Terahertz radiation driven by two-color laser pulses at near-relativistic intensities: Competition between photoionization and wakefield effects.

X Davoine,P González De Alaiza Martínez,L Bergé,A Debayle,L Gremillet

doi:10.1038/srep26743

X Davoine, P González De Alaiza Martínez + Show 3 more

Open Access

https://doi.org/10.1038/srep26743

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Abstract

We numerically investigate terahertz (THz) pulse generation by linearly-polarized, two-color femtosecond laser pulses in highly-ionized argon. Major processes consist of tunneling photoionization and ponderomotive forces associated with transverse and longitudinal field excitations. By means of two-dimensional particle-in-cell (PIC) simulations, we reveal the importance of photocurrent mechanisms besides transverse and longitudinal plasma waves for laser intensities >1015 W/cm2. We demonstrate the following. (i) With two-color pulses, photoionization prevails in the generation of GV/m THz fields up to 1017 W/cm2 laser intensities and suddenly loses efficiency near the relativistic threshold, as the outermost electron shell of ionized Ar atoms has been fully depleted. (ii) PIC results can be explained by a one-dimensional Maxwell-fluid model and its semi-analytical solutions, offering the first unified description of the main THz sources created in plasmas. (iii) The THz power emitted outside the plasma channel mostly originates from the transverse currents.

Highlights

Profiles produce a low-frequency current responsible for THz wave emission[36]
By increasing the laser intensity from 1015 to 3 × 1017 W/cm[2], we reveal the enhanced action of longitudinal plasma waves inside the plasma channel, as the ion charge number is increased in the interval 2 ≤Z* ≤ 8
As a matter of fact, we show that the magnetic field component parallel to the laser polarization axis does provide a reliable measurement of THz emissions by plasma ponderomotive forces, while the electric field polarized in the same direction mostly arises from photocurrent sources

Summary

Introduction

Profiles produce a low-frequency current responsible for THz wave emission[36]. The robustness of this so-called photocurrent scenario at intensities above 1015 W/cm[2] is, an open issue. Two-dimensional (2D) particle-in-cell (PIC) simulations display evidence that the photocurrent mechanism is mostly responsible for THz pulse generation at laser intensities above 1016 W/cm[2] and produces GV/m field strengths in argon. As a matter of fact, we show that the magnetic field component parallel to the laser polarization axis does provide a reliable measurement of THz emissions by plasma ponderomotive forces, while the electric field polarized in the same direction mostly arises from photocurrent sources

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