Optics and Photonics Journal, 2011, 1, 11-14
doi:10.4236/opj.2011.11003 Published Online March 2011 (http://www.SciRP.org/journal/opj)
Copyright © 2011 SciRes. OPJ
Silica Nano-Networks as Stretches on Segmented SU8
Rods for Sub-Wavelength Photonics
Francois Doré1, Bruno Bêche1, Nolwenn Huby1, F. Artzner1, Lionel Camberlein2, Etienne Gaviot2
1Institute of Physics of Rennes, UMR CNRS 6251, University of Rennes 1, Rennes, France
2Laboratory of Acoustics, UMR CNRS 6613, University of Maine, Le Mans, France
E-mail: bruno.beche@univ-rennes1.fr, francois.dore@univ-rennes1.fr
Received January 20, 2011; revised March 4, 2011; accepted March 8, 2011
We report an original approach based on a fluidic mechanism involving silica nano-particules that al-
lowed us to design a elaborate set of segmented-optical structures such as arranged clusters of pillars and
cross-tapered-waveguides. We show that the association of such complex segmented pre-formed struc-
tures c an be sp eci fical ly sh ap ed, by way of coup ling nano-fluidics and dry ing mechanisms. Th e formation
of specific silica nano-patterns or organized networks of silica nano-ridges may be predictable and in
perfect agreement with the theory of minimal surfaces dealt with physics of fluids. The interest of such
nano-photonic coupling mechani sms has been clearly highlighted thanks to their ab ilities to build original
nano-silica-networks and the specific development of new filters based on resonant tunnelling effects
between multi-nano-ridges.
Keywords: Integrated Photonics, Nano-Connections, Silica Nano-Rib Waveguides, Nano-Optical Coupling,
Nano-Network, Fluidic Mechanisms
1. Introduction
The main purpose of nano-optical-connections is to con-
vey optical signals with a quite higher wavelength than
the dimensions of a given optical waveguide arranged
several wavelengths in the distance. Due to their notice-
ably high shape ratio (length/lateral-dimension), na-
no-waveguides structures as nanowires [1,2], nanotubes
[3], nanoridges [4] and so on, present promising proper-
ties for optical components, especially light routing, na-
no-connections and networks. Thus, sub-wave-
length waveguides are of great interest for investigation
on sub-micronic propagation and optical nano-coupling
mechanism between specific structures [5]. Regarding
silica materials, various techniques based on new pro-
ductions in materials science together with original
processes proved the adequacy to develop hybrid inte-
grated photonics and to obtain a noticeable nano-sized
confinement marked with a spatial resolution around
twenty times smaller than the proceeding wavelength
[1-4]. Then, the ability to prepare silica nano-connections
may open new opportunities for implementing low-di-
mensional silica materials. In a previous work [4], we
have presented a fluidic mechanism coupled with silica
nanoparticules so as to design single nano-ridge waveguides
with two classical optical coupling configurations be-
tween organic straight-rib waveguides. In this paper, we
highlight the great interest to process with such kind of
nano-fluidic and dynamic dry devices. Indeed, being
widened they come up as suitable to develop various and
complex nano-connections with multi-sub- lambda
branches and nano-networks between adequate segmented
elements (tapers, pillars, rods). One of the key points is
clearly that the pattern of such nano-networks as
stretches on the abovementioned segmented elements is
directly predictable and in perfect agreement with the
theory of minimal surfaces dealt with classical physics of
fluids [6]. Moreover, in such a complex nano-network,
we have characterized the expected nano-optical coupl-
ing with a sub-wavelength routing propagation regime
directly on the integr ated chip.
2. Silica Sub-Wavelength Connections and
Networks Based on a Fluidic Approach
Based on a Minimal Surface at Nanoscale
Considering our processes and materials, relevant devic-
es can be developed: They basically rely on a guided-
Copyright © 2011 SciRes. OPJ
wave proceeding on a (100) silicon substrate coated with
a specific SiO2 layer first obtained by thermal oxidation
of the silicon wafer, yielding 1 µm in thickness with an
index value nSiO2 close to 1.45 at visible and IR wave-
lengths. The organic SU8 film (whose higher refractive
index 1.56 at such wavelengths is most suited for a
guiding layer ranging around [1-10] µm in thickness), is
deposited by spin coating and cured to remove the sol-
vent according to convenient steps of temperature [7]. By
way of UV-lithography, 40 s with UV light source (200
mJ/ cm2) and a similar baking modus to cross-link the
polymer, adequate taper-waveguides and rod-pillars pat-
terns defined on a mask are copied on the SU8 guiding
layer. Then, a generic development process with the spe-
cific SU8 developer (MicroChem®) allows us to obtain
versatile structures prior to operate a mandatory post-
baking (200-2 h) so as to stabilize the SU8 global pat-
terns several micrometers in thickness.
A specific mixture made of a sodium-dodecyl-sulfate
surfactant (SDS) plus water is prepared with a concentra-
tion in silica beads (from Ludox®) ranging around 5%.
The global solution of SDS with silica beads is deposited
by way of a µl-syringe onto a thin cover glass plate (de-
voted to optical microscopy). Then, the whole structure
is turned down and carefully deposited onto the SU8
tapers and rod-pillars waveguides making up the optical
chip patterns. Then, the nano-silica fluid can be strained
between the elements of organic-structures: To this end,
a drying mechanism is operated several minutes so as to
shape a complex nano-network of silica nano-ridges,
typically ranging around [50-100] nm in width, whose
configuration is totally defined by the physics of fluids.
Figures 1(a) and 1(b) represent optical microscopy
images respectively obtained by differential interference
contrast (DIC) and scanning electron treatments of the
nano-silica-connections after being stretched by drying
fluid processes between the segmented SU8-tapers and
-pillars. It may be noted as a key point, that with such a
nanoscale scope, the formation of the specific silica na-
no-patterns or organized networks of silica nano- ridges
is in perfect agreement with the theory of minimal sur-
faces provided with the mechanics of fluids. So, the na-
no-shape between the fourth rod pillar defined by two
points and the couple of the three nano-ridges that
emerge directly from both points at an angle of 120°
(Figure 1(b)) may be observed as a typical example: It is
a specific shape accounting for a minimal surface [6] as
regards a straight nano-ridge, if we consider the radii of
the local curvature of the interface that tends to infinity
with Laplace's equation. The interest of such processes
and nano-fluidic mechanisms is clearly highlighted.
Thanks to their ability to shape original nano- sili-
ca-networks, with a significant building potential in the
third direction between more complex 3D-organic-
Figure 1. Optical microscopy images obtained by differen-
tial interference contrast (DIC) and scanning electron
treatment of the nano-silica-connections stretched by dry-
ing fluid processes between segmented SU8-waveguides
(tapers and pillars). The nano-pattern surrounded in white
dashed line between the four SU8 pillars corresponds to the
minimal surface shape obtained by fluidic mechanisms; as a
result all the angles between straight nanoridge-waveguides
that shape the nano-network are actually equal to 120°: (a)
general layout of the photonic structures; (b) silica nano-
photonic network.
preforms obtained by advanced selective lithographic
processes. Indeed 3D-structures may come up as far dif-
ferent from simple planar nano-networks.
3. Nano-optical Coupling Imaging and Sub-
wave-length Propagation
Relevant photonic characterizations have been achieved
by way of micro-injection processing with a specific opt-
ical bench so as to assess the performance of the obtained
nano-network design related to the nanofluidic and mi-
nimal surface theory. Such a micro-optical injection
bench consists of a laser source operating at 670 nm and
micromanipulators associated with objectives so as to
drive an upstream monomode optical field in the seg-
mented SU8 taper operating as an integrated source con-
nected with the downstream nano-network ridges. Hence,
the excitation of the optical mode of the first SU8 ta-
per-waveguide structure with its relevant nano-optical
Copyright © 2011 SciRes. OPJ
Nano-opt i cal coupl i n g area
Injection (=670 nm)
Cross-secti onal
2) 3)
Optical fields
in nanoridges
End of taper
Figure 2. Photograph of a nano-optical coupling observed
between the pre-formed SU8 tapers-pillars highlighting the
sub-wavelength propagation regime located inside the silica
nano-ridges for injection laser wavelengths 670 nm. The
image and graphs in the lower part depict the
cross-sectional view and light intensity profiles of the opti-
cal field detected by a camera CCD respectively at the end
of the SU8 waveguide opposite to the taper-injection, and
with both perpendicular silica nano-ridges.
coupling area,
sub- propagat ion
SU8 tap e rs
and pi l lars
Figure 3. Upper-view and (zoom) of the nano-optical
coupling located on the nano-patterns and sub-wavelength
propagation area; the minimal surface or silica-nano-con-
nections are shown with the black solid lines between the
coupling is verified, as well as the sub-wavelength prop-
agation through the si l i ca nanori dge-network (Figure 2).
A microscope and micro-beam profiler (MBP-100-
USB series from Newport®) pitched on upper-view with
its specific software allows us to characterize all the
propagation mechanisms into the whole nano-ridges net-
work. Cross-sectional views 2) and 3) in Figure 2 stand
for the measurements and profile of the optical field into
two given silica nano-rigdes.
The extremity of the bench is fitted with a (Pulnix-
PE)-camera, together with a video system so as to visual-
ize and confirm the output single mode optical signal at
the terminal of the SU8 taper (Figure 2, cross-sec-
tional view 1) accounting for the sub-wavelength optical
coupling and the propagation within the nano-network.
Shown in Figure 3 is the nano-optical coupling area with
sub-wavelength propagation through the minimal surface
silica network depicted in Figures 1(a) and 1(b). Then,
such effective propagation and nano-coupling mechanisms
have been validated into complex silica nano-net works
fabricated with nanofluidi c pr ocesses.
4. Conclusion
We have validated the experimental mainstays regard-
ing a hybrid-organic/inorganic-materials approach, based
on combining a system of specific fluids loaded with
nano-particles together with adequate organic inte-
grated optical processes: Then we have demonstrated
the ability to shape complex silica 2D-nano- networks
as stretches arranged on segmented pre- formed SU8
rods-pillars and tapers devoted to sub- wavelength
photonics. The formation of such specific silica pat-
terns at nanoscale, or organized networks of silica na-
no-ridges, may be totally predictable and in total
agreement with the so-called theory of minimal sur-
faces addressed by physics of fluids.
Considering micro-optical injection, specific coupling
mechanisms allowed us to observe a sub-wavelength
propagation regime into operative silica nano-connec-
tions, a network being arranged upon a set of pre-
formed-patterned rods/tapers. Such reproducible tech-
nologies come up as a low-cost and interesting solution
to shape versatile and complex 3D-nano-netwoks for
future nano-optical routing schemes by using advanced
and selective technical-lithographies on previously pre-
formed organics.
5. Acknowledgements
The authors would like to express their gratitude Rennes
Métropole, Région Bretagne and ANR-08-JCJC-
0136-01 programs for their financial support. The au-
thors gratefully acknowledge Pr. I. Cantat (IPR UMR
CNRS 6251, France) for their kind discussions on flui-
dic processes.
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