Formation of subwavelength periodic structures on
tungsten induced by ultrashort laser pulses
Q. Z. Zhao,* S. Malzer, and L. J. Wang
Institute of Optics, Information and Photonics, Max-Planck Research Group and University Erlangen-Nuremberg,
91058 Erlangen, Germany
*Corresponding author: email@example.com
Received March 14, 2007; revised May 3, 2007; accepted May 9, 2007;
posted May 14, 2007 (Doc. ID 81016); published June 25, 2007
The evolution of surface morphology of tungsten irradiated by single-beam femtosecond laser pulses is in-
vestigated. Ripplelike periodic structures have been observed. The period of these ripples does not show a
simple relation to the wavelength and angle of incidence. The orientation of ripples is aligned perpendicu-
larly to the direction of polarization for linearly polarized light. Surprisingly, we find that the alignment of
the ripple structure turned left or right by 45° with respect to the incident plane when using right and left
circularly polarized light, respectively. The period of the ripple can be controlled by the pulse energy, the
number of pulses, and the incident angle. We find a clear threshold for the formation as a function of pulse
energy and number of pulses. The mechanism for the ripple formation is discussed, as well as potential ap-
plications in large-area structuring of metals. © 2007 Optical Society of America
OCIS codes: 140.3390, 220.4000, 190.4350, 160.3900.
Periodic microstructuring or nanostructuring of ma-
terials has found wide applications in different tech-
nical implementations. A well-defined surface struc-
ture will give the surface a completely new property.
Common techniques used to generate periodic sur-
face modification at the microscale or nanoscale in-
clude treatments by plasmas; electron, ion, or proton
beams; and laser beams. Among them, two-beam and
induced periodic microstructures have attracted in-
creasing interest because they do not need masks
and photoresists and can inscribe periodic structures
on almost any material directly [1–3]. On the other
hand, ripplelike periodic structures induced by a
single laser beam have been found on semiconductors
, metals, and dielectrics [5–7]. More recently, the
formation of nanoripples inside transparent dielec-
trics also was observed by using single-beam fs laser
irradiation . In previous studies, the single-beam
laser-induced periodic structures have shown two
main features: (1) the orientation of the induced
ripples is perpendicular to the direction of laser po-
larization, and circular polarization cannot form pe-
riodic structures, and (2) the structures normally
contain a period ?=?/?1±sin ??, where ? is the laser
wavelength and ? the incident angle measured from
the surface normal. The physical mechanism of the
formation of ripples has already been explained
In this Letter, we report the evolution of surface
morphology of tungsten irradiated by a single-beam
fs laser. Ripples with spatial periods up to half the la-
ser wavelength were observed with different incident
angles, and they still can be formed by using circu-
larly polarized light. Finally, we demonstrate the
writing of large-area periodic structures with a single
beam, which is more efficient than conventional li-
thography and fs laser interference inscription.
A multipass amplified Ti:sapphire mode-locked la-
ser system with 800 nm wavelength, 33 fs pulse du-
ration, ?1 mJ average energy, and 1 kHz repetition
rate is used as the irradiation source. After passing
through a variable neutral-density filter and a me-
chanical shutter, the beam is guided into a micro-
scope and is focused on the sample surface by a
computer-controlled three-dimensional (3D) stage.
The number of laser pulses applied to the sample is
controlled by a frequency control system. All experi-
ments are performed in air under atmospheric pres-
sure. The morphology of induced structures is in-
spected by using an optical microscope and an
ultrahigh-resolution field-emission scanning electron
microscope (FE-SEM). Tungsten foils with thickness
of 50 ?m are used in this study.
The SEM analysis (Fig. 1) indicates that the mor-
with thehelp ofa
tures on tungsten induced by fs laser with varied energy
and pulse numbers. (a)–(r) Induced by vertical polarization;
(s), (t) induced by left and right circular polarization, re-
spectively. Inset, schematic of the momentum conservation
condition of wave vectors of incident light ?ki?, plasma
wave ?kp?, and ripple structure ?kr?.
(Color online) Morphological evolution of struc-
OPTICS LETTERS / Vol. 32, No. 13 / July 1, 2007
0146-9592/07/131932-3/$15.00 © 2007 Optical Society of America
phology of fs laser-induced surface structures de-
pends on both laser energy and the number of ap-
plied pulses. Figures 1(q) and 1(r) show that
nanoscale surface defects begin to form even after a
single pulse exposure at energies 3.2 and 1.6 ?J, re-
spectively. When the pulse energy is less than 1.6 ?J,
no surface morphological changes are observed by
SEM for single-pulse irradiation.After 10-pulse expo-
sure at energies 3.2 and 1.6 ?J [Figs. 1(n) and 1(o)],
the surface defects with some boundaries are already
clearly formed. When the number of pulses is in-
creased to 100, the ripples begin to occur [Figs.
1(k)–1(m)]. In fact, when the number of pulses is up
to about 15, the ripples already occur at energies of
3.2 and 1.6 ?J (not shown in this figure). As can be
seen in Fig. 1(k), after 100 pulses, the periodic struc-
tures in the middle of the beam spot have been de-
stroyed to form flowerlike structures as a result of
material redeposition during laser ablation. How-
ever, ripples can still be observed on the periphery of
the irradiated spot, where the Gaussian beam inten-
sity is low enough for periodic structures formation
at energy of 3.2 ?J. With a lower energy of 0.8 ?J, the
periodic structures can be generated over the entire
spot [Fig. 1(m)]. As the number of pulses up to 103is
increased, holes are generated at energies of 3.2, 1.6,
and 0.8 ?J due to laser ablation [Figs. 1(f)–1(h)];
ripples still can be observed on the periphery of the
ablated spot. At an energy of 0.4 ?J [Fig. 1(i)], the
ripples are formed in the middle of the spot, but on
the periphery some damage defects occur. More inter-
estingly, the phenomenon shows a thresholdlike be-
havior. For example, at a low pulse energy of 0.2 ?J,
irradiation by 103pulses cannot produce the ripple-
like structure [Fig. 1(j)].
Previous studies [7,11] showed that ripples could
be produced with a single laser pulse. In our case, a
single pulse, even 10 pulses, cannot form periodic
structures. This phenomenon was also observed in
. In our case, when the number of pulses is up to
about 15, the ripples can be generated at a pulse en-
ergy of 1.6 ?J. If the pulse energy is lower than
1.6 ?J, the formation of ripples requires much more
pulse accumulation; for example, at a laser energy of
0.8 ?J, after 50 pluses, a ripple can be formed. This
indicates that the threshold for formation of ripples
by successive irradiation apparently decreases with
an increasing pulse number . As for the situation
of Fig. 1(j), even for up to 103pulses no ripple was
generated. This is because the incident laser inten-
sity is too low to melt the tungsten surface.
The threshold dependence of formation of ripples
on pulse fluence and pulse number was measured
and is shown in Fig. 2(a). We further plot the inverse
of the fluence as a function of pulse numbers [Fig.
2(b)]. The threshold shows a linear dependence. This
indicates that the product of fluence and pulse num-
ber can be used as the measure for the threshold of
We further measured the dependence of the period
of ripples on the laser energy, the number of pulses,
and the incident angle. We observed the decrease of
the period with an increase of the number of pulses
for fixed laser energy [Fig. 3(a)]. The dependence of
structural period on laser energy for fixed pulse num-
ber is shown in Fig. 3(b). An increase of the struc-
tural period with laser energy was observed. At a
pulse energy of 0.6 ?J and 200 pulses, the depen-
dence of the ripple period on the incident angle is
shown in Fig. 3(c). We also plot the graph of the con-
ventional equation ?=?/?1+sin ??. As can be seen,
the periods of ripples decrease from 528 to 364 nm
with the incident angle from 0° to 70°. Obviously, the
experimental result does not follow the conventional
We also investigated the influence of laser polariza-
tion on the formation of ripples. The ripples are
aligned perpendicularly to the direction of laser po-
larization when we used the linearly polarized beam,
in agreement with previous results [5,9]. When we
apply an elliptically polarized beam, the ripples are
aligned perpendicularly to the elongated axis, which
is similar to observation in . Interestingly, when
we use a circularly polarized beam, the ripple struc-
ture still can be produced. The orientation of ripples
is +45° and −45° for left and right circularly polar-
ized beams with respect to the incident plane of the
beam, respectively [Figs. 1(s) and 1(t)]. This behavior
was not observed in previous studies to our knowl-
bers required to form periodic structures. Only above-the-
line ripple structures can be observed. (b) Inverse of the
fluence as a function of pulse number.
(Color online) (a) Laser energy versus pulse num-
Fig. 3. (Color online) Dependence of period of ripples on (a) the number of pulses, (b) the pulse energy, and (c) the incident
angle. The solid curve in (c) is plotted to guide the eye.
July 1, 2007 / Vol. 32, No. 13 / OPTICS LETTERS
edge. We further observe the formation of ripples by
two consecutive series of pulses. First, we expose the
sample to a series of pulses to form ripples along the
direction perpendicular to the polarization. Second,
we apply a second series of pulses with polarization
orthogonal to the first series. One may think that a
two-dimensional grid will be formed. Experimentally,
we found that the original ripples were washed out
and at the same place new ripples formed with the
orientation now perpendicular to the polarization of
the second group of pulses.
Our results are quite different from those one could
have expected from the conventional explanation, at-
tributing the ripple formation to the interference of
incident light with scattered light. The mechanism
proposed by Shimotsuma et al.  on the formation of
ripples inside bulk glass appears to offer some hint.
In their case, the interference between the incident
light field and the bulk electron plasma wave pro-
duces a periodic modulation of electron plasma den-
sity, temperature, and structural change inside the
glass. They developed an analytical expression  of
the ripple period depending on the electron tempera-
ture and density and showed that the ripple period
increases when the electron temperature and the
electron density increase. Our result is somewhat
similar to the observation of Shimotsuma et al. How-
ever, the incident light now excites surface plasma
waves rather than bulk plasma waves. Such a
plasma density wave could couple with the incident
light wave only if it propagates in the plane of light
polarization . The interference between the inci-
dent light and the plasma density wave generates a
periodically modulated intensity field, which will
heat the sample surface nonuniformly, causing a non-
uniform melting, evaporation, or ablation. After laser
irradiation, the interference fringe is inscribed on the
target surface. The inset of Fig. 1 shows the sche-
matic of the momentum conservation condition of
wave vectors of incident light ?ki?, plasma wave ?kp?,
and ripple structure ?kr?. The mechanism of the for-
mation of ripples induced by a circularly polarized
beam is still unclear.
Finally, if we continuously scan the sample to con-
nect every spot well, we can produce a line with
ripples inside. By repeatedly scanning the sample,
we can obtain a large-area structure with ripples
In summary, the evolution of surface morphology of
tungsten under a single-beam fs laser irradiation has
been systematically studied. The generation of
ripplelike periodic structures is decided by applied la-
ser energy, pulse numbers, incident angle, and laser
polarization; the period of ripples can be controlled
by these parameters. Our observation has potential
application in large-area structuring of metals, such
as spectrally selective devices  and highly direc-
tional radiation sources .
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area periodic structures. (a) Schematic of two writing
schemes. Dashed arrow, laser polarization direction; solid
arrow, laser scanning direction. Inset, optical microscope
image of horizontal and vertical scanning. (b) SEM image
of large-area ripple structures. Inset, magnification.
(Color online) Demonstration of writing of large-
OPTICS LETTERS / Vol. 32, No. 13 / July 1, 2007