High spatial frequency periodic structures induced on metal surface by femtosecond laser pulses

Abstract

The high spatial frequency periodic structures induced on metal surface by femtosecond laser pulses was investigated experimentally and numerically. It is suggested that the redistribution of the electric field on metal surface caused by the initially formed low spatial frequency periodic structures plays a crucial role in the creation of high spatial frequency periodic structures. The field intensity which is initially localized in the grooves becomes concentrated on the ridges in between the grooves when the depth of the grooves exceeds a critical value, leading to the ablation of the ridges in between the grooves and the formation of high spatial frequency periodic structures. The proposed formation process is supported by both the numerical simulations based on the finite-difference time-domain technique and the experimental results obtained on some metals such as stainless steel and nickel.

Full text

High spatial frequency periodic structures
induced on metal surface by
femtosecond laser pulses
Jian-Wu Yao,1 Cheng-Yun Zhang,1 Hai-Ying Liu,1 Qiao-Feng Dai,1 Li-Jun Wu,1
Sheng Lan,1,* Achanta Venu Gopal,2 Vyacheslav A. Trofimov,3 and Tatiana M. Lysak3
1Laboratory of Photonic Information Technology, School of Information and Optoelectronic Science and
Engineering, South China Normal University, Guangzhou 510006, China
2Department of Condensed Matter Physics and Material Science, Tata Institute of Fundamental Research,
Homi Bhabha Road, Mumbai 400005, India
3Department of Computational Mathematics and Cybernetics, M. V. Lomonosov Moscow State University,
Moscow 119992, Russia
*slan@scnu.edu.cn
Abstract: The high spatial frequency periodic structures induced on metal
surface by femtosecond laser pulses was investigated experimentally and
numerically. It is suggested that the redistribution of the electric field on
metal surface caused by the initially formed low spatial frequency periodic
structures plays a crucial role in the creation of high spatial frequency
periodic structures. The field intensity which is initially localized in the
grooves becomes concentrated on the ridges in between the grooves when
the depth of the grooves exceeds a critical value, leading to the ablation of
the ridges in between the grooves and the formation of high spatial
frequency periodic structures. The proposed formation process is supported
by both the numerical simulations based on the finite-difference time-
domain technique and the experimental results obtained on some metals
such as stainless steel and nickel.
©2012 Optical Society of America
OCIS codes: (220.4241) Nanostructure fabrication; (350.3390) Laser materials processing;
(160.3900) Metals.
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1. Introduction
In the last two decades, laser induced periodic surface structures (LIPSSs) on the surfaces of
various materials (including metals, semiconductors and dielectrics) with periods
approximately equal to the laser wavelength have been investigated extensively by using
nanosecond and picosecond lasers [1–5]. In recent years, this study has been extended to
femtosecond (fs) lasers and it has been known that LIPSSs (e.g., ripples) can be created on the
surface of a material when the laser fluence approaches the damage threshold of the material
[6–15]. For different materials, the periods of LIPSSs induced by fs laser pulses are found to
be smaller than laser wavelength. The interference between the incident laser light and the
surface scattered wave was proposed many years ago to explain the formation of LIPSSs [3].
Recently, it is suggested that the surface plasmon polaritons (SPPs) excited by fs laser
irradiation play a crucial role in the formation of LIPSSs [16–18]. In recent years, LIPSSs
with periods significantly smaller than laser wavelength have attracted considerable interest
because of their potential applications [11–15]. While the LIPSSs with periods approximately
equal to the laser wavelength are called low spatial frequency LIPSSs (LSFLs), the LIPSSs
with periods much smaller than the laser wavelength are referred to as high spatial frequency
LIPSSs (HSFLs). Apparently, HSFLs induced by fs laser pulses cannot be interpreted by
using the physical models described above [3,16–19]. So far, several mechanisms have been
proposed to explain the formation of HSFLs induced by fs laser pulses, such as self-
organization [20], second harmonic generation [11,21,22], excitation of SPPs [23], and
Coulomb explosion [24] etc. However, the actual physical mechanism responsible for the
generation of HSFLs is still debated. In addition, very few works related to the formation of
HSFLs have been carried out on metals by using fs laser pulses. Very recently, the effects of
various structures intentionally created on the surface on the fs laser ablation of material
surface have been investigated. It clearly indicated that the structures created on the surface
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Received 9 Nov 2011; revised 27 Nov 2011; accepted 27 Nov 2011; published 4 Jan 2012
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16 January 2012 / Vol. 20, No. 2 / OPTICS EXPRESS 906

would significantly influence the field intensity distribution on the surface and the subsequent
ablation process [25].
In this article, we investigated experimentally and numerically the formation of HSFLs on
metal surface by fs laser pulses. It is suggested that the redistribution of the electric field
intensity due to the formation of LSFLs plays a crucial role in the creation of HSFLs. The
proposed formation process of HSFLs is supported by both the numerical simulations based
on the finite-difference time-domain (FDTD) technique and the experimental results obtained
on metals such as stainless steel and nickel.
2. Experimental
In experiments, we used a Ti: sapphire fs amplifier (Legend, Coherent) that delivers 800 nm,
90 fs pulses at a repetition rate of 1 kHz. The metals we studied were 301 L stainless steel and
nickel which are widely applied in industry. They were mechanically polished and cleaned in
an ultrasonic cleaner with acetone. The surface roughness was less than 10 nm for the
stainless steel sample and less than 3 nm for the nickel sample. The sample was fixed on a
three-dimensional motorized translation stage. The horizontally polarized laser beam was
focused normally on the sample by using a lens with a focusing length of 150 mm. A half-
wave plate in combination with a polarizer was employed to control the laser ñuence. The
diameter of the laser spot on the sample surface was measured to be ~40 µm. The laser
scanning was carried out by translating the sample in the plane parallel to the sample surface.
The surface morphology after fs laser ablation was examined by scanning electron
microscope (SEM).
3. Results and discussion
3.1 HSFLs formed on metal surface
Recently, several research groups have reported the observation of HSFLs on different
materials with periods much smaller than laser wavelength [11–15]. However, there are few
reports on the formation of HSFLs on metals induced by fs laser pulses. In our experiments,
we studied the formation and evolution of LIPSSs on metal surface by scanning fs laser beam
across the metal surface with appropriate irradiation conditions. The scanning of laser beam
ensures the uniform distribution of laser irradiation along the scanning line. Otherwise, the
sample will be greatly ablated at the center of the ablation crater and HSFLs are only
observed at the edge of the ablation crater [21]. It has been recognized that the morphology of
LIPSSs depends strongly on the laser fluence and the number of irradiated pulses. Therefore,
these two parameters were carefully controlled and adjusted. In our case, we fixed the laser
fluence at 0.16 J/cm2 which is slightly above the ablation threshold of the stainless steel.
Then, the average number of fs laser pulses irradiated on the excitation spot was adjusted by
varying the scanning speed of the laser beam from 2.0 to 0.25 mm/s. By doing so, the average
number of fs laser pulses irradiated on the excitation spot ranges from 20 to 160. With these
irradiation conditions, the surface of the stainless steel was processed by scanning the fs laser
beam in the horizontal direction. The SEM images for the scanned lines obtained by using
different scanning speeds are shown in Fig. 1. With a fast scanning speed of 2.0 mm/s, one
can see a LSFL with a period of ~650 nm, as shown in Fig. 1(a). The LSFL consists of
multiple grooves aligned in the horizontal direction. The formation of multiple grooves
originates from the ripple-like electric field distribution on the surface. When the average
number of irradiated pulses was increased to 40 (scanning speed = 1.0 mm/s), secondary
grooves with a length of ~1 μm and a width of ~200 nm were created on the ridges of the
primary grooves, as can be clearly seen in Fig. 1(b). With a further increase in the average
number of irradiated pulses to 80 (scanning speed = 0.5 mm/s), it was found that the length of
the primary grooves was almost doubled. In addition, it was noticed that the secondary
grooves were significantly elongated and became equal to the primary grooves, as shown in
Fig. 1(c). In this case, one cannot distinguish the primary grooves initially formed in the
LSFL and the secondary grooves formed on the ridges of the primary ones. As a result, a
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HSFL with a period equal to half of the LSFL was obtained. When the average number of
irradiated pulses was increased to 160 (scanning speed = 0.25 mm/s), regular HSFL with a
period of about 300 nm was created, as shown in Fig. 1(d).
Fig. 1. SEM images of the scanned lines on the surface of stainless steel by using different
scanning speeds: (a) 2.0 mm/s, (b) 1.0 mm/s, (c) 0.5 mm/s, and (d) 0.25 mm/s. The laser
fluence was fixed at 0.16 J/cm2.
Although LIPSSs have been studied extensively in the past, there is no report on the
LIPSSs with secondary grooves sitting on the ridges of the primary grooves, as shown in Fig.
1(b). In fact, this kind of LIPSSs was observed not only on the surface of stainless steel but
also on the surface of nickel. Figure 2 shows such a LIPSS formed on the surface of nickel by
scanning fs laser beam with a fluence of 0.16 J/cm2 and a scanning speed of 2.0 mm/s. The
irradiation condition corresponds to an average number of irradiated pulses of 20 on the
excitation spot. Under this condition, the length of the secondary grooves is slightly shorter
than that of the primary ones. One can easily distinguish the initially and newly formed
grooves. Therefore, the evolution of the LIPSS with increasing number of irradiated pulses
clearly reveals a possible process for the generation of HSFLs, at least in some metals. As
compared with the stainless steel, the formation of HSFLs was induced on the surface of
nickel with a smaller average number of irradiated pulses. It is thought that the lager
absorption coefficient of nickel at the laser wavelength is responsible for this behavior.
Fig. 2. SEM image of the LIPSS formed on the surface of nickel by scanning fs laser beam
with a fluence of 0.16 J/cm2 and a scanning speed of 2.0 mm/s.
3.2 Effect of LSFLs on the generation of HSFLs
As described at the beginning, several mechanisms have been proposed to explain the
formation of HSFLs induced by fs laser pulses [11,20–24]. However, the actual mechanism of
HSFL formation is still debated. It is thought that the physical mechanism for HSFL
formation may depend strongly on the physical properties of materials. Different from
semiconductors or dielectrics, metals are highly absorptive to laser light and the excitation of
SPPs and localization of light may play an important role in the formation and evolution of
LIPSSs. In fact, the interaction of the incident light with the metal surface will be modified
after the formation of LSFLs, leading to the redistribution of the field intensity on the surface
of metals. For shallow ripples (or grooves) formed on the surface of metals, it has been found
that the incident light will be localized in the grooves, resulting in a deepening and an
elongation of the grooves [18]. When the depth of the grooves exceeds a certain value,
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however, it is expected that the distribution of electric field on the surface will be greatly
changed, significantly affecting the subsequent ablation process. This behavior is confirmed
by the numerical simulations described in the following.
3.3 Numerical simulation based on the FDTD technique
In order to gain a deep insight into the effect of LSFLs on the generation of HSFLs on metal
surface, we have simulated the electric field distribution on the surface of a metal with
initially formed ripples (or grooves) by using the FDTD technique. The simplified model for
the structured surface used in our numerical simulation is schematically shown in Fig. 3. The
V-shaped grooves created on the surface are described by four structural parameters l, w, h,
and d, which denote the length, width, depth, and period of the grooves, respectively. Based
on experimental observations and numerical simulations, it is found that the variations of l, w,
and d may affect the electric field distribution in the grooves. However, the variations in these
parameters do not induce obvious change in the electric field distribution on the surface of the
metal. In other words, it is the depth of the grooves that plays an important role in the
redistribution of the electric field or the formation of HSFLs. Based on experimental
observation, we chose l = 6.0 μm, w = 0.2 μm, d = 0.65 μm and varied h to see its effect on
the distribution of the electric field. In the numerical simulations, the Drude model is
employed to describe the physical properties of metals. The real and imaginary parts of the
dielectric constant of nickel at 800 nm are chosen to be εr = 1.3 and εi = 2.17, respectively
[26]. The behavior of the stainless steel is expected to be similar to that of nickel because of
the similar dielectric constant. Some typical simulation results are presented in Fig. 4.
Fig. 3. Simplified model for the structured surface used in the numerical simulations.
For shallow grooves with h ~80 nm, the numerical simulation shows that the electric field
is mainly localized in the grooves. This behavior has been reported previously [18]. It results
in the deepening and elongation of the grooves. With a further increase of h to 150 nm,
however, an obvious change is observed in the electric field distribution. One can easily find
that the local maxima of the electric field occur on the ridges in between the two neighboring
grooves. Although the field intensity of these maxima is still weaker than that localized in the
grooves, they appear on the surface of the metal and affect the subsequent ablation of the
surface. As expected, the intensities of these maxima increase with the deepening of the
grooves, as shown in Figs. 4(c) and 4(d) for h = 200 and 300 nm, respectively. The intensities
of these maxima even exceed the field intensities in the grooves for h = 300 nm. Once the
field intensities of these maxima exceed the ablation threshold of the metal, shorter grooves
will be created in between the initially formed grooves, as shown in Fig. 1(b). As a result, the
incident light will be redistributed among the initially and newly formed grooves and some
energy will be deposited on the newly formed grooves. This results in the deepening and
elongation of the newly formed grooves. This process continues, leading to the formation of
HSFLs, as shown in Figs. 1(c) and 1(d). When the depth of the grooves becomes larger, the
field intensities in the grooves become weaker while those on the ridges become stronger. In
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Received 9 Nov 2011; revised 27 Nov 2011; accepted 27 Nov 2011; published 4 Jan 2012
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Fig. 4. Simulated electric field distribution on the surface of nickel (x-y plane) with grooves of
different depths: (a) h = 80 nm, (b) h = 150 nm, (c) h = 200 nm, and (d) h = 300 nm.
Fig. 5. Simulated electric field distributions in the x-z plane for grooves with different depths.
addition, the field distribution in the grooves is also changed, as shown in Figs. 4(a)–4(d). In
Fig. 4(d), it is found that the separation between the local field maxima in the grooves is
approximately equal to the laser wavelength (0.8 μm). Therefore, it is thought the deep
grooves behave as plasmonic waveguides for light and the local field maxima originate from
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Received 9 Nov 2011; revised 27 Nov 2011; accepted 27 Nov 2011; published 4 Jan 2012
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the standing wave formed in the waveguides. We have also simulated the electric field
distributions in the x-z plane for grooves with different depths. The simulation results shown
in Fig. 5 reveal that the ablation of the grooves is alleviated and eventually stopped with the
increase in the depth of the grooves. When h = 170 nm, the field intensity localized in the
groves becomes weaker. For h > 200 nm, the field intensity is concentrated on the ridges in
between the grooves, leading to the creation of new grooves. In addition, the field intensity in
the initially formed grooves becomes very weak, implying that no ablation occurs in these
grooves.
5. Conclusion
In summary, we have suggested a formation process for HSFLs on metal surface. Based on
the FDTD simulation, it is revealed that the field intensity distribution on metal surface will
be dramatically changed when the depth of the initially formed grooves exceeds a critical
value. The field intensity which is localized in the grooves becomes concentrated on the
ridges in between the grooves, leading to the creation of new grooves and thus the HSFLs.
The proposed formation process was supported by the experimental results observed on
stainless steel and nickel. It will be helpful for the fabrication and application of HSFLs on
metal surface.
Acknowledgments
The authors acknowledge the financial support from the National Natural Science Foundation
of China (Grant Nos. 10974060, 51171066 and 11111120068) and the project for high-level
professionals in the universities of Guangdong province, China.
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Received 9 Nov 2011; revised 27 Nov 2011; accepted 27 Nov 2011; published 4 Jan 2012
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