By fabricating an array of tiny metallic nanostructures onto the end facet of an optical fibre, researchers in the USA have devised a new type of a sensor called a plasmonic optical-antenna fibre probe. Elizabeth Smythe and co-workers from Harvard University say the probe could potentially perform in situ chemical and biological detection and surface-enhanced Raman spectroscopy, SERS (Opt. Express 15, 7439–7447; 2007).

The idea is that one end of the probe is connected to both a spectrometer and a light source and the other end is inserted into a fluid or biological specimen. Changes in the spectrum of the light reflected from the end of the fibre in the specimen, and picked up by the probe, are then monitored. The probe is influenced by the interaction between the nanoparticles and their surroundings. In particular shifts in the nanoparticles' plasmon resonance are induced by changes in the refractive index of the surrounding media.

Credit: © 2007 OSA

In their initial studies, the team fabricated an array of gold nanorods onto the end of a multimode silica fibre. A 3-nm-thick layer of titanium and a 35-nm-thick layer of gold were evaporated onto the fibre and then etched by an ion beam to create the array. The array was carefully positioned to cover the core of the fibre to maximize the interaction between the light in the fibre and the nanorods.

To optimize the performance of such a probe, the team has explored the effects of changing the dimensions and spacing of the nanorods. The results reveal that the resonance of the nanorods is highly tunable and is most sensitive to changes in rod length, enabling different designs of probe to be customized for particular measurements or samples.

The researchers also say that if a smaller interaction area and more efficient use of the incident light were required, such a nanorod array could also be fabricated onto the facet of a single-mode fibre.