Plasmonic Nanodevices

Plasmonic nanostructures are developed for achieving high enhanced Raman signals in a very narrow area, beyond the diffraction limit of light.   A plasmonic effect is generated when a metallic structure properly designed is invested by an incident light: the electrons of the metal begin to oscillate with a frequency near to that of the native plasmonic resonance, forming a localized electromagnetic field which, in some cases, could be so strong to allow single molecule detection. The applications of this phenomenon are very interesting overall in the field of spectroscopy, where the use of metallic structures as substrates for sample analyses allows a huge enhancement of electric signal from the analytes. As an example Surface Enhanced Raman Scattering (SERS) effect in Raman spectroscopy is based on plasmon resonances in spatially confined metal nanoparticles.
In our laboratory we produce metallic nanostructures with combining lithographic technique and a self-assembling method for metal deposition, in particular gold or silver electroless deposition. The fabricated devices are constituted by self-similar quasi-spherical nano-aggregates and give the possibility to obtain Raman spectra of largely diluted solutions (aptoM). These devices are theoretically predicted to largely enhance the electromagnetic field in hot-spots close to the smallest nano-spheres [1] and with a spot-szie much smaller than the diffraction limit imposed by light propagation. For this reason this kind of nano-structures has been named “nanolenses”, and they can theoretically produce an enhancement of the Raman signal up to 1012 see SEM images below).


        

Besides the large enhancement factors that plasmonic nanostructures produce in Raman spectroscopy, the interest that surface plasmons is attracting is also due to their ability to concentrate and propagate light in subwavelenght structures, thus overwhelming the optical diffraction limit and leading to nano-optics.
It has been shown that surface plasmons, excited by far-field laser sources, can propagate in properly designed nano-waveguides shaped as a cone with a very sharp tip (curvature radius of few nanometers at the cone tip). The electromagnetic field carried by the surface plasmons is so concentrated in very narrow regions of space, much smaller than the diffraction limit. This is due to the fact that properly designed waveguides produce an adiabatic compression of the surface plasmons, which leads to their accumulation and to a high enhancement of the electromagnetic field at the far tip of the waveguide [2].
In this way, the nano-waveguide can be used as plasmonic nanolens to be combined with Raman spectroscopy for Raman mapping of the investigated samples with high-spatial-resolution (down to 10nm scale) and high-signal-enhancement. In our laboratory we produced tapered silver nano-waveguides with tips having less than 10nm of curvature radius. The tips are fabricated on top of AFM cantilevers, thus allowing for the simultaneous topographical and Raman mapping of the samples. Experimental measurements have shown as best result a spatial resolution of 7nm for Raman mapping. Below it is possible to see SEM images of Nanocones on AFM cantilevers, with a photonic-crystal optical cavity for coupling the far field laser light with the base of the cone.


REFERENCES

1) K. Li, M.I. Stockman, D. Bergman, "Self-Similar Chain of Metal Nanospheres as an Efficient Nanolens", Phys. Rev. Lett. 91, 227402 (2003).  
2) M.I. Stockman, "Nanofocusing of Optical Energy in Tapered Plasmonic Waveguides", Phys. Rev. Lett. 93, 137404 (2004).  
3) F. De Angelis, G. Das, P. Candeloro, M. Patrini, M. Galli, A. Bek, I. Maksymov, C. Liberale, L.C. Andreani, E. Di Fabrizio, "Nanoscale chemical mapping using three-dimensional adiabatic compression of surface plasmon polaritons", Nature Nanotechnology 5, 67-72 (2010).  
4) F. De Angelis, M. Patrini, G. Das, I. Maksymov, M. Galli, L. Businaro, L.C. Andreani, E. Di Fabrizio, "A Hybrid Plasmonic-Photonic Nanodevice for Label-Free Detection of a Few Molecules", Nano Letters 8, 2321 (2008).