High Power Laser Science and Engineering, (2014), Vol. 2, e32, 7 pages.
© Author(s) 2014. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/
licences/by/3.0), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
doi:10.1017/hpl.2014.32
High harmonic generation and ionization effects in
cluster targets
M. Aladi, I. M
´
arton, P. R
´
acz, P. Dombi, and I.B. F
¨
oldes
Wigner Research Centre for Physics of the Hungarian Academy of Sciences, Association EURATOM HAS,
H-1121 Budapest, Konkoly-Thege u. 29-33, Hungary
(Received 19 March 2014; revised 4 July 2014; accepted 4 August 2014)
Abstract
High harmonic generation in gas jets was investigated in different gases up to more than 14 bar backing pressure. The
observation of increase of harmonic intensity with increasing pressure and laser intensity shows evidence of the presence
of clusters in Xe with an increased efficiency compared with He, whereas Ar is an intermediate case for which clusters
will start to dominate above a certain backing pressure. Spectral investigations give evidence for tunable harmonic
generation in a broad spectral range. A spectral shift of opposite signature caused by the free electrons in the focal
volume and the nanoplasmas inside the cluster was observed.
Keywords: clusters; high harmonic generation; nanoplasmas; ultrashort pulses
1. Introduction
The most flexible tool for generating coherent extreme
ultraviolet (EUV) radiation by an intense laser pulse is
the generation of its high harmonics. The harmonics of
ultrashort laser pulses in gases cover practically the full
spectral range from the visible to kilo-electronvolt x-ray. In
the interaction of an intense laser pulse with a gas consisting
of atoms, molecules or – in the present case – clusters,
optical ionization may occur as a result of the distortion of
the Coulomb potential by the intense electric field
[1, 2]
. The
freed electron is driven by the laser electric field, and may
recombine with its parent ion, emitting the excess energy
in the form of a high energy photon. As this process is
repeated every half-cycle, the temporal periodicity of the
process leads to the appearance of discrete spectral lines at
harmonics of the laser frequency. This half-cycle periodicity
is the reason why in most cases odd harmonics are generated
in gases up to a limit defined by the laser intensity and the
ionization potential of the gas, giving an upper limit for the
generated photon energies. A great deal of effort has been
expended towards increase of the conversion efficiency of
harmonics for better usability. High harmonic generation
(HHG) from clusters seems to be a possible candidate for
an efficient light source giving higher emission frequencies
and higher conversion efficiency
[3, 4]
.
Although the higher conversion efficiency and higher
observable harmonics are advantageous, the investigation
Correspondence to: I. B. F
¨
oldes, Wigner Research Centre for Physics,
H-1525 Budapest, POB 49, Hungary. Email: foldes.istvan@wigner.mta.hu
of HHG from clusters is not strongly prevalent because of
two main reasons. The first reason is that although the
conversion can be higher than in atomic gases, the ionization
of clusters sets an upper limit for the obtainable intensity,
similarly to that of atoms. Therefore, most of the recent
efforts aim to use loose focusing
[5–7]
and consequently a
long homogeneous target material for harmonic genera-
tion. Clearly, it is not easy to realize an elongated cluster
source. The other obstacle for a broader application is
that the mechanism of HHG from clusters is still a subject
of intense debate
[8]
. Apart from the traditional three-step
model which is based on the recombination to the same
atom, models have been developed in which recombination
is considered to the neighbouring atoms
[9, 10]
, which may
even produce incoherent radiation as there will be no phase
locking between the two atomic wavefunctions
[11]
. Ruf
et al.
[8]
suggests an alternative with tunnel ionization from a
partly delocalized electron wavefunction and recombination
to this wavefunction, i.e., to the cluster itself. In experiments
the difficulty of separation of harmonics from the monomer
atoms and from clusters makes it difficult to determine the
actual mechanism of harmonic generation.
In the present work we aim to progress towards the
ionization limit, i.e., we are investigating the limit where
the ionization sets in with the signature of free electrons
in the free space and inside the cluster as well. HHG
is investigated in different gases, namely in He which is
purely atomic, in Xe which generally forms clusters, and
in transitory Ar where significant cluster generation sets in
1
