NEWS &ES if antiferromagnetic, would be geometrically frustrated contain copper. Because its superconductivity can easily by the hexagonal configuration of the cobalts; Fig. 2), in be destroyed by removing the water molecules, and thus view of the significantelectron-electron repulsion decreasing the interlayer separation, Nao.35 CoO2.1.3H,O typically observed for octahedrally coordinated 3 uld also provide insight into the relationship between transition metals So if this view is correct, the cobalt dimensionality andsuperconductivity Contrasts and uperconductor could have some of the ingredients that similarities between it and the copper oxides may shed make the copper oxide behaviour so interesting: further light on the challenging issue of understanding proximity to a Mott insulator-to-metal transition igh-T superconductivity The electronic, magnetic, and coupled with spin-1/2 magnetism. The high structural properties of this new cobalt oxide are likely to thermoelectric power observed for the more bethe subject ofi sodium-rich Naos CoO2 compound, which is not typical of simple metals and makes it attractive for References thermoelectricenergy conversion applications, is 1. Sleight,A.W.sckn242,1519-1527(1988) in this class of materials.8. For all of these reasonsthe a further evidence that something unusual is happenin 2. Orenstein, L& Millis, A I Science 288, 468-174(2000). 3. Takada, K ct. Nature 422, 53-55(2003). sodium cobalt bronzes were predicted to be good 4. Fouassicr, C, Mate ka, G, Reau, J -M. Hagenmuller, P J. Solid State Chen. 6. candidates for investigation as superconductors%, lo. Whatever its relationship to the high-TC oxides 5. Terasaki, I, Sasago, Y. Uchinokura, K Phys. Rev. B56, 12685-12687(1997). may be, the discovery of the Naa3s CoO2 1.3H20 6. Balsys, R L& Davis, R L. Solid State lonic 93. 279-382( 1996). superconductor represents a major advance, because 8. Singh, D . Phrs. Ren. B6). 13397-13402(200) there are so few examples of layered transition metal 9. Ray.R, Ghoshray, A, Ghoshra, K.& Nakamura, S. Phys.Rev. B59 oxide superconductors and relatively few examples of oxide superconductors in general that do not O.Tanaka, T, Nakamura, S& lida, S Jpn JAppl. Phys. 233, 1581-1582(1994) NANOPARTICLE WAVEGUIDES Watching energy transfer There may be plenty of room at the bottom, but the size of conventional optical elements is restricted by the diffraction limit of light. Plasmon waveguides made from metal nanoparticle chains may allow a drastic reduction in the size of photonic devices JoACHIMR. KRenn is at the Institute for As they reporton page 229 of this issue, Stefan Maier and Experimental Physics and Erwin Schrodinger Institute for colleagues have now directly observed energy transfer Nanoscale Research, Karl-Franzens-University Graz along a nanoparticle plasmon waveguide. These metal A-8010 Graz, Austria. nanoparticle chains may becomea centralcomponent e-mait: joachim. krenneuni-grazat of nanoscale photonic devices. Near-field optics, proposed around 20 years ago, M odern technologyrelies heavilyon photonic provides a neat way to circumvent the diffraction limit2. and optoelectronic devices for signal Instead of imaging with a system of lenses, a glass fibre transmission androuting, chemical analysis with a submicroscopic tip can be used to probe the light and sensorapplications Asis thecase forelectronics, fields close to a sample surface. Because the distance increasing the speed and sensitivity of photonicdevices between tip and sample is much smaller than the light relies on further miniaturization of photonicelements. wavelength, non-propagating light fields that are But thereisa fundamentallimit to theminimum size of bound to the sample surface can be detected. These conventional opticalelements such as dielectric evanescent fields decay in intensity within a fraction of waveguides. This limit is set by diffraction to beabout ving away from the half the wavelength oflight. For light, this surface, and carry information about sample features translateinto aminimumelement size of a few hundred smaller than the value set by the diffraction limit. nanometres-too large for fabricating nano-and Owing to their non-propagating character, evanescent molecular-scale photonic devices. Near-fieldoptics, fields can only be detected by a local probe immersed whichexploits non-propagating(evanescent)fields into the near-field of the sample. The corresponding of experimental device is called a near-field scanning rcumventing the diffraction(and thereforesize)limit. optical microscope(NSOM) naturematerialsivol2iaPril2003iwww.natUre.com/naturematerials @2003 Nature Publishing Group
NEWS & VIEWS if antiferromagnetic,would be geometrically frustrated by the hexagonal configuration of the cobalts; Fig. 2),in view of the significant electron–electron repulsions typically observed for octahedrally coordinated 3d transition metals8 .So if this view is correct,the cobalt superconductor could have some of the ingredients that make the copper oxide behaviour so interesting: proximity to a Mott insulator-to-metal transition coupled with spin-1/2 magnetism.The high thermoelectric power observed for the more sodium-rich Na0.5CoO2 compound,which is not typical of simple metals and makes it attractive for thermoelectric energy conversion applications,is further evidence that something unusual is happening in this class of materials5,8.For all of these reasons the sodium cobalt bronzes were predicted to be good candidates for investigation as superconductors5,9,10. Whatever its relationship to the high-Tc oxides may be, the discovery of the Na0.35CoO2⋅1.3H2O superconductor represents a major advance, because there are so few examples of layered transition metal oxide superconductors and relatively few examples of oxide superconductors in general that do not contain copper. Because its superconductivity can easily be destroyed by removing the water molecules,and thus decreasing the interlayer separation,Na0.35CoO2⋅1.3H2O could also provide insight into the relationship between dimensionality and superconductivity. Contrasts and similarities between it and the copper oxides may shed further light on the challenging issue of understanding high-Tc superconductivity.The electronic,magnetic,and structural properties of this new cobalt oxide are likely to be the subject of intense investigation for some time. References 1. Sleight, A. W. Science 242, 1519–1527 (1988). 2. Orenstein, J. & Millis, A. J. Science 288, 468–474 (2000). 3. Takada, K.et al. Nature 422, 53–55 (2003). 4. Fouassier, C., Matejka, G., Reau, J.-M. & Hagenmuller, P.J. Solid State Chem. 6, 532–537 (1973). 5. Terasaki, I., Sasago, Y. & Uchinokura, K. Phys. Rev. B 56, 12685–12687 (1997). 6. Balsys, R. J. & Davis, R. L. Solid State Ionics 93, 279–382 (1996). 7. Choy, J. H., Kwon, S. J. & Park, G. S. Science 280, 1589–1592 (1998). 8. Singh, D. J. Phys. Rev. B 61, 13397–13402 (2000). 9. Ray, R., Ghoshray, A., Ghoshray, K. & Nakamura, S. Phys. Rev. B 59, 9454–9461 (1999). 10.Tanaka, T., Nakamura, S. & Iida, S.Jpn. J. Appl. Phys. 2 33, L581–L582 (1994). 210 nature materials | VOL 2 | APRIL 2003 | www.nature.com/naturematerials NANOPARTICLE WAVEGUIDES Watching energy transfer There may be plenty of room at the bottom, but the size of conventional optical elements is restricted by the diffraction limit of light. Plasmon waveguides made from metal nanoparticle chains may allow a drastic reduction in the size of photonic devices. JOACHIM R. KRENN is at the Institute for Experimental Physics and Erwin Schrödinger Institute for Nanoscale Research,Karl-Franzens-University Graz, A-8010 Graz,Austria. e-mail: joachim.krenn@uni-graz.at Modern technology relies heavily on photonic and optoelectronic devices for signal transmission and routing,chemical analysis and sensor applications.As is the case for electronics, increasing the speed and sensitivity of photonic devices relies on further miniaturization of photonic elements. But there is a fundamental limit to the minimum size of conventional optical elements such as dielectric waveguides.This limit is set by diffraction to be about half the wavelength of light.For visible light,this translates into a minimum element size of a few hundred nanometres — too large for fabricating nano- and molecular-scale photonic devices.Near-field optics, which exploits non-propagating (evanescent) fields rather than propagating light waves,provides a way of circumventing the diffraction (and therefore size) limit. As they report on page 229 of this issue,Stefan Maier and colleagues1have now directly observed energy transfer along a nanoparticle plasmon ‘waveguide’.These metal nanoparticle chains may become a central component of nanoscale photonic devices. Near-field optics,proposed around 20 years ago, provides a neat way to circumvent the diffraction limit2 . Instead of imaging with a system of lenses,a glass fibre with a submicroscopic tip can be used to probe the light fields close to a sample surface.Because the distance between tip and sample is much smaller than the light wavelength,non-propagating light fields that are bound to the sample surface can be detected.These ‘evanescent’fields decay in intensity within a fraction of the light wavelength when moving away from the surface,and carry information about sample features smaller than the value set by the diffraction limit. Owing to their non-propagating character,evanescent fields can only be detected by a local probe immersed into the near-field of the sample.The corresponding experimental device is called a near-field scanning optical microscope (NSOM). © 2003 Nature Publishing Group
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 Group
NEWS & 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
