INEWS& ES Near-field optics is rapidly developing in new directions, owing to the realization that exploitation of vanescentlight fields may pave the way towards nanoscale photonic devices. One of the most successful NSOM approaches so far relies on materials not readily ssociated with optics: noble metals. Metal nanoparticles can sustain resonant collective oscillations of their onduction electrons. When driven by an external light field, theseelectron oscillations couple to the optical excitation to form surface plasmon polaritons(SPPs) bound to the nanoparticles. The intensity of SPP fields maximized at the nanoparticle surface and decays fields, which can exceed the optical excitation intensity by several orders of magnitude. Silver and gold are of SPP near-field and because their SPP resonances lie in the visible spectral range. Theenhancement is well known and exploited in techniques such as surface-enhanced applications that SPPs could be used for, such as Far-field detection nanoscale optical devices relying on SPP near-fields Due to the evanescent character of the involved optica fields, these would not be hindered in miniaturization by the diffraction limit. These considerations led Quinten et al. to propose optical waveguide composed of a linear chain of fluorescence from nanospheres placed along the length Figure 1 Excitation and closely packed silver nanoparticles. An external field f the nanoparticle chain, the authors were able to detection of energy transportin excites the first particle of the chain, giving rise to ar estimate the SPP propagation length to be a few metal nanoparticle chains by intense SPP near-field. If the second particle is situated hundred nanometres. near-field optical microscopy within this near-field, it picks up the optical excitation, The good news is that SPP propagation along the The nanoparticle'waveguide'is and so on along the chain. The SPP fields are well nanoparticle chain has been observed. The bad news is locally excited by light confined to the nanoparticles in the direction that theobserved propagation length confirms th nanting from the tipof an perpendicular to the chain -itis only along the chain theoretical predictions, showing extremely strong NSOM. The electromagnetic along a nanoparticle chain is limited to a few hundred optical addressing of individual nanoparticles or even fluorescent nanosphere sitting nanometres at best. Sorather than acting as waveguides molecules can still be envisaged, with nanoparticle on top of the nanoparticles. in the conventional sense, which support practically chains acting as the front end of conventional The NSOM tip is scanned along lossless light propagation, nanoparticle chains should waveguides. The enhanced SPP near-field could also be the nanoparticle chain, and the be seen as local devices focusing optical fields down to d in data storage and near- field microscopy, and fluorescence intensity for nanoscale volumes metal nanoparticles offer easy integration with other varying tip positions along the The first direct experimental demonstration ofSPP metal structures. SPPs exist not only as particle-boun particle chain is collected in the coupling between two individual nanoparticles was excitations, but also as propagating waves along metal far-field. The energy transport to followed by spectroscopic investigations of nanoparticle wires with micro-or nanoscale cross-sections For such the nanosphere manifests itself chains and further theoretical considerations eometries,SPP propagation lengths one to two orde in an increase in width of the Now, Stefan Maier and his colleagues report the first of magnitude larger than for nanoparticle chains have direct measurement of SPP propagation along a silver been reported, leaving open the possibility of entirely fibre tip of an NSOM asalocallight source spP a the SPP-based optical devices. nanoparticle chain!. To excite SPPs, the group us propagation along the nanoparticle chain was probed References by measuring the fluorescence intensity(collected in the I. far-field) from fluorescent polystyrene nanospheres 2. Pohl, D w& Courjon, D (eds)Near-Field Optics Vol. 242 of NATO Series E: that have been deposited on top of the particle chains ed Sciences(Kluwer Academic, Dordrecht, 1993). (Fig. 1). Maier and colleagues found that the 3. Quinten, M, Leitner, A, Krenn. ]. R &Aussenegg E. ROpt Lett. 23, 1331-1333 fluorescence from nanospheres sitting on top of metal anoparticles was significantly broader than that from 4. Krenn, I R. Phys. Rev. B60, 5029-5033(1999). isolated nanospheres. This fluorescence broadening is 6 Brongersma, M. L. Hartman, I w&Atwater, H A. Phys. Ren. B62, indicativeofSPP propagation along the nanoparticle 6356-16539(2000 chain to the fluorescent nanosphere By mapping the renn,J.R. al. Europlys Lett 60, 663--669(2002). 211 @2003 Nature Publishing GroupNEWS & VIEWS Near-field optics is rapidly developing in new directions,owing to the realization that exploitation of evanescent light fields may pave the way towards nanoscale photonic devices.One of the most successful approaches so far relies on materials not readily associated with optics:noble metals.Metal nanoparticles can sustain resonant collective oscillations of their conduction electrons.When driven by an external light field,these electron oscillations couple to the optical excitation to form surface plasmon polaritons (SPPs) bound to the nanoparticles.The intensity of SPP fields is maximized at the nanoparticle surface and decays exponentially away from it. What makes SPPs so attractive for near-field optics? It is partly because of their highly intense evanescent fields,which can exceed the optical excitation intensity by several orders of magnitude.Silver and gold are of particular interest due to their high field enhancement, and because their SPP resonances lie in the visible spectral range.The enhancement effect is well known and exploited in techniques such as surface-enhanced Raman scattering.But there are possibly more applications that SPPs could be used for,such as nanoscale optical devices relying on SPP near-fields. Due to the evanescent character of the involved optical fields,these would not be hindered in miniaturization by the diffraction limit. These considerations led Quinten et al. 3 to propose an optical waveguide composed of a linear chain of closely packed silver nanoparticles.An external field excites the first particle of the chain,giving rise to an intense SPP near-field.If the second particle is situated within this near-field,it picks up the optical excitation, and so on along the chain.The SPP fields are well confined to the nanoparticles in the direction perpendicular to the chain — it is only along the chain that light propagates (Fig. 1).But this approach has its limitations.Even for silver and gold,SPP propagation along a nanoparticle chain is limited to a few hundred nanometres at best.So rather than acting as waveguides in the conventional sense,which support practically lossless light propagation,nanoparticle chains should be seen as local devices focusing optical fields down to nanoscale volumes. The first direct experimental demonstration of SPP coupling between two individual nanoparticles4was followed by spectroscopic investigations of nanoparticle chains5 and further theoretical considerations6 . Now,Stefan Maier and his colleagues report the first direct measurement of SPP propagation along a silver nanoparticle chain1 .To excite SPPs,the group used the fibre tip of an NSOM as a local light source.SPP propagation along the nanoparticle chain was probed by measuring the fluorescence intensity (collected in the far-field) from fluorescent polystyrene nanospheres that have been deposited on top of the particle chains (Fig. 1).Maier and colleagues found that the fluorescence from nanospheres sitting on top of metal nanoparticles was significantly broader than that from isolated nanospheres.This fluorescence broadening is indicative of SPP propagation along the nanoparticle chain to the fluorescent nanosphere.By mapping the fluorescence from nanospheres placed along the length of the nanoparticle chain,the authors were able to estimate the SPP propagation length to be a few hundred nanometres. The good news is that SPP propagation along the nanoparticle chain has been observed.The bad news is that the observed propagation length confirms the theoretical predictions,showing extremely strong damping of the SPP propagation (around –3 dB per 100 nm).However,applications such as the short-range optical addressing of individual nanoparticles or even molecules can still be envisaged,with nanoparticle chains acting as the front end of conventional waveguides.The enhanced SPP near-field could also be used in data storage and near-field microscopy,and metal nanoparticles offer easy integration with other metal structures.SPPs exist not only as particle-bound excitations,but also as propagating waves along metal wires with micro- or nanoscale cross-sections.For such geometries,SPP propagation lengths one to two orders of magnitude larger than for nanoparticle chains have been reported7 ,leaving open the possibility of entirely SPP-based optical devices. References 1. Maier, S. A.et al. Nature Mater. 2, 229–232 (2003). 2. Pohl, D. W. & Courjon, D. (eds) Near-Field OpticsVol. 242 of NATO Series E: Applied Sciences (Kluwer Academic, Dordrecht, 1993). 3. Quinten, M., Leitner, A., Krenn, J. R. & Aussenegg, F. R. Opt. Lett. 23, 1331–1333 (1998). 4. Krenn, J. R.et al. Phys. Rev. B 60, 5029–5033 (1999). 5. Maier, S. A., Kik, P. G. & Atwater, H. A.Appl. Phys. Lett. 81, 1714–1716 (2002). 6. Brongersma, M. L., Hartman, J. W. & Atwater, H. A. Phys. Rev. B 62, 16356–16539 (2000). 7. Krenn, J. R.et al. Europhys. Lett. 60, 663–669 (2002). nature materials | VOL 2 | APRIL 2003 | www.nature.com/naturematerials 211 Figure 1 Excitation and detection of energy transport in metal nanoparticle chains by near-field optical microscopy1 . The nanoparticle ‘waveguide’ is locally excited by light emanating from the tip of an NSOM.The electromagnetic energy is transported along the waveguide towards a fluorescent nanosphere sitting on top of the nanoparticles. The NSOM tip is scanned along the nanoparticle chain,and the fluorescence intensity for varying tip positions along the particle chain is collected in the far-field.The energy transport to the nanosphere manifests itself in an increase in width of the nanosphere fluorescence. Fluorescent nanosphere Intensity Far-field detection NSOM Ag nanoparticle SPP near-field © 2003 Nature Publishing Group