Chirp-control of resonant high-order harmonic generation in indium ablation plumes driven by intense few-cycle laser pulses.

Z Abdelrahman,T Witting,J P Marangos,M A Khokhlova,J W G Tisch,A Zair,V V Strelkov,D J Walke

doi:10.1364/oe.26.015745

Abstract

We have studied high-order harmonic generation (HHG) in an indium ablation plume driven by intense few-cycle laser pulses centered at 775 nm as a function of the frequency chirp of the laser pulse. We found experimentally that resonant emission lines between 19.7 eV and 22.3 eV (close to the 13th and 15th harmonic of the laser) exhibit a strong, asymmetric chirp dependence, with pronounced intensity modulations. The chirp dependence is reproduced by our numerical time-dependent Schrödinger equation simulations of a resonant HHG by the model indium ion. As demonstrated with our separate simulations of HHG within the strong field approximation, the resonance can be understood in terms of the chirp-dependent HHG photon energy coinciding with the energy of an autoionizing state to ground state transition with high oscillator strength. This supports the validity of the general theory of resonant four-step HHG in the few-cycle limit.

Highlights

High-order harmonic generation (HHG) in ablation plumes has been studied extensively in recent years [1,2,3]
We have studied high-order harmonic generation (HHG) in an indium ablation plume driven by intense few-cycle laser pulses centered at 775 nm as a function of the frequency chirp of the laser pulse
We have experimentally studied the effect of laser chirp on VUV resonant high harmonics generated in an indium ablation plume by few-cycle pulses with a center wavelength of 775 nm

Summary

Introduction

High-order harmonic generation (HHG) in ablation plumes has been studied extensively in recent years [1,2,3]. In this process, coherent [4] short-wavelength radiation is generated through the interaction of an intense femtosecond laser pulse with matter ablated from a surface by a separate laser pulse. A interesting feature of this process is the observation of resonantlyenhanced coherent radiation which we shall refer to as “resonant harmonics” These resonant harmonics can be enhanced significantly (by up to 2 order of magnitude [1, 2]) compared to neighbouring non-resonant ones. This increased harmonic flux is of potential importance for applications of the radiation, such as coherent diffractive imaging [5] and attosecond pulse generation [6,7,8,9,10]

Methods

Results

Conclusion