Principal frequency of an ultrashort laser pulse

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 a 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 independent of the carrier envelope phase. This is confirmed by solving the time-dependent Schrodinger 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 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.