Ultrafast science, attosecond and femtosecond spectroscopies

Schematic of typical strong-field experimental configuration. Shows the finite interation volume where the target and focused laser pulse overlap, as well as the macroscopic- and microscopic-scale responses associated with different measurements.

High-harmonic generation and ionization spectroscopies

Because of their spatial and temporal coherence, laser pulses are a means of choice to manipulate and probe the electronic structure of matter at the scales atoms and molecules,  and they have become a keystone of ultrafast science over the past few decades. Both in University labs and large/mid-scale Facilities, experimental capabilities are now in the golden age of light sources, with ever shorter, stronger, and better-controlled pulses. Here we show a sketch of typical ultrafast science experiments: (background frame) A very short and intense laser pulse is focused onto an atomic or molecular target. (foreground frame) The strong-laser-matter interaction can result in single or multiple ionization of the target. These freed electrons can further interact and be accelerated by the laser, leading to a coherent build-up of broadband electromagnetic radiation called high-harmonic generation. In this framework, both ionization and high-harmonic generation are intrinsically coherent processes and can therefore be used to probe the target's electronic structure, eventually with sub-femtosecond resolution.

Related publications -- high-harmonic spectroscopy:

Related publications -- ionization spectroscopy:

Decorative -- reproduced from Journal of Physics B 49, 10LT01 (2016)

Circularly polarized high-harmonic generation with a bicircular laser field

The standard experimental configuration for high-harmonic generation (HHG) -- the nonlinear up-conversion process by which a gas of atoms or molecules irradiated by a strong laser pulse emits an electromagnetic radiation composed of integer multiples of the driving field -- consists of a linearly polarized source. Yet, with the progresses in laser technologies and advances in our understanding of such strong laser-matter interactions, it was soon realized that alternative polarizations/laser configurations could be used as means to probe and control the physical processes at play. In this perspective, here we investigate the question of high-harmonic generation with a bicircular laser pulse, i.e., composed of two circularly polarized color components. Applied to molecular systems, instead of the standard electric dipole selection rules, which cannot be invoked because of the absence of angular momentum conservation for the field-free dynamics, we show that spectral properties are connected to discrete dynamical symmetries. Compared with numerical simulations, we show that molecular systems offer a robust framework for circularly polarized harmonics, of which the orders and helicities (left or right polarized) are accurately predicted with our theory.

Picture from:  Journal of Physics B 49, 10LT01 (2016)

Related publications: