Direct imaging of the acoustic waves generated by femtosecond filaments in air

Abstract

We present time-resolved measurements of the gas acoustic dynamics following interaction of spatial single- and higher-mode 50 fs, 800 nm pulses in air at 10 Hz and 1 kHz repetition rates. Results are in excellent agreement with hydrodynamic simulations. Under no conditions for single filaments do we find on-axis enhancement of gas density; this occurs only with multifilaments. We also investigate the propagation of probe beams in the gas density profile induced by a single extended filament. We find that light trapping in the expanding annular acoustic wave can create the impression of on-axis guiding in a limited temporal window.

Demonstration of Long-Lived High-Power Optical Waveguides in Air

“…[25], that might explain their recent report of guiding [25], an issue further discussed in Ref. [26]. At the longer delays of tens of microseconds and beyond, the thermal gas-density hole acts as a negative lens, as seen in our earlier experiments [23].…”

“…Taking η ¼ 0.02, p ¼ 1 atm, and α ¼ 2 × 10 −8 cm −1 [12] gives P g Δt=A < ∼1.5 × 10 5 J=cm 2 as the energy flux limit for thermal blooming. For example, for a 1.5-mm diameter airwaveguide core formed from an azimuthal array of filaments [26], the limiting energy is P g Δt ∼ 2.7 kJ. Note that we use a conservative value for α at λ ∼1 μm that includes both molecular and aerosol absorption for maritime environments [12], which contain significantly higher aerosol concentrations than dry air.…”

“…While the acoustic superposition guide is a promising approach, future experiments will need filaments generated by very well-balanced multilobe beam profiles, an example of which is seen in Ref. [26].…”

From Gyroscopic to Thermal Motion: A Crossover in the Dynamics of Molecular Superrotors

“…1 circular interference fringes, corresponds to the position of the centrifuge beam. Because of the centimeter-long confocal parameter, the fringe pattern stems from the diffraction of probe pulses inside the long density depression channel and does not reflect the true distribution of light intensity (e.g., wave guiding) or refractive index [29]. Yet, owing to the linearity of the weak probe propagation and its extended length, the fringe contrast serves as a sensitive indicator of the centrifuge-induced changes in the refractive index of the gas, Δn.…”

Sound emission from the gas of molecular superrotors

“…The exact power is not universal and typically varied between 4 and 5 in our experiments with nitrogen, and between 3 and 4 with oxygen. According to the theory of laserinduced pressure waves in dense media [22], as well as numerical simulations of hydrodynamic expansion [7], the sound wave amplitude is expected to scale linearly with the amount of laser energy deposited in the sample. In the case of an impulsive rotational excitation, most of the energy is transferred via a single two-photon Raman transition, suggesting a quadratic dependence on the pulse energy [4], which has been recently verified experimentally [6,11].…”