Long range propagation of ultrafast, ionizing laser pulses in a resonant nonlinear medium

TL;DR

This study investigates how ultrafast, high-power laser pulses propagate through rubidium vapor, revealing nonlinear effects like self-focusing and plasma channel formation, with experimental results supported by computer simulations.

Contribution

First experimental demonstration of long-range propagation of ultrafast, ionizing laser pulses in resonant rubidium vapor with a validated propagation model.

Findings

Observation of laser pulse absorption at low energy

Transverse confinement of laser beam at higher energies

Beam broadening due to medium ionization

Abstract

We study the propagation of 0.05-1 TW power, ultrafast laser pulses in a 10 meter long rubidium vapor cell. The central wavelength of the laser is resonant with the $D_2$ line of rubidium and the peak intensity in the $10^{12}-10^{14} ~W/cm^2$ range, enough to create a plasma channel with single electron ionization. We observe the absorption of the laser pulse for low energy, a regime of transverse confinement of the laser beam by the strong resonant nonlinearity for higher energies and the transverse broadening of the output beam when the nonlinearity is saturated due to full medium ionization. We compare experimental observations of transmitted pulse energy and transverse fluence profile with the results of computer simulations modeling pulse propagation. We find a qualitative agreement between theory and experiment that corroborates the validity of our propagation model. While the quantitative differences are substantial, the results show that the model can be used to interpret the observed phenomena in terms of self-focusing and channeling of the laser pulses by the saturable nonlinearity and the transparency of the fully ionized medium along the propagation axis.