Attosecond metrology sensitive to sub-optical-cycle electronic and structural dynamics is opening up new avenues for ultrafast spectroscopy of condensed matter. Using intense lightwaves to precisely control the fast carrier dynamics in crystals holds great promise for next-generation petahertz electronics and devices. The carrier dynamics can produce high-order harmonics of the driving field extending up into the extreme-ultraviolet region. Here, we introduce polarization-state-resolved high-harmonic spectroscopy of solids, which provides deeper insights into both electronic and structural sub-cycle dynamics. Performing high-harmonic generation measurements from silicon and quartz, we demonstrate that the polarization states of the harmonics are not only determined by crystal symmetries, but can be dynamically controlled, as a consequence of the intertwined interband and intraband electronic dynamics. We exploit this symmetry-dynamics duality to efficiently generate coherent circularly polarized harmonics from elliptically polarized pulses. Our experimental results are supported by ab-initio simulations, providing evidence for the microscopic origin of the phenomenon.

Recent studies have demonstrated that the polarization states of high harmonics from solids can differ from those of the driving pulses. To gain insights on the microscopic origin of this behavior, we perform one-particle intraband-only calculations and reproduce some of the most striking observations. For instance, our calculations yield circularly polarized harmonics from elliptically polarized pulses that sensitively depend on the driving conditions. Furthermore, we perform experiments on ZnS and find partly similar characteristics as reported from silicon. Comparison to our intraband-only calculations shows reasonable qualitative agreement for a below-band-gap harmonic. We show that intraband dynamics predict depolarization effects for higher field strengths. For harmonics above the band gap, interband dynamics become important and the high-harmonic response to elliptical excitation looks systematically different. Our work proposes a method to distinguish between different high-harmonic generation mechanisms and it could pave the way to compact solid-state high-harmonic sources with controllable polarization states.

Femtosecond X-ray laser pulses synchronized with an optical laser were employed to investigate the reaction dynamics of the photooxidation of CO on the anatase TiO 2 (101) surface in real-time. Our time-resolved soft X-ray photoemission spectroscopy results provide evidence of ultrafast timescales and, coupled with theoretical calculations, clarify the mechanism of oxygen activation that is crucial to unraveling the underlying processes for a range of photocatalytic reactions relevant to air purification and selfcleaning surfaces. The reaction takes place between 1.2 -2.8 (± 0.2) ps after irradiation with an ultrashort laser pulse leading to the formation of CO 2 , prior to which no intermediate species were observed on a picosecond time scale. Our theoretical calculations predict that the presence of intragap unoccupied O 2 levels leads to the formation of a charge-transfer complex. This allows the reaction to be initiated following laser illumination at a photon energy of 1.6 eV (770 nm), taking place via a proposed mechanism involving the direct transfer of electrons from TiO 2 to physisorbed O 2 .