APPLIED PHYSICS LETTERS 86, 121911 共2005兲
Short-pulse-laser-induced optical damage and fracto-emission
of amorphous, diamond-like carbon films
Klaus Sokolowski-Tintena兲 and Wolfgang Ziegler
Institut für Optik und Quantenelektronik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1,
07743 Jena, Germany
Dietrich von der Linde
Institut für Experimentelle Physik, Universität Duisburg-Essen, 45117 Essen, Germany
Michael P. Siegal and D. L. Overmyer
Sandia National Laboratories, Albuquerque, New Mexico 87185
共Received 29 December 2004; accepted 3 February 2005; published online 16 March 2005兲
Short-pulse-laser-induced damage and ablation of thin films of amorphous, diamond-like carbon
have been investigated. Material removal and damage are caused by fracture of the film and ejection
of large fragments. The fragments exhibit a delayed, intense and broadband emission of
microsecond duration. Both fracture and emission are attributed to the laser-initiated relaxation of
the high internal stresses of the pulse laser deposition-grown films. © 2005 American Institute of
Physics. 关DOI: 10.1063/1.1888037兴
Ablation of solids by ultrafast lasers attracts increasing
interest, in particular with respect to potential technological
applications for high precision material processing.1 In a
number of experiments advantages of ultrashort laser pulses
have been demonstrated. However, the understanding of the
fundamental physical processes leading to material removal
is still incomplete. For the case of linearly absorbing semiconductors and metals we have demonstrated1 that nearthreshold ablation with single ultrashort laser pulses exhibits
a material-independent, universal behavior. Removal of material is brought about by hydrodynamic expansion of the
laser-generated hot, pressurized matter followed by its decomposition into a two-phase, liquid-gas mixture.2,3
However, for thin films laser-induced ablation can be
distinctively different as compared to bulk materials. Often
the primary mechanism of material removal is not the transformation of the irradiated solid material to a volatile phase,
but for example spallation or adhesion failure to the substrate
caused by high tensile stresses after fast laser heating of the
film.4,5 These effects have important technological ramifications since they influence the optical damage resistance of
coatings6 but may also open new possibilities for the controlled structuring or removal of thin films without damaging
the substrate.7,8
In this letter we report on the distinct ablation behavior
of thin films of amorphous, diamond-like carbon after irradiation with single femtosecond laser pulses. We find that the
lowest threshold damage mechanism corresponds to fracture
of the film followed by ejection of large 共⬎10 ␮m兲 fragments. These fragments exhibit a strong and broadband
fracto-emission9 of microsecond duration, which is attributed
to the relaxation of high internal stresses characteristic for
the as-deposited films.
Amorphous diamond-like carbon films of 60 nm thickness have been grown by pulsed laser deposition 共PLD兲. Carbon was ablated from a pyrolytic graphite target at fluences
a兲
Author to whom correspondence should be addressed; electronic mail:
sokolowski@ioq.uni-jena.de
0003-6951/2005/86共12兲/121911/3/$22.50
of about 25–50 J / cm2 with a 10 Hz KrF-excimer laser 共17
ns, 248 nm兲 and deposited onto single crystalline silicon and
fused silica substrates at room temperature.10 Detailed characterization of these films has been carried out using a variety of techniques indicating a high degree of fourfold coordination and a density of about 90% of that of diamond.
These films were irradiated in ambient atmosphere with
120 fs laser pulses at 620 nm 共p-polarized, angle of incidence 45°兲 delivered by a 10 Hz amplified colliding-pulse
mode-locked 共rhodamine 6G/ DODCI兲 dye laser.11 To follow
the evolution of the surface morphology we employed optical microscopy, either by using a second probe laser pulse as
illumination, or in a configuration where we imaged the irradiated surface area using light emission originating from
the ablating material 共see below兲. For the imaging a highresolution long-distance microscope objective was used. The
optical micrographs were recorded with the help of a charge
coupled device 共CCD兲 camera in conjunction with a computer controlled video digitizer.
Typical experimental results about the final surface morphology are shown in the micrographs in the left panel of
Fig. 1. The visual impression of the damage morphology
indicates 共mechanical兲 fracture as the damage mechanism. In
the vicinity of the well-defined damage threshold of
0.25 J / cm2 the material seems only slightly elevated from
the substrate and forms a periodic surface structure. For
higher fluences the film is completely removed and large
fragments, tens of micrometers in size, are ejected and deposited in and around the ablation region.
Most surprisingly we found that these fragments are the
source of a strong light emission, as can be seen in the right
panel of Fig. 1. The emission was easily visible to the bare
eye and appeared bright white like a spark. Preliminary measurements with different color filters show that the emitted
light covers a broad spectral range 共from the blue to the near
infrared兲.
To investigate the temporal evolution of the emission we
have carried out explicit time-dependent, but spatially integrating measurements using in the imaging setup a photodiode in conjunction with an oscilloscope as a detector instead
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© 2005 American Institute of Physics
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Sokolowski-Tinten et al.
Appl. Phys. Lett. 86, 121911 共2005兲
FIG. 3. Temporally integrated emission signal as a function of pump fluence
as measured using two different color filters 共BG 3 and RG 645兲.
FIG. 1. Left panel: optical micrographs of the final surface morphology for
four different fluences. Right panel: corresponding images recorded using
the self-emission of the irradiated material. Frame size: 270 ␮m ⫻ 190 ␮m.
of the CCD. Typical time traces 共␮s scale兲 of the emission
are depicted in Fig. 2 for three different fluences, from close
to threshold to twice the threshold value. The inset in Fig. 2
shows for the same fluences the behavior at early times on a
sub-␮s-scale. The small peak at very early times marked by
an arrow is not related to emission from the surface, but
corresponds to scattered pump light and can be used as a
time marker.
The temporal behavior exhibits a rather complex structure consisting of a “fast” peak at early times with a duration
FIG. 2. Temporal evolution of the fracto-emission of the ablated material for
three different fluences above the damage threshold 共0.25 J / cm2兲. Inset:
expanded view of the early time behavior; the small peak marked by an
arrow corresponds to scattered pump light.
of 100–200 ns followed by a much “slower” second peak
after 0.5–1 ␮s. The emission eventually decays with a time
constant of a few microseconds. While the intensity ratio
between fast and slow component increased with increasing
fluence the decay time of the fast peak decreased 共see inset兲.
Two additional remarks should be made. First, although the
slow component in the examples shown in Fig. 2 seems to be
single peaked and exhibits a nearly exponential decay, this is
not always the case. Instead a complex multi-peak structure
is observed in some cases. Second, the slow component
makes the main contribution to the emission visible in the
time-integrated images 共right panel of Fig. 1兲 since it contains roughly 90% of the totally emitted light.
To underline the close relation between damage of the
film and emission we have plotted in Fig. 3 the total emission signal as obtained from an integral of time traces 共as
those shown in Fig. 2兲 measured for a number of different
fluences. The squares and circles have been measured using
different filters/filter combinations 共Schott BG 3 + KG 3; RG
645兲 in front of the photodiode. They represent the “blue”
共300 nm⬍ ␭ ⬍ 450 nm兲 and “red” 共␭ ⬎ 650 nm兲 part of the
emission spectrum, respectively. While below the damage
threshold 共0.25 J / cm2, marked by an arrow兲 absolutely no
emission can be detected, the 共sharp兲 increase of the emission signal occurs only for fluences above threshold.
The data presented so far give clear evidence that the
emission is directly related to the damage and fragmentation
process. Emission of a laser-generated plasma can be excluded since the fluences used in the experiments correspond
to peak intensities well below 1013 W / cm2 and were thus
below both the breakdown threshold of air as well as below
the threshold of surface plasma formation. Instead, the results of the explicit time-dependent measurements and the
fact that the spatial distribution of the emission closely resembles the final damage morphology demonstrate that the
emission occurs mainly after fracture of the film and deposition of the fragments. This is in clear contradiction to direct
plasma emission as well as simple thermal emission due to
laser heating since both should be strongest just after the
excitation when the temperature of the material is highest.
Moreover, the onset of emission is delayed with respect to
the laser excitation and also temporally separated from the
usual ablation of bulk solids which occurs in hundreds of
picoseconds to few nanoseconds time-scale2 and is attributed
to a rapid liquid-gas transition.
Although the measurements have been performed in air,
a chemical reaction, i.e., oxidation, can be excluded as an
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Appl. Phys. Lett. 86, 121911 共2005兲
Sokolowski-Tinten et al.
explanation for the emission because it was also observed
during the irradiation under vacuum 共10−3 mbar兲. To explain
our observations we propose instead that the peculiar damage behavior is related to the high internal compressive stress
of the films due to the PLD-growth process10,12 which is of
the order of a few GPa. Impulsive and thus isochoric heating
with an ultrashort laser pulse sufficiently reduces the adhesion forces 共already decreased due to the internal stress兲 to
the substrate and the film is removed from the substrate without transition to the gas phase. The internal stress leads then
also to the destruction of the film and ejection of the fragments. Within this context the fast peak of the emission may
correspond to the initial detachment and breakup of the film,
while the slow component may be related to the final relaxation of the internal stress after formation of the individual
fragments.
Finally, it should be noted that for higher peak fluences,
above ⬇0.4 J / cm2, we do not observe fragmentation and
fracto-emission in the center of the laser-excited region, but
the material-independent universal ablation behavior reported in Ref. 1.
K.S.T. acknowledges support by the Erich–Becker foundation and by the University of Essen 共Forschungspool兲.
Sandia is a multiprogram laboratory operated by Sandia Cor-
poration, a Lockheed Martin Company, for the United States
Department of Energy under Contract No. DE-AC0494AL85000.
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We do not expect any particular wavelength dependence of the effect and,
in fact, similar results have been obtained using sub-100 fs pulses at 800
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1