Vol.3, No.6, 488-495 (2011) Natural Science
http://dx.doi.org/10.4236/ns.2011.36068
Thermal response and optical absorptance of metals
under femtosecond laser irradiation
Anatoliy Y. Vorobyev, Chunlei Guo
The Institute of Optics, University of Rochester, Rochester, USA; [e-mail], [e-mail]
Received 15 March 2011; revised 10 April 2011; accepted 27 April 2011.
ABSTRACT
A detailed study on correlation between residual
thermal response of a sample and its optical
absorptance change due to laser-induced sur-
face structural modifications in multi-shot fem-
tosecond laser irradiation is performed. Ex-
periments reveal an overall enhancement for
residual thermal coupling and absorptance in air.
Surprisingly, residual thermal coupling in air
shows a non-monotonic dependence on pulse
number and reaches a minimum value after a
certain number of pulses, while these behaviors
are not seen in absorptance. In vacuum, how-
ever, both suppression and enhancement are
seen in residual energy coupling although ab-
sorptance is always enhanced. From these ob-
servations, it appears that air plasma plays a
dominant role in thermal coupling at a relatively
low number of applied pulses, while the forma-
tion of craters plays a dominant role at a high
number of pulses.
Keywords: Femtosecond Laser; Ablation; Residual
Energy; Absorptance; Surface Structures
1. INTRODUCTION
Many applications of femtosecond lasers, such as
high-precision materials machining [1,2], nanotechnol-
ogy [3-7], modification of optical properties of materials
[8-12], thin film deposition [13], modification of wetting
properties of solids [14-17], producing diamond-like
materials [18], laser plasma thrusters [19], and biomedi-
cine [17,20], are based on laser ablation of solids. Al-
though femtosecond laser ablation have been extensively
studied in the past [21-27], many of the physical mecha-
nisms remain unclear. For example, recent studies have
shown that the residual thermal energy in an irradiated
metal sample abruptly increases following both sin-
gle-pulse and pulse-train femtosecond laser ablation in a
gas medium when the laser fluence is above a certain
threshold [28,29]. In the case of pulse-train ablation [29],
it was found that one contributing factor to the enhanced
thermal coupling following a large number of applied
pulses is an increase of sample absorptance due to sur-
face structural modifications. However, we have shown
that the surface modifications alone cannot fully explain
the enhanced thermal energy deposition [29]. To further
study the effects of the enhanced thermal coupling to
metals, in this paper, we perform a shot-to-shot detailed
study on how residual energy correlates to the absorp-
tance change due to surface structural modifications. We
observe an overall enhancement for residual thermal
coupling and absorptance in air. Surprisingly, residual
thermal coupling in air shows a non-monotonic depend-
ence on pulse number and reaches a minimum value
after a certain number of pulses, while these behaviors are
not seen in absorptance. In vacuum, however, both sup-
pression and enhancement are seen in residual energy
coupling although absorptance is always enhanced. To
explain these observations, we suggest that air plasma
plays a dominant role in thermal coupling at a relatively
low number of applied pulses while the formation of cra-
ters plays a dominant role at a high number of pulses.
2. EXPERIMENTAL SETUP
The laser used in the experiment is an amplified
Ti:sapphire system generating 65-fs pulses of about 1.5
mJ/pulse at a 1-kHz repetition rate with the central wa-
velength at 800 nm. Our experimental setup is shown in
Figure 1. To produce ablation, the laser beam is focused
onto a platinum (Pt) bulk sample at normal incidence.
The Pt sample is mechanically polished before experi-
ments. The number of laser pulses, N, delivered to the
sample is controlled by a fast electromechanical shutter.
Following pulse irradiation, some directly absorbed
laser energy will be carried away by the ablated material,
while a fraction of the absorbed pulse energy will remain
in the surface layer of the sample, dissipate into the bulk
sample due to heat conduction, and remain in the sample
as residual thermal energy. The remaining energy fol-

A. Y. Vorobyev et al. / Natural Science 3 (2011) 488-495 489
Figure 1. Experimental setup for studying both residual ther-
mal coupling and absorptance change of a metal due to surface
structural modifications following pulse irradiation.
lowing irradiation will cause the bulk sample tempera-
ture to rise. Furthermore, there can be indirect mecha-
nisms contributing to the residual heating of the sample,
such as re-deposition of the hot ablated material back
onto the sample and exothermic chemical reactions. To
characterize the total residual thermal energy coupling to
the sample, we define a residual energy coefficient
(REC), K, as K = ER/EI, where ER is the total residual
energy remaining in the sample following pulse irradia-
tion and EI is the pulse energy incident onto the sample.
To measure ER, we apply a calorimetric technique that
has been described in our previous studies [28,29].
Briefly, we measure the bulk temperature rise ΔTR with a
thermocouple battery following ablation and determine
ER calorimetrically as ER = mCΔTR, where m is the mass
and C is the known specific heat capacity of the sample.
To measure EI, a fraction of the incident pulse energy is
split off by a beamsplitter and diverted to a pyroelectric
joulemeter. This calorimetric technique can also be used
to study the change of the sample absorptance due to
laser-induced surface modifications by applying the fol-
lowing procedure that has also been described in our
previous work [9]. To determine the absorptance of a
damaged material following pulse irradiation, we at-
tenuate the laser fluence with neutral density filters to a
level much below the ablation threshold and apply the
same calorimetric technique as used in determining REC.
Briefly, after ablation we will irradiate the ablated spot
using a train of low-fluence laser pulses that will not
produce further surface modifications. The absorbed
fraction of this low-fluence pulse train energy, EA, will
dissipate into the sample due to heat conduction and
cause the bulk temperature of the whole sample to rise
by ΔTA. We measure ΔTA with the thermocouple battery
and determine calorimetrically the energy EA absorbed
by the sample. Having also measured the total energy of
the low-fluence pulse train, EIT, the absorptance of the
sample following laser ablation is determined as A =
EA/EIT. Thus, our calorimetric technique allows us to
measure both REC and absorptance of the ablated sam-
ple and to determine the relationship between REC and
absorptance. Our experimental procedure is as follows.
We apply the first laser pulse to an undamaged Pt sam-
ple with fluence above the ablation threshold and deter-
mine REC. We then attenuate the beam with the neutral
density filters and measure the absorptance of the irradi-
ated spot. Next, we remove the filters, apply the second
high-fluence pulse, and determine REC; we then reinsert
the filters and measure the absorptance A following
two-pulse ablation. This measurement is repeated at the
same sample spot, so we can determine both REC and A
as a function of pulse number N. Our experiment is car-
ried out in both 1-atm air and in a vacuum at a base
pressure of 6  10–3 Torr. The number of applied laser
shots is varied from 1 to 1000. Following pulse irradia-
tion, the irradiated spots are examined under a scanning
electron microscope (SEM).
3. EXPERIMENTAL RESULTS
Following femtosecond laser irradiation, the residual
energy coefficient K(N) and low-fluence absorptance
A(N) are studied as functions of the number of applied
laser pulses, N, at single-pulse laser fluence of F = 9.0
and 3.6 J/cm2 in both air and vacuum. The single-pulse
ablation threshold Fabl for the mechanically polished Pt
surface is determined to be Fabl = 0.15 J/cm2. The num-
ber of pulses required to drill through a 0.25-mm-thick
Pt sample in air is determined to be 920 and 1530 at F =
9.0 and 3.6 J/cm2, respectively. This gives an average
ablation rate of 272 and 163 nm/pulse for the two flu-
ence levels. As a reference, we also measure the low-flu-
ence absorptance A0(N) for an undamaged surface at
fluence much below the ablation threshold. This A0(N) is
shown with the cross symbols in Figure 2.
One can see that A0 is a constant value of about 0.34
and is independent of the number of applied shots, indi-
cating that this laser fluence is indeed low enough and
no heat accumulation effects are involved. This value of
A0 (~0.34) agrees with the table value of the absorptance
for a mechanically polished Pt at  = 800 nm [30]. The
dependences of REC and absorptance on the number of
ablation shots at F = 9.0 J/cm2 in 1-atm air are shown in
Figure 2. We notice two distinct regions in the behavior
of K(N) and A(N). For N < 12, REC decreases with N
while for N > 12, REC increases with N. An important
feature in the behavior of REC is that the highest REC
value (K = 0.76) is reached by only one single pulse ab-
lation although the absorptance of the undamaged sur-
face before the first pulse irradiation is the lowest (A =

A. Y. Vorobyev et al. / Natural Science 3 (2011) 488-495 490
110100 1000
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0 2
Ablation in air at F = 9.0 J/cm
REC
Absorptivity after ablation
Absorptivity of undamaged surface
K, A, A0
Number of shots
Figure 2. The pulse number dependences of residual energy
coefficient and absorptance for platinum following ablation at
F = 9.0 J/cm2 in 1-atm air.
0.34). Following the second pulse irradiation, REC de-
creases from 0.76 to 0.64 although the sample absorp-
tance (A = 0.5) before the second pulse is much higher
than the undamaged surface value. For 2 < N < 12, REC
decreases while the absorptance remains virtually the
same. These data clearly show that for low N the en-
hanced absorptance compared to the undamaged surface
is not the dominant factor for the enhanced residual
thermal heating. For N > 20, REC starts to increase with
N and reaches a saturation level of about 0.75 at N ≈ 200.
Therefore, in the range of N studied in this experiment,
about 50% - 75% of incident pulse energy remains in the
sample as residual thermal energy (thermal load). The
A(N) data in Figure 2 show that the absorptance en-
hances from 0.34 (undamaged surface) to 0.5 following
single-pulse ablation, then remains approximately con-
stant for N < 30. To determine the surface structures as-
sociated with the absorptance enhancement, we take
SEM images of the sample surface. As shown in Figure
3, the enhanced surface absorptance mainly results from
the laser-induced micro- and nano-roughness [9]. With
further increasing N, we observe the formation of an
ablation-induced crater (see Figure 4(a)) and both REC
and absorptance increase as the cavity develops. From
Figure 2, we can see that A surpasses K when N > 600.
The behaviors of A0(N), A(N), and K(N) measured in a
vacuum are shown in Figure 5. We can see that the
low-fluence absorptance A0(N) of an undamaged surface
is about the same as the value measured in air. The over-
all behavior of A(N) in vacuum is similar to that in air,
although the absorptance enhancement due to surface
structural modifications is greater in vacuum compared
to that in air. However, the dependence of K on N in va-
cuum is very different from that in air. We can see that
Figure 3. SEM images of the surface morphology of the ir-
radiated spot after one-shot ablation of platinum at F =
9.0.J/cm2 in 1-atm air: (a) the general view of the ablated
spot; (b) micro- and nano-structural features on the periphery
of the ablated spot; (c) spherical nanoparticles re-deposited in
the central area of the ablated spot; (d) magnified view of the
peripheral surface structures shown in (b). The diameter of
the irradiated spot is about 100 m.
Figure 4. SEM images showing the onset of
the cavity formation following 20-shot ab-
lation at F = 9.0 J/cm2 in air (a) and in vac-
uum (b).
following the first pulse ablation, REC reaches a value
of 0.15 in vacuum, well below the value of absorptance
of the undamaged surface. This low REC value in vac-
uum contrasts with a much higher REC value we ob-
serve in air (K = 0.76 higher than the absorptance value).
For N between 2 and about 20 shots, REC remains to be
nearly constant at 0.15 despite the absorptance has in-
creased from 0.44 to 0.74. For N > 20, REC increases
significantly and reaches a value of about 0.85 at a large

A. Y. Vorobyev et al. / Natural Science 3 (2011) 488-495 491
110100 1000
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
2
Ablation in vacuum
at F = 9.0 J/cm
REC
Absorptivity after ablation
Absorptivity of
undamaged surface
K, A, A0
Number of shots
Figure 5. The pulse number dependences of residual en-
ergy coefficient and absorptance following ablation at F
= 9.0 J/cm2 in a vacuum at a base pressure of 6  10–3
Torr.
N. In the entire range of N, we notice that K < A for ab-
lation in vacuum, which contrasts with the relationship
of K > A in air. We also take SEM images of surface
morphology following ablation in vacuum under the
same ablation conditions as in air and some representa-
tive images are shown in Figure 6. A comparative anal-
ysis of the images taken for ablation in air versus in va-
cuum shows that the central area of the ablated spot in
vacuum is free of redeposited nanoparticles (compare
Figure 6(c) with Figure 3(c)) although a small amount
of redeposited particles may be observed on the periph-
eral area of the ablated spot. Figure 5 shows that both
K(N) and A(N) begin to increase significantly for N in
the range of 10 - 20 shots in vacuum. The SEM study
reveals that these enhancements correlate with the cavity
formation (see Figure 4(b)). Therefore, for ablation both
in air (Figures 2 and 4(a)) and in vacuum (Figures 5
and 4(b)), the onset of the significant REC and absorp-
tance enhancements occurring at N ≈ 10 - 30 corre-
sponds to the formation of a crater.
To study the effects of laser fluence on REC and ab-
sorptance in multipulse ablation, we also measure K(N)
and A(N) following ablation at lower fluence of 3.6
J/cm2. The obtained results are plotted in Figures 7 and
8. The comparison of these lower-fluence data with the
higher-fluence data in Figures 2 and 5 shows that the
lower laser fluence mostly affects K(N), and we observe
the relationship of K < A at F = 3.6 J/cm2 as opposed to
K > A at F = 9.0 J/cm2. To further understand the effects
of laser fluence on residual thermal coupling, we meas-
ure the fluence dependence of K following single-pulse
(N = 1) ablation in air, neon, and vacuum by irradiating a
fresh spot on the Pt sample after each shot. The behavior
of K(F) following a single femtosecond pulse ablation of
an undamaged surface is shown in Figure 9. We see that
Figure 6. SEM images of surface morphology of the irradiated
spot after one-shot ablation of platinum at F = 9.0 J/cm2 in
vacuum: (a) the general view of the ablated spot; (b) micro-
and nano-structural features on the periphery of the ablated
spot; (c) the magnified view of the central area of the ablated
spot (there is no re-deposited nanoparticle as compared to Fig.
3(c)); (d) the magnified view of the peripheral surface struc-
tures shown in (b). The diameter of the irradiated spot is 100
m.
110100 1000
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9 2
Ablaion in air at F = 3.6 J/cm
2
REC
Absorptivity after ablation
Absorptivity of undamaged surface
K, A, A0
Laser fluence, J/cm
Figure 7. The pulse number dependences of residual en-
ergy coefficient and absorptance following ablation at F =
3.6 J/cm2 in 1-atm air.
the three curves are virtually overlap on each other for F
< 1.2 J/cm2. However, these three curves start to branch
out for F > 1.2 J/cm2, i.e., REC increases both in air and
neon but decreases in vacuum. This measurement shows
that the specific type of ambient gas plays very little role
in affecting REC, but the presence of an ambient gas is
the dominant factor for the enhanced REC. To further
understand the ambient gas pressure effects on residual
thermal energy deposition, we measure the pressure de-
pendence of K and the data are shown in Figure 10. We
can see that REC decreases as the ambient air pressure is
lowered. This pressure dependence becomes insignifi-
cant in the low-pressure range, i.e., P < 0.03 Torr.

A. Y. Vorobyev et al. / Natural Science 3 (2011) 488-495 492
110100 1000
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0 2
Ablation in vacuum at F = 3.6 J/cm
REC
Absorptivity after ablation
K, A
Number of shots
Figure 8. The pulse number dependences of residual energy
coefficient and absorptance following ablation at F = 3.6
J/cm2 in a vacuum at a base pressure of 6  10–3 Torr.
0.01 0.1110
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
2
Ablation threshold
1-atm air
Vacuum
1.08-atm Ne
Residual energy coefficient
Laser fluence, J/cm
Figure 9. Residual energy coefficient for platinum as a func-
tion of laser fluence in 1-atm air, 1.08-atm Ne, and a vacuum
at a base pressure of 6  10–3 Torr.
0.010.1110100 1000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
2
F = 10 J/cm
Residual energyu coefficient
Air pressure, Torr
Figure 10. Residual energy coefficient for platinum as a
function of air pressure at F = 10 J/cm2.
4. DISCUSSIONS
Thermal response of metals following high-fluence
laser irradiation has been studied in the past using mi-
crosecond [31] and nanosecond [32] laser pulses, and
enhanced heat deposition in metals has been found in
those experiments when plasmas are generated. To ex-
plain those enhanced thermal energy coupling, it has
been suggested that energy transferring to a metal sam-
ple from laser-induced plasmas plays a key role [31].
Previously, we have found that the behavior of residual
thermal energy coupling shows a similar general trend
between nanosecond- and femtosecond-laser pulse abla-
tion [28], and this suggests that the physical mechanism
used to explain the long-pulse REC enhancement may
also account for the behaviors of residual heating in
femtosecond laser ablation. In general, two types of plas-
mas can be produced during a femtosecond laser pulse: 1)
solid-density plasma in the skin layer of the sample and
2) ambient gas plasma. Solid-density plasma can be
produced in both air and vacuum; in our experiment,
however, the enhanced thermal coupling occurs only in
air and therefore, we believe the solid-density plasma
does not play a key role in the enhanced energy coupling.
Therefore, we will subsequently focus our discussion on
the effects of ambient air plasma.
The experimental data shown in Figure 2 are obtained
with an intensity of about 1014 W/cm2. At this high in-
tensity, direct ionization of the ambient air during the
femtosecond laser pulse easily occurs by means of mul-
tiphoton and tunneling ionization even without the pres-
ence of a metal sample [33]. In the presence of a metal
surface, the threshold of air ionization in front of the
metal surface can be significantly reduced due to: 1) air
exposing to the additional laser light reflected from the
metal surface besides the incident light [34], 2) energetic
electrons escaping from the metal surface due to mul-
tiphoton photoelectron and thermoionic emission [35],
and 3) UV radiation from laser-induced solid-density
plasma, if any. The above three processes may addition-
ally contribute to air ionization besides the dominant
multiphoton ionization [34] from the incident laser light.
It should be emphasized that this air plasma is produced
prior to the hydrodynamic motion of the ablated material
from the sample that occurs on the nanosecond time
scale [36,37]. We note that the generation of air plasma
prior to the hydrodynamic motion of the ablated material
has been observed in the past in picosecond laser abla-
tion [38]. The difference between plasmas produced in
air and in vacuum can clearly be seen from the plasma
plume images in Figure 11, which are taken using an
open-shutter camera [28]. We can see that the size of hot
plasma generated following ablation in air is larger than
that in vacuum. Since material ejection in vacuum is not

A. Y. Vorobyev et al. / Natural Science 3 (2011) 488-495 493
constrained by ambient gas, plume formed in vacuum
should have a larger size than that in air if they are
formed only by the ejected metal particles. The fact that
we see a larger plume size in air rather than in vacuum
indicates that there must be air plasma generated by a
high-intensity laser pulse. The formation of air plasma
during femtosecond pulse can enhance thermal energy
coupling to a sample due to the following two pathways:
1) transferring the energy stored in the air plasma to the
sample [34], 2) enhanced re-deposition of the ablated
material back onto the sample due to the presence of a
high-pressure on the sample surface from the ambient
gas plasma long after the femtosecond laser pulse. The
decay time of the air plasma pressure,
 , is given by [39]
p
ra

, where r is the radius of the irradiated spot and
ap is the sound speed in the plasma. For our ablation
experiment in air, we estimate

to be about 10 ns with
r = 50 m and ap = 5 km/s [39]. In our experiment, we
observe a greater amount of particle re-deposition within
the ablation spot in air following a single pulse ablation,
see e.g., SEM images in Figures 3(c) and 6(c). From the
SEM images in Figure 12, we again see a greater
amount of material re-deposition around the crater in air
compared to that in vacuum following multi-pulse abla-
tion. Therefore, an enhanced re-deposition of the ablated
materials in air following both single-pulse and mul-
ti-pulse ablation is confirmed experimentally.
By taking into account the above-mentioned enhanced
thermal coupling assisted by air plasma, the behavior of
REC in Figure 2 can be explained as follows. For the
low N (N < 12) where REC decreases from 0.76 after
Figure 11. Open-shutter photographs
of the plasma generated by single-pulse
ablation at F = 3.3 J/cm2 in 1-atm air
and vacuum (P = 7 × 10–3 Torr). The
laser beam is incident normally onto
the sample from left.
Figure 12. SEM images of the re-deposited
materials on a platinum sample following
1500-pulse ablation at F = 3.3 J/cm2 in (a)
air and (b) vacuum. There is clearly more
material re-deposition around the crater in
air than in vacuum.
the first pulse to 0.65 after 12 pulses, laser ablation
mainly produces surface roughness (Figure 3) and no
crater is formed. This surface roughness will diffuse the
surface backscattering, reduce the amount of reflected
light absorbed by air plasma, and subsequently reduce
the air plasma-assisted heating of the sample. For N > 12,
a cavity starts to develop (Figure 4(a)), and this causes
REC to increase with N. The cavity effects on the en-
hanced REC can be more clearly seen from the REC
data in vacuum (Figure 5) where the air plasma effects
are excluded: we can see from the figure that, for N < 10,
REC is very low (K ~ 0.15) in vacuum without the con-
tribution of air plasma, but increases significantly with N
for N >10 when a cavity is formed (Figure 4(b)). The
results obtained here show a significant effect of the
reflected light on air plasma. However, a full under-
standing of this effect requires further experimental and
theoretical studies.
5. CONCLUSIONS
In this paper, we perform a shot-to-shot detailed study
on how residual thermal coupling depends on the ab-
sorptance change due to laser-induced surface structural
modifications. Surprisingly, our study shows that, with a
small number of applied pulses in air, the enhanced re-
sidual thermal coupling following the first-pulse ablation
decreases with pulse number despite of an enhanced
absorptance. Both residual thermal coupling and absorp-
tance increase with pulse number following a large
number of applied pulses. To explain our observation at

A. Y. Vorobyev et al. / Natural Science 3 (2011) 488-495 494
a low pulse number, we propose a plasma-assisted me-
chanism for femtosecond laser ablation in a gas medium.
Supported by our experimental results, we believe that
the formation of ambient gas plasma enhances thermal
energy coupling to a sample due to the following two
pathways: energy transfer from the air plasma to the
sample and enhanced re-deposition of hot ablated parti-
cles back onto the sample due to the presence of high
pressure from the ambient gas plasma. These plas-
ma-assisted effects can only play a role in a gas medium
and are most important following a relatively low num-
ber of applied shots when only surface roughness but not
a crater develops. At a large number of applied pulses,
the residual heating of the sample is mainly governed by
the formation of craters that enhance thermal energy
coupling to the sample both in air and in vacuum. Al-
though further studies are needed to better understand
the physical mechanisms of thermal energy coupling to
solids, our study provides new guidelines to determine
the optimal thermal-loading conditions for achieving
high-quality material processing using femtosecond laser
pulses.
6. ACKNOWLEDGMENTS
The authors acknowledge R. Grzegorzak for assistance with vacuum
equipment. The research was supported by the National Science Foun-
dation and US Air Force Office of Scientific Research.
REFERENCES
[1] Pronko, P.P., Dutta, S.K., Squier, J., Rudd, J.V., Du, D.
and Mourou G. (1995) Machining of sub-micron holes
using a femtosecond laser at 800 nm. Optics Communi-
cations, 114 , 106-110.
doi:10.1016/0030-4018(94)00585-I
[2] Chichkov, B. N., Momma, C., Nolte, S., von Alvensleben,
F. and Tunnermann, A. (1996) Femtosecond, picosecond
and nanosecond laser ablation of solids. Applied Physics
A, 63, 109-115. doi:10.1007/BF01567637
[3] Pereira, A., Cros, A., Delaporte, P., Georgiou, S., Ma-
nousaki, A., Marine, W. and Sentis, M. (2004) Surface
nanostructuring of metals by laser irradiation: Effects of
pulse duration, wavelength and gas atmosphere. Applied
Physics A, 79, 1433-1437.
doi:10.1007/s00339-004-2804-x
[4] Nolte, S., Chichkov, B.N., Welling, H., Shani, Y., Lie-
bermann, K. and Terkel, H. (1999) Nanostructuring with
spatially localized femtosecond laser pulses. Optics Let-
ters, 24, 914-916. doi:10.1364/OL.24.000914
[5] Amoruso, S., Ausanio, G., Bruzzese, R., Vitello, M. and
Wang, X. (2005) Femtosecond laser pulse irradiation of
solid targets as a general route to nanoparticle formation
in a vacuum. Physical Review B, 71, 033406.
doi:10.1103/PhysRevB.71.033406
[6] Koch, J., Korte, F., Bauer, T., Fallnich, C., Ostendorf, A.
and Chichkov, B.N. (2005) Nanotexturing of gold films
by femtosecond laser-induced melt dynamics. Applied
Physics A, 81, 325-328.
doi:10.1007/s00339-005-3212-6
[7] Vorobyev, A.Y. and Guo, C. (2006) Femtosecond laser
nanostructuring of metals. Optics Express, 14, 2164-2169.
doi:10.1364/OE.14.002164
[8] Wu, C., Crouch, C.H., Zhao, L., Carey, J.E., Younkin, R.,
Levinson, J.A., Mazur, E., Farrell, R.M., Gothoskar, P.
and Karger, A. (2001) Near-unity below-band-gap ab-
sorption by microstructured silicon. Applied Physics Let-
ters, 78, 1850-1852. doi:10.1063/1.1358846
[9] Vorobyev, A.Y. and Guo, C. (2005) Enhanced absorp-
tance of gold following multi-pulse femtosecond laser
ablation. Physical Review B, 72, 195422.
doi:10.1103/PhysRevB.72.195422
[10] Vorobyev, A.Y. and Guo, C. (2008) Colorizing metals
with femtosecond laser pulses. Applied Physics Letters,
92, 041914. doi:10.1063/1.2834902
[11] Kaakkunen, J.J.J., Paivasaari, K., Kuittinen, M. and
Jaaskelainen, T. (2009) Morphology studies of the metal
surfaces with enhanced absorption fabricated using in-
terferometric femtosecond ablation. Applied Physics A,
94, 215-220. doi:10.1007/s00339-008-4895-2
[12] Paivasaari, K., Kaakkunen, J.J.J., Kuittinen, M. and
Jaaskelainen, T. (2007) Enhanced optical absorptance of
metals using interferometric femtosecond ablation. Op-
tics Express, 15, 13838-13843.
doi:10.1364/OE.15.013838
[13] Ausano, G., Barone, A.C., Iannotti, V., Lanotte, L., Amo-
ruso, S., Bruzzese, R. and Vitiello M. (2004) Magnetic
and morphological characteristics of nickel nanoparticles
films produced by femtosecond laser ablation. Applied
Physics Letters, 85, 4103-4105. doi:10.1063/1.1815065
[14] Baldacchini, T., Carey, J.E., Zhou, M. and Mazur, E.
(2006) Superhydrophobic Surfaces Prepared by Micro-
structuring of Silicon Using a Femtosecond Laser.
Langmuir, 22, 4917-4919. doi:10.1021/la053374k
[15] Zorba, V., Persano, L., Pisignano, D., Athanassiou, A.,
Stratakis, E., Cingolani, R., Tzanetakis, P. and Fotakis, C.
(2006) Making silicon hydrophobic: Wettability control
by two-lengthscale simultaneous patterning with femto-
second laser irradiation. Nanotechnology, 17, 3234-3238.
doi:10.1088/0957-4484/17/13/026
[16] Vorobyev, A.Y. and Guo, C. (2009) Metal pumps liquid
uphill. Applied Physics Letters, 94, 224102.
doi:10.1063/1.3117237
[17] Fadeeva, E., Schlie, S., Koch, J., Chichkov, B.N., Vo-
robyev, A.Y. and Guo, C. (2009) Femtosecond laser-in-
duced surface structures on platinum and their effects on
hydrophobicity and fibroblast cell proliferation. Contact
Angle, Wettability and Adhesion, 6, 163-171.
[18] Qian, F., Cracuin, V., Singh, R.K., Dutta, S.D. and Pron-
ko, P.P. (1999) High intensity femtosecond laser deposi-
tion of diamond-like carbon films. Journal of Applied
Physics, 86, 2281-2290. doi:10.1063/1.371043
[19] Luke, J.R., Phipps, C.R. and McDuff, G.G. (2003) Laser
plasma thruster. Applied Physics A, 77, 343-348.
[20] Polte, T.R., Shen, M., Karavitis, J., Montoya, M., Pendse,
J., Xia, S., Mazur, E. and Ingber, D.E. (2007) Nanostruc-
tured magnetizeable materials that switch cells between
life and death. Biomaterials, 28, 2783-2790.
doi:10.1016/j.biomaterials.2007.01.045
[21] Anisimov, S.I. and Luk'yanchuk, B.S. (2002) Selected

A. Y. Vorobyev et al. / Natural Science 3 (2011) 488-495 495
problems of laser ablation theory. Physics—Uspekhi, 45,
293-324. doi:10.1070/PU2002v045n03ABEH000966
[22] Stoian, R., Rosenfeld, A., Ashkenasi, D., Hertel, I.V.,
Bulgakova, N.M. and Campbell, E.E.B. (2002) Surface
charging and impulsive ion ejection during ultrashort
pulsed laser ablation. Physical Review Letters, 88,
097603. doi:10.1103/Phy sRevLett.88.097603
[23] Bulgakova, N.M., Stoian, R., Rosenfeld, A., Hertel, I.V.
and Campbell, E.E.B. (2004) Electronic transport and
consequences for material removal in ultrafast pulsed la-
ser ablation of materials. Physical Review B, 69, 054102.
doi:10.1103/PhysRevB.69.054102
[24] Ivanov, D.S. and Zhigilei, L.V. (2003) Combined atom-
istic-continuum modeling of short-pulse laser melting
and disintegration of metal films. Physical Review B, 68,
064114. doi:10.1103/PhysRevB.68.064114
[25] Zhigilei, L.V., Lin, Z. and Ivanov, D.S. (2009) Atomistic
Modeling of Short Pulse Laser Ablation of Metals: Con-
nections between Melting, Spallation, and Phase Explo-
sion. The Journal of Physical Chemistry C, 113, 11892-
11906. doi:10.1021/jp902294m
[26] Povarnitsyn, M.E., Itina, T.E., Sentis, M., Khishchenko,
K.V. and Levashov, P.R. (2007) Material decomposition
mechanisms in femtosecond laser interactions with met-
als. Physical Review B, 75, 235414.
doi:10.1103/PhysRevB.75.235414
[27] Gamaly, E.G., Rode, A.V., Luther-Davies, B. and Ti-
chonchuk, V.T. (2002) Ablation of solids by femtosecond
lasers: Ablation mechanism and ablation thresholds for
metals and dielectrics. Physics of Plasmas, 9, 949-957.
doi:10.1063/1.1447555
[28] Vorobyev, A.Y., Kuzmichev, V.M., Kokody, N.G., Kohns,
P., Dai, J. and Guo, C. (2006) Residual thermal effects in
Al following single ns- and fs-laser pulse ablation. Ap-
plied Physics A, 82, 357-362.
doi:10.1007/s00339-005-3412-0
[29] Vorobyev, A.Y. and Guo, C. (2005) Direct observation of
enhanced residual thermal energy coupling to solids in
femtosecond laser ablation. Applied Physics Letters, 86,
011916. doi:10.1063/1.1844598
[30] Kaye, G.W.C. and Laby, T.H. (1956) Tables of Physical
and Chemical Constants. 11th Edition, Longmans, Lon-
don.
[31] McKay, J. A., Bleach, R. D., Nagel, D. J., Schriemph, J.
T., Hall, R. B., Pond, C. R. and Manlief, S. K. (1979)
Pulsed-CO2-laser interaction with aluminum in air:
Thermal response and plasma characteristics. Journal of
Applied Physics, 50, 3231-3240. doi:10.1063/1.326361
[32] Vorobyev, A.Y. (1985) Reflection of the pulsed ruby laser
radiation by a copper target in air and in vacuum. Soviet
Journal of Quantum Electronics, 15, 490-493.
doi:10.1070/QE1985v015n04ABEH006963
[33] Milchberg, H. M., Clark, T. R., Durfee, C. G., Antonsen,
T. M. and Mora, P. (1996) Development and applications
of a plasma waveguide for intense laser pulses. Physics
Plasmas, 3, 2149-2155. doi:10.1063/1.871668
[34] Bulgakova, N.M., Zhukov, V.P., Vorobyev, A.Y. and Guo,
C. (2008) Modeling of residual thermal effect in femto-
second laser ablation of metals. role of gas environment.
Applied Physics A, 92, 883-889.
doi:10.1007/s00339-008-4568-1
[35] Fujimoto, J. G., Liu, J.M. and Ippen, E.P. (1984) Femto-
second Laser interaction with Metallic Tungsten and
Nonequilibrium Electron and Lattice Temperatures. Ap-
plied Physics Letters, 53, 1837-1840.
doi:10.1103/PhysRevLett.53.1837
[36] König, J., Nolte, S. and Tünnermann, A. (2005) Plasma
evolution during metal ablation with ultrashort laser
pulses. Optics Express, 13, 10597-10607.
doi:10.1364/OPEX.13.010597
[37] Breitling, D., Ruf, A., Berger, P.W., Dausinger, F.H.,
Klimentov, S.M., Pivovarov, P.A., Kononenko, T.V. and
Konov, V.I. (2003) Plasma effects during ablation and
drilling using pulsed solid-state lasers. Proceedings of
SPIE, 5121, 24-33. doi:10.1117/12.513766
[38] Mao, S.S., Mao, X., Greif, R. and Russo, R.E. (2000)
Dynamics of an air breakdown plasma on a solid surface
during picosecond laser ablation. Applied Physics Letters,
76, 31-33. doi:10.1063/1.125646
[39] McKay, J.A., Schriemph, J.T., Cronburg, T.L., Eninger,
J.E. and Woodroffe, J.A. (1980) Pulsed CO2 laser inter-
action with a metal surface at oblique incidence. Applied
Physics Letters, 36, 125-127. doi:10.1063/1.91403