Figure 7
Optical-harmonic generation in a mid-infrared filament.
(a) A high-power ultrashort mid-infrared driver pulse induces and ultrafast ionization of atmospheric air, giving rise to a radial beam profile of electron density. Scattering off the ionization-induced changes in the refractive index of the gas gives rise to a complex beam pattern behind the filament. While the central, most intense part of the beam forms a filament, a peripheral fraction of the beam, where the field intensity is much lower, undergoes scattering off the electron density profile, giving rise to a ring structure of the far-field beam profile. (b) Ionization-induced blue shift of the driver pulse. An input driver field (top) induces an ultrafast buildup of the electron density, ρ(τ), giving rise to a time-dependent plasma change in the refractive medium of the gas, δnp(τ) ≈ −ρ(τ)/(2ρcr). The refractive index thus decreases with τ = t − zn0/c, from the leading edge of a driver to its trailing edge. As a result, the trailing edge of the driver propagates with a higher phase velocity, undergoing a Doppler-like blue shift (bottom).
