Principal frequency of an ultrashort laser pulse

Abstract

We introduce an alternative definition of the main frequency of an ultrashort laser pulse---the principal frequency ${\ensuremath{\omega}}_{P}$. This parameter is complementary to the most accepted and widely used carrier frequency ${\ensuremath{\omega}}_{0}$. Given the fact that these ultrashort pulses, also known as transients, have a temporal width comprising only a few cycles of the carrier wave, corresponding to a spectral bandwidth $\mathrm{\ensuremath{\Delta}}\ensuremath{\omega}$ covering several octaves, ${\ensuremath{\omega}}_{P}$ describes, in a more precise way, the dynamics driven by these sources. We present examples where, for instance, ${\ensuremath{\omega}}_{P}$ is able to correctly predict the high-order harmonic cutoff independent 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 subcycle electron dynamics is better described using ${\ensuremath{\omega}}_{P}$. The concept of ${\ensuremath{\omega}}_{P}$, however, can be applied to a large variety of scenarios, not only within the strong-field physics domain.