Attosecond metrology with linearly polarized light pulses is the basis of a highly successful research area. An even broader impact can be expected from a generalized metrology that covers two-dimensional polarization states, enabling notably the study of chiroptical phenomena on the electronic time scale. Here, we introduce and demonstrate a comprehensive approach to the generation and complete characterization of elliptically to circularly polarized attosecond pulses. The generation relies on a simple plug-in device of unprecedented simplicity. For the characterization, we introduce SPARROW (Stokes-Parameter and Attosecond Resolved Reconstruction of Optical Waveforms), which encodes the attosecond-metrology information into the photoemission angle in the polarization plane and accesses all four Stokes parameters of the attosecond pulses. Our study demonstrates a physically transparent scheme for attosecond metrology with elliptical to fully circular polarizations, applicable to both table-top and accelerator-based light sources, which will unlock studies of chiral molecules, magnetic materials and novel chiroptical phenomena on the most fundamental time scales.
A Fano resonance arises from the pathway interference between discrete and continuum states, playing a fundamental role in many branches of physics, chemistry and material science. Here, we introduce the concept of a laserassisted Fano resonance, created from two interferometric pathways that are coupled together by an additional laser field, which introduces a controllable phase delay between them and results in a generalized Fano lineshape that can be actively controlled on the attosecond time scale. Based on our experimental results of unprecedented resolution, we dynamically image a resonant electron wave packet during its evolution directly in the time domain, extracting both the amplitude and the phase, which allows for the measurement of the resonant photoionization time delay. Ab-initio calculations and simulations employing a physically transparent two-level model agree with our experimental results, laying the groundwork for extending our concepts into attosecond quantum control of complex systems.
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