Supporting Information
Femtosecond laser induced tunable surface ablation aided by square grids
diffraction
Weina Han,1 Lan Jiang,1 Xiaowei Li,1,a) Yang Liu1 and Yongfeng Lu2
1
Laser Micro/Nano-Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology,
Beijing 100081, PR China
2
Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588-0511, USA
A. Diffraction-aided fs laser ablated structures with a diffraction distance
around 20 μm
Identical experiment is performed with the copper grids tightly placed on the surface of the
sample with a distance D around 20 μm which is measured by the optical microscope. As
expected, reduced number of micro-peak dots is formed. By increasing the scanning speed,
the surface transformation from 3 striped ripple lines (Fig. S1(a)) to a 3 × 3 lattice
distribution of ripples (Fig. S1(b)-(f)) is achieved. The magnified 3 × 3 lattice arrangement of
Fig. S1(b) is shown in Fig. S1(g). The corresponding calculated relative intensity distribution
with the case of D = 20 μm is shown in Fig. S1(h). It can be noted that, the separation width
and number of the micro-peak intensity both vary in the horizontal and vertical direction. A
close correspondence between the position of the multipledots and the pattered surface is
observed. This illustrates that the energy redistribution caused by the grids aperture
diffraction plays a key role in the periodic arrangement of the ablated ripple structures.
a)
Electronic mail: lixiaowei@bit.edu.cn.
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FIG. S1. Diffraction-aided fs laser ablated surface structures through the copper grids in
different scanning speeds. (a) 100 μm/s; (b) 200 μm/s; (c) 300 μm/s; (d) 400 μm/s; (e) 500
μm/s; (f) 600μm/s. The inset (g) shows a magnification of 3 × 3 lattice dots in (b); (h) the
corresponding calculated normalized irradiated intensity (along the horizontal line with
maximum intensity) with a 3 × 3 lattice. The red double-headed arrow and blue arrow
indicate the polarization direction and the scanning direction, respectively. The polarization
direction and the scanning direction is fixed from (a) to (f), and (a)-(f) share the same scale
bar.
2
B. Two-step experiment based on the laser spot overlap
The following is a series of two-step induced diffracted ripple dots, aiming to explain the
influence of the initially induced structures during the overlap by dynamic laser scanning
process on the subsequent ripple dots formation. Fig. S2(a) is the schematic diagram of the
experimental process. The polarization direction is along the y axis. For the first step, the
sample surface is pretreated by 100 laser shots on point A (the distance between spot center
and the edge of the grid is 4 μm), and then for the second step, another 100 laser pulses are
shot on point B or C by moving the translation stage. The distance between AB, AC, AD,
AE, and AF are about 2 μm, 4 μm, 6 μm, 8 μm, and 10 μm, respectively. Fig. S2(b)-S2(f) are
SEM images of the two-step experimental results. In Fig. S2(b), about half regions of the
grid-covered area are overlapped, it can be seen that 4 stripes of ripples are formed due to the
concentric energy deposition. In Fig. S2(c), the center distance between two spots is
increased to about 4 μm. Under this condition, nearly all the grid-covered area is overlapped,
we can find an increased number of 5 stripes formation, as shown in Fig. S2(d). Keep
increasing the center distance, 6 stripes can be found (Fig. S2(e)-S2(f)).
The formation of the multiple dots can be considered as the repetition of the two-step process.
We deduce that the control of the number of the multiple dots in a lattice by scanning speed is
mainly attributed to the overlap during the scanning process. When the fs laser is irradiated
on sample surface through the copper grid, initially seeding diffracted multiple ripple dots is
induced. The overlap areas and positions between the seeding ripple dots and irradiation
beam change with the scanning speed, which could affect the subsequent multiple dots
distribution under certain conditions (Fig. S2(b)-S2(c)). This seeding effect plays a dominant
role for the subsequent multiple dots formation. Moreover, it may be also related with the
surface plasmon polaritons (SPP). Previous researches reveal that initially structured surface
could affect the generation and propagation of the SPP. Hence, when the laser beam irradiates
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on certain positions of the initial diffracted structure, the coupling between the specially
excited SPP and the irradiation laser beam may result in the subsequent optical field
redistribution. However, the exact mechanism for this phenomenon is still unclear and needs
more investigations.
FIG. S2. (a) Schematic diagram of the two-step experimental process. (b)-(f) are SEM images
of the stripes of ripple structures induced by the two-step method. The scale bar is 5 μm.
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C. Polarization-dependent ablation effect
It is expected that the interaction of the laser with the material surface will be changed after
the formation of initial ripples, which leads to the redistribution of the incident laser that may
significantly affect the subsequent ablation process.S1 The initially formed ripples, which
behave as a surface grating, facilitate the coupling between the incident laser light and the
SPPs. According to our previous study, directional SPP scattering by the initially formed
surface ripple structures can be achieved under linearly polarized fs laser irradiation. Given
SPP launching is strongly limited to TM incidence because of the intrinsic TM polarization
state of SPP fields. Thus, the SPP launching and coupling is perpendicular to the ripple
structures (along the polarization direction of the linearly polarized fs laser). Fig. S3 shows a
schematic illustration for the directional SPP launching and scattering for different relative
angles. The ablation of the orthogonal arranged ripple structures reaches the maximum by the
overlap of the directional SPP launching and coupling when the incident laser pulse is along
the orthogonal edge of the square aperture.
(a)
v
(b)
θ = 0°
v
θ = 45°
FIG. S3. Schematic illustration of the SPPs launching and scattering based on the initially
formed ripple structures. The blue arrow indicates the scanning direction and the red doubleheaded arrow indicates the SPPs propagation.
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D. Striped ripple lines connection with orthogonal polarization directions
Compared with that in Fig. 1(a) in the text, more striped lines are formed with a lower
deposited energy. Two situations: 1) the polarization direction is parallel to the scanning
direction (θ = 0°); and 2) the polarization direction is perpendicular to the scanning direction
(θ = 90°) is performed in the research. More stripes are formed as the pulse fluence reduces.
It can be easily found that the direction of the stripes is along one edge of the grid. This is
because of the symmetrically orthogonal microdots lattice induced by the diffraction-aided
energy distribution effect.
v
(a)
6 stripes
E
v∥E
v
(b)
6 stripes
E
v⊥E
v
(c)
7 stripes
E
v⊥E
FIG. S4. Striped ripple lines formation at relatively low scanning speed (v = 100 μm/s) with
pulse fluence (a)-(b) F = 0.75 J/cm2; (c) F = 0.65 J/cm2. (a) The polarization direction is
parallel to the scanning direction. (b)-(c) The polarization direction is perpendicular to the
scanning direction. The blue arrow indicates the laser scanning direction and the red arrow
indicates the laser polarization direction. The scale bar is 10 μm.
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E. The mechanism of the polarization-dependent striped ripple lines
connection between the two orthogonal directions
The mechanism of the ripples formation is still a subject of discussion and is usually related
to the optical or mechanical reaction of irradiated surfaces, which may involve, e.g.,
collective electronic behavior, selective energy deposition, or the capillarity phenomena.
According to a previous study,S1 the initially formed ripples, which behave as a surface
grating, may facilitate the coupling between the incident laser light and the surface plasmon
wave. Meanwhile, Garrelie et al. experimentally demonstrated the effect of an initial
structured surface on the role of SPP, which highlights its important role in ripple
formation.S2 Furthermore, research shows that for linearly polarized fs irradiation, directional
SPP scattering is anticipated,S3,S4 strengthening the subsequent ripple fabrication. Fig. S5
shows a schematic illustration for the diffraction-aided fs laser induced anisotropic ripple
formation with orthogonal polarization directions. Elongated ripple structures are fabricated
along the polarization direction due to the enhanced SPP scattering. It illustrates that a
reduced adjacent spacing along the vertical/horizontal direction can be achieved when the
polarization direction is around the vertical/horizontal direction, leading to the directional
ripples connection.
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v
(a)
θ = 90° E
v
(b)
θ = 0° E
FIG. S5. Schematic illustration of selective connection between the the formation processes
of striped ripple lines at relatively low or high scanning speeds.
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F. Fresnel diffraction model for a square aperture
The schematic diagram of the electric field distribution at different planes is shown in Fig.
S6. For the given case, we consider the plane of the sample as an observation plane. And the
square grids can be simplified as a single square aperture with a 19-μm length. The sample is
located at the focus plane, and the square aperture is placed above the sample with a distance
of D. The field of a spatially dispersed Gaussian laser pulse can be expressed as:
E1 (x, y)  E0 exp[
x2  y 2
]
2 w2
(S1)
where E0 is a field amplitude, w is the incident beam waist before the focusing lens.
According to the experimental parameters, the beam waist is w = 1.5 mm in our situation.
Using the slow varying envelope approximation, the field after the objective can be written
as:
E2 ( x, y)  E1 exp[ik
x2  y 2
]
2f
(S2)
where k = 2 /λ (λ is the laser wavelength of 800 nm) and f is the focal length of the objective.
Under the paraxial approximation, the laser field near the square aperture plane can be
obtained by the use of the Fresnel diffraction formula below:
E3 ( x, y, z ) 
exp[i k ( f  D)] 
(( x   ) 2  ( y   ) 2
E
(

,

)

exp[i
k
]d d ,
2
i ( f  D)  
2( f  D)
(S3)
The laser field after the square aperture can be expressed as:

1,


E4 ( x, y )  E3 
0,


a
a
x ;
2
2
a
a
a
a
x   ,x  ,y   ,y  ;
2
2
2
2

(S4)
where a is the length of the square aperture, a = 19 μm. Thus, the laser field on the surface of
the sample plane can be obtained by the secondary Fresnel diffraction:
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(( x   )2  ( y   ) 2
E5 ( x, y, z )  E4   E4 ( , )  exp[i k
]d d ,

2D

(S5)
Thus, the intensity distribution of the sample plane can be obtained as follows:
I ( x, y, z )  E5 ( x, y, z ) .
2
(S6)
E1
E2
Lens
Square
aperture
Substrate
E3
E4
E5
D
FIG. S6. Schematic diagram of the electric field distribution at different planes.
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S1
M. Huang, F. Zhao, Y. Cheng, N. Xu, and Z. Xu, “Origin of laser-induced near-
subwavelength ripples: interference between surface plasmons and incident laser,” ACS
Nano 3, 4062–4070(2009).
S2
F. Garrelie, J. P. Colombier, F. Pigeon, S. Tonchev, N. Faure, M. Bounhalli, S. Reynaud,
and O. Parriaux, “Evidence of surface plasmon resonance in ultrafast laser-induced ripples,”
Opt. Express 19, 9035–9043(2011).
S3
X. Li, Q. Tan, B. Bai, and G. Jin, “Experimental demonstration of tunable directional
excitation of surface plasmon polaritons with a subwavelength metallic double slit,” Appl.
Phys. Lett. 98, 251109(2011).
S4
W.N. Han, L. Jiang, X.W. Li, P.J. Liu, L. Xu, and Y.F. Lu, “Continuous modulations of
femtosecond laser induced periodic surface structures and scanned line-widths on silicon by
polarization changes”, Opt. Express 21, 15505-15513(2013).
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