Numerical study of the thermal ablation of wet solids by ultrashort laser pulses
Danny Perez,
1,
* Laurent Karim Béland,
1
Delphine Deryng,
1
Laurent J. Lewis,
1,†
and Michel Meunier
2,‡
1
Département de Physique et Regroupement Québécois sur les Matériaux de Pointe (RQMP), Université de Montréal, Case Postal 6128,
Succursale Centre-Ville, Montréal, Québec, Canada H3C 3J7
2
Laboratoire de Procédés par Laser, Département de Génie Physique et Regroupement Québécois sur les Matériaux de Pointe (RQMP),
École Polytechnique de Montréal, Case Postal 6079, Succursale Centre-Ville, Montréal, Québec, Canada H3C 3A7
Received 1 June 2007; revised manuscript received 26 October 2007; published 24 January 2008
The ablation by ultrashort laser pulses at relatively low fluences i.e., in the thermal regime of solids wetted
by a thin liquid film is studied using a generic numerical model. In comparison with dry targets, the liquid is
found to significantly affect ablation by confining the solid and slowing down the expansion of the laser-heated
material. These factors affect the relative efficiency of the various ablation mechanisms, leading, in particular,
to the complete inhibition of phase explosion at lower fluences, a reduced ablation yield, and significant
changes in the composition of the plume. As a consequence, at fluences above the ablation threshold, the size
of the ejected nanoclusters is lower in presence of the liquid. Our results provide a qualitative understanding of
the effect of wetting layers on the ablation process.
DOI: 10.1103/PhysRevB.77.014108 PACS numbers: 61.80.Az, 78.20.Bh
I. INTRODUCTION
Laser ablation—the collective ejection of material from a
target following irradiation by short, intense bursts of
light—is a technology widely used in many applications
such as thin film deposition and cleaning, surface microma-
chining, laser surgery, mass spectrometry, etc.
1
It is also an
efficient method for the controlled production of nanopar-
ticles. In this respect, one can distinguish between the direct
ablation of a solid target in vacuum or in a gaseous environ-
ment, where the target can expand freely, and “confined”
ablation, where the target is immersed in a liquid such as
water. In the latter case, depending on the nature of the target
and the liquid, clusters of various sizes are more or less
dispersed in the liquid, forming a colloidal solution which
could have potential biomedical applications see Refs. 2 and
3 for a general overview of the subject. Ultrafast lasers have
recently been used to produce colloidal suspensions of very
fine a few nanometers gold nanoparticles in pure water,
which are very difficult to fabricate by other methods.
4–6
While it has been shown experimentally that the chemical
nature and the thickness of the immersing liquid influence
the morphology, composition, and size distribution of the
clusters, these effects are not well understood.
2
The physics of ablation in vacuum has been studied in
detail both experimentally
7–9
and theoretically;
10–20
recent
numerical models by Perez and co-workers
14,16–18
and
Lorazo et al.,
15,19,20
in particular, have provided a compre-
hensive picture of the mechanisms underlying ablation in the
thermal regime. It has been demonstrated that ablation can
occur through different processes, viz., spallation, phase ex-
plosion whereby a thermodynamically metastable homoge-
neous liquid decomposes into a mixture of liquid droplets
and gas, fragmentation disintegration of a homogeneous
material into clusters under the action of large strain rates,
or vaporization, as a function of increasing fluence.
16
Also, it
has been shown that, in the case of very long nanosecond
pulses in molecular solids, the route to ablation is largely
determined by the degree of local confinement, i.e., depth
into the target;
18
this turns out to be of utmost relevance to
the present study. In contrast, the corresponding problem in
presence of a liquid layer remains largely unexplored: there
exists, to our knowledge, only a few numerical studies of
“wet” targets at subthreshold fluences i.e., below the thresh-
old for ablation;
21,22
these evidently do not cover the abla-
tion regime which is of importance for understanding the
formation of clusters.
In order to assess the effect of the presence of a liquid
film on the ablation process, we report in this paper the re-
sults of a numerical study of ultrafast femtosecond laser ab-
lation in a solid covered by a thin liquid layer, and compare
the results with the case of a dry target. We limit the study to
relatively low fluences where the thermal regime is predomi-
nant and ignore any role of the resulting plasma which would
become important at higher fluences. Anticipating our re-
sults, we find that the main effect of the liquid is to confine
the solid target over long time scales and to subsequently
slow down its expansion. This severely restricts the effi-
ciency of some of the ablation mechanisms, in particular,
phase explosion. Changes in the relative importance of the
different mechanisms in turn reduce the ablation yield,
strongly affecting the properties of the plume. We also show
that the dynamics of the liquid film is largely dominated by
the propagation of the pressure waves emitted at the interface
with the solid target.
II. COMPUTATIONAL DETAILS
For the present study we employ the model proposed by
Perez and Lewis,
14,16
which was successfully used to inves-
tigate ablation in a variety of situations and, in particular, to
unveil a new mechanism for ablation, viz., fragmentation.
This model is based on a two-dimensional system of
Lennard-Jones atoms whose evolution in time is followed
using molecular-dynamics MD simulations. The laser
pulse, Gaussian in time and of duration t, is modeled as a
sequence of discrete photons absorbed by the target accord-
ing to the Beer-Lambert law, Iz = I
0
e
-z
, where the depth z
PHYSICAL REVIEW B 77, 014108 2008
1098-0121/2008/771/0141089 ©2008 The American Physical Society 014108-1