Nanomodification of gold surface by picosecond soft x-ray laser pulseGenri Norman, Sergey Starikov, Vladimir Stegailov, Vladimir Fortov, Igor Skobelev et al. Citation: J. Appl. Phys. 112, 013104 (2012); doi: 10.1063/1.4731752 View online: http://dx.doi.org/10.1063/1.4731752 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v112/i1 Published by the American Institute of Physics. Related ArticlesUltrafast laser induced breakdown spectroscopy of electrode/electrolyte interfaces Appl. Phys. Lett. 100, 234101 (2012) Application of the Z-scan technique to determine the optical Kerr coefficient and two-photon absorptioncoefficient of magnetite nanoparticles colloidal suspension J. Appl. Phys. 111, 113509 (2012) Terahertz time-domain spectroscopy of anisotropic complex conductivity tensors in silicon nanowire films Appl. Phys. Lett. 100, 211102 (2012) Time resolved pump-probe scattering in MnAs/GaAs(001): A look into the dynamics of α-β stripe domains Appl. Phys. Lett. 100, 211905 (2012) Comment on “Observation of anomalous acoustic phonon dispersion in SrTiO3 by broadband stimulated Brillouinscattering” [Appl. Phys. Lett. 98, 211907 (2011)] Appl. Phys. Lett. 100, 206101 (2012) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors
Nanomodification of gold surface by picosecond soft x-ray laser pulse
Genri Norman,1,2 Sergey Starikov,1,2,a) Vladimir Stegailov,1,2 Vladimir Fortov,1,2
Igor Skobelev,1 Tatyana Pikuz,1,3 Anatoly Faenov,1,3,b) Sataoshi Tamotsu,4 Yoshiaki Kato,5
Masahiko Ishino,3 Momoko Tanaka,3 Noboru Hasegawa,3 Masaharu Nishikino,3
Toshiuki Ohba,3 Takeshi Kaihori,3 Yoshihiro Ochi,3 Takashi Imazono,3 Yuji Fukuda,3
Masaki Kando,3 and Tetsuya Kawachi31Joint Institute for High Temperatures, Russian Academy of Sciences, Moscow 125412, Russia2Moscow Institute of Physics and Technology, Dolgoprudny 141700, Russia3Quantum Beam Science Directorate, Japan Atomic Energy Agency, Kyoto 619-0215, Japan4Graduate School of Humanities and Science, Nara Women’s University, Nara 630-8506, Japan5Graduate School for the Creation of New Photonics Industries, Hamamatsu 431-1202, Japan
(Received 23 January 2012; accepted 30 May 2012; published online 3 July 2012)
We show experimentally the possibility of nanostructuring (about 20 nm) of gold surface by
picosecond soft x-ray single pulse with low fluence of �20 mJ/cm2. The nanometer-scale changes
of the surface structure are due to the splash of molten gold under fluence gradient of the laser
beam. In addition, the ablation process occurs at slightly higher fluence of �50 mJ/cm2. The
atomistic model of ablation is developed which reveals that the low threshold fluence of this
process is due to the build-up of the high electron pressure and the comparatively low electron-ion
energy relaxation rate in gold. The calculated ablation depths as a function of the irradiation
fluence are in good agreement with the experimental data measured for gold surface modification
with ultra-short duration soft x-ray and visible lasers. VC 2012 American Institute of Physics.
[http://dx.doi.org/10.1063/1.4731752]
I. INTRODUCTION
The laser-induced ablation of materials is receiving
growing attention because it opens new possibilities to pro-
vide precise microprocessing and fabrication of nano-
structures on the surfaces of dielectrics, semiconductors, and
metals. During past two decades, particular interest is on the
use of ultra-short pulse lasers for modification of material
surfaces due to the significance both in practical1,2 and theo-
retical aspects.3,4 Under intense pulse excitation, it is possi-
ble to create highly non-equilibrium state of matter,4,5 which
can significantly reduce the laser fluence for beginning of
surface modification process. Furthermore, new exciting pos-
sibilities have been opened with development of short pulse
transient collisional soft x-ray lasers (SXRL)6 and the x-ray
free electron lasers (XFEL)7 which are becoming available
now for various applications.
The ablation thresholds of dielectrics (for example, LiF
crystals)8,9 irradiated by nanosecond (ns) laser pulses are
higher by an order of magnitude as compared with the
threshold under pico- (ps) and femtosecond (fs) laser pulse
irradiation. The observed dependences of the ablation thresh-
old of the dielectrics on the duration of the laser pulses could
be explained by existence of three various mechanisms of
ablation. At subpicosecond laser pulse with rather high laser
fluence, the ablation takes place like Coulomb explosion.10
At picosecond laser pulse, the spallative ablation mechanism
occurs when the delivery of laser energy to the material is
faster than the elastic reaction to the heating but is slower
than the typical electron response times.9,11 As the acoustic
relaxation time of the dielectrics is of the order of 10 ps,
such ablation mechanism is valid for any laser wavelengths
if the laser pulse duration is shorter than this value. At nano-
second laser pulses, the ablation takes place like thermal pro-
cess when the absorbed energy density is comparable to the
cohesive energy of the condensate state. The presence of var-
ious ablation mechanisms explains the experimentally
observed difference of the ablation thresholds of dielectrics
for ns and ultra-short laser pulses.
Already previous measurements have demonstrated that
the ablation thresholds for SXRL irradiation of dielectrics are
quite different compared with the ablation thresholds for the
visible lasers. The decrease of the irradiation laser wavelength
from infrared (IR) and visible to soft x-ray (SXR) causes two
orders of magnitude decrease of the ablation threshold and
such result is not dependent on the laser pulse duration. The
photon absorption and collisional relaxation processes are
determined by the photon energy and the electron structure.
The multiphoton ionization is necessary for the absorption
and the creation of conduction-band electrons at IR and visi-
ble radiation.12,13 In this case, the attenuation length of the
laser pulse is longer or comparable to the laser wavelength
(i.e., �micrometer). On the contrary, the soft x-ray pulses are
absorbed in length of about 10 nm (Refs. 9 and 11) and the
absorbed energy is accumulated in a very narrow surface
layer. Therefore, the comparatively low fluences may produce
the ablation. This is why the ablation threshold for dielectrics
is so different for IR and visible radiation compared with that
of x-rays despite the same duration of the laser pulses.
The laser ablation mechanism for the metals remains
unresolved, because the existing experimental data are con-
tradictory and inconsistent with theoretical predictions. This
is especially true for ultra-short pulse laser interaction with
a)Electronic mail: [email protected])Electronic mail: [email protected].
0021-8979/2012/112(1)/013104/8/$30.00 VC 2012 American Institute of Physics112, 013104-1
JOURNAL OF APPLIED PHYSICS 112, 013104 (2012)
metals. Among various metals, gold is known to have com-
paratively low rate of energy transfer from the electron sub-
system (ES) to the ion subsystem (IS) after energy
deposition.4,14 For this reason, experiments with gold will
allow studying the two-temperature effects (electron temper-
ature Te� ion temperature Ti) in the ablation of metals. The
attenuation lengths of optical and SXR pulses in gold are
comparable up to about 1 nm wavelength. This fact allows to
perform the study of ablation with optical and SXRL pulses
in scope of the common model.
The absorptance of metals for an optical pulse is much
different from it for an x-ray pulse. The gold absorptances
for optical pulses at ablation are equal to 13% and 10% in
Refs. 15 and 16, respectively. We take the gold absorptance
to be 13% for optical pulses. In this work, all experimental
values of the absorbed fluences in gold for optical pulses are
taken as the published fluences multiplied by 0.13. For x-ray
pulses in the gold, an absorbed fluence is equal to incident
irradiation fluence. The comparison of the ablation thresh-
olds is performed for absorbed fluences as it has key
significance.
There is considerable uncertainty in the experimental
data of single-pulse laser ablation of gold. At the laser abla-
tion with short (about 0.1 ps) optical pulse,15 the threshold
absorbed fluence Fabs and the crater depth d have been meas-
ured to be about 9 mJ/cm2 and 10 nm, respectively. In
another experiment17 also with short optical pulse, these val-
ues were 150 mJ/cm2 and 50 nm, respectively. With short
duration soft x-ray pulse, the threshold value for the surface
modification of gold was determined as Fabs� 20 mJ/cm2.18
The ablation thresholds in multi-pulse subpicosecond re-
gime are also ambiguous. The threshold values of Fabs in the
multi-pulse regime with 1 kHz repetition rate are about 6 and
20 mJ/cm2 in Refs. 15 and 19, respectively. In an experiment
in Ref. 20 with 10 Hz repetition rate, the ablation threshold
of Fabs was determined equal to about 70 mJ/cm2. The diffe-
rence between single- and multi-pulse regimes was investi-
gated in Refs. 15 and 21. Many facts point that the ablation
threshold Fabs at such repetition rates should be similar to
values at single-pulse regime. However, the question about it
remains open.
In this work, we experimentally study the nanometer
scale modification of solid gold surfaces irradiated by single
x-ray laser pulse. To describe the ablation process near the
threshold fluence, we propose two-temperature atomistic
model with electron-temperature-dependent (ETD) inter-
ionic potential. The use of this potential makes it possible to
take into account the effect of the electron pressure on the
behavior of ions and to clarify the experimental data on mod-
ifications of gold surface measured with the ultra-short dura-
tion SXRL and visible lasers.
II. EXPERIMENTAL SETUP AND RESULTS
The SXRL irradiation experiment was carried out by use
of the SXRL facility at Japan Atomic Energy Agency
(JAEA).22,23 The experimental setup was composed of three
elements: the SXRL source, optics, and the sample. A sche-
matic diagram of the experimental setup is shown in Fig. 1(a).
The soft x-ray seed pulse generated by the first Ag
plasma was amplified with the second Ag plasma and finally
had a wavelength of 13.9 nm, bandwidth of narrower than
10�4, the duration time of 7 ps, and the beam divergence of
0.35 mrad� 0.3 mrad in the horizontal and vertical direc-
tions, respectively. The output energy of the SXRL pulse
was varied in each shot, with the average of 200–300 nJ.23,24
The SXRL pulse was focused on the surface of an Au speci-
men by using a spherical Mo/Si multilayer coated mirror
having a 1000-mm radius of curvature, which had been opti-
mized for soft x-rays of 13.9 nm at an incidence angle of 2�.
FIG. 1. (a) Schematic diagram of experimen-
tal setup for irradiation of a sample by focus-
ing a SXRL pulse. The sample stage with LiF
crystal and Au targets moves in two direc-
tions: X-stage for delivering fresh targets, and
Y-stage for changing focusing position of
SXRL. (b) The luminescent image of a LiF
crystal surface and visible (in DIC mode)
images of LiF and Au surfaces damaged by
irradiation of SXRL beam with energy of
�48 nJ.
013104-2 Norman et al. J. Appl. Phys. 112, 013104 (2012)
This mirror was placed at a distance of 2637 mm from the
SXRL output. A Zr filter of 0.2 mm thickness was placed in
front of the spherical mirror to reduce the scattered optical
radiation from the laser produced Ag plasmas. The transmit-
tance of the Zr filter and the reflectivity of the Mo/Si mirror
at 13.9 nm were approximately 48% and 50%, respectively.
Therefore, the total energy of the SXRL beam on the Au sur-
face was 48–72 nJ.
The mechanically polished Au plate with a thickness of
0.5 mm was fixed on the surface of a LiF crystal. This Au-
LiF side-by-side targets was mounted on the holder having
two movable directions (see Fig. 1(a)). The target on the
holder was moved in vacuum after each laser shot along the
SXRL beam propagation direction and also perpendicular to
it, in order to record the beam patterns at different focusing
distances and to use the fresh LiF crystal or Au surfaces. All
target surfaces were carefully aligned to assure equal focus-
ing conditions on them. The Au and LiF targets were illumi-
nated at various focusing positions of the SXRL beam.
Usually for each focusing position at the beginning of exper-
imental run we monitored some shots using LiF crystal.
Then, we did shots on Au surface and after irradiation of Au
we have once more checked the focused laser intensity using
LiF detector. It allows determining the SXRL intensity distri-
bution in each focusing position using the procedure
described in Refs. 8 and 25 with the LiF crystal soft x-ray de-
tector. Measured variation of the focal spot non uniformity
was 640% along the focal spot for the lowest laser fluencies
and 615% for higher one.
After irradiation of the LiF crystal by the SXRL, the
photoluminescence patterns of the color centers (CCs) in LiF
were observed by using a confocal fluorescence laser micro-
scope (OLYMPUS model FV300). A 488 nm Ar laser was
used in the microscope to excite CCs, which then emitted lu-
minescence in the spectral range of 500–800 nm with the
peaks at 530 nm for F3 and 670 nm for F2, respectively.
Typical luminescent image of the SXRL beam, recorded
with the LiF crystal irradiated by the SXRL beam with
energy of �48 nJ is presented in Fig. 1(b). Due to the high
sensitivity and the large dynamic range of the LiF crystal de-
tector, high-quality images have been observed in a single
SXRL shot with different pulse energies and not only at the
best focal position, but also away from it. Thus, these pro-
vide the information on the best focusing position with high
accuracy and controllably at varied SXRL fluences on the
surfaces of the irradiated targets. High spatial resolution
(about 700 nm) and high dynamic range of the LiF crystal
detector8,25 allowed to record clearly resolved detailed struc-
tures in the intensity distribution of the SXRL beam (see
Fig. 1(b)), together with the aberrations and broad scatterings
of the XRL beam. With this diagnostic method, we found
that the SXRL energy fraction in the best focusing spots,
where the surface damage on LiF crystal was observed,
reached 60% in this experimental series.
Different imaging techniques were used to investigate
the damage structures on the surfaces of the Au plates and
the LiF crystals and to measure the shapes and depths of the
nanostructures on the surface. The sample surfaces were
observed by a visible microscope (BX60, OLYMPUS Cor-
poration) with the differential interference contrast (DIC)
mode. The detailed structures of the modified surfaces,
which could not be seen with a visible microscope, were
observed by a scanning electron microscope (SEM, JSM-
6380LVN, JEOL Ltd.) (Fig. 2). The cross sectional profiles
of the induced patterns were measured by an atomic force
microscope (AFM, Explorer, TopoMetrix Corporation)
(Fig. 3). These images demonstrate that the surfaces of the
Au samples were damaged in a single SXRL shot. The beam
spot sizes at the threshold for nanostructure formation on the
sample surfaces by the SXRL beam irradiation were found
to be about 300 lm2 for LiF crystal and about 50 lm2 for Au
targets. If we take into account the real intensity distribution
of the SXRL beam in the focusing spot, which has been
measured by the LiF crystal, the nanostructuring threshold
for LiF crystal is �9.6 mJ/cm2 and �21 mJ/cm2 for Au. It is
necessary to mention that Au surface modification was not
observed for the SXRL fluence of less than �15 mJ/cm2.
Figs. 2 and 3 show the SEM and AFM images of the
irradiated Au surfaces by a single laser shot of the SXRL
beam obtained near the threshold for nanostructure forma-
tion. Nanoscale surface modification is already clearly seen
at the SXRL fluence of Fabs¼ 21 6 5 mJ/cm2. According to
the analysis of surface structures, the metal is redistributed
on a surface but is not ablated. These changes are probably
due to the splash of the molten gold under intensity gradient
(i.e., pressure/temperature gradients) across the laser beam.
In this case, the surface compression wave propagating in
liquid layer plays the key role in modification of the sample.
The typical obtained depth of the surface irregularities
(nanoscale roughness) is of the order of 20 nm. Stronger
change in the surface modification could be seen in Figs.
2(b) and 3(b). With increase of Fabs to �60 mJ/cm2, the
depth of the surface modification reaches to 50–80 nm. These
changes most probably are determined by ablation process.
FIG. 2. SEM images of gold surfaces after
irradiation by a single laser shot of the SXRL
beam at different fluences of (a) Fabs¼ 21 6 5
mJ/cm2 and (b) Fabs¼ 60 6 15 mJ/cm2,
respectively. Magnified yellow boxes show
parts of the SEM images with higher spatial
resolution.
013104-3 Norman et al. J. Appl. Phys. 112, 013104 (2012)
Even larger depth modifications of �100–150 nm were
observed in the hot spots of the SXRL beam when the fluence
exceeds 100 mJ/cm2. It is necessary also to stress that in all
experimental AFM images presented in Fig. 3 large amount of
material deposition on the metal surface is clearly seen.
III. SIMULATION
A. Model
During ablation, the laser irradiation initially leads to
the excitation of the ES. One of the main theoretical difficul-
ties for construction of a model is the fact that the electron-
ion relaxation time is comparable to the time-scale of the
ablation itself and the accompanying processes (heat trans-
fer, phase transitions, etc.) One way for modeling of this pro-
cess is continuum approximation with two-temperature
equation of state.26,27 This methodology, however, does not
take into account of the phenomena at the atomic level (met-
astable phase decay, nucleation, etc.) that are essential for
the description of ablation. Note that the similar situation
takes place in various phenomena with high electron temper-
ature: electrical explosion of conductors, formation of swift
heavy ion tracks in a nuclear materials, etc.28–30
Molecular dynamics (MD) models of the two-
temperature (2T) system proposed in Refs. 31–33 consist in
considering the ES as a continuum. The energy transfer from
the ES to the IS is implemented using a Langevin thermostat.
This approach can be used to describe many features of abla-
tion but it gives significant overestimation of the crater depth
d as was shown for Cu (Ref. 31) and Au. (Ref. 34) This 2T
model does not take into account the effects of electron pres-
sure build-up35,36 and the changes of interionic forces as a
result of the ES excitation.37–39
Au is a noble metal with one s-electron and ten
d-electrons per atom in the valence shell. A combined
description of the localized d- and delocalized s-electrons is
needed in order to address the actual electron pressure of this
material. The total energy of ES also contains localized and
delocalized parts.40,41 The delocalized energy of ES is
determined only by the electron concentration and tempera-
ture like the energy of an ideal gas of free electrons. The
pressure Pdeloce of the delocalized electron energy may be
taken into account in the ion dynamics by the blast force
�rðPdeloce Þ � rðT2
e Þ.35,36 On the contrary, the localized
energy of ES is determined by the positions of the ions paripassu with Te.
In the present work, an atomistic 2T model with the
interionic ETD potential is proposed for the description of
the ablation process. The ETD-potential allows us to take
into account an important part of the electron pressure of the
localized electron energy in addition to the blast force of the
delocalized electron energy.
Our model is implemented as a modification of the 2T
model in the LAMMPS code.42,43 The evolution of the ES is
described by the equation
CedTe
dt¼ rðkerTeÞ � GðTe � TiÞ �
IðtÞe�x=l
l; (1)
where Ce is the electron heat capacity,5 ke is the electron ther-
mal conductivity,32 G is the coupling constant for the
electron-ion interaction,32 I is the absorbed laser intensity of
the rectangular pulse of width s, and l is the attenuation
length. In our model, the SXR pulse is differed from the opti-
cal pulse only by l. The model does not take into account the
absorption mechanism. The approximation is used that ther-
malization in ES instantly occurs. In Ref. 44, the time of the
electron thermalization at laser excitation in gold is estimated
to be equal to several hundred femtoseconds. It is faster than
the considered processes in IS at the present model.
The creation of the ETD-potential of gold for the atom-
istic simulation is performed with force-matching method.45
This method is used for the development of the potential as
implemented in the PotFit code.45 The method provides a
way to construct physically justified interparticle potentials
without referring to experimental data. The idea is to adjust
the interparticle potential to optimally reproduce per-atom
forces computed at the ab initio level (e.g., density func-
tional theory) for a fine-tuned set of small reference
FIG. 3. The AFM images of the gold surfaces
after irradiation by a single laser shot of the
SXRL beam with different fluencies of (a)
Fabs¼ 21 6 5 mJ/cm2 and (b) Fabs¼ 60 6 15
mJ/cm2, respectively.
013104-4 Norman et al. J. Appl. Phys. 112, 013104 (2012)
structures. The reference data are calculated using the
Vienna ab initio simulation package (VASP) code40 (plane-
wave basis cut-off energy is 500 eV, projector augmented-
wave (PAW) pseudopotential, linear density approximation
(LDA) xc-functional, 2� 2� 2 Monkhorst-Pack k-mesh).
We use 32 reference structures for the fitting database with
2929 atoms altogether. These structures represent 32 various
states of gold (liquid and FCC-lattice at different densities).
The calculations of the reference data are performed at three
different Te of 0.1, 3, and 6 eV that is set as a parameter of
the Fermi-Dirac distribution for partial occupancies of elec-
tron bands. The three interionic potentials (for each Te) are
created with the force-matching procedure. The potentials
have the embedded atom method (EAM) form. The full
ETD-potential is created using quadratic polynomial interpo-
lation with respect to Te of the parameters of those three ref-
erence EAM potentials.
Fig. 4 shows the dependence of the total pressure (calcu-
lated by the VASP code) and the pressure of localized electron
energy (calculated in the MD model with using of the ETD-
potential and virial theorem) on Te. The difference in pres-
sures corresponds to the pressure Pdeloce of delocalized elec-
tron energy which cannot be described by ETD-potential.
The ion equations of motion are
midvi
dt¼ FiðTeÞ þ F
Langi ðTe; TiÞ �
5ðPdeloce Þni
; (2)
where FiðTeÞ is the interionic force of the ETD-potential,
FLangi is the Langevin force that models the electron-ion
energy transfer,33 the last term in Eq. (2) is the blast
force,35,36 and ni is the locally averaged ion concentration.
Note that local variations of the ETD-potential with Te
change the local properties of the IS (e.g., the melting tem-
perature).37 The values of Fabs are determined by the change
of the total energy of the ion subsystem during the two-
temperature stage.
The size of the simulation box is 2100� 28.6� 8.2 nm
in the x, y, and z directions. The periodic boundary conditions
in y and z directions are used. Au ions form a crystal in one
half of the simulation box (at 1050< x< 2100 nm). During
the simulation at t< s, the Te near surface layers may reach
3 eV and the total pressure may reach 70 GPa (see Fig. 5).
Fig. 6 shows the typical snapshots of the subsequent IS evolu-
tion. The crystal structure of ions melts and expands as the
energy transfers from ES to IS. The liquid layer near surface
forms during about 100 ps. Meanwhile, the zone of voids
forms because the mechanical and thermal relaxations of the
system lead to the creation of the region with negative pres-
sure. The typical profiles of ion pressure along x-directions
are shown in Fig. 7. The depth d of ablation crater is calcu-
lated as: d ¼ Nr=neqS, where Nr is number of all removed
ions during ablation, neq is equilibrium ion concentration, and
S is surface area. The melt depth is calculated as depth of
formed liquid layer: Nl=neqS, where Nl is number of ions in
liquid state (including the removed ions). Two variants of
laser pulses are considered: s¼ 7 ps and l¼ 18 nm are chosen
in order to model the x-ray pulses considered in this work,
s¼ 0.1 ps and l¼ 6 nm are used for the modelling of experi-
ments with optical lasers.15,17,20
B. Results
Different mechanisms of ablation are found from the
results of the simulation. At s¼ 0.1 ps, the two ablation
regimes can be distinguished (see the curve 5 in Fig. 8). If
Fabs< 130 mJ/cm2, ablation is due to the electron pressure
build-up (similar to the electron-driven ablation mechanism
described in Ref. 26). Fig. 5 illustrates this ablation mecha-
nism. At the initial moment of time, a high pressure is created
in the near-surface layer due to the increase of a high electron
FIG. 4. Dependence of the total pressure (red solid line) and the pressure of
localized electron energy (dashed black line) on Te. The difference between
these pressures determines the pressure Pdeloce of the delocalized electron
energy.
FIG. 5. The temporal change of the total pressure P with the ion concentra-
tion ni. The arrow schematically shows the evolution of state (taking into
account Pdeloce ) near surface layer during the ablation at s¼ 0.1 ps, l¼ 6 nm
(optical pulse) and Fabs¼ 121 mJ/cm2: (a) initial normal conditions; (b)
rapid heating of the electron subsystem (Te increases to 2.95 eV) by a laser
pulse; (c) relaxation due to expansion and Te decreasing; (d) final state that
corresponds to P��5.4 GPa when void formation starts.
013104-5 Norman et al. J. Appl. Phys. 112, 013104 (2012)
temperature (isochoric process). The formation of a layer
with negative pressure takes place as a result of the joint
action of the two pressure-reducing processes: the mechanical
expansion in vacuum and the decrease of Te due to relaxation
processes. At Fabs> 125 mJ/cm2, the fast electron-driven
ablation with depth of tens of nm is present as well. However,
in this case, such ablation cannot be distinguished from much
deeper ablation with depth of hundreds of nm due to the rare-
faction wave formation (like in Refs. 17 and 34) which starts
to be realized. Therefore, the dependence d(Fabs) has the sec-
ond ablation threshold. Such a peculiarity of the dependence
d(Fabs) may be the reason of the overestimation of the abla-
tion threshold and depth in the previous models of sub-
picosecond pulses.17,34
At the pulse duration of s¼ 7 ps, the ablation depth dsmoothly grows together with Fabs (curve 7 in Fig. 8). In this
case, the separation of two mechanisms is impossible (the
time of electron-ion relaxation is comparable with s and the
electron-driven ablation does not occur explicitly). The
effect of spallation takes place as a result of the joint action
of mechanisms involved in both types of ablation. Note that
the situation with two ablation threshold for sub-picosecond
laser pulses and one ablation threshold for picosecond laser
pulses was observed in multi-pulse regimes for copper in
Ref. 46 and for gold in Ref. 19. The ablation mechanism
associated with boiling is not investigated in this work. The
fluence for realization of considerable boiling must be larger
than examined fluences.
The direct atomistic simulation of the modification of
surface by splash of the molten gold is difficult. Such a simu-
lation demands the large computational resources. Several
FIG. 6. Ablation patterns given by our atomistic model: (a) and (b)—optical
pulses with s¼ 0.1 ps at Fabs¼ 65 mJ/cm2 and Fabs¼ 150 mJ/cm2; (c) and
(d)—SXR pulses with s¼ 7 ps at Fabs¼ 65 mJ/cm2 and Fabs¼ 130 mJ/cm2;
(e)—various state zones near the surface at SXR pulse in (c) case, s¼ 200 ps.
FIG. 7. The ion total pressure profiles
along x-directions in various moments of
simulation. Parameters of the simulation:
(left) s¼ 0.1 ps, l¼ 6 nm, and Fabs
¼ 150 mJ/cm2; (right) s¼ 7 ps, l¼ 18 nm,
and Fabs¼ 65 mJ/cm2. Simulation
moments: 1—s¼ 0.1 ps; 2—s¼ 3.5 ps for
(optical pulse) and 7.0 ps for (SXR pulse);
3—s¼ 20 ps; 4—s¼ 70 ps.
FIG. 8. The dependence of modification depth on Fabs (the various proc-
esses). The optical sub-picosecond pulses: 1, 2, and 3—results of experi-
ments in Refs. 15, 17, and 20 respectively; 4—ablation depth d in MD
calculation without electron pressure;34 5—calculated ablation depth d for
optical pulses with s¼ 0.1 ps in this work. The SXR pulses with s¼ 7 ps
(this work): 6—experimental modification depth; 7—ablation depth d in
simulation; 8—calculated melt depth.
013104-6 Norman et al. J. Appl. Phys. 112, 013104 (2012)
billions of atoms are necessary for MD-simulation of similar
process. However, the depth of surface modification dm may
be estimated by the melt depth. The melt depth slightly
depends on s. In Fig. 8, the melt depth for s¼ 7 ps is shown.
It is visible that the dependence of the melt depth on Fabs
agrees with our experimental data on nanomodification of
surface with SXR pulses. In addition, the ablation depth
reaches the melt depth at Fabs� 80 mJ/cm2. This fact con-
firms that the first changes of the surface structure probably
are due to the splash of molten gold. However, at higher val-
ues of Fabs, the ablation process determines nanomodifica-
tion. Thus, the various mechanisms of the surface
modification are disclosed. The dm may be determined by
one of such mechanisms (e.g., melting and splash at low flu-
ences) or by several ablation mechanisms.
The most close agreement of the simulation results for
optical pulses is observed with the experiment on ablation of
gold in Ref. 20 where multi-pulse regime with very low
(10 Hz) repetition rate was used. The second threshold of
ablation conforms to data of Ref. 17. The discrepancy is
observed between the simulation results and experiment in
Ref. 15. One of possible reasons is that the modification of
surface may be induced by the melting and splash of metal at
laser irradiation.
IV. CONCLUSION
Our experiments and modeling demonstrate that material
irradiated by short SXR pulses with Fabs� 20–50 mJ/cm2
allows the nanostructuring of gold surfaces in depth of
�20 nm. The experimental energy threshold of nanostructur-
ing (20 mJ/cm2) by SXRL pulse in this work is most probably
related to the laser-induced melting and splash of gold. This
threshold of nanostructuring is slightly smaller than the
threshold of ablation (about 50 mJ/cm2) at which a metal
layer is removed. For optical pulses, the similar threshold of
ablation is defined in the atomistic simulation in our investi-
gation. This simulation results agree with other studies20 of
gold ablation by sub-ps optical pulses. In addition, the abla-
tion threshold in Refs. 17 and 34 may be the second threshold
associated with rarefaction wave formation. The existence of
the second threshold of ablation at sub-picosecond pulse also
conforms to results of other studies.19,46
Such result is rather different from the visible and
SXRL ablation of dielectrics, where the distinction in the
wavelength of lasers causes �2 orders of magnitude differ-
ence in the value of the ablation threshold. Such fact is
ascribed to the different electron band structures of metals
and dielectrics, their electron-ion relaxation stages, and dif-
ferent absorption mechanisms for visible and SXR photons.
The main distinction between metal and dielectric is the
presence of conduction-band electrons. For metals, the
attenuation lengths are comparable for SXRL and optical
pulses. In addition, the role of the electron pressure is very
high in metals. We can conclude that for metals, and espe-
cially for gold, the relatively slow electron-ion relaxation
time results in maintaining of the high electron pressure in
the near surface region for several picoseconds, that is
sufficiently long for the development of the hydrodynamic
response that causes the formation of the negative pressure
region and the ablation of a thin surface layer.
ACKNOWLEDGMENTS
The clusters of Moscow Institute of Physics and Tech-
nology (MIPT60), Moscow Joint Supercomputer Center
(MVS-50K), and Moscow State University (“Lomonosov”)
were used for calculations. The work was financially sup-
ported by the Programs for Basic Research of the Presidium
of the RAS No. 2 (coordinator V. E. Fortov) and 23 (coordi-
nator is N.F. Morozov), the RFBR Grants 12-02-00947 and
12-08-00666, the President RF Grants MK-7192.2012.8,
SNL under the US DOE/NNSA ASC program and Japan
basic research foundation (KIBAN B) No. 2136052 of JSPS.
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