High Power Laser Science and Engineering, (2019), Vol. 7, e36, 9 pages.
© The Author(s) 2019. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/
licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
doi:10.1017/hpl.2019.20
Enhancement of the surface emission at the
fundamental frequency and the transmitted
high-order harmonics by pre-structured targets
K. Q. Pan
1
, D. Yang
1
, L. Guo
1
, Z. C. Li
1
, S. W. Li
1
, C. Y. Zheng
2,3
, S. E. Jiang
1
, B. H. Zhang
1
,
and X. T. He
2,3
1
Laser Fusion Research Center, China Academy of Engineering Physics, Mianyang 621900, China
2
Center for Applied Physics and Technology, Peking University, Beijing 100871, China
3
Institute of Applied Physics and Computational Mathematics, Beijing 100088, China
(Received 22 November 2018; revised 27 March 2019; accepted 22 April 2019)
Abstract
Laser interaction with an ultra-thin pre-structured target is investigated with the help of both two-dimensional and three-
dimensional particle-in-cell simulations. With the existence of a periodic structure on the target surface, the laser seems to
penetrate through the target at its fundamental frequency even if the plasma density of the target is much higher than the
laser’s relativistically critical density. The particle-in-cell simulations show that the transmitted laser energy behind the
pre-structured target is increased by about two orders of magnitude compared to that behind the flat target. Theoretical
analyses show that the transmitted energy behind the pre-structured target is actually re-emitted by electron ‘islands’
formed by the surface plasma waves on the target surfaces. In other words, the radiation with the fundamental frequency
is actually ‘surface emission’ on the target rear surface. Besides the intensity of the component with the fundamental
frequency, the intensity of the high-order harmonics behind the pre-structured target is also much enhanced compared
to that behind the flat target. The enhancement of the high-order harmonics is also related to the surface plasma waves
generated on the target surfaces.
Keywords: high-order harmonics generation; high power laser; laser–plasma interaction; particle-in-cell simulation
1. Introduction
Laser–solid interactions have been widely investigated
in recent decades because of their wide applications,
such as charged particle acceleration
[1–5]
and radiation
generation
[6–14]
. For example, the vacuum heating mech-
anism and J × B heating mechanism
[2]
can be used to
accelerate electrons. Also, the radiation pressure acceleration
(RPA) mechanism
[15–18]
or the breakout afterburner (BOA)
mechanism
[19–22]
can be used to efficiently accelerate
protons or even heavy ions. It is shown that the most efficient
target thickness for RPA or BOA is l
0
= a
0
λ
0
n
c
/2πn
e
,
where a
0
= eE
0
/m
e
ω
0
c is the normalized vector potential,
m
e
and e are the electron mass and charge, λ
0
and ω
0
are
the laser wavelength and frequency, n
c
= m
e
ω
2
0
/4πe
2
is the
critical density and n
e
is the electron density of the solid. For
Correspondence to: S. E. Jiang, Laser Fusion Research Center,
Mianyang 621900, China, Email: jiangshn@vip.sina.com; X. T. He, Center
for Applied Physics and Technology, Peking University, Beijing 100871,
China, Email: xthe@iapcm.ac.cn
example, for a
0
= 5 and n
e
= 50n
c
, the optimal thickness is
only about 0.016λ
0
, and so thin a target cannot absorb much
of the laser energy. As to the radiation generation by laser–
solid interactions, synchrotron radiation
[6–8]
and high-order
harmonic generation
[23–25]
are always useful mechanisms.
Synchrotron radiation generated by laser–solid interactions
may have a high energy conversion efficiency. Some
previous works have reported that even more than 40% of
the laser energy could be converted into radiation energy
[26]
.
However, such a high energy conversion efficiency needs a
super-intense laser, which is not achievable in the laboratory
at present. When the laser intensity is low, the energy
conversion efficiency is also low. As to the high-order
harmonics generated by laser–solid interactions, it is known
that the laser can only penetrate to the skin depth, so only a
few electrons will participate in the radiation process and the
energy conversion efficiency from the laser to the radiation is
also very low. Researchers have made efforts to improve the
energy conversion efficiency of the laser–solid interactions.
Fortunately, using a periodic structure on the target surface
1
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