Generation of elliptically polarized high-order harmonics using few-cycle pulses with one or two carrier frequencies

Abstract

Using the strong-field approximation, we study the generation of elliptically polarized high-order harmonics driven by few-cycle laser pulses with one or two carrier frequencies. In the few-cycle regime, sub-cycle electron dynamics strongly modifies the harmonic spectra and their polarization, leading to pronounced dependence on the pulse duration, carrier-envelope phase (CEP), and driving-field polarization. For monochromatic pulses, elliptically polarized harmonics are produced by elliptically polarized field. High-harmonic yield for elliptically polarized laser field rapidly decreases for atomic targets. For molecular targets harmonic ellipticity can be substantial even for linearly polarized driving pulses. Notably, the harmonic ellipticity displays abrupt changes for molecular orientations associated with destructive interference minima, providing robust, CEP-insensitive structural signatures. For bichromatic pulses, we demonstrate enhanced control of spectral and polarization properties: linearly polarized \(\omega \)–\(2\omega \) field generates highly CEP-dependent multi-plateau spectra, whereas counterrotating elliptically polarized fields yield harmonics with significant ellipticity and moderate intensity. Our results establish few-cycle and two-carrier-frequency driving schemes as versatile tools for polarization control and polarization-resolved high-harmonic spectroscopy.