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Micro-structuring of gold coated plates with LIPSS for localized plasmonic sensors

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Proceedings of LPM2020 the 21st International Symposium on Laser Precision Microfabrication
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Proceedings of LPM2020 the 21st International Symposium on Laser Precision Microfabrication
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Micro-structuring of gold coated plates with LIPSS for localized
plasmonic sensors
Julien Dupuy, Marc Lamblin, Fabian Dortu, Damien Bernier, Yves Hernandez
Multitel a.s.b.l, Parc Initialis, Mons, BE 7000
Localized surface plasmon resonance (LSPR) or particle plasmon resonance (PPR) consists in
a specific arrangement of metallic nanostructures in order to enhance their plasmonic response. This
technique can lead to the development of high sensitivity bio-sensors but faces various difficulties
on the fabrication side.
We investigate Laser Induced Periodic Surface Structuring (LIPSS) in order to produce sub
micron periodic patterns on gold layer for localized plasmonic sensors fabrication. LIPSS are peri-
odic structures that are generally created on a material surface by accumulation of ultra-short pulses
exhibiting energies close to the material ablation threshold. Although LIPSS physical mechanisms
are still under investigation, they have been induced in a wide variety of materials (metals, semicon-
ductors, polymers, etc) for numerous applications such as realization of hydrophobic/hydrophilic
surfaces, control of surface reflection or realization of low friction / high adhesion surfaces. Here we
used ultrashort pulses (500 fs) at 515 nm to induce LIPSS structures on thin gold layers for LSPR
sensors.
Keywords: LIPSS, Plasmonic, Gold, LSPR
1. Introduction
Localized Surface Plasmon Resonance (LSPR) is a
powerful technique for sensitive bio-sensing. LSPR in met-
als strongly depends on the size and shape of the sensor
and can be enhanced with a nanoscale pattern. LSPR sen-
sors can be applied in many applications like chemical,
biological sensing… In our case we are targeting the de-
velopment of a label-free biosensor for plant diseases eval-
uation at points-of-care [1]. In this work, our team has de-
veloped an original microfabrication method involving
gold gratings synthesis by pulsed laser writing.
Different other methods have been explored in literature for
generating micro and nano periodic structures on gold [2,
3,11] like UV lithography, interferometry, nanosphere li-
thography, nanoimprint [4,5]. Another solution we are in-
vestigating is the use of Laser Induced Periodic Surface
Structuring (LIPSS) in order to produce sub-micron period-
ic patterns on gold for localized plasmonic sensors fabrica-
tion.
LIPSS are periodic structures that are generally created on
a material surface by accumulation of ultra-short pulses
exhibiting energies close to the material ablation threshold.
High resolution structures proportional to the laser operat-
ing wavelength, typically ripples of hundreds of nanome-
ters, can be generated [6].
2. Description of the experiment
2.1 Laser and micro-machining setup
The micromachining set-up used for the experiments is
described in Figure 1. As we can see, the system is made of
a laser system followed by three optical modules.
The femtosecond laser used in this experiment operates
at a wavelength of 1030nm and delivers optical pulses of
energy up to 40µJ at a repetition rate in the range of 10 kHz
to 300 KHz. The optical beam generated by this laser will
be modified through three different modules before reach-
ing the sample.
The first module permits to change the wavelength of
the laser. A second harmonic generation set-up, based on a
LBO crystal, is employed in order convert the optical
wavelength down to 515nm. This wavelength is more suit-
able than infrared because of the absorption spectra of Gold
and it will permit achieving smaller LIPSS structures.
The second module permits to control the power/energy
of the laser beam and its profile. We use a halfwave plate
(HWP) and a polarization beamsplitter cube for power con-
trol and a diffractive optical element (DOE) for beam shap-
ing. The DOE converts the input beam into a square top-hat
beam shape (4x4mm) with a homogeneous energy distribu-
tion. A second HWP is placed to control the linear laser
polarization orientation.
The third module consists of the scanning and focusing
elements. We used here a 2D scanning galvanometric head
equipped with a 160 mm f-theta lens, and coupled with a
X-Y-Z translation stages platform to position precisely the
sample under laser beam. With this configuration, two
techniques are possible for dynamic irradiation of the sam-
Fig. 1 Laser experiment for LIPSS structuring
Proceedings of LPM2020 the 21st International Symposium on Laser Precision Microfabrication
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ples: move the sample using the translation stages, keeping
a fixed point of incidence for the focalized laser, or fix the
sample position and scan the surface with the galvanomet-
ric head. The focused spot size (square shape) is estimated
around 25µm.
2.2 Samples description
The samples are made of a thin layer of gold (35 nm)
deposited on glass by vapor phase evaporation, with an
intermediate 1 nm adhesion layer of Chromium. The Glass
is a 1mm thick standard microscope slide. The LIPSS pro-
cess permitted to generate regular patterns with a periodici-
ty of about 500 nm. Due to the thin layer of gold, careful
manipulations and a precise control of the laser energy de-
posited on the material are needed. Initial surface rough-
ness of a sample is Sa=2nm.
2.3 Characterization
The analyses of the irradiated samples were performed
with a 3D confocal laser microscope equipped with 20X
and 150X objectives, and also with a SEM microscope.
The first microscope is useful to have rapid views of the
surface and precise estimation of the profile and roughness
of the LIPSS generated structures; and the second one is
mandatory to see details within the nanometer scale.
3. Main results
3.1 Ablation threshold
The first step of the work was to define the minimum
deposited laser energy density on the sample required for
removing the thin gold layer, i.e. the ablation threshold.
Therefore, we studied the ablation behavior of the material
as a function of the laser fluence for a high scanning speed
fixed at 1000 mm/s, and for a pulse frequency of 100kHz.
This condition makes it possible to separate the laser im-
pacts on the surface and to evaluate an ablation threshold
for a single pulse. We then defined the threshold fluence as
the minimum value for which an ablation profile is meas-
urable. The ablation threshold was estimated to 0.072
J/cm², which corresponds to an average power of 50mW,
which is in-line with literature [6,7]. The main difficulties
to evaluate this ablation threshold were that we often have
a “peeling off effect” of the gold layer even at very low
laser fluence. Experiences conducted us to use the thin
Chromium layer as a mandatory adhesive layer.
3.2 Fluence effect
Dynamic studies were performed by adjusting the la-
ser polarization with the HWP so that the ripples produced
by LIPSS are oriented perpendicularly to the machining
scan lines, and by fixing the laser frequency at 100kHz for
three different fluences for a single pulse energy of 0.072
J/cm², 0.058 J/cm² and 0.042 J/cm². This corresponds to
average powers of 50, 40 and 30 mW respectively.
This time, we used a scanning speed in the range 10-
100 mm/s, giving us an overlap for each laser pulse corre-
sponding to 99-95%
To evaluate the impact of the three fluences on the
generated LIPSS structures, we measured the surface
roughness Sa on the center of the scanned line (yellow
mark on Fig.7)
Depending on the scanning speed, the overlap is more
or less important and the number of pulses varies between
1000 and 10000 pulses/mm. In all presented cases the
LIPPS patterns are very regular. The best parameter found
is 25mm/s with 30mW average power (0,042J/cm² - 100
pulses). Even if a 10X factor of deposited energy is tested
(related to the speed process) and the average value of the
surface roughness slightly decreases with the speed, we can
see that the highest speed of 100mm/s give us a good pat-
tern structure and also a faster process to generated LIPSS
on cm² surface area.
3.3 Laser Frequency effect
We also tried different laser frequencies with different
scanning speeds in order to improve the quality and the
depth of the LIPSS structure. We compared for the same
fluence of a single laser shot of 0.042J/cm² (we adapted the
Fig. 3 Ablation of the thin gold layer obtained with a flu-
ence of 0.09 J / cm²
Fig.4 Analysis of LIPPS surface roughness as a function
of the scanning speed, for 3 different laser powers
@100kHz
Fig. 2 Samples scheme
Proceedings of LPM2020 the 21st International Symposium on Laser Precision Microfabrication
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average power of the laser) the repetition rates of 100, 200
and 300kHz.
The trend observed in the previous paragraph is also
valid if the frequency is doubled or tripled (Fig.4) with a
slight decrease of the surface roughness, i.e the depth of the
LIPSS pattern, with the increase of the speed.
This study highlights two different behaviors of the
gold thin film under femtosecond irradiation. Within the
limits of the parameters studied, the most important param-
eter is the single pulse fluence since it is this one which
determines the evolution that matter will undergo. In a sec-
ond step, it is the number of overlapped pulses or cumula-
tive energy that allows these micro-structures to evolve
(Fig 6).
4. Discussion
We present the analysis of the two best results obtained
within this work. We present a case of low spatial period
LIPSS (ripples on the surface) but also one LIPSS parame-
ter allowing a periodic ablation of the thin gold layer.
4.1 Best Low Frequency LIPSS obtained
We identified different parameters giving the best rip-
ples for the generated LIPSS, i.e the most periodic, regular
and contrasting ones. We measured a periodicity in the or-
der of 520nm, same range than the laser wavelength. These
experiments were done with the same single pulse laser
fluence around 0.042J/cm², and only the scanning speed
and laser frequency change (Fig 5.)
The light intensity distribution on the focused laser spot,
although having certain homogeneity thanks to the DOE, is
not so clean particularly at low fluence. You can see some-
times different ripples and patterns from LIPSS at the bor-
ders of the scanning lines (Fig 7.)
The SEM analysis reveals that the periodic structures of
the LIPSS are made of an arranged structure of bubbles and
holes.
4.2 Periodic ablation
Here we present the laser and scanning parameter for
which we observed a complete ablation of the layers of
gold and chromium. Compared with the previous experi-
ments, we worked this time above the ablation threshold,
and with high speed, trying to limit the effects of total abla-
tion on LIPSS generated structure.
Fig.8 SEM analysis for LIPSS with 0,042J/cm², 80mm/s
300kHz
Fig.7 Analysis for LIPSS with 0,042J/cm² with a laser
scanning speed of 80mm/s, 300kHz
Fig.5 Analysis of LIPPS surface roughness as a function
of scanning speed, for 3 different laser repetition
rates
Fig.6 Analysis of LIPPS surface roughness as a function
of cumulative energies
Proceedings of LPM2020 the 21st International Symposium on Laser Precision Microfabrication
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We observe that the gold streaks, perpendicular to the laser
scanning direction, are homogeneous. By analyzing their
profiles (Fig 9), we observed that their depth is very close
to the thickness of the gold and chromium layers, i.e. a
depth of about 35nm.
We measured also a periodicity in the order of 520nm.
Nevertheless, a SEM analysis of the sample reveals that the
lines are not very homogeneous, which affects the periodic-
ity analysis.
5. Conclusion
We have studied the different steps of the self-
organization of matter resulting from dynamic irradiation
by a femtosecond laser with Yb doped fiber doubled along
a line on the surface of a thin layer of gold of 35nm. The
formation of LIPSS oriented perpendicular to the radiation
lines was carried out.
The analysis of these samples with a scanning electron
microscope reveals that the reliefs of the wavelets are
peaks and aligned holes. The explanation advanced by the
literature to explain the weak clarity of LIPSS on gold is
the too weak coupling electron-phonon which results in the
diffusion of the electrons before they transferred their ener-
gy to the matrix. [6,7]
The main obstacles encountered during this work were
the extreme sensitivities of the parameters. Indeed, a small
variation of the thickness of the gold layer, or of the laser
pulses energy, leads to a large variation in the morphology
of the observed structures. We would next try to carry out
specific periodic structures with points spaced from each
other by the same distance. For these realizations, circular
polarization or the creation of cross patterns with perpen-
dicular wavelet directions, are promising lines of research.
Another technique will be Laser-induced reorganization
[11] using our LIPSS process, but in this case our samples
need to be adapted to this technique.
Acknowledgments
The samples were provided by the research center
MATERIA NOVA in the frame of the FWVL INTERREG
project BIOSENS (SMARTBIOCONTROL).
This work was carried out within the frame of the project
LASER4SURF, funding from the European Union's Hori-
zon 2020 research and innovation program under grant
agreement No768636.
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Fig. 10 Periodic ablation for LIPSS with 0,086J/cm²,
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Fig. 11 SEM analysis for Periodic ablation for LIPSS with
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Fig. 9 Linear Roughness Ra vs scanning speed for 6 dif-
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Proceedings of LPM2020 the 21st International Symposium on Laser Precision Microfabrication
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Article
Full-text available
Laser-induced reorganization and simultaneous fusion of nanoparticles is introduced as a versatile concept for pattern formation on surfaces. The process takes advantage of a phenomenon called laser-induced periodic surface structures (LIPSS) which originates from periodically alternating photonic fringe patterns in the near-field of solids. Associated photonic fringe patterns are shown to reorganize randomly distributed gold nanoparticles on a silicon wafer into periodic gold nanostructures. Concomitant melting due to optical heating facilitates the formation of continuous structures such as periodic gold nanowire arrays. Generated patterns can be converted into secondary structures using directed assembly or self-organization. This includes for example the rotation of gold nanowire arrays by arbitrary angles or their fragmentation into arrays of aligned gold nanoparticles.
Article
Full-text available
An optical sensor based on the coupling between the plasmonic and photonic resonance modes in metallic photonic crystals is investigated. Large-area metallic photonic crystals consisting of periodically arranged gold nanostructures with dimensions down to sub-100 nm are fabricated using solution-processible gold nanoparticles in combination with interference lithography or interference ablation, which introduces a variety of fabrication techniques for the construction of this kind of sensor device. Sensitivity of the plasmonic response of the gold nanostructures to the changes in the environmental refractive index is enhanced through the coupling between the narrow-band photonic resonance mode and the relatively broad-band plasmon resonance, which is recognized as a Fano-like effect and is utilized to explore sensors. Theoretical modeling shows the characterization and the optimization of the sensitivity of this kind of sensor device. Theoretical and experimental results are demonstrated for the approaches to improve the sensitivity of the sensor device.
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J. Hastanin & Al., "Compact multichannel spectroscopic label-free biosensor platform for plant diseases point-of-care testing (POCT)," Proc. SPIE 11361, Biophotonics in Point-of-Care, 113610P (April 2020)
Localized surface plasmon resonance: Nanostructures, bioassays and biosensing-A review
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Automated polarization control for the precise alignment of laser-induced self organized nanostructures
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U.Hermens & Al., "Automated polarization control for the precise alignment of laser-induced self organized nanostructures" -Optics and lasers in engineering-(2017)