined as a chirp. The calculation illustrated that even for pulses with a constant carrier wavelength (λ(0) = 0.5 µm) and pulse-filling coefficient (l(0) = 1.98), the efficiencies for extinction and scattering differ very much between the carrier wave and the different chirped pulses. The slowly varying background of the extinction and the scattering curves is damped by the chirp. When the pulse is deeply chirped, the maxima and minima of the background curves reduce to the point where they disappear, and the efficiency curves illustrate a steplike dependence on the sphere size. Another feature is that the chirped-pulse scattering seems blind: it depends only on the amount of chirp (|b|), regardless of upchirp (b > 0) or downchirp (b< 0).