Chiral light-matter interactions have been investigated for two centuries, leading to the discovery of many chiroptical processes used for discrimination of enantiomers. Whereas most chiroptical effects result from a response of bound electrons, photoionization can produce much stronger chiral signals that manifest as asymmetries in the angular distribution of the photoelectrons along the light-propagation axis. We implemented self-referenced attosecond photoelectron interferometry to measure the temporal profile of the forward and backward electron wave packets emitted upon photoionization of camphor by circularly polarized laser pulses. We measured a delay between electrons ejected forward and backward, which depends on the ejection angle and reaches 24 attoseconds. The asymmetric temporal shape of electron wave packets emitted through an autoionizing state further reveals the chiral character of strongly correlated electronic dynamics.

Adiabatic evolution, an important dynamical process in a variety of classical and quantum systems providing a robust way of steering a system into a desired state , was introduced only recently to frequency conversion . Adiabatic frequency conversion allowed the achievement of efficient scalable broadband frequency conversion and was applied successfully to the conversion of ultrashort pulses, demonstrating near‐100% efficiency for ultrabroadband spectrum . The underlying analogy between undepleted pump nonlinear processes and coherently excited quantum systems was extended in the past few years to multi‐level quantum systems, demonstrating new concepts in frequency conversion, such as complete frequency conversion through an absorption band . Additionally, the undepleted pump restriction was removed, enabling the exploration of adiabatic processes in the fully nonlinear dynamics regime of nonlinear optics . In this article, the basic concept of adiabatic frequency conversion is introduced, and recent advances in ultrashort physics, multi‐process systems, and the fully nonlinear dynamics regime are reviewed.

Laser-driven high-order harmonic generation 1,2 (HHG) provides tabletop sources of broadband extreme-ultraviolet (XUV) light with excellent spatial 3 and temporal 4 coherence. These sources are typically operated at low repetition rates, f rep 100 kHz, where phase-matched frequency conversion into the XUV is readily achieved 5,6 . However, there are many applications that demand the improved counting statistics or frequency-comb precision afforded by operation at high repetition rates, f rep > 10 MHz. Unfortunately, at such high f rep , phase matching is prevented by the accumulated steady-state plasma in the generation volume 7-11 , setting stringent limitations on the XUV average power. Here, we use gas mixtures at high temperatures as the generation medium to increase the translational velocity of the gas, thereby reducing the steady-state plasma in the laser focus. This allows phase-matched XUV emission inside a femtosecond enhancement cavity at a repetition rate of 77 MHz, enabling a record generated power of ∼2 mW in a single harmonic order. This power scaling opens up many demanding applications, including XUV frequency-comb spectroscopy 12,13 of few-electron atoms and ions for precision tests of fundamental physical laws and constants 14-20 .The highly-nonlinear HHG process requires peak laser intensities around 10 14 W/cm 2 , which necessitates large laser pulse energies 10 µJ, and short pulse durations 100 fs, as typically reached with low repetition rate, chirped-pulse amplified 21 laser systems. However, high repetition rates are desirable for applications such as photoelectron spectroscopy 22-24 and microscopy 25 as well as electron-ion coincidence spectroscopy 26,27 , which are limited by counting detection or space-charge effects to few XUV ionization events per shot. Most notably, precision frequency-comb spectroscopy 12,13 requires f rep 10 MHz in order to stabilize the comb. Recent efforts allowed HHG to be directly driven at f rep 1 MHz, using either the direct output of a high-power oscillator 22,28 or the coherent combination of several fibre amplifiers 29,30 . Achieving the necessary intensity for HHG with f rep 10 MHz requires lasers with average power in the kW range. Apart from one demonstration at 20 MHz, where the measured XUV power was extremely low 31 , higher repetition rates up to 250 MHz 32 have been facilitated only by using passive enhancement cavities, which store ∼10 kW of laser power, where a gas jet is introduced at an intracavity focus 7,10-12,33-35 .In a macroscopic extended medium, efficient HHG requires matching the phase velocities of the generating laser and the generated fields. This can be achieved by balancing neutral and plasma dispersion, the geometric phase shift due to focusing (the Gouy phase), and the HHG intrinsic dipole phase 5,36 . Achieving this balance becomes increasingly challenging as the repetition rate increases above ∼10 MHz. The reason for this difficulty is that the plasma generated by one pulse does not have time to clear the focal volume before t...

Ultrafast strong-field physics provides insight into quantum phenomena that evolve on an attosecond time scale, the most fundamental of which is quantum tunneling. The tunneling process initiates a range of strong field phenomena such as high harmonic generation (HHG), laser-induced electron diffraction, double ionization and photoelectron holography—all evolving during a fraction of the optical cycle. Here we apply attosecond photoelectron holography as a method to resolve the temporal properties of the tunneling process. Adding a weak second harmonic (SH) field to a strong fundamental laser field enables us to reconstruct the ionization times of photoelectrons that play a role in the formation of a photoelectron hologram with attosecond precision. We decouple the contributions of the two arms of the hologram and resolve the subtle differences in their ionization times, separated by only a few tens of attoseconds.