High order harmonics from relativistic electron spikes

Takashi Imazono,Hajime Kotaki ,Yuji Fukuda ,T Zh Esirkepov ,P S Foster ,Toyoaki Kimura,Takashi Kameshima,P P Rajeev ,Hajime Okada,S V Bulanov ,I Daito ,M Borghesi ,A Ya Faenov ,P Mckenna ,M Kando ,M Mori ,James Green ,T A Pikuz ,N Nishimori ,K Kawase ,Hiroyuki Daido,Kiminori Kondo,Hiromitsu Kiriyama,M Coury ,Yoshiaki Kato,D R Symes ,Yukio Hayashi,C M Brenner ,A Sagisaka ,James Koga ,Tetsuya Kawachi,Alexander S Pirozhkov ,T Tajima ,H Ahmed ,D Neely ,Koichi Ogura,B Dromey ,Е Н Рагозин ,Liming Chen ,P Gallegos

doi:10.1088/1367-2630/16/9/093003

Abstract

A new regime of relativistic high-order harmonic generation has been discovered (Pirozhkov 2012 Phys. Rev. Lett. 108 135004). Multi-terawatt relativistic-irradiance (>1018 W cm−2) femtosecond (∼30–50 fs) lasers focused to underdense (few × 1019 cm−3) plasma formed in gas jet targets produce comb-like spectra with hundreds of even and odd harmonic orders reaching the photon energy of 360 eV, including the ‘water window’ spectral range. Harmonics are generated either by linearly or circularly polarized pulses from the J-KAREN (KPSI, JAEA) and Astra Gemini (CLF, RAL, UK) lasers. The photon number scalability has been demonstrated with a 120 TW laser, producing 40 μJ sr−1 per harmonic at 120 eV. The experimental results are explained using particle-in-cell simulations and catastrophe theory. A new mechanism of harmonic generation by sharp, structurally stable, oscillating electron spikes at the joint of the boundaries of the wake and bow waves excited by a laser pulse is introduced. In this paper, detailed descriptions of the experiments, simulations and model are provided and new features are shown, including data obtained with a two-channel spectrograph, harmonic generation by circularly polarized laser pulses and angular distribution.

Highlights

High-order harmonic generation is one of the most fundamental processes of nonlinear optics
[26] we discovered a new high-order harmonic generation regime in relativistic laser plasma, explained by a new mechanism based on catastrophe theory [27, 28]
In this paper we describe new features of the high-order harmonic generation regime in underdense relativistic laser plasma, first described in [26], including data obtained with a twochannel spectrograph, harmonic generation by circularly polarized laser pulses, spatial coherence and angular distribution

Summary

Introduction

High-order harmonic generation is one of the most fundamental processes of nonlinear optics. Numerous nonlinearities produce high-order harmonics in various laser–matter interaction regimes. At irradiances of the order of 1018 W cm−2 and higher, corresponding to relativistic plasma dynamics [1, 2], several regimes of high order harmonics generation are known, including laser–solid target interaction [3,4,5,6,7,8], harmonics generation in a plasma channel [9], and the second harmonic from electro-optic shocks [10, 11]. Nonlinear Thomson scattering [12] by plasma electrons [13,14,15,16,17] and electron beams [18, 19] produces harmonic spectra.

Methods

Results

Discussion

Conclusion