Appl. Phys. A 63, 103 — 107 (1996)
Pulse-width influence on the laser-induced
structuring of CaF (111)
2
D. Ashkenasi*, H. Varel, A. Rosenfeld, F. Noack, E.E.B. Campbell
Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Postfach 1107, D-12474 Berlin, Germany
(Fax: #49-30/6392-1229, E-mail: CAMPBELL@mbi.fta-berlin.de)
Received: 26 February 1996/Accepted: 27 February 1996
Abstract. We have investigated the morphology of CaF
2
(111) irradiated by 780 nm laser pulses of varying pulse
width (200 fs — 8 ns) with fluences above the damage threshold. Large differences can be observed which we relate
to the mechanisms and dynamics of defect production in
this wide band gap material. The best defined and most
controllable ablation is obtained for laser pulse widths of
a few picoseconds. For nanosecond and femtosecond
pulses strong fracturing of the crystal is observed with
damage outside the laser irradiated zone. This has a thermal origin for nanosecond pulses but a non-thermal origin for pulse widths below approximately 1 ps.
PACS: 81.40.!z; 81.60.!j
The interaction of laser pulses with optically transparent
materials has been the subject of some considerable interest for many years [1 — 3]. For practical reasons, it is
important to determine the damage thresholds and the
damage processes for materials which are extensively used
e.g. in laser optics. With the increasing availability of
ultra-short laser pulses (with pulse widths on the femtosecond to picosecond time range) it may also be of
practical interest to determine whether it is possible to
produce controllable and well-defined micrometer-sized
structures in transparent materials by means of pulsed
laser ablation. This is generally not possible with standard
nanosecond lasers. Fluoride crystals are among the most
studied transparent materials. One reason for this is their
practical importance as optical components in UV laser
optics, which is a consequence of their very wide band
gaps. Another more fundamental reason is the relative
simplicity of the crystal structures which makes them ideal
model systems for investigating the basic mechanisms
*Also at: Fachbereich Physik, Freie Universität Berlin, Arnimallee
14, D-14195 Berlin, Germany
involved in the laser damage (or ablation) process. Many
fundamental studies on defect formation have been carried out on alkali halides and alkaline earth fluorides
[4 — 8] using laser pulses longer than a few tens of
picoseconds. The presence of defects and surface states
strongly influences the optical properties of the materials
leading to enhanced photon absorption, vacancy creation,
and finally laser ablation [9]. Two recent studies have
investigated the damage threshold for CaF as a function
2
of laser pulse width (q) for photon energies much lower
than the band gap [10, 11]. In these studies different
behaviour could be observed for three different time regimes: for t520 ps the damage threshold was linearly
proportional to q1@2, as one would expect from a thermally
dominated process [10]; a plateau was reached for
24q45 ps and, for even shorter pulses, the damage
threshold decreased further [11]. In this paper we report
morphological studies of the surface damage and ablation
structures produced on laser irradiation with pulse widths
in these three different time regimes. Quite dramatic differences can be observed which we attribute to different
mechanisms related to the dynamics of defect production
and relaxation in the crystal.
1 Experimental setup and results
The laser used in our experiments was a Ti:Sapphire
oscillator-amplifier system based on the chirped-pulseamplification technique (CPA). CPA allowed us to vary
the pulse width without significantly changing other important parameters. The laser pulse at a wavelength of
790 nm was focused by an f"58 mm lens onto the sample
surface giving an irradiated area of approximately
500 lm2. The experiments were carried out under vacuum
((10~5 mbar) and for single-shot as well as for multipleshot irradiation per site at a repetition rate of 1 and 50 Hz.
The CaF samples (Korth, Germany) were polished on
2
both sides. The polished samples have a uniform distribution of mechanically-induced surface defects which ensures, as well as possible, equal conditions for every new
104
Fig. 1. Electron microscope
pictures of polished CaF
2
(111) after 50 on 1 shot
ablation in vacuum
((10~5 mbar) with laser
pulses of different pulse
widths (200 fs, 580 fs, 1.8 ps,
3.2 ps and 4.7 ps) at
a wavelength of 790 nm and
a fluence of 7 J/cm2
.
105
spot. If cleaved samples are used one can not guarantee
the homogeneity of the surface and one has enhanced
damage at or near steps [12]. We employed two complementary methods for analysis of the surface after irradiation: electron microscopy which can give detailed information on changes that have occured to the
sample surface and atomic force microscopy which can
provide detailed information on the depth and height of
structures.
Figure 1 shows some typical electron microscope pictures obtained after irradiation of the sample with 50 laser
shots at a fluence of 7 J/cm2. This fluence is approximately
a factor 2 — 3 higher than the threshold damage fluence for
pulse widths in the range shown (5 ps — 200 fs) [11]. For
the two shortest pulse widths (200 fs and 580 fs) one can
see strong fracturing at the edge of and beyond the irradiated area with no indication of melting. On increasing
the pulse width to a few picoseconds one obtains a much
smoother, well-defined ablation in the area exposed to the
laser pulse. This area is delineated by a narrow melt edge
which can be clearly seen in the enlargement of the 1.8 ps
structure in Fig. 1. There are no signs of fracture except for
the small ((2 lm) rectangular shaped sub-structures in
the centre and a small crack at the left-hand side of the
irradiated area for t"1.8 ps. These effects seem to decrease on going to slightly longer pulses. Figure 2 shows
the situation with 8 ns pulses. A fluence of 50 J/cm2 was
used which is close to the damage threshold fluence for
this pulse width. Here one sees very strong fracturing with
damage occuring far beyond the irradiated area (the geometrical pulse profiles were very similar over the entire
pulse width range investigated). Again, as for the fs pulses,
we see no evidence of melting. The nanosecond results are
in good agreement with previous single-shot studies using
248 nm excimer laser pulses (14 ns) [12]. In this work
extensive fracturing along the natural cleavage planes
could be observed leading to the formation of large tiles of
various shapes which may be partly or entirely removed
Fig. 2. Electron microscope pictures of polished CaF (111) after 50
2
on 1 shot ablation in vacuum ((10~5 mbar) with laser pulses of
8 ns at a wavelength of 790 nm and a fluence of 50 J/cm2
from the surface and which show no signs of melting.
Dickinson observed similar effects with ns pulses for
single-shot damage of MgO and NaCl [13]: fracture with
no signs of melting.
Atomic force pictures of the ablated structures obtained after 5 laser shots are shown in Fig. 3 for two
different pulse widths (200 fs and 4.7 ps). The fluence was
chosen in each case to be a factor of three higher than the
in [11] determined threshold damage fluence. The much
smoother structure for 4.7 ps is immediately apparent and
seems to closely mirror the geometrical profile of the laser
pulse. For 200 fs pulses the damage around the hole is less
apparent than in the electron microscope pictures but can
still be seen.
2 Discussion
CaF is a so-called type II material with very strong
2
electron-phonon coupling which implies a high rate for
self-trapping of excitons and/or holes after laser excitation
[8, 14, 15]. There are two main excitation mechanisms
using photons with energies well below the band gap: (a)
multi-photon absorption leading to the formation of defects (via electron-hole pairs) such as self-trapped excitons
and Frenkel defects and (b) single photon absorption
between occupied and empty defect states or of free electrons in the conduction band [2, 16]. The latter will lead
to most of the local heating of the crystal. In our case of
surface damage it is generally believed that single photon
absorption in the conduction band is more likely to lead
to photoexcitation than to efficient local heating [2, 17].
The mechanisms leading to macroscopic material damage
and ablation for different laser pulse widths will thus
depend very much on the kind of defects produced, the
time range for their production and their lifetime.
The nanosecond results of Gogoll et al. [12] were
interpreted as being due to the build-up of shear stress due
to lateral temperature variations in the irradiated volume.
This leads to thermoelastic displacement along the cleavage planes which gives rise to the strong fracturing observed in the experiments. No melting is observed at the
fluences investigated because the defect formation rate
and thus the heat input is too low for melting. The temperature rise is, however, still large enough to produce the
shear stress leading to fracturing.
For excitation with ps pulses there is a much larger
probability for the formation of defects (electron-hole
pairs) due to multiphoton absorption. These electron-hole
pairs are very efficiently converted to self-trapped excitons
(STE) on a time range of about 1 ps [8]. These have
a lifetime of a few ls [9, 15]. If the critical defect density
for strong single photon absorption (approx. 1021 cm~3
[10]) is reached during the first part of the laser pulse
(1 — 2 ps) the remainder of the pulse can efficiently excite
between excitonic states which, followed by rapid conversion of electronic to vibrational energy, leads to a rapid
heating of the crystal producing melting and vapourisation of the material. Since we have a Gaussian laser beam,
the temperature rise at the edges is lower than in the
centre giving the characteristic melt edge (Fig. 1). Due to
the low heat conductivity in these materials there is no
106
Fig. 3. Atomic force microscope pictures as surface height profile (left) and as section analysis (right) of polished CaF (111) after 5 on 1 shot
2
ablation in vacuum ((10~5 mbar) with laser pulses at a wavelength of 790 nm: pulse width: 200 fs, fluence 10 J/cm2 (top); 4.7 ps, 15 J/cm2
(bottom)
observable thermal damage outside the area that directly
interacts with the laser beam.
For excitation with laser pulses with q41 ps one will
have an even larger probability for the multiphoton excitation of electron-hole pairs during the laser pulse. These
will convert to self-trapped excitons on a timescale of 1 ps
as discussed above, i.e. for very short pulses the self-trapped excitons will be formed after the pulse. There is no
remaining laser pulse to produce rapid heating of the
crystal via single photon absorption from the defect states.
However, the localised excitons produced after the laser
pulse will weaken the bonds in the crystal [9] and strongly
deform the lattice. If they have been produced with sufficient density without being thermally removed, the lattice
deformation will cause immense stress in the crystal thus
inducing severe fracturing and explosive non-thermal removal of material.
Perhaps still somewhat speculative, the above considerations are able to explain the experimental findings very satisfactorily. Based on our present knowledge
on defect formation, the above considerations should be
valid for similar type-II materials, e.g. MgF . It is
2
interesting to note, that the optically induced defect
formation does not only account for simple damage but
with a proper choice of pulse width and fluence may in
future play a major role for high quality structuring of
wide-gap materials. Further experiments are in progress
to test the suggested mechanisms for ablation in the ps
and fs regimes.
Acknowledgements. We would like to thank S. Gogoll and Prof. E.
Matthias (both of the F.U. Berlin) for some stimulating discussions.
Financial support from the BMBF (13N6591/1) is gratefully
acknowledged.
107
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