Functional silica nano-connections based onfluidic approach for integrated photonics

B. Beche, A. Jimenez, L. Courbin, L. Camberlein, F. Artznerand E. Gaviot

A practical concept is reported based on reproducible fluidic mechan-isms coupled with silica nano-particles for the development of nano-optical-connections directly on organic integrated photonic chips.Silica nano-rib waveguides have been shaped with various widthsranging between 50 and 300 nm and about 100 mm in length. An effec-tive nano-photonic coupling mechanism has been demonstrated and asub-wavelength propagation regime obtained between two organic ribtapers and waveguides with a perpendicular and a parallel configur-ation, respectively. The specific silica nano-rib-waveguide structuresshow off optical losses propagation ranging around 37–68 dB/mmat visible and infrared wavelengths. Such flexible devices offer versatilefabrication control by changing, respectively, nano-particles andsurfactant concentrations. Thus, they present great potential regardingfuture applications for shaping nano-connections and high-densitynetwork integrations between original optical segmented circuitssuch as plots, lines or any pre-formed photonic structures.

Introduction: Researchers in nano-photonics are eager to control thepropagation of light in the visible and infrared (IR) range into eversmaller spaces. In particular, the main purpose of nano-optical-connec-tions is to transmit such electromagnetic radiation with a quite higherwavelength than the dimensions of a given optical waveguide arrangedseveral wavelengths in the distance. Concerning silica materials, varioustechniques based on new productions in materials science and originalprocesses have allowed development of hybrid integrated photonics[1, 2] and the obtaining of a noticeable nano-sized confinementmarked with spatial resolution around ten times smaller than the pro-ceeding wavelength [3–5]. The ability to prepare silica nano-connec-tions may open new opportunities for implementing low-dimensionalsilica materials. In this Letter, we highlight the great interest toprocess with SiO2 nano-particles solutions together with fluidic anddynamic dry devices as a generic integrated photonic approach so asto develop a specific technology of nano-connections with sub-lambdabranches and nano-networks onto optical chips. Such reproduciblehybrid processes allow us to get the formation of uniform sub-wave-length silica nano-ribs with various width values up to several tens ofnanometres directly onto an organic photonic chip. Moreover, we havecharacterised the expected nano-optical coupling with a sub-wavelengthpropagation regime about a 100 wavelengths in the distance in both per-pendicular and parallel configurations, called respectively cross- andpara-coupling, directly on the integrated chip.

Silica nano-rib waveguides: Such devices basically rely on a guided-wave proceeding on a (100) silicon substrate coated with a specificSiO2 layer first obtained by thermal oxidation of the silicon wafer, yield-ing 1.2 mm thickness with an index value nSiO2

close to 1.45 at visibleand IR wavelengths. Then, an organic SU8 film, the higher refractiveindex of which is most suited for a guiding layer 20 mm in thickness,is deposited by spin coating and cured to remove the solvent accordingto convenient steps of temperature [6]. The development process withthe specific SU8 developer (from MicroChemw) allows us to obtainphotonic structures fitted with two optical-configurations as waveguidesand tapers, respectively, 6 and 1 mm in width; such pair-waveguides andtapers are set apart with a 100 mm air gap. Then, a global mixture is pre-pared with a concentration in silica beads ranging around 2 to 10%. Asan example, so as to obtain a global mixture 5% in silica beads concen-tration, a 167 ml of colloidal silica 30 wt % suspension in water (fromLudoxw) is diluted in 833 ml pure water prior to being added to 2 to3 mg of a sodium-dodecyl-sulfate surfactant (SDS) solution with therequired concentration. Such an SDS surfactant allows us to stabilisethe pH condition of the mixture with a view to enhancing the reprodu-cibility of the process as compared with pure water. Then, the global sol-ution of SDS and silica beads is deposited by way of a ml-syringe onto athin cover glass layer for optical microscopy. Then, the whole is turneddown and carefully deposited onto the apt area of the performed opticalchip. Then, the nano-silica fluid is strained between both waveguide-structure-configurations so as to shape a liquid film that can beremoved along the vertical faces, respectively, defined by the twofacing rib waveguides and the tapers so as to allow the formation of

thin bridges made of silica solution (Figs. 1a and b). According to athorough drying that brings out self-assembled silica nanoparticleswith concentrations ranging around 2–10%, various silica nanorib-structures have been shaped with widths typically ranging between 50and 300 nm, as can be observed with scanning electron microscopy.

a

b

t=0s

t=12s

airair fluid

SU8 rib-waveguides

silica nano-rib-waveguide

SiO2 fluid air

SU8 tapers

air

SiO2 nano-rib

t=0s

t=7s

Fig. 1 Formation of silica nano-rib waveguides by successive withdrawingand drying fluid processes in, respectively, both configurations

a Perpendicular between both SU8 waveguidesb Parallel between two SU8 tapers facing each other

Measurements and results: Considering the nanorib structures depictedin Figs. 2a and b, remarkable configurations, respectively, 70 and270 nm in width, and 700 and 900 nm in height can be clearly observedfor nano-optical cross- and para-coupling. Specific photonic characteris-ations have been achieved by way of a micro-injection process with anoptical bench so as to test the performance of the obtained nano-connec-tion design. This micro-optical injection bench consists of a laser sourceoperating at 670 and 980 nm fitted with an enhanced control in tempera-ture together with associated objectives as detailed in [6]. Hence, theexcitation of the optical mode of, respectively, the first SU8 rib wave-guide and the taper structures together with a relevant optical couplingand propagation with both silica nanorib configurations are verified.The extremity of the bench is fitted with a camera (Pulnix-PE), togetherwith a video system so as to visualise the output optical signal at the endof both sections of the SU8 waveguides: then the effective opticalcoupling is validated via the whole SU8-rib/silica-nano-rib/SU8-ribphotonic structure in both cross-coupling (Fig. 3) and para-couplingconfigurations, respectively (Fig. 4). Moreover, a microscope andmicro-beam profiler (MBP-100-USB series from Newportw) pitchedon the upper-view with its specific software allows us to characteriseboth SU8 waveguides linked with the silica nano-ribs; indeed, thenano-optical propagation losses can be assessed at visible and IR wave-lengths via the Beer-Lambert law expressed as Id ¼ I0 exp(2a d), witha(mm21) the linear absorption coefficient of energy and d ¼ 0.1 mm.Figs. 3 and 4 show such operative nano-optical cross- and para-couplings between, respectively, the first excited-arm and the otherfacing waveguide (tapers) via the silica nano-ribs. Then, the sub-wavelength propagation and coupling mechanism are validated intosuch silica nano-connections. According to about ten measurements,a relevant average of the nano-optical losses defined as u ¼ 10log[exp(2a d)] have been estimated at 37 and 68 dB/mm with both670 and 980 nm wavelengths, respectively (Fig. 3). It can be notedthat such optical propagation values stemming from the leaky modesaspects, should depend on the occurrence of the Si substrate properties(with high permittivity) and may be considered as a 3-D anti-waveguideconfiguration. Experimental results are in good agreement with thegeneral tenets of the optical modes coupling theory; the latter predicts,in a large variety of cases, a k-coupling-coefficient inversely

Techset CompositionLtd, Salisbury Doc: {EL}ISSUE/46 -5/Pagination/EL59135.3dOrganic and inorganic circuits anddevices

ELECTRONICS LETTERS 4th March 2010 Vol. 46 No. 5

proportional to the wavelength (k / 1/l) considering given dimensionsand permittivities of the waveguides and their relevant overlap fieldintegrals.

a

b

10 mm

1 mm

70 nm nano-rib

SiO2 sub-lwaveguide

SU8 rib

270 nm

1 µm

waveguides

Fig. 2 Scanning electron microscopy images of global photonic structures

a First perpendicular configurationb Second parallel configuration

injection (l=670 nm)

cross-sectional detection

nano-opticalcoupling

nano-rib optical transmission (u.a.)

100

0

80

60

40

20

37dB/mm

d=100 mm

d=100 mm

(l=670 nm)

nano-rib optical transmission (u.a.)

100

80

60

40 20

68dB/mm

d=100 mm

(l=980 nm)

Fig. 3 Photograph of perpendicular nano-optical coupling, called crosscou-pling, between both organic waveguides and sub-wavelength propagationregime inside silica nano-rib for 670 nm wavelength

Measurement of optical losses at, respectively, 670 and 980 nm

injection SU8 taper

sub- wavelength propagation

coupling via silica nano-rib

l=670 nm

Fig. 4 Photograph of nano-optical coupling and sub-wavelength propa-gation regime inside silica nano-rib for 670 nm wavelength in second con-figuration of two SU8 tapers facing each other (para-coupling)

Conclusion: Experiments based on a hybrid organic/inorganicmaterials approach have demonstrated the ability to shape varioussilica nano-rib waveguides as ridged-like top shape by combining asystem of nano-particles into specific fluids with organic integratedoptical processes. Such a promising technique can be generalised withvarious substrates. Moreover it is clear that as the surface energy ofthe system organic material/inorganic fluid can be controlled, e.g.with a proper plasma treatment of the organic material or with an opti-mised concentration of silica nano-particles and surfactant, it is possibleto adjust the superficial tension and the drying mechanism of the fluidbetween the optical structures. Micro-optical injection and couplingallowed us to observe a sub-wavelength propagation regime into operat-ive nano-rib connections arranged between two configurations as,respectively, perpendicular and parallel patterns. The propagation mech-anism features optical losses ranging between 37 and 68 dB/mm atvarious visible and near-IR wavelengths. Such coupled and reproducibletechnologies are low-cost and interesting solutions to develop versatiledesigns of nano-connected optical circuits onto original photonic struc-tures such as segmented elements circuits for nano-optical routingschemes and networks.

# The Institution of Engineering and Technology 20109 September 2009doi: 10.1049/el.2010.2569One or more of the Figures in this Letter are available in colour online.

B. Beche, A. Jimenez, L. Courbin and F. Artzner (Institute of Physics ofRennes, UMR CNRS 6251, University of Rennes 1, Rennes 35042,France)

E-mail: bruno.beche@univ-rennes1.fr

L. Camberlein and E. Gaviot (Laboratory of Acoustics, UMR CNRS6613, University of Le Mans, Le Mans 72000, France)

References

1 Sundar, V.C., Yablon, A.D., Grazul, J.L., Ilan, M., and Aizenberg, J.:‘Fibre-optical features of a glass sponge’, Nature, 2003, 424, p. 899

2 Duval, D., Tarabout, C., Artzner, F., Gaviot, E., Renault, A., and Beche,B.: ‘Development of new practical approach to integrated photonicsbased on biomimetic molecular self-assembled nanotubes’, Electron.Lett., 2008, 44, p. 1134

3 Tong, L., Gattass, R.R., Ashcom, J.B., He, S., Lou, J., Shen, M.,Maxwell, I., and Mazur, E.: ‘Subwavelength-diameter silica wires forlow-loss optical wave guiding’, Nature, 2003, 426, p. 816

4 Tong, L., Lou, J., Gattass, R., He, S., Chen, X., Liu, L., and Mazur, E.:‘Assembly of silica nanowires on silica aerogels for microphotonicdevices’, Nano Lett., 2005, 4, p. 259

5 Tong, L., Lou, J., Ye, Z., Svacha, G.T., and Mazur, E.: ‘Self-modulatedtaper drawing of silica nanowires’, Nanotechnology, 2005, 16, p. 1445

6 Beche, B., Pelletier, N., Gaviot, E., Hierle, R., Goullet, A., Landesman,J.P., and Zyss, J.: ‘Conception of optical integrated circuits on polymers’,Microelectron. J., 2006, 37, p. 421

ELECTRONICS LETTERS 4th March 2010 Vol. 46 No. 5