High-order harmonic generation from the dressed autoionizing states

M A Fareed,F Légaré,S Mondal,N Thiré,B E Schmidt,V V Strelkov,T Ozaki

doi:10.1038/ncomms16061

Abstract

In high-order harmonic generation, resonant harmonics (RH) are sources of intense, coherent extreme-ultraviolet radiation. However, intensity enhancement of RH only occurs for a single harmonic order, making it challenging to generate short attosecond pulses. Moreover, the mechanism involved behind such RH was circumstantial, because of the lack of direct experimental proofs. Here, we demonstrate the exact quantum paths that electron follows for RH generation using tin, showing that it involves not only the autoionizing state, but also a harmonic generation from dressed-AIS that appears as two coherent satellite harmonics at frequencies ±2Ω from the RH (Ω represents laser frequency). Our observations of harmonic emission from dressed states open the possibilities of generating intense and broadband attosecond pulses, thus contributing to future applications in attosecond science, as well as the perspective of studying the femtosecond and attosecond dynamics of autoionizing states.

Highlights

In high-order harmonic generation, resonant harmonics (RH) are sources of intense, coherent extreme-ultraviolet radiation
For some media, the intensity of a specific harmonic order could be enhanced by one to two orders of magnitude compared with the neighbouring harmonics, which are generated owing to conventional three-step process
In step 3, the electron accelerated in the continuum is trapped by the parent ion, so that the system lands in the autoionizing state (AIS), and in step 4 relaxes to the ground state by emitting the harmonic photon

Summary

Introduction

In high-order harmonic generation, resonant harmonics (RH) are sources of intense, coherent extreme-ultraviolet radiation. The present study shows that RH generation process involves the AIS for coherent harmonic emission at resonant energy via the microscopic response.

Results

Conclusion