Principal frequency of an ultrashort laser pulse

TL;DR

This paper introduces the principal frequency $\,\omega_P$ as a new, more accurate parameter for describing ultrashort laser pulses, especially in high-order harmonic generation, improving predictions of electron dynamics and spectral features.

Contribution

The paper proposes the principal frequency $\,\omega_P$ as an alternative to the carrier frequency, providing a better description of ultrashort pulse dynamics and high-order harmonic cutoff predictions.

Findings

$\,\omega_P$ accurately predicts high-order harmonic cutoffs.

$\,\omega_P$ describes sub-cycle electron dynamics more effectively.

The concept applies broadly beyond strong field physics.

Abstract

We introduce an alternative definition of the main frequency of an ultrashort laser pulse, the principal frequency $\omega_P$. This parameter is complementary to the most accepted and widely used carrier frequency $\omega_0$. Given the fact that these ultrashort pulses, also known as transients, have a temporal width comprising only few cycles of the carrier wave, corresponding to a spectral bandwidth $\Delta\omega$ covering several octaves, $\omega_P$ describes, in a more precise way, the dynamics driven by these sources. We present examples where, for instance, $\omega_P$ is able to correctly predict the high-order harmonic cutoff independently of the carrier envelope phase. This is confirmed by solving the time-dependent Schr\"odinger equation in reduced dimensions, supplemented with the time-analysis of the quantum spectra, where it is possible to observe how the sub-cycle electron dynamics is better described using $\omega_P$. The concept of $\omega_P$, however, can be applied to a large variety of scenarios, not only within the strong field physics domain.