Optical Measurement of Photorecombination Time Delays

Measuring the photoionization time delay between electrons from different orbitals is one of the most important accomplishments of attosecond science. These measurements are typically done using attosecond pulses to photoionize a target inside a photoelectron spectrometer. In such experiments, the measured delay corresponds to the superposition of all possible paths to ionization and can include multiple sources of delay. These effects can be difficult to deconvolve. Here, we exploit the collision physics nature of recollision and show that, by perturbing recollision dynamics, photorecombination time delays due to electron dynamics and structure can be measured entirely optically and without obfuscation from molecular structure and propagation effects. While we concentrate on photorecombination delays in argon around the Cooper minimum our approach is general. Therefore, our work holds the potential to fundamentally change how attosecond measurement is performed and paves the way for the entirely optical measurement of ultrafast electron dynamics and photorecombination delays due to electronic structure, multielectron interaction, and strong-field driven dynamics in complex molecular systems and correlated solid-state systems.

Characterizing multielectron dynamics during recollision

Measuring the delay for an electron to emerge from different states is one of the major achievements of attosecond science. This delay can have two origins - the electron wave packet is reshaped during departure by the electrostatic field of the ionizing medium or it is modified by dynamic interaction with the remaining electrons. Most experiments have observed the former, but confirmation requires a complex calculation. A direct measurement of multielectron dynamics is needed. Photo-recombination - the inverse of photoionization - occurs naturally during electron recollision and can be measured by combining a perturbing beam to modify the recollision electron before recombination. These in situ methods allow us to unambiguously isolate multielectron dynamics - the reference being the spectral phase of an attosecond pulse simultaneously measured in spectral regions without multielectron interaction. Here, we measure the group delay of the recollision electron caused by plasmonic resonance dynamics in Xe, simulate the in situ measured spectral phase of a recollision electron generated in the presence of the plasmonic resonance in C$_{60}$ and present a corresponding semi-classical theory based on the strong-field approximation. Our results suggest that in situ techniques, together with 300 eV recollision electrons, will allow the ultimate time response of electronic matter to be measured.

Near-field imaging of dipole emission modulated by an optical grating

Multiphoton-ionized electrons are born into a strong light field that will determine their short-term future. By controlling the infrared beam, we enable atoms or molecules to generate extreme ultraviolet (XUV) pulses and synthesize attosecond pulses - the shortest controlled events ever produced. Here we show that a weak obliquely incident beam imposes an optical grating on the fundamental beam, resulting in a spatially modulated attosecond pulse. We observe the modulation on a spectrally resolved near-field XUV image, encoding all information of the spectral phase of the recollision electron and, therefore, the attosecond pulse produced by structureless atoms. Near-field imaging is an efficient method for measuring the duration of attosecond pulses, especially important for soft X-ray pulses created in helium. For more complex systems, it includes auto ionization and giant plasmon resonances.

Characterizing Fano Resonances During Recollision

When intense light irradiates a quantum system, an ionizing electron recollides with its parent ion within the same light cycle and, during that very brief (few femtosecond) encounter, its kinetic energy sweeps from low to high energy and back. Therefore, recollision offers unprecedented time resolution and it is the foundation on which attosecond science is built. For simple systems, recolliding trajectories are shaped by the strong field acting together with the Coulomb potential and they can be readily calculated and measured. However, for more complex systems, multielectron effects are also important because they dynamically alter the recolliding wave packet trajectories. Here, we theoretically study Fano resonances, one of the most accessible multielectron effects, and we show how multielectron dynamics can be unambiguously isolated when we use in situ measurement. The general class of in situ measurement can provide key information needed for time-dependent ab initio electronic structure theory and will allow us to measure the ultimate time response of matter.

Control of $text{N}_2^+$ Air Lasing

A near-infrared laser generates gain on transitions between the $text{B}^{text{2}} Sigma_{text{u}}^{text{+}}$ and $text{X}^{text{2}} Sigma_{text{g}}^{text{+}}$ states of the nitrogen molecular cation in part by coupling the $text{X}^{text{2}} Sigma_{text{g}}^{text{+}}$ and $text{A}^{text{2}} Pi_{text{u}}$ states in the V-system. Traditional time resolved pump-probe measurements rely on post-ionization coupling by the pump pulse to initialize dynamics in the $text{A}^{text{2}} Pi_{text{u}}$ state. Here we show that a weak second excitation pulse reduces ambiguity because it acts only on the ion independent of ionization. The additional control pulse can increase gain by moving population to the $text{A}^{text{2}} Pi_{text{u}}$ state, which modifies the lasing emission in two distinct ways. The presence of fast decoherence on $text{X}^{text{2}} Sigma_{text{g}}^{text{+}}$ to $text{A}^{text{2}} Pi_{text{u}}$ transitions may prevent the formation of a coherent rotational wave packet in the ground state in our experiment, but the control pulse can reverse impulsive alignment by the pump pulse to remove rotational wave packets in the $text{B}^{text{2}} Sigma_{text{u}}^{text{+}}$ state.