Particle-in-cell simulations of electron acceleration from relativistic interaction of mid-infrared laser interactions with near solid density matter

Gregory K Ngirmang,John T Morrison,W M Roquemore,Scott Feister,Enam A Chowdhury,Chris Orban

doi:10.1063/1.4995669

Abstract

Advances in ultra-intense laser technology are enabling, for the first time, relativistic intensities at mid-infrared (mid-IR) wavelengths. Anticipating further experimental research in this domain, we present high-resolution two dimensional Particle-in-Cell (PIC) simulation results using the Large-Scale Plasma (LSP) code that explores intense mid-IR laser interactions with near solid density targets. We present the results of thirty PIC simulations over a wide range of intensities ( 0.03<a0<40) and wavelengths ( λ= 780 nm, 3 μm, and 10 μm). Earlier studies [Orban et al., Phys. Plasmas 22, 023110 (2015) and Ngirmang et al., Phys. Plasmas 23, 043111 (2016)], limited to λ= 780 nm and a0∼1, identified super-ponderomotive electron acceleration in the laser specular direction for normal-incidence laser interactions with dense targets. We extend this research to mid-IR wavelengths and find a more general result that normal-incidence super-ponderomotive electron acceleration occurs provided that the laser intensity is not highly relativistic ( a0≲1) and that the pre-plasma scale length is similar to or longer than the laser wavelength. Under these conditions, ejected electron angular and energy distributions are similar to expectations from an analytic model used in Ngirmang et al. [Phys. Plasmas 23, 043111 (2016)]. We also find that, for a0∼1, the mid-IR simulations exhibit a classic ponderomotive steepening pattern with multiple peaks in the ion and electron density distribution. Experimental validation of this basic laser-plasma interaction process should be possible in the near future using mid-IR laser technology and optical interferometry.

Highlights

Anticipating further experimental research in this domain, we present high-resolution two dimensional Particle-in-Cell (PIC) simulation results using the LargeScale Plasma (LSP) code that explores intense mid-IR laser interactions with near solid density targets
We extend this research to mid-IR wavelengths and find a more general result that normal-incidence super-ponderomotive electron acceleration occurs provided that the laser intensity is not highly relativistic (a0Շ1) and that the pre-plasma scale length is similar to or longer than the laser wavelength
In anticipation of future experiments utilizing ultraintense, mid-infrared laser pulses and their interaction with dense targets, we used LSP 2D(3v) simulations to explore these interactions over a range of intensities and wavelengths

Summary

Introduction

While advances in laser technology have allowed ultraintense laser interactions at near-infrared (near-IR) wavelengths to be thoroughly explored, it is only more recently that ultra-intense laser interactions at mid-IR wavelengths have become experimentally possible. A variety of groups are beginning to examine what may be learned from experiments at these wavelengths and how phenomena observed in the near-IR may scale to longer wavelengths. Some of this interest stems from the existence of atmospheric “windows” in the mid-IR, while other groups consider how the longer length scale of mid-IR interactions and lower critical density [defined in Eq (1)] of such lasers allows subtle phenomena to be more probed, and recently, mid-IR laser filaments have been demonstrated in air. Another interesting value of intense mid-IR interactions is in examining the physics of laser damage.9,10To the best of our knowledge, despite recent interest in mid-IR ultra-intense laser interactions, the literature has not focused much attention on short-pulse, intense mid-IR laser interactions with dense (i.e., solid or liquid density) targets.These interactions are interesting for a variety of reasons, among them is the favorable scaling of the pondpeffirffioffiffiffimffi otive electron energy with laser wavelength (a0 / Ik2 / k). A variety of groups are beginning to examine what may be learned from experiments at these wavelengths and how phenomena observed in the near-IR may scale to longer wavelengths.4 Some of this interest stems from the existence of atmospheric “windows” in the mid-IR, while other groups consider how the longer length scale of mid-IR interactions and lower critical density [defined in Eq (1)] of such lasers allows subtle phenomena to be more probed, and recently, mid-IR laser filaments have been demonstrated in air.. Some of this interest stems from the existence of atmospheric “windows” in the mid-IR, while other groups consider how the longer length scale of mid-IR interactions and lower critical density [defined in Eq (1)] of such lasers allows subtle phenomena to be more probed, and recently, mid-IR laser filaments have been demonstrated in air.8 Another interesting value of intense mid-IR interactions is in examining the physics of laser damage.. With experimental capabilities still growing in the mid-IR, the present work offers a simulation survey of energetic electron ejection from mid-IR laser irradiated dense targets and may guide future experiments

Results

Discussion

Conclusion