High-order harmonic generation and Fano resonances

TL;DR

This paper develops a theory for high harmonic generation near autoionizing resonances, revealing Fano-like line shapes and phase variations, with implications for attosecond pulse production.

Contribution

It generalizes the strong-field approximation to include resonant effects and predicts phase behavior of resonant harmonics, aligning with recent experimental data.

Findings

Resonant harmonic line shape is a product of Fano factor and non-resonant harmonic.

Harmonic phase varies rapidly near the resonance.

Theory supports phase-locking for attosecond pulse generation.

Abstract

We present a high harmonic generation theory which generalizes the strong-field approximation to the resonant case, when the harmonic frequency is close to that of the transition from the ground to an autoionizing state of the generating system. We show that the line shape of the resonant harmonic is a product of the Fano-like factor and the harmonic line which would be emitted in the absence of the resonance. The theory predicts rapid variation of the harmonic phase in the vicinity of the resonance. The calculated resonant harmonic phase is in reasonable agreement with recent measurements. Predicting the phase-locking of a group of resonantly-enhanced harmonics, our theory allows to study the perspectives of producing attosecond pulse train using such harmonics.