Energetic, tunable, highly elliptically polarized higher harmonics generated by intense two-color counter-rotating laser fields

Abstract

In this work, we demonstrate experimentally the efficient generation and tunability of energetic highly elliptical high harmonics in Ar gas, driven by intense two-color counter-rotating laser electric fields. A bichromatic beam tailored by a Mach-Zehnder-Less for Threefold Optical Virginia spiderwort (MAZEL-TOV) apparatus generates high-order harmonic generation (HHG), where the output spectrum of the highly elliptical HHG radiation can be tuned for an energy range of $\mathrm{\ensuremath{\Delta}}E\ensuremath{\approx}150$ meV in the spectral range of $\ensuremath{\sim}20$ eV with energy per pulse ${E}^{\mathrm{XUV}}\ensuremath{\approx}400$ nJ at the source. Furthermore, we employ time-dependent density-functional simulations to probe the dependence of the harmonic ellipticity and the strength of the attosecond pulses on the driving-field parameters and demonstrate the robustness of the HHG with the bichromatic field. We show how, by properly tuning the central frequency of the second harmonic, the central frequency of the extreme ultraviolet (XUV) high-harmonic radiation is continuously tuned. The demonstrated energy values largely exceed the output energy from many other laser-driven attosecond sources reported so far and prove to be sufficient for inducing nonlinear processes in an atomic system. We envisage that such tunable energetic highly elliptical HHG spectra can remove the facility restrictions from requirements of few-cycle driving pulses for isolated circular attosecond-pulse generation.