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Introduction

The “Lab on Fiber” technology envisages the integration of highly functionalized materials at nano and micro scale within a single optical fiber and aims to develop a novel generation of miniaturized and advanced “all-in-fiber” devices for both communication and sensing applications. Those devices taking advantages from the intrinsic miniaturization and mechanical flexibility and cost effectiveness could provide effective auto-diagnostic features useful in many application fields. However, the realization of highly integrated optical fiber devices requires that several micro and nano structures be fabricated, embedded and connected all together in order to achieve the necessary light-matter interaction and physical connection. As a consequence, a critical issue to be addressed consists in the definition of a reliable fabrication onto unconventional substrates such as the optical fiber tip. Recently, promising approaches in this direction were proposed by various groups following both indirect-write [1] approach and direct-write [2] approach. The first one relies on nanoimprint and transfer technique in which master nanostructures are generated by electron-beam lithography (EBL) on standard substrates, and successively transferred to the fiber tip. Although, the methods presented so far rely on well assessed fabrication process on planar substrates, the last transferring step plays a fundamental role in determining both the fabrication yield and the performance of the final device. To overcome the complex transferring step, alternative approaches based on direct-write patterning of the fiber tip have been explored.

In this work we report on a reliable fabrication process based on EBL method and spin coating technique truly adapted to a flat cleaved tip of optical fibers, that can directly pattern dielectric and metallic nanostructures of arbitrary shape and periodicity. In particular we report on a fiber probe based on hybrid metallo-dielectric nanostructures supporting LSPRs able to work both as chemical sensor.

Experimental Results and Discussion

Fabrication Process

The fabrication process essentially consists of three main technological steps: (i) spin-coating deposition of electron-beam resist with accurate thickness control and flat surface over the fiber core region; (ii) EBL nano-patterning and (iii) superstrate deposition of different functional materials. Spin coating technique produces edge beads around the perimeter of the substrate and this effect is particularly relevant when very small substrates are considered. Here the edge bead can be as large as the substrate itself thus preventing the use of standard lithographic techniques. Our approach relies on the capability to deposit layers onto the cleaved end tip of standard optical fibers using the spin coating technique, producing controllable thickness and flat surface area centered around the fiber core. To this aim, we use a customized chuck for optical fibers and a selection of suitable process parameters, obtaining repeatable polymer resist layer with different thickness varying in the range from 100 to 400 nm with uniform areas over the fiber core of about 50–60 μm in diameter [3]. The further step relies on the layer sub-wavelength patterning by EBL, using special settings in the exposure procedure to reduce the total exposure time. The integration of additional functional superstrates, either dielectric or metallic, on the patterned areas is made using standard deposition techniques in Ultra High Vacuum systems. Overall, the proposed fabrication process allows rapid prototyping, with a 90 % yield, thanks to the reliable spin coating process which make the substantial difference with the other fabrication processes reported so far, furthermore our nanostructures show a good adhesive strength also resulting in reusable devices.

Sensor Design and Refractive Index Measurements

To test the capability of our process, we fabricated a hybrid metallo-dielectric nanostructure supporting LSPRs. It consist of a two-dimensional (2D) structure of a ZEP dielectric layer with a thickness t ZEP , patterned in a square lattice of holes and covered with a gold film of variable thickness t GOLD deposited on both the ridges and the grooves. A schematic cross section view of our device is shown in Fig. 65.1a. Gold was chosen because of its good plasmonic properties, biocompatibility and resistance to oxidation degradation. Resonant wavelengths can be tailored for the specific application by a suitable choice of the device geometrical parameters such as the lattice period, filling factor, dielectric and metal thickness. In this work we firstly studied a structure characterized by the following set of parameter: t ZEP  = 200 nm, the pitch a = 900 nm, the hole radius r = 225 nm and t GOLD  = 20 nm. Figure 65.1b, c shows the SEM images of the 2D structure on optical fiber tip.

Fig. 65.1
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(a) Schematic cross section view of the 2D metallo-dielectric structure; (b) Scanning Electron Micrograph of a patterned optical fiber tip overview of the patterned area of 100 × 100 μm2; (c) zoom in of the 2D array; r = 225 nm, a = 900 nm

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The localized surface plasmon resonance excited by the in-fiber direct light illumination of this structure has been discussed in ref [3]. The resonances are very sensitive to variations of the surrounding refractive index, thus a change in the local or bulk refractive index around the fiber tip device gives rise to a wavelength shift of the resonant dip due to a change of the phase matching condition. In particular we analyzed the influence of the gold thickness on the surrounding refractive index (SRI) sensitivity of the fiber device, the results showed that the sensitivity increase when the gold layer thickness is reduced [3]. Following these results, we choose for our sensor a gold layer thickness of 20 nm.

The reflection spectra were collected using a broadband optical source covering the wavelength range 1,200–1,700 nm. The light was coupled to the a single mode optical fiber via a 3 dB directional coupler and redirecting the reflected light (via a 2 × 1 directional coupler) to an optical spectrum analyzer (Ando AQ6317C). The sample was then immersed in different liquid solutions such as water (n = 1.333), ethanol (n = 1.362) and isopropyl alcohol (n = 1.378). The experimental results are shown in Fig. 65.2, in which is evident the typical red-shift of the curves with increasing values of the surrounding refractive index (SRI). In particular, in the inset of Fig. 65.2 we plot the relative wavelength shifts of the reflection dips as a function of the SRI. The graph demonstrates a sensitivity of ~125 nm/RIU for detecting changes in the bulk refractive indices of different chemicals surrounding the fiber tip device.

Fig. 65.2
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Experimental reflectance spectra of the fiber sensor in various solvent; (inset) relative wavelength shift of the reflection dips as a function of the SRI

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Conclusions

We have demonstrated that by a suitable customization of standard nanoscale deposition and patterning techniques, typically used for planar devices, we were able to define a fabrication process enabling the integration of functional dielectric and metallic nanoscale structures directly on the tip of standard single mode fibers. The effectiveness of the proposed technological process has been confirmed through the realization of a miniaturized fiber tip device based on a 2D hybrid metallo-dielectric nanostructure supporting LSPR used as chemical sensor and showing a sensitivity of 125 nm/RIU.