Resonances occur throughout science and engineering, from guitar strings to NMR scanners. Nearly all resonances follow the well-known Lorentzian lineshape – i.e. the intensity of the resonance is symmetric when it is plotted as a function of the driving frequency. In 1961, however, the Italian physicist Ugo Fano discovered a new kind of asymmetric resonant lineshape which is now named in his honour. The Fano resonance occurs when two oscillators – one with a sharp resonance and one with a much broader one - interact together. Surface plasmons (which are hybrids between photons and electrons, created when nanoscale holes in metal films are irradiated by light whose wavelength exceeds the dimensions of the holes) tend to show rather broad resonances and so have been widely used in the last few years to study Fano physics.
Now the LCN’s Paul Warburton and his student Edward Osley, along with colleagues at UCL’s Department of Electrical Engineering, have observed a tuneable plasmon Fano resonance. Here the strength of the interaction between a broad plasmon resonance and a much narrower molecular resonance can be tuned to allow tailoring of the precise shape of the resultant Fano resonance. In their experiments (recently published in Physical Review Letters, volume 110, article number 087402) infrared light was shone onto an array of nanoscale cross-shaped apertures in a gold film. This gold film is coated with a polymer containing a carbonyl bond which resonates when excited at a wavelength of 5.8 microns. One of the two arms of each cross is engineered to create plasmons resonant at that same wavelength, thereby creating the Fano resonance. But simply by rotating the polarisation of the incident infrared light it is possible to control the strength of the Fano interaction between the plasmon resonance and the molecular resonance.
Since the Fano resonance is very sharp it is suited to a range of applications in biological and chemical sensing, fields in which surface plasmons already play an important role. The tuneability of the Fano resonance as reported by Osley et al. allows tailoring of the resonant Fano lineshape. Hence a single plasmonic sensor could be easily tuned to make it sensitive to a range of different molecular resonances.
Journal link: http://prl.aps.org/abstract/PRL/v110/i8/e087402
- electron micrograph of the array of cross-shaped apertures in a gold film coated with a polymer film
- magnified view of four apertures
- infra-red reflection spectra from the polymer-coated apertures. The resonance of the carbonyl bond is visible at 5.8 microns. The Fano lineshape changes as the polarisation of the incident infra-red light changes from 0 (brown) to 90° (purple).