Two statistical regimes in the transition to filamentation

TL;DR

This paper experimentally studies the spectral fluctuations of ultrashort laser pulses near the filamentation threshold, revealing two distinct regimes that challenge the traditional notion of a clear filamentation boundary.

Contribution

It identifies two different spectral transition regimes in laser filamentation, providing new insights into the complex nature of the transition process.

Findings

Spectral broadening increases continuously at the spectrum center.

Edge spectra exhibit bimodal probability distributions during transition.

Dual behavior complicates defining a precise filamentation threshold.

Abstract

We experimentally investigate fluctuations in the spectrum of ultrashort laser pulses propagating in air, close to the critical power for filamentation. Increasing the laser peak power broadens the spectrum while the beam approaches the filamentation regime. We identify two regimes for this transition: In the center of the spectrum, the output spectral intensity increases continuously. In contrast, on the edges of the spectrum the transition implies a bimodal probability distribution function for intermediate incident pulse energies, where a high-intensity mode appears and grows at the expense of the original low-intensity mode. We argue that this dual behavior prevents the definition of a univoquial threshold for filamentation, shedding a new light on the long-standing lack of explicit definition of the boundary of the filamentation regime.