Functional silica nano-connections based on fluidic approach for integrated photonics B. Be ˆche, A. Jimenez, L. Courbin, L. Camberlein, F. Artzner and 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 widths ranging between 50 and 300 nm and about 100 mm in length. An effec- tive nano-photonic coupling mechanism has been demonstrated and a sub-wavelength propagation regime obtained between two organic rib tapers and waveguides with a perpendicular and a parallel configur- ation, respectively. The specific silica nano-rib-waveguide structures show off optical losses propagation ranging around 37– 68 dB/mm at visible and infrared wavelengths. Such flexible devices offer versatile fabrication control by changing, respectively, nano-particles and surfactant concentrations. Thus, they present great potential regarding future applications for shaping nano-connections and high-density network integrations between original optical segmented circuits such as plots, lines or any pre-formed photonic structures. Introduction: Researchers in nano-photonics are eager to control the propagation of light in the visible and infrared (IR) range into ever smaller spaces. In particular, the main purpose of nano-optical-connec- tions is to transmit such electromagnetic radiation with a quite higher wavelength than the dimensions of a given optical waveguide arranged several wavelengths in the distance. Concerning silica materials, various techniques based on new productions in materials science and original processes have allowed development of hybrid integrated photonics [1, 2] and the obtaining of a noticeable nano-sized confinement marked 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-dimensional silica materials. In this Letter, we highlight the great interest to process with SiO 2 nano-particles solutions together with fluidic and dynamic dry devices as a generic integrated photonic approach so as to develop a specific technology of nano-connections with sub-lambda branches and nano-networks onto optical chips. Such reproducible hybrid processes allow us to get the formation of uniform sub-wave- length silica nano-ribs with various width values up to several tens of nanometres directly onto an organic photonic chip. Moreover, we have characterised the expected nano-optical coupling with a sub-wavelength propagation regime about a 100 wavelengths in the distance in both per- pendicular and parallel configurations, called respectively cross- and para-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 specific SiO 2 layer first obtained by thermal oxidation of the silicon wafer, yield- ing 1.2 mm thickness with an index value n SiO 2 close to 1.45 at visible and IR wavelengths. Then, an organic SU8 film, the higher refractive index of which is most suited for a guiding layer 20 mm in thickness, is deposited by spin coating and cured to remove the solvent according to convenient steps of temperature [6]. The development process with the specific SU8 developer (from MicroChemw) allows us to obtain photonic structures fitted with two optical-configurations as waveguides and tapers, respectively, 6 and 1 mm in width; such pair-waveguides and tapers 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%. As an 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 (from Ludoxw) is diluted in 833 ml pure water prior to being added to 2 to 3 mg of a sodium-dodecyl-sulfate surfactant (SDS) solution with the required concentration. Such an SDS surfactant allows us to stabilise the 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 a thin cover glass layer for optical microscopy. Then, the whole is turned down and carefully deposited onto the apt area of the performed optical chip. Then, the nano-silica fluid is strained between both waveguide- structure-configurations so as to shape a liquid film that can be removed along the vertical faces, respectively, defined by the two facing rib waveguides and the tapers so as to allow the formation of thin bridges made of silica solution (Figs. 1a and b). According to a thorough drying that brings out self-assembled silica nanoparticles with concentrations ranging around 2–10%, various silica nanorib- structures have been shaped with widths typically ranging between 50 and 300 nm, as can be observed with scanning electron microscopy. a b t=0s t=12s air air fluid SU8 rib-waveguides silica nano- rib-waveguide SiO 2 fluid air SU8 tapers air SiO 2 nano-rib t=0s t=7s Fig. 1 Formation of silica nano-rib waveguides by successive withdrawing and drying fluid processes in, respectively, both configurations a Perpendicular between both SU8 waveguides b Parallel between two SU8 tapers facing each other Measurements and results: Considering the nanorib structures depicted in Figs. 2a and b, remarkable configurations, respectively, 70 and 270 nm in width, and 700 and 900 nm in height can be clearly observed for nano-optical cross- and para-coupling. Specific photonic characteris- ations have been achieved by way of a micro-injection process with an optical bench so as to test the performance of the obtained nano-connec- tion design. This micro-optical injection bench consists of a laser source operating at 670 and 980 nm fitted with an enhanced control in tempera- ture together with associated objectives as detailed in [6]. Hence, the excitation of the optical mode of, respectively, the first SU8 rib wave- guide and the taper structures together with a relevant optical coupling and propagation with both silica nanorib configurations are verified. The extremity of the bench is fitted with a camera (Pulnix-PE), together with a video system so as to visualise the output optical signal at the end of both sections of the SU8 waveguides: then the effective optical coupling is validated via the whole SU8-rib/silica-nano-rib/SU8-rib photonic structure in both cross-coupling (Fig. 3) and para-coupling configurations, respectively (Fig. 4). Moreover, a microscope and micro-beam profiler (MBP-100-USB series from Newportw) pitched on the upper-view with its specific software allows us to characterise both SU8 waveguides linked with the silica nano-ribs; indeed, the nano-optical propagation losses can be assessed at visible and IR wave- lengths via the Beer-Lambert law expressed as I d ¼ I 0 exp(2a d), with a(mm 21 ) 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 other facing waveguide (tapers) via the silica nano-ribs. Then, the sub- wavelength propagation and coupling mechanism are validated into such silica nano-connections. According to about ten measurements, a relevant average of the nano-optical losses defined as u ¼ 10 log[exp(2a d)] have been estimated at 37 and 68 dB/mm with both 670 and 980 nm wavelengths, respectively (Fig. 3). It can be noted that such optical propagation values stemming from the leaky modes aspects, should depend on the occurrence of the Si substrate properties (with high permittivity) and may be considered as a 3-D anti-waveguide configuration. Experimental results are in good agreement with the general tenets of the optical modes coupling theory; the latter predicts, in a large variety of cases, a k-coupling-coefficient inversely TechsetCompositionLtd,Salisbury Doc:{EL}ISSUE/46-5/Pagination/EL59135.3d Organic andinorganiccircuits anddevices ELECTRONICS LETTERS 4th March 2010 Vol. 46 No. 5