In our recent research, we propose and validate a novel fabrication process based on a 'direct writing' approach taking advantage of conventional deposition and nanopatterning techniques, typically used for planar substrates and suitably adapted to directly operate on optical fiber tip," says Cusano.
In essence, the team's fabrication process consists of three main technological steps: 1) dielectric overlay deposition, with flat surface and controlled thickness over the fiber core region, by means of a properly customized spin coating process; 2) nanoscale patterning of the deposited overlay by an electron-beam lithography (EBL) tool; and 3) superstrate overlay deposition, where different functional materials – metallic or non-metallic – can be deposited by various techniques (e.g. sputtering, thermal evaporation etc…), properly customized to operate with optical fibers.

As the researchers point out, a distinguished feature of this process is the use of a customized spin-coater chuck which allows a flat and reproducible resist layer deposited onto optical fiber tips; onto which then nanostructured arrays are directly written using an EBL system. Importantly, this fabrication process follows almost ordinary lithographic techniques – here adapted to operate on fiber facets – allowing rapid prototyping with a 90% yield and the ability to produce robust and reusable devices.
To demonstrate the effectiveness of their proposed methodology, the team fabricated a miniaturized fiber tip device based on a 2D hybrid metallo-dielectric nanostructure supporting localized surface plasmon resonances (LSPR).
We carried out both experimental and full-wave numerical analyses to characterize the resonant phenomena" says Cusano. "The measured Q-factors were higher than those observed in typical plasmonic crystal configurations. We have also shown that the LSPR can be easily tuned by adjusting the physical and geometrical parameters of the crystal nanostructure and can be designed to be very sensitive to modifications of the surrounding medium."
With a view towards possible applications, the researchers have also shown some preliminary results on the capability of their platform to be used for label-free chemical and biological sensing.
"Moreover, we have also shown the surprising capability of our device to detect acoustic waves, taking advantage of the low elastic modulus of the patterned polymer" notes Cusano. The research team, driven by the enormous potentiality of lab-on-fiber technology, is devoting their efforts to optimizing the fabrication process in terms of reliability and throughput as well as enlarging the set of functional materials to be integrated.
Our dream is to get this technology to a point where it can compete with the already better established lab-on-chip technology – especially taking advantage of the higher versatility in integrating complex lighting systems as well as sophisticated optoelectronics components, devices and systems," says Cusano.

By Michael Berger. Copyright © Nanowerk