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: starikov@ihed.ras.ru.b)Electronic mail: anatolyf@hotmail.com.

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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