Femtosecond laser microstructuring of nickel foil
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38 CHINESE OPTICS LETTERS / Vol. 8, No. 1 / January 10, 2010
Femtosecond laser microstructuring of nickel foil
Wei Jia (___ %%%)
∗
, Bin Zhou (±±± QQQ), Xun Li (ooo |||), Lu Chai ( ´´´),
Ruobing Zhang (ÙÙÙeeeXXX), and Chingyue Wang ()
Ultrafast Laser Laboratory, School of Precision Instruments and Opto-Electronics Engineering, Tianjin University,
Key Laboratory of Opto-Electronic Information Technical Science,
Ministry of Education of China, Tianjin 300072, China
∗
E-mail: [email protected]
Received February 18, 2009
We observe the morphological change and grain structure of Ni foil when it is ablated with femtosecond laser
pulses. Scanning electron microscopy and field emission transmission electron microscopy are used to study
the nature of the morphology and grain structure of nickel foil and determine the essential features. The
results indicate that there are many random nanostructures in the center of the ablated region composed of
nano crystalline grains as well as some core-shell structures. The observed morphologies seem to suggest that
phase explosion and extremely high cooling rate are the most probable physical mechanisms responsible
for the formation of surface nanostructures.
OCIS co des: 140.7090, 140.3390, 320.7130, 160.3900.
doi: 10.3788/COL20100801.0038.
As ultrashort pulse duration confines heat diffusion, the
femtosecond laser pulses provide a promising tool for the
alteration of surface morphology and texture without
significant damage to the underlying material. Unique
materials modification and surface texturing effects may
be achieved through the controlled use of laser irradiation
parameters, such as pulse energy and pulse count
[1−5]
.
Physically, these correlate to thermal gradient structure
and quenching rates. Thus, it is possible to develop
unique phases and surface microstructures through the
appropriate combination of laser processing parameters.
Her et al. have found that silicon surfaces develop an
array of sharp conical spikes when irradiated with fem-
tosecond laser pulses
[6,7]
. Wu et al.
[8−10]
reported that
the linearly polarized femtosecond laser pulses produce
fine periodic structures with periodicity shorter than the
laser wavelength. The studies of Vorobyev et al. have
shown that random surface nanostructures can be pro-
duced following ablation of metals by femtosecond laser
pulses
[11−14]
.
In this letter, the morphology and grain structure of
nickel ablated with femtosecond laser pulses is stud-
ied, using scanning electron microscopy (SEM) and field
emission transmission electron microscopy (FETEM) to
determine the essential features and their relationships.
Based on the thermal evolution of the ablation with fem-
tosecond laser pulses, a mechanism for the formation of
the unique surface texturing is proposed.
All samples were processed in the air using a com-
mercially available femtosecond laser micromachining
workstation (Clark-MXR, UMW-2110i) equipped with a
Ti:sapphire chirped pulse amplification (CPA) system.
The laser emitted pulses of linearly polarized light at a
central wavelength of 775 nm. The laser pulse width and
repetition rate were 130 fs and 1 kHz, respectively, and
the maximum energy was 1 mJ per pulse. The laser beam
was focused by a long working distance microscopic ob-
jective (numerical aperture NA = 0.14). The laser beam
was normally focused onto the sample surface. The fo-
cused beam diameter was measured to be about 11.5 µm.
A commercially available Ni foil (99.9% purity) was
used as the target. All samples had dimensions of 20
× 20 × 0.03 (mm). The morphology of the machined
surface was examined by a SEM (Philips XL30E) at an
operating voltage of 20 kV. A FETEM (Philips Tecnai
G2 F20) with an accelerating voltage of 200 kV was used
to characterize the structural properties of the ablated
sample. FETEM samples of ab out 3 mm in diameter
were prepared using focused ion beam (FIB) etching
with 5-kV voltage and tilt angle of 10
◦
. In order to avoid
the residual ionization effect, the FETEM samples were
machined using a FIB etching firstly.
Figure 1 shows the evolution of the modification at
near-ablation-threshold fluence of 2.6 J/cm
2
after in-
creasing numbers of incident laser shots. In the center of
the ablated region, there are large numbers of randomly
oriented protrusions of nanoscale dimension, which is a
foamlike structure, and periodic surface structures (the
well-known ‘periodic ripples’) in the peripheral of the
ablated region. Outside the laser-ablated region, there
is a large quantity of debris caused during laser ablation
and dep osited on the surface of the Ni sample. As the
shot number increases, the extent of modification area
expands and the periodic ripples grow. This indicates
the accumulation behavior of the ablation. If one con-
siders the Gaussian-shaped intensity profile, it becomes
clear that the lower intensity outer-edges of the beam
cause no ablation, but cause periodic ripples due to the
accumulation in the outer regions of the irradiated area.
The morphology in the center of the ablated region
is similar to the random nanostructures reported by
Vorobyev
[12−14]
and consistent with rapid expulsion of
liquid and vapour droplets which cool quickly
[15]
and
resolidify. This suggests that phase explosion
[16]
is the
most probable physical mechanism responsible for the
random nanostructures. For high fluence and sufficiently
short pulses, melted material at the irradiated surface is
unable to boil because the time scale does not permit
the necessary heterogeneous nuclei to form. Instead, it is
superheated significantly past the normal boiling point to
1671-7694/2010/010038-03
c
° 2010 Chinese Optics Letters
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