JOURNAL OF APPLIED PHYSICS 108, 113302 共2010兲
Multidiagnostic analysis of ultrafast laser ablation of metals with pulse pair
irradiation
S. Amoruso,1,2,a兲 R. Bruzzese,1,2 X. Wang,2 G. O’Connell,3 and J. G. Lunney3
1
Dipartimento di Scienze Fisiche, Università degli Studi di Napoli Federico II, Complesso Universitario di
Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy
2
CNR-SPIN, Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy
3
School of Physics, Trinity College–Dublin, Dublin 2, Ireland
共Received 31 August 2010; accepted 16 October 2010; published online 3 December 2010兲
Copper targets are irradiated in the ablation regime by pairs of equal, time-delayed collinear laser
pulses separated on a timescale going from ⬇2 ps to ⬇2 ns. The ablation plume is characterized by
ion probe diagnostic, fast imaging, and temporally and spatially resolved optical emission
spectroscopy. The variation in the ablation efficiency with the delay between the pulses is analyzed
by measuring the ablation crater profile with a contact profilometer. The second laser pulse modifies
the characteristics of the plasma plume produced by the first pulse and the ablation efficiency. The
different mechanisms involved in double pulse ultrafast laser ablation are identified and discussed.
The experimental findings are interpreted in the frame of a simple model of the interaction of the
second pulse with the nascent ablation plume produced by the first pulse. This model yields
consistent and quantitative agreement with the experimental findings predicting the observed
experimental trends of the ablation depth reduction and ion yield increase with the delay between
the pulses, as well as the characteristic timescale of the observed changes. The possibility of
controlling the characteristics of the plumes produced during ultrafast laser ablation via an efficient
coupling of the energy of the second pulse to the various ablation components produced by the first
pulse is of particular interest in ultrafast pulsed laser deposition and microprobe analyses of
materials. © 2010 American Institute of Physics. 关doi:10.1063/1.3516491兴
I. INTRODUCTION
Ultrafast laser ablation 共ULA兲 is attracting a lot of attention as a unique material processing technique that covers
several contiguous application areas: e.g., nanoparticle 共NP兲
generation, film growth, material processing, and laserinduced breakdown spectroscopy 共LIBS兲. This has stimulated interest in both deeper physical understanding of the
ULA process and strategies aiming to modify or control the
properties of the ablated species and ablation craters.1–5
The ULA process has been extensively studied both
theoretically and experimentally.6–13 In ULA of metals, the
pulse energy is absorbed by the conduction band electrons
with a rapid increase in their temperature and pressure. Then,
for times t lower than the electron-lattice relaxation time el
共typically ⬇10 ps for elemental metals兲, the energy is progressively transferred to the lattice which increases its temperature and melts. At time t of the order of el, a sharp
pressure profile is generated in a narrow zone near the target
surface. This leads to the generation of an intensive compressive wave that moves into the irradiated material, and of an
associated rarefaction wave whereby material flows away
from the target surface. The target decomposition is generally examined by following the thermodynamic trajectories
of different layers in the phase diagram of the target
material.6–9 The surface layers of the target reach temperatures well above the critical temperature of the material and
a兲
Author to whom correspondence should be addressed. Electronic mail:
amoruso@na.infn.it.
0021-8979/2010/108共11兲/113302/10/$30.00
then relax directly through a rapid expansion into an atomic
plume. The deeper layers, instead, relax following thermodynamic trajectories which pass near or below the critical point
of the material, finally, decomposing into a plume of NPs
through mechanical fragmentation and phase explosion.
These last decomposition mechanisms describe the major
part of the ablated material.2,6,7,14
This scenario is coherent with the experimental findings
of a recent study on the early stages 共ⱕ30 ns兲 of the ULA of
an Al metallic target.15 By using ultrafast time-resolved x-ray
absorption spectroscopy 共time resolution= 30 ps兲, this study
observed that material ejection from the target surface starts
within the first 30 ps and lasts for few tens of ns. The different ablation species 共ions, atoms and NPs兲 are ejected sequentially from the surface, forming a structured nascent
plume with the faster atomic species followed by a mixed
phase containing hot-liquid NPs which are moving more
slowly. The velocities of these populations are consistent
with those of the atomic and NP plumes predicted by molecular dynamics simulations and measured at few mm from
the target surface for ULA of an Al target in vacuum.16
The above comments indicate that the use of temporally
shaped pulses or irradiation with a pair of equal laser pulses
关double pulse 共DP兲兴 with controlled temporal separation may
offer interesting routes to modify the properties of the ULA
process thanks to the effects brought about by the interactions with a modified target surface or with the different
components of the nascent ablation material. These two approaches are complementary. Shaped pulses within a temporal window of few tens of picoseconds allow variation in the
108, 113302-1
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113302-2
Amoruso et al.
excitation sequence of the target on the timescale of the
electron-phonon relaxation time. DP irradiation facilitates
tuning of the properties of the ablation plume, depending on
the temporal delay of the second pulse with respect to the
ablation pulse. Some of these studies have shown that it is
possible to: 共i兲 modify the composition and shape of the
laser-produced plasma;1–4 共ii兲 control the size distribution of
silicon NPs or the ionic/neutral emission ratio of a metallic
Al plasma;17,18 共iii兲 change the characteristics of the ablation
craters;1,5 and 共iv兲 improve signal intensity and reproducibility in LIBS.19 Thus, an effort is underway to get deeper understanding of the different physical mechanisms involved in
these processes with the aim of improving the control of
laser-induced ablation and plasma formation processes of interest in methods like laser microprocessing, LIBS, pulsed
laser deposition, and so forth.
DP ULA experiments for different metals 共e.g., Ag, Al,
Au, Cu, and Ni兲 have also shown a surprising reduction in
the ablation efficiency with the increase in the delay between
the laser pulses.1,2,20,21 This effect was observed both under
vacuum and in air, at fluence levels in the range from few to
ten times the ablation threshold. When the delay between the
pulses, ⌬t is much shorter than el, the ablation efficiency is
almost independent of the delay. For ⌬t ⱖ el, the ablation
efficiency decreases monotonically, finally, reaching a plateau regime for ⌬t ⬇ 100 ps 共⌬t Ⰷ el兲, where it can reach
values even smaller that that obtained by a single pulse 共SP兲
only. Recently, the suppression of the ablation efficiency has
been analyzed by detailed hydrodynamic modeling for the
case of DP ULA of a copper target irradiated by 100 fs pulses
at 800 nm, and related to the formation of a second shock
wave induced by the interaction of the second laser pulse
with the expanding target material which suppress the rarefaction wave created by the first pulse, thus lowering the
ablation yield.22 This study also showed that for delay ⌬t
⬇ 50 ps the second pulse can be efficiently absorbed 共an
absorption coefficient of ⬇35% was estimated兲 by a front
layer of the nascent ablation material located at a distance of
⬇100 nm from the original target surface, in agreement with
previous experimental considerations.1,20 Therefore, it is possible to selectively couple more or less energy to one of the
various ablated components, which can be of interest in various fields.
The investigations carried out so far have enabled some
understanding of DP ULA, although most of the experiments
examine specific features of the process, such as ablated
depth, ablation plume dynamics, or plume emission enhancement, sometimes on a reduced range of delays between the
pulses. It seems that an experiment that examines the simultaneous behavior of several features may be useful. In a recent letter,23 we reported how DP irradiation of Cu, with
delays ⌬t from 2 ps to 2 ns, leads to interesting modifications
of the plume properties arising from the coupling of the second pulse with the nascent ablation material. Here, we aim to
extend our previous analysis to further illustrate and discuss
the role of the delay between the two pulses on the various
characteristics of the ULA process. The DP ablation plume
was studied by exploiting time- and space-resolved optical
emission spectroscopy, fast imaging, and ion probe tech-
J. Appl. Phys. 108, 113302 共2010兲
niques, while the ablation efficiency was characterized by
analyzing the crater depth profile with a contact profilometer.
II. EXPERIMENTAL METHODS
DP ULA experiments were carried out with a laser
source emitting ⬇250 fs pulses at 527 nm. These laser pulses
were obtained by the second harmonic-generation and pulse
compression of the fundamental output 共i.e., 1055 nm, ⬇0.9
ps兲 of a Nd:glass laser system. The temporal profile of the
laser pulse was monitored by a single shot background free
autocorrelation technique. The emitted energy per pulse was
of the order of ⬇1 mJ at 527 nm at a typical repetition rate of
33 Hz. The average fluence F was obtained as E / S, where E
is the incident laser energy and S is the spot area estimated
by measurement of the laser impact region as a function of
the laser pulse energy.24 During the experiments the repetition rate can be reduced by means of a mechanical shutter. A
Michelson interferometer was used to split the beam in two
different arms with equal energy, one of them equipped with
a micrometer resolution motorized translation stage which
introduces a controlled time delay, ⌬t, between the laser
pulses within the range 0–2000 ps. Interference fringes were
used to determine the condition ⌬t = 0. The two collinear
laser pulses from the interferometer were focused on a copper target 共99.9%兲 at an incidence angle of 45°, in high
vacuum 共10−7 mbar兲. The target was mounted on a rotating
holder to avoid local drilling.
The ion plasma produced during the DP ULA process
was monitored by using a negatively biased planar Langmuir
probe.25 The probe was oriented to face the target spot and
located at a distance of 38 mm from the target along the
normal of the target surface. The probe collecting area was a
2.5⫻ 5 mm2 square copper plate, insulated on the rear side.
The plasma plume optical emission was imaged on the
entrance slit of an imaging spectrograph equipped with a 300
grooves/mm grating coupled to an intensified-chargecoupled-device 共ICCD兲. Two-dimensional 共2D兲 single shot
images of the plume were registered by operating the grating
in reflection with the entrance slit fully open. Spectrally resolved, one-dimensional 共1D兲 images of the plume emission,
in the range 370–630 nm, were obtained by orienting the
entrance slit of the spectrograph along the plume expansion
axis. The spectrally resolved images were acquired by accumulation over 20 laser shots, to compensate the lower signal
level and to reduce the noise. The ICCD was equipped with
a 1024⫻ 1024 array and operated in time-gated detection
mode. To reduce the noise, a 2⫻ binning was operated during image acquisition. ICCD gain and gate width were adjusted in order to achieve both a good accuracy of the plume
propagation, and a reasonable signal-to-noise ratio of the
plume images.
The ablation efficiency at some fixed delays between the
pulses was found by irradiation of a stationary target with a
fixed number of pulses 共N = 3300兲, and measuring the ablation crater depth profile with a contact profilometer.
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Amoruso et al.
J. Appl. Phys. 108, 113302 共2010兲
0
-5
-10
Δt (ps)
-15
2
5
20
200
1500
-100
-50
0
50
Depth (nm/pulse)
Depth (nm/pulse)
113302-3
0
-5
d
-10
100
-50
Distance (µm)
0
50
Distance (µm)
FIG. 1. 共Color online兲 Typical depth profiles for copper ablation using DPs
with various delays. The depth per pulse was obtained by dividing the measured depth by the number of pulses N.
FIG. 2. 共Color online兲 Crater depth profile showing the maximum and minimum depths of the crater bottom 共arrows兲 from which d was estimated as an
average.
III. EXPERIMENTAL RESULTS AND ANALYSIS
dent a clear dependence of the ablation efficiency on ⌬t. For
short delay 共⌬t ⱕ 5 ps兲, d is of the order of 15 nm. As ⌬t is
increased the ablation depth progressively decreases, approximately leveling off at a value of about 6 nm for ⌬t
ⱖ 100 ps. A suppression of the ablation efficiency similar to
that observed here was reported earlier for various elemental
metallic targets 共copper, aluminum, nickel, gold, and silver,
e.g.兲1–3,21,22 thus indicating the existence of a common physical effect. In Fig. 2, the variation in the ablation depth d with
the delay ⌬t is compared to an exponential function
A. Ablation efficiency
In order to analyze the ablation efficiency, a stationary
target was irradiated with a fixed number of pulses N at some
selected delays ⌬t, and the depth profile along the minor
radius of the elliptical crater produced on the target was measured. Some typical crater depth profiles are presented in Fig.
1. In all the cases the aspect ratio of the crater 共i.e., the depth
to diameter ratio兲 was below 1, thus minimizing any effect of
deep-hole confinement on the estimate of the ablation rate
per pulse. It has been shown earlier that in this condition the
crater depth varies approximately linearly with the number
of laser pulses,26 therefore, the dependence of the ablation
rate on the delay ⌬t was obtained by estimating the ablation
depth per pulse d. As illustrated in Fig. 2, for each profile d
was estimated by considering the average value between the
maximum and minimum depths 共arrows兲 of the crater bottom, and their difference was regarded as error bar. It is also
worth noting that a very good consistency between the ablation depth and the crater volume was observed earlier in our
experimental conditions,1 therefore, we consider the ablation
depth as a reliable measure of the ablation efficiency.
The variation in the maximum ablation depth with ⌬t is
reported in Fig. 3; the dashed line shows the maximum ablation depth for SP irradiation. From Figs. 1 and 3 it is evi-
d共⌬t兲 = d0 − ⌬d共1 − e−⌬t/c兲,
共1兲
where d0 ⬇ 15 nm corresponds to the ablation depth at zero
delay, i.e., for a SP of twice the energy, ⌬d ⬇ 9 nm is the
total depth variation and c ⬇ 20 ps is a characteristic time
constant of the process. Time constants for the ablation depth
reduction c ⬇ 12 ps for copper and c ⬇ 30 ps for gold were
reported by Noel and Hermann during DP ULA with Ti:sapphire pulses at a fluence of 2 J / cm2 per each pulse.2 This
was related to a change in the electron heat conductivity
induced by the progressive increase in the lattice temperature
occurring for delays larger than the electron-lattice thermalization time. Recently, Roberts et al.21 studying DP ULA of
silver foils observed that the time lag from the short delay
region 共⌬t ⬍ el兲 to the plateau at large delay critically depends on the laser pulse fluence, decreasing from ⬇100 ps at
a fluence level of about 1 J / cm2 to few picoseconds at
⬇10 J / cm2. This experimental observation might suggest
that this characteristic time is not only related to the properties of the target but also to other effects such as energy
Ablation depth d (nm)
Prior to the DP experiment we measured the fluence
threshold Fth for ablation of copper with a SP in our experimental conditions, estimating a value of Fth ⬇ 0.15 J / cm2.
The data presented below were obtained carrying out DP
ULA experiments at a laser fluence of each pulse F
⬇ 0.6 J / cm2, which corresponds to ⬇4 times the ablation
threshold of copper.
In the following we will illustrate the effects of the delay
⌬t on various characteristics of the DP ULA process. First,
Sec. III A we will discuss the variation in the ablation efficiency with ⌬t for our experimental conditions. Then, we
will address the effects of DP irradiation on the characteristics of the atomic and NP ablation plumes. In particular, the
modifications of the DP irradiation on the ion features will be
presented in Sec. III B, while in Sec. III C the atomic and NP
plumes will be characterized by optical time-gated imaging
and time- and space-resolved emission spectroscopy.
15
10
SP
5
1
10
100
1000
Delay ∆t (ps)
FIG. 3. 共Color online兲 Maximum ablation depth d as a function of delay ⌬t
between pulses. The solid curve is a fit to Eq. 共1兲. The dashed line indicates
the maximum ablation depth for SP irradiation.
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J. Appl. Phys. 108, 113302 共2010兲
Amoruso et al.
150
(a)
(a)
Δt (ps)
2
KE (eV)
1
10
100
1000
1
0
50
8
0
2
4
6
8
10
εtot (10
Time (µs)
50
Ion Yield
10
2
(10 ions/cm )
100
eV)
Ion current (mA)
3
12
113302-4
(b)
6
4
2
40
(b)
1
10
100 1000
Delay ∆t (ps)
30
20
1
10
100 1000
Delay Δt (ps)
FIG. 4. 共Color online兲 共a兲 Ion TOF signals for different delays between the
two laser pulses. 共b兲 Ion yield as a function of the delay between the pulses.
The solid line is a fit to Eq. 共2兲.
absorption in the nascent laser-produced plasma, as invoked
in Refs. 1 and 20. Therefore, the interpretation of these experimental findings can benefit from the analysis of other
features of the DP ULA process, as for instance the atomic
and NP plume characteristics, which will be discussed hereafter.
B. Ion probe results
The influence of a second delayed pulse on the properties of the ion plasma produced during the DP ULA process
was investigated by recording ion time-of-flight 共TOF兲 signals, for different delays ⌬t, with a Langmuir probe. Figure
4共a兲 reports some characteristic ion profiles for various values of ⌬t. Both amplitude and TOF at the peak vary with the
delay. In particular, as ⌬t increases from 1 ps to a few hundred picoseconds we observe a gradual rise of the signal
amplitude which peaks at shorter TOF, thus indicating an
increase in the ion yield and velocity. Conversely, at longer
delays 关see e.g., the profile at ⌬t = 1000 ps in Fig. 4共a兲兴 the
peak of the ion TOF profiles is gradually delayed while its
amplitude progressively decreases. This behavior points to
the existence of different interaction regimes as ⌬t varies in
a temporal window of several hundred picoseconds.
The variation in the collected ion yield was obtained by
integration of the ion profiles of Fig. 4共a兲. This is plotted in
Fig. 4共b兲, which includes additional data points corresponding to delays not shown in Fig. 4共a兲. At ⌬t ⬇ 1 – 2 ps the DP
ion yield is almost constant, it progressively increases reaching a maximum at ⌬t ⬇ 200 ps, and then decreases for
longer delays.
In Fig. 4共b兲, the growth of the ion yield Y for 1 ps
⬍ ⌬t ⬍ 200 ps is well described by an exponential function23
FIG. 5. 共Color online兲 共a兲 Average kinetic energy KE of the ions as a
function of the delay ⌬t between the pulses. 共b兲 Total energy of the ion flux
tot as a function of the delay ⌬t between pulses.
Y共⌬t兲 = Y 0 + ⌬Y共1 − e−⌬t/c兲,
共2兲
where Y 0 ⬇ 27⫻ 1010 ions/ cm2 corresponds to the ion yield
at zero delay, i.e., for a SP of twice the energy, ⌬Y ⬇ 16.5
⫻ 1010 ions/ cm2 is the total variation in the ion yield, and
c ⬇ 20 ps is a characteristic time constant of the process of
ion increase induced by the DP irradiation. The fact that the
exponential dependences of both the ion yield in Fig. 4 and
the ablation depth in Fig. 3 are characterized by a very similar time constant c ⬇ 20 ps, supports the idea that the observed changes are related to the same physical mechanisms
for 1 ps⬍ ⌬t ⬍ 200 ps. At longer delays 共⌬t ⬎ 200 ps兲, instead, the ion yield drops down while the ablation efficiency
seems to remain almost constant, thus indicating a progressive transition to another regime of the DP ULA process.
From the TOF signals in Fig. 4共a兲, the average ion kinetic energy 共KE兲 was calculated and plotted in Fig. 5共a兲. For
DP at ⌬t ⬇ 1 – 2 ps KE⬇ 100 eV, then it increases as the
delay is increased, such that at ⌬t = 50 ps it is ⬇130 eV. By
further increase in the delay, KE decreases reaching values of
⬇50–60 eV which is even smaller than that observed at very
short delays. From Figs. 4共b兲 and 5共a兲, we observe that the
maximum of the ion yield and average energy is reached at
two different delays, namely, ⬇200 ps and ⬇50 ps. Therefore, we analyzed the dependence on ⌬t of the total energy
of the ion flux collected by the probe tot共⌬t兲 = Y共⌬t兲
⫻ KE共⌬t兲 ⫻ S p, where S p is the ion probe area. This is shown
in Fig. 5共b兲. tot shows a strong dependence on ⌬t: while
there is little change for short delays 共⌬t ⬇ 1 – 2 ps兲, an increase in almost a factor of 2 is observed from short delay to
⌬t ⬇ 50 ps, where it reaches a maximum. For still longer
delays 共⌬t ⬎ 50 ps兲, tot gradually reduces reaching values
even lower that the one observed at the shortest delays of
1–2 ps when ⌬t ⱖ 650 ps as a consequence of the combined
reduction in the average energy of the ions KE and of its
yield Y. The significant reduction in the average kinetic energy at long delays seems to be more related to the presence
of ions with lower kinetic energy than to a decrease in the
ion yield 关see Figs. 4共b兲 and 5共a兲兴. To clarify this aspect, Fig.
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113302-5
Amoruso et al.
J. Appl. Phys. 108, 113302 共2010兲
Δt (ps)
Relative intensity (arb. units)
1
5
50
Δt (ps)
1
500
1500
0
2
4
6
8
Time (µs)
FIG. 6. 共Color online兲 Examples of ion TOF profiles normalized to their
corresponding area 共relative intensity兲 at some delays showing the variation
in the relative contribution of fast 共small TOF, upper panel兲 and slow 共large
TOF, lower panel兲 ions to the TOF profile. In the lower panel the TOF
profile at ⌬t = 1 ps is shown in gray to facilitate the comparison.
6 shows some typical ion profiles from Fig. 4共a兲 normalized
to the same area, from which the variation in the fast 共small
TOF兲 and slow 共large TOF兲 ion contributions can be appreciated. Tacking ⌬t = 1 ps as a reference TOF signal, in Fig. 6
one can notice: 共i兲 an increase in the faster component with
increasing delay up to ⬇50 ps and 共ii兲 a progressive reduction in the fast ions with a simultaneous increase in slow ions
for long delays 共e.g., ⌬t = 500 ps and 1500 ps兲.
FIG. 7. 共Color online兲 共a兲 Snapshots of the atomic plume emission at different delays ⌬t, acquired 200 ns after the ablating laser pulse with a gate
width of 50 ns. 共b兲 Snapshots of the NP plume emission at different delays
⌬t, acquired ⬇10 s after the ablating laser pulse with a gate width of
5 s. The delay ⌬t is reported on the top of each image. The number in
parenthesis in the top-left corner of the image indicates that the intensity has
been multiplied by that factor. z = 0 marks the position of the target surface.
The intensity is plotted in linear scale.
7共a兲. The intensity is displayed on a logarithmic scale to
compensate for the very different emission signal of the ablated species. In particular, in Fig. 8 we can identify:27 共i兲 an
intense signal at ⬇490 nm located at the atomic plume front,
which corresponds to the characteristic emission of excited
ions 共Cu+兲 resulting from the overlap of different Cu+ strong
C. Optical imaging and spectroscopy
The effects of the second delayed pulse on the properties
of the atomic and NP plumes produced during the DP ULA
process was investigated by recording 2D single shot images
of the plumes emission at various delays ⌬t. Figure 7共a兲
shows 2D maps of the atomic plume emission registered 200
ns after the ablating laser pulse. The region of high intensity
clearly visible at 2 and 20 ps close to the target surface is the
NP plume which has not yet fully expanded. The 2D maps of
the NP plume emission after its expansion are shown in Fig.
7共b兲 which reports images registered ⬇10 s after the ablating laser pulse. Figure 7共a兲 indicates a progressive increase in the atomic plume emission intensity with the delay
⌬t, which is accompanied by a shortening of the plume extension along the direction z normal to the target surface.
Moreover, the region of maximum intensity of the atomic
plume moves toward the target surface as ⌬t increases. In
Fig. 7共a兲, one can also observe that a distinct spatial separation between the atomic and NP plumes is present at short
delays 共⌬t = 2 and 20 ps兲. This separation gradually reduces
as ⌬t increases, the two ablation components 共atoms and
NPs兲 becoming nearly connected for larger values of ⌬t. A
strong effect of the delay ⌬t on the NP plume characteristics
is evidenced in Fig. 7共b兲: both the NP plume emission intensity and spatial extension along the z-axis are significantly
reduced as the delay ⌬t increases.
Figure 8 shows 1D spectrally resolved images of the
atomic plume emission corresponding to the images of Fig.
FIG. 8. 共Color online兲 1D spectrally resolved images of the atomic plume
emission at different delays ⌬t, acquired 200 ns after the ablating laser pulse
with a gate width of 50 ns. The delay ⌬t is reported in the top-right corner
of each image. z = 0 marks the position of the target surface. The intensity is
plotted in logarithmic scale.
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113302-6
Amoruso et al.
J. Appl. Phys. 108, 113302 共2010兲
Optical emission yield (arb. units)
50
FIG. 9. 共Color online兲 1D spectrally resolved images of the atomic plume
emission at different delays ⌬t, acquired 2 s after the ablating laser pulse
with a gate width of 1 s. The delay ⌬t is reported in the top-right corner
of each image. z = 0 marks the position of the target surface. The intensity is
plotted in logarithmic scale. The intensity scale is two times that used in Fig.
7.
lines, as well as a faint Cu+ emission at 505.18 nm; 共ii兲 some
characteristic, persistent emission lines at 510.55, 515.32,
and 521.82 nm due to excited neutrals 共Cuⴱ兲 contributing to
the very intense core of the atomic plume emission of Fig.
7共a兲 and some other weaker Cuⴱ emission lines 共e.g., 406.2
nm and 578.2 nm兲; 共iii兲 the typical structureless, broadband
emission of a slow population of NPs following the faster
atomic plume 共see the upper panel at ⌬t = 2 ps, e.g.兲,28 which
are still very close to the target surface at 200 ns after the
ablating laser pulse 关see Fig. 7共a兲兴.
The spectra of Fig. 8 shows a continuous increase in the
Cuⴱ emission intensity with the delay ⌬t, while the variation
in the Cu+ emission signal is nonmonotonic, first rising up
and reaching a maximum at ⌬t ⬇ 100 ps, and then declining
at larger delays. Besides, the NP broadband emission significantly decreases with the delay ⌬t. This is still more evident
in Fig. 9, where the 1D spectrally resolved images registered
2 s after the ablating pulse are reported. Moreover, the
regions of maximum emission of Cuⴱ and Cu+ in Fig. 8
gradually move toward the target surface as ⌬t increases, the
atomic emission lines approaching the NP region of emission
close to the target surface 共z = 0兲 at the larger delays, as already observed before in the images of Fig. 7共a兲. In the spectra of Fig. 9, the presence of slower Cuⴱ atoms at increasing
delays can also be ascertained, thus suggesting that a component of low-energy excited neutrals is formed in the
atomic plume at longer delays, similarly to what has been
already observed for the ions in Sec. III B.
A more quantitative characterization of the dependence
of the three different plume components 共Cu+, Cuⴱ, and NPs兲
with the delay ⌬t between the pulses was obtained by recording the overall emission spectra of the atomic and NP
plumes, and spatially and spectrally integrating the different
Cu
(a)
+
SP (x3)
0
200
(b)
NPs
100
0
1500
SP
(c)
Cu*
1000
500
0
SP (x3)
1
10
100 1000
Delay ∆t (ps)
FIG. 10. 共Color online兲 Optical emission yield of the various plume components as a function of the time delay ⌬t between the pulses: 共a兲 Cu+; 共b兲
NPs; and 共c兲 Cuⴱ. The dashed lines show the values corresponding to SP
irradiation. The solid lines are fits to an exponential dependence of the
emission intensity on ⌬t 关see Eqs. 共2兲 and 共3兲兴.
contributions. The results are summarized in Fig. 10; the
dashed-dotted line in each plot shows the value corresponding to the SP irradiation. The optical emission yield of Cu+
obtained by integrating the signal at ⬇490 nm and reported
in Fig. 10共a兲 shows a behavior very consistent with that observed for the ions yield collected by the Langmuir probe in
Fig. 4共b兲. The curve in Fig. 10共a兲 is a fit of the Cu+ optical
emission yield to Eq. 共2兲 for 1 ps⬍ ⌬t ⬍ 200 ps 共Y 0 ⬇ 6.7
and ⌬Y ⬇ 41兲 which yields the same characteristic time constant c ⬇ 20 ps already observed for the collected ion yield
in the interval 1 ps⬍ ⌬t ⬍ 200 ps. Figure 10共b兲 reports the
NPs emission yield versus ⌬t. The NP signal is significantly
reduced for 1 ps⬍ ⌬t ⬍ 200 ps, following a trend which is
well described by an exponential dependence similar to Eq.
共1兲, i.e.,
Y NP共⌬t兲 = Y NP,0 − ⌬Y NP共1 − e−⌬t/c兲,
共3兲
with a characteristic time constant c ⬇ 20 ps 关solid curve in
Fig. 10共b兲兴. At larger delays 共⌬t ⱖ 300 ps兲 the data are somewhat scattered, but still remain at rather low values being
approximately described by a plateau regime 关dashed curve
in Fig. 10共b兲兴. Interestingly, the same functional behavior of
the ablation efficiency and NP optical yield suggests that
decrease in the NP emission signal is mainly due to a reduced ablation efficiency of DP ULA, consistent with the
observation that the ablated material of an ULA process
mainly decomposes into a nanoparticulate form.2,6,7,14 The
reduction in the NP optical yield and ablation efficiency for
1 ps⬍ ⌬t ⬍ 200 ps is also very well correlated with the enhancement of the ion yield. This strict association can be
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J. Appl. Phys. 108, 113302 共2010兲
Amoruso et al.
NPs yield (arb. units)
113302-7
200
1
2
150
5
10
100
20
50
100
1500
200
1750 1250
800
1000
2000
650 500
50
0
0
20
40
60
+
Cu yield (arb. units)
FIG. 11. 共Color online兲 NP vs Cu+ optical emission yields. The straight line
is a linear fit showing a strict correlation between the two quantities for
1 ps⬍ ⌬t ⬍ 200 ps 共the correlation coefficient is r = −0.984兲.
clearly appreciated by plotting the values of Cu+ and NP
optical emission yield against each other as in Fig. 11. The
data are very well correlated in the range 1 ps⬍ ⌬t
⬍ 200 ps 共the correlation coefficient is r = −0.984兲, thus suggesting that their opposite variation on ⌬t is due to the same
underlying physical mechanisms. Instead, for ⌬t ⱖ 300 ps,
the link between these two physical quantities is progressively reduced being finally lost as ⌬t increases further.
The optical emission yield of Cuⴱ, obtained by integrating the signals of the 510.55, 515.32, and 521.82 nm emission lines, is shown in Fig. 10共c兲. The atomic emission is
also well described by an exponential dependence 关Eq. 共2兲兴
for ⌬t ⱕ 800 ps but with a larger characteristic time constant
c ⬇ 400 ps. This exponential trend is then followed by another regime characterized by a further increase in the emission signal. It is likely that this different behavior of the
optical emission yield of the excited neutrals results from the
generation of the low-energy component whose contribution
was observed to progressively increase with the delay ⌬t.
Therefore, the optimal conditions for the enhancement of
excited neutrals seem to be rather different than those for the
ions, in our experimental conditions.
IV. DISCUSSION
The experimental results illustrated in the previous section show the existence of different regimes of interaction in
DP ULA of a metallic target resulting from the interplay
between the different mechanisms involved in the process.
Particularly, interesting is the correlation between the reduction in the ablation efficiency and NP plume emission intensity and the increase in the ion yield for delays ⌬t
ⱕ 100 ps. Besides, at larger delay it is noticeable that both
ablation efficiency and NP plume emission tends to a plateau
while the optical emission from neutrals is further enhanced
and the ions signal decreases. These experimental observations refer to a fluence per pulse of ⬇4 times the ablation
threshold, a regime of interest for a deeper understanding of
the mechanisms involved in the DP ULA process, as well as
for possible interesting outcomes in a class of ULA-based
applications, e.g., femtosecond laser machining, LIBS, NP
synthesis, plasma generation, and film growth through ultrafast pulsed laser deposition.
A reasonably clear picture of SP ULA of metals has now
emerged thanks to an extensive experimental work,7,10,11,15,16
and a number of theoretical analyses of the decomposition
mechanisms.5–9,12,13 However, DP ULA experiments and
modeling are still rather limited. In DP ULA the effects
brought about by the irradiation with a second pulse will
depend critically on how the near-surface region of the target
has evolved since the delivery of the first pulse and thus on
the delay ⌬t between the pulses. In particular, for a delay ⌬t
much less than the electron-lattice relaxation time el, the DP
irradiation is expected to be rather similar to one SP with the
same overall fluence. On the other hand, for a delay larger
than el the second pulse can interact either with a modified
target surface or with the complex, structured material progressively released from the target surface which finally
gives rise to the two distinct ablation plumes observed in the
far field: a faster-moving atomic plume and a slower-moving
NPs plume.15,28 For copper el is of the order of 10 ps.22
Povarnitsyn et al.22 have carried out hydrodynamic modeling of DP ULA of copper mainly highlighting the effect of
the delay ⌬t between the pulses on the target decomposition
mechanisms, which finally results in the decrease in the crater depth observed experimentally.1,2,20 This study confirmed
that when ⌬t is much shorter than el, both pulses contribute
to ablation, similar to the case of one pulse with twice the
fluence. For ⌬t of the same order of el, the second pulse
interacts with the front of the nascent ablation plume reheating the ablated material and generating a high-pressure region in the vicinity of the decomposing target surface. This,
in turn, produces a shock wave that reduces the intensity of
the ablating rarefaction wave in the target, thus leading to a
decrease in the ablation efficiency. Finally, for ⌬t much
longer than el, the ablation crater is mainly formed by the
first pulse while the second pulse interacts with the ablating
material without inducing any additional removal of material
from the target. This description is consistent with the behavior of the ablation efficiency of Figs. 1 and 3. In particular, in
Fig. 3 we observed a ⬇30% reduction in the ablation depth
at ⌬t ⬇ el, and a plateau for ⌬t ⬎ 100 ps.
The experimental results of Secs. III B and III C reveal
the changes induced in the ULA plumes by the interaction of
the second pulse with the ablating material, at the different
stages of the target decomposition and ablation process. Partial absorption of the second pulse in the front layer of the
nascent ablation plume explains the observed increase in
both the yield 关Figs. 4 and 10共a兲兴 and energy 共Fig. 5兲 of the
ions for ⌬t ⱕ 100 ps. This process seems to reach an optimal
state at ⬇50 ps, in our experimental conditions. The correlated reduction in the ablation efficiency and NP optical
emission 共see Figs. 3, 10, and 11兲 is also consistent with a
decrease in the energy coupling to the target. In this respect,
it is worth noting that in Ref. 22 an average absorption coefficient A ⬇ 0.35 for the second pulse was estimated at a
delay of ⬇50 ps. This absorption occurs in a front layer of
the nascent ablation plume with a thickness ⌬z ⬇ 200 nm,
which corresponds to an average linear absorption coefficient
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113302-8
Amoruso et al.
J. Appl. Phys. 108, 113302 共2010兲
␣ = −⌬z−1 ⫻ ln共1 − A兲 ⬇ 2 ⫻ 104 cm−1. By taking into account
that the average density n of the absorbing layer in Ref. 22 is
⬇7%–8% of the solid density 共for copper ns = 8.46
⫻ 1022 cm−3兲, we can estimate an absorption cross section of
the nascent ablation plume abs共=␣ / n兲 of the order of few
⫻ 10−18 cm2, at ⌬t ⬇ 50 ps.
In an attempt to clarify the optical properties of the front
layer produced in the first stages of the ULA process, we
have calculated the total absorption cross section of a highdensity, high-temperature material whose properties resemble the nascent front layer produced during ULA of metals. Molecular dynamics simulations and hydrodynamic
modeling of ULA of metals show that, at a delay of 20–100
ps after the ablation pulse, the front layer density n and temperature T are of the order of 共0.01– 0.1兲 ⫻ ns and several
thousand kelvin.6,7,9,22,29 The total optical absorption cross
section was obtained by using a spectral synthesis codes
共e.g., PRISMSPECT兲 共Ref. 30兲 modeling plasma ionization,
level populations, line broadening, and spectral emission and
absorption for a plasma with temperature and density given
above. The estimated absorption cross section abs at 527 nm
共2.35 eV兲 is always of the order of few⫻ 10−18 cm2 in the
density range 1019 – 1022 cm−3, at a temperature of several
thousand kelvin, in agreement with the prediction of Ref. 22.
This indicates a total absorption cross section of the front of
the ablation material which is only slightly reduced and of
the same order of magnitude of that for solid copper at room
temperature 共7.8⫻ 10−18 cm2兲. At an average fluence level
of few times the ablation threshold, which corresponds to
⬇1018 photons/ cm2, there is a significant probability
共⬇0.8–1.0兲 that the atoms of the front layer absorb photons
from the second pulse producing a temperature rise with a
consequent increase in the ionized and excited species in the
atomic plume, as experimentally observed.
To a first approximation it can be assumed that the observed increase in the ions and excited neutrals ⌬Y is proportional to the fraction of energy absorbed from the second
pulse, Eabs
⌬Y共⌬t兲 ⬀ Eabs ⬀ 1 − exp关− Aⴱ共⌬t兲兴,
共4兲
where Aⴱ共⌬t兲 is the absorbance of the nascent plume at delay
⌬t. Therefore, the yield Y共⌬t兲 at delay ⌬t can be expressed
as Y共⌬t兲 = Y 0 + ⌬Y共⌬t兲, Y 0 corresponding to the yield at short
delays 共⌬t Ⰶ el兲. Since the energy diffusion length into the
target is small and el is short, to a first approximation the
physical phenomenon occurring at the target surface is rather
similar to the one occurring after an impulsive heating of a
thin near-surface layer of a semi-infinite solid medium.29,31
In this case, a compression or shock wave propagates into
the bulk of material, while an unloading rarefaction wave
forms leading to material release and expansion from the
original target surface into vacuum. The front of the rarefaction wave expands with a characteristic velocity uexp while
the density profile n共z , ⌬t兲 of the released material is characterized by an exponential profile31,32
冉
n共z,⌬t兲 = n0 exp −
z
uexp⌬t
冊
,
共5兲
where n0 is the density at the position z = 0 of the original
surface and is given by n0 = ns / e, where ns is the atomic
density of the target material and e is the Neper number. In
this respect, it is worth observing that an approximately exponential density distribution of the atomic front layer of the
ablation material produced during ULA was observed by ultrafast time-resolved x-ray absorption spectroscopy in Ref.
15, though the measurements reported in the paper refer to
much longer delays 共ⱖ500 ns兲. In ULA, as time ⌬t elapses
after the laser pulse, the rarefaction wave continues to add
material to the nascent plume. The average absorbance
Aⴱ共⌬t兲 is given by
Aⴱ共⌬t兲 =
冕
⬁
n共z,⌬t兲abs共z,⌬t兲dz ⬇ n0absuexp⌬t,
共6兲
0
where an average value abs has been considered due to the
slowly varying dependence of the absorption cross section
on the layer physical properties. By using Eqs. 共6兲 and 共4兲
the observed exponential dependence of Eq. 共2兲, which fits
fairly well the experimental data of Figs. 4共b兲 and 10共a兲, is
finally obtained. An estimate of the characteristic time constant of such dependence can be obtained by considering
Aⴱ共兲 ⬇ 1, which yields ⬇ 共n0absuexp兲−1. Both experimental
characterization15 and modeling29 of the initial stages of
ULA of a metallic target show an expansion velocity of the
nascent ablation plume of the order of uexp
⬇ 105 – 106 cm/ s, while the velocity of the density maximum can be about ten times less. Therefore, for n0 ⬇ few
⫻ 1022 and abs ⬇ few⫻ 10−18 cm2, values of the characteristic time constant of the order of ⬇10–100 ps are fully
justified and indeed experimentally observed by us and
others.2,23 The fact that Eq. 共2兲 describes rather well the experimental data for ⌬t ⱕ 100 ps also indicates that the properties of the front of the nascent ablation plume vary slowly
in this regime, probably as a consequence of the continuous
supply of material from the target over this time interval.
In the regime where the second laser pulse is efficiently
absorbed 共⌬t ⱕ 100 ps兲, the ablation efficiency falls significantly 共see Fig. 3兲, as a consequence of both a reduced energy transfer into the target and of the effects brought about
by the pressure wave induced by the interaction of the second laser pulse with the expanding material, as discussed in
Ref. 22. Since in ULA the deeper layers of the original target
mainly decompose in the form of nanoaggregates, the reduced ablation efficiency is also manifested by a reduction in
the NP emission signal intensity 关Fig. 10共b兲兴. Moreover, the
absorption of the second laser pulse also causes the observed
increase in ionization, which explains the strict correlation
observed in this regime between the NP plume emission intensity and the ion yield 共Fig. 11兲. By considering that the
contribution of the second pulse to the ablation efficiency is
proportional to the fraction of its energy reaching the target
one can derive the dependence reported in Eq. 共1兲, which fits
rather well the experimental data of Fig. 3. In this respect, it
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113302-9
Amoruso et al.
is also worth noting that Eq. 共1兲 also describes fairly well the
theoretical predictions of the ablation depth reported in Ref.
22.
As the delay ⌬t increases beyond a value around 100 ps,
both the density and temperature of the absorbing front layer
progressively decreases and its absorption cross section at
the laser wavelength drops down. Moreover, in some cases
hydrodynamic modeling also predicts a separation of the
front layer from the rest of ablating material after a certain
delay, indicating the end of the material supplied to this front
component of the ablation plume. Therefore, the atomic front
layer becomes gradually more transparent to the second laser
pulse, which explains the reduction in the ions yield at larger
delays observed in Figs. 4共a兲 and 10共a兲. Consequently, an
increasingly larger fraction of the second pulse reaches the
inner regions of the ablating material. Since the deeper regions decompose in the form of a hot-liquid-phase material
which relaxes into a nascent NP plume expanding at lower
velocity,7,22,29 this interaction eventually produces further atomization and ionization through NP break-up processes,
thus leading to the occurrence of an atomic emission increasing over a longer time interval with respect to ions, as observed in Fig. 10. At very long delays 共⌬t ⬎ 500 ps兲, the
additional increase in this emission evidences a still more
efficient coupling of the second pulse with the inner regions
of the ablating material. It seems also that in our experimental conditions this interaction does not produce a simultaneous increase in ionized species. This can be understood if
one compares this situation to SP ULA in the same experimental conditions,23 where only an extremely faint ion emission signal is observed at the same fluence level. A similar
behavior of the ions and excited neutrals emission was instead observed at comparable delays for Ti in Ref. 4 and for
Cu in Ref. 33. This seems to suggest a rather significant
influence of the specific experimental conditions 共e.g., laser
pulse fluence, wavelength, and target physical properties兲 on
the interaction between the second pulse and the nascent ablation material, where the coupling of the second laser pulse
to the hot-liquid-phase material ejected later in time can be
strongly mediated by the specific properties of the nascent
ablation plume. This is indeed supported by the reduction in
the DP delay needed to approach the SP ablation depth at
larger fluences experimentally observed in DP ULA of silver
targets.21
Finally, in this regime 共200ⱕ ⌬t ⱕ 2 ns兲 both the ablation efficiency 共Fig. 2兲 and NP emission 关Fig. 10共b兲兴 remain
low, indicating that the second pulse does not cause any additional emission of material from the target. This is probably due to the interaction of the second pulse with the decomposing hot-liquid-phase material which hinders efficient
transport of energy to the rest of the target. Efficient coupling
of the second laser pulse energy to the target is predicted to
occur only after delays of the order of tens of nanoseconds or
more,22 when the material ablated by the first pulse becomes
completely transparent, a timescale which has not yet been
achieved in DP experiments.
J. Appl. Phys. 108, 113302 共2010兲
V. CONCLUSION
In this investigation, experiments aiming to explore various aspects of DP ULA of a metallic target were carried out
with the aim of addressing the changes to the ablation efficiency and to the properties of the ablation plumes as a function of the delay ⌬t between the pulses. Our experimental
results confirm the reduction in the ablation efficiency with
the delay ⌬t reported earlier by us and others,1,2,20,21 interpreted in terms of the interaction of the second pulse with the
front of the nascent ablation plume reheating the ablated material and to the effects brought about by such interaction.
The analysis of the ablated species allowed us to identify the
other effects of this interaction and the characteristic timescale of the induced changes. In particular, an interesting
increase in the ion yield was observed for ⌬t ⱕ 100 ps,
which is well correlated with the reduction in the NP yield
and ablation depth.
A simple model based on impulsive heating of a thin
near-surface layer of a semi-infinite solid medium and on the
exponential density profile of the released material corroborates the experimentally observed dependencies, also providing estimate of the characteristic time constant in fairly good
agreement with the experimental findings. The good consistency of the model predictions and experimental observations suggests that this relatively simple model describes the
main features of DP ULA of metallic targets.
We consider that several of the features observed in the
present investigation on a copper target can be considered as
rather general aspects of the DP ULA of metals at moderate
laser intensities. The experimental results obtained and interpretation will be particularly helpful in the design of strategies to improve the control and/or manipulate the characteristics of the plumes of material generated in ULA processes.
This, in turn, is important for the various applications of
ULA in fields like material deposition and film growth, NP
generation, material microprobe analysis with LIBS, laser
produced plasmas, and so forth.
ACKNOWLEDGEMEMTS
We acknowledge financial support from Science Foundation Ireland under Grant No. 09/RFP/PHY2422.
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