Spatio-temporal coupling of attosecond pulses

TL;DR

This paper investigates the complex interplay between spatial and temporal properties of broadband attosecond pulses, revealing chromatic aberrations that impact their focusing and application potential.

Contribution

It introduces a simple analytical Gaussian optics model and experimental measurements to analyze the spatio-temporal coupling in attosecond pulses, highlighting effects often neglected.

Findings

Harmonic divergence depends strongly on frequency.

Chromatic aberrations significantly affect pulse focusing.

Spatio-temporal coupling must be considered in applications.

Abstract

The shortest light pulses produced to date are of the order of a few tens of attoseconds, with central frequencies in the extreme ultraviolet range and bandwidths exceeding tens of eV. They are often produced as a train of pulses separated by half the driving laser period, leading in the frequency domain to a spectrum of high, odd-order harmonics. As light pulses become shorter and more spectrally wide, the widely-used approximation consisting in writing the optical waveform as a product of temporal and spatial amplitudes does not apply anymore. Here, we investigate the interplay of temporal and spatial properties of attosecond pulses. We show that the divergence and focus position of the generated harmonics often strongly depend on their frequency, leading to strong chromatic aberrations of the broadband attosecond pulses. Our argumentation uses a simple analytical model based on Gaussian optics, numerical propagation calculations and experimental harmonic divergence measurements. This effect needs to be considered for future applications requiring high quality focusing while retaining the broadband/ultrashort characteristics of the radiation.