Chiral broadband High Harmonic Generation Source by Vectorial Time-Polarization-Gating
Chiral (highly helical) extreme ultraviolet (XUV) sources are pivotal for investigating chiroptical phenomena on the ultrafast electronic timescale. Table-top, coherent High Harmonic Generation (HHG)-based sources are particularly well-suited for these studies. However, chiral materials, such as organic chiral molecules and solid-state magnetic materials, exhibit fine spectral features which necessitate broadband radiation for their complete interrogation. The generation of radiation that is both broadband and helical through HHG presents a seemingly paradoxical challenge: while chiral HHG emission requires at least two recollisions occurring along different directions in the polarization plane, the Floquet limit might already be reached with as few as three recollisions, resulting in a sparse spectrum characterized by pronounced discrete harmonic peaks. Here we propose a straightforward scheme that enables the interrogation of fine spectral features, in principle restricted only by the resolution of the XUV spectrometer, with chiral XUV light. Our method is based on using a vectorial two-color driver with close central-frequencies with slight symmetry breaking. It integrates the time-gating and polarization-gating techniques to generate a vectorial driver which induces well-controlled bursts of recollisions, occurring along different directions in the polarization plane. The method satisfies the dual requirements of an XUV source which is both broadband and helical. We perform polarization scan and demonstrate that the broadband XUV radiation exhibits rapid modulations in its spectral ellipticity, and fast alternation in its spectral helicities. The phase of modulations could be controlled by introducing a slight symmetry breaking. This allows us to control and modulate the XUV polarization state, which should enable the detection of chiroptical signals with enhanced sensitivity.