LETTERS ocal detection ot electromagnetic energy transport below the diffraction limit in metal nanoparticle plasmon waveguides STEFAN A MAIER*1 PIETER G KIK HARRY A ATWATER SHEFFER MELTZER2 ELAD HAREL2 BRUCE E. KOEL2 AND ARIA. G. REQUICHA2 Thomas J Watson Laboratory of Applied Physics, California Institute of Technology, Mail Stop 128-95, Pasadena, California 91125, USA Laboratory for Molecular Robotics, Computer Science Department, University of Southem California, Los Angeles, Califomia 90089, USA *e-mail stmaier @caltech. edu Published online: 2 March 2003: doi: 10. 1038/nmat852 chieving control of light-material interactions for photonic 0.06 device applications at nanoscale dimensions will require △ Nanoparticle chain ctures that guide electromagnetic energy with a lateral 日 Single particles mode confinement below the diffraction limit of light. This cannot be achieved by using conventional waveguides' or photonic crystals Ithas been suggested that electromagneticenergy can beguided below the diffraction limit along chains of closely spaced metal surG particles that convert the optical mode into non-radiating ce plasmons. A variety of methods such as electron beam lithography and self-assembly have been used to construct metal transport along plasmon waveguides has proved elusive. Here we 审:4 present observations of electromagnetic energy transport from localized subwavelength source to a localized detector over distances of about 0.5 um in plasmon waveguides consisting of closely spaced silver rods. The waveguides are excited by the tip of a near-field 0.00 scanning optical microscope, and energy transport is probed by using Energy (ev) fluorescent nanosphere The transport of electromagnetic energy in plasmon waveguides ting of closely spaced metal nanoparticles relies on near-field Figure 1 Far-field extinction spectrum of Ag nanoparticle chains and single ling between surface plasmon-polariton modes of adjacent particles. The far-field extinction spectrum of a plasmon waveguide consistingofAg particles. This type of guiding due to near-field coupling has recently nanorods with a 3: 1 aspect ratio and a surface-to-surface spacing of 50nmbetween been demonstrated experimentally in macroscopic analogues operating adjacent particles shows a plasmon resonance peak shift to higher energies(red triangles in the microwave regime. At the submicrometre scale, plasmon and Lorentz fit compared with the extinction spectrumof isolated, non-interacting waveguides were theoretically and numerically analysed, allowing for the particles(black squares and Lorentz fit). The exciting light was polarized along the long axis ivestigation of inter-particle interactions,, and the dispersion relation of the nanorods, perpendicular to the particle chain axis. The inset shows ascannin and group velocity for energy transport. The characteristics of optical- electron micrographof the plasmon waveguide layoutunder study pulse propagation in plasmon waveguides consisting of spherical or pheroidal nanoparticles were determined by using finite-difference time-domain simulations. Far-field polarization spectroscopy experiments on ordered two-dimensional arrays of Au and Ag the particle arrays that are dependent on inter-particle distances 3, l4 nanoparticles with submicrometre inter-particle spacing have It has further been shown that the resonance energy shifts that occur in confirmed that electromagnetic interactions between the particles are one-dimensional arrays consisting of closely spaced Au particles enable present, revealing energy shifts of the collective plasmon resonances of estimation of the group velocity and energy attenuation length of @2003 Nature Publishing Group
LETTERS Achieving control of light-material interactions for photonic device applications at nanoscale dimensions will require structures that guide electromagnetic energy with a lateral mode confinement below the diffraction limit of light.This cannot be achieved by using conventional waveguides1 or photonic crystals2 . It has been suggested that electromagnetic energy can be guided below the diffraction limit along chains of closely spaced metal nanoparticles3,4 that convert the optical mode into non-radiating surface plasmons5 . A variety of methods such as electron beam lithography6 and self-assembly7 have been used to construct metal nanoparticle plasmon waveguides. However, all investigations of the optical properties of these waveguides have so far been confined to collective excitations8–10, and direct experimental evidence for energy transport along plasmon waveguides has proved elusive. Here we present observations of electromagnetic energy transport from a localized subwavelength source to a localized detector over distances of about 0.5 µm in plasmon waveguides consisting of closely spaced silver rods. The waveguides are excited by the tip of a near-field scanning optical microscope,and energy transport is probed by using fluorescent nanospheres. The transport of electromagnetic energy in plasmon waveguides consisting of closely spaced metal nanoparticles relies on near-field coupling between surface plasmon–polariton modes of adjacent particles. This type of guiding due to near-field coupling has recently been demonstrated experimentally in macroscopic analogues operating in the microwave regime6,11. At the submicrometre scale, plasmon waveguides were theoretically and numerically analysed,allowing for the investigation of inter-particle interactions3 , and the dispersion relation and group velocity for energy transport4 . The characteristics of opticalpulse propagation in plasmon waveguides consisting of spherical or spheroidal nanoparticles were determined by using finite-difference time-domain simulations12. Far-field polarization spectroscopy experiments on ordered two-dimensional arrays of Au and Ag nanoparticles with submicrometre inter-particle spacing have confirmed that electromagnetic interactions between the particles are present, revealing energy shifts of the collective plasmon resonances of the particle arrays that are dependent on inter-particle distances13,14. It has further been shown that the resonance energy shifts that occur in one-dimensional arrays consisting of closely spaced Au particles enable estimation of the group velocity and energy attenuation length of nature materials | VOL 2 | APRIL 2003 | www.nature.com/naturematerials 229 Local detection of electromagnetic energy transport below the diffraction limit in metal nanoparticle plasmon waveguides STEFAN A. MAIER*1 , PIETER G. KIK1 , HARRY A. ATWATER1 , SHEFFER MELTZER2 , ELAD HAREL2 , BRUCE E. KOEL2 AND ARI A.G. REQUICHA2 1 Thomas J.Watson Laboratory of Applied Physics,California Institute of Technology,Mail Stop 128-95,Pasadena,California 91125,USA 2 Laboratory for Molecular Robotics,Computer Science Department,University of Southern California,Los Angeles,California 90089,USA *e-mail:stmaier@caltech.edu Published online:2 March 2003; doi:10.1038/nmat852 2.0 2.1 2.2 2.3 2.4 2.5 2.6 0.00 0.02 0.04 0.06 x 10 Extinction (a.u.) Energy (eV) Nanoparticle chain Single particles Figure 1 Far-field extinction spectrum of Ag nanoparticle chains and single particles.The far-field extinction spectrum of a plasmon waveguide consisting of Ag nanorods with a 3:1 aspect ratio and a surface-to-surface spacing of 50 nm between adjacent particles shows a plasmon resonance peak shift to higher energies (red triangles and Lorentz fit) compared with the extinction spectrum of isolated,non-interacting particles (black squares and Lorentz fit).The exciting light was polarized along the long axis of the nanorods,perpendicular to the particle chain axis.The inset shows a scanning electron micrograph of the plasmon waveguide layout under study. © 2003 Nature Publishing Group
LETTERS b Intensity Topography d 品品 Figure 2 Excitation and detection of energy transport in plasmon waveguides by near-field optical microscopy. a, Sketch of the experiment Light emanating from the tip of an illumination-mode near-field scanning optical microscope(NSOM) locally excites a plasmon waveguide. The waveguide transports the electromagnetic energy toafuorescent nanosphere, and the fluorescence intensity for varying tip positions is collected in the far-field. b, Scanning electron micrograph of a 100 umx 100 um grid consisting of Ag plasmon waveguides.c, d, Topography and fluorescent NSOM scan of a 6umx6um area consisting of single fluorescent nanospheres(c, area highlighted by red box in b)and an area containing isolated fluorescent nanospheres in the vicinity of plasmon waveguides (d, area highlighted by green box in b). The fluorescent intensity of two single nanospheres(grey circles and data cuts Aand B) can be compared with the intensity distribution of nanosphereson top of plasmon waveguides(red circles, data cuts WG1 and wG2) by using cuts(grey and red lines) through the fluorescent intensity scan along the waveguide axis @2003 Nature Publishing Group
LETTERS 230 nature materials | VOL 2 | APRIL 2003 | www.nature.com/naturematerials Topography Fluorescence B WG1 WG2 A Dye Intensity Far-field detection a c d b 0 0 1 1 2 2 3 3 4 4 5 5 µm 250 50Height (nm) 0 0 1 1 2 2 3 3 4 4 5 5 µm 425 –179NSOM intensity (kHz) 0 0 1 1 2 2 3 3 4 4 5 5 µm 250 50Height (nm) 0 0 1 1 2 2 3 3 4 4 5 5 µm 332 –215NSOM intensity (kHz) Figure 2 Excitation and detection of energy transport in plasmon waveguides by near-field optical microscopy. a,Sketch of the experiment.Light emanating from the tip of an illumination-mode near-field scanning optical microscope (NSOM) locally excites a plasmon waveguide.The waveguide transports the electromagnetic energy to a fluorescent nanosphere,and the fluorescence intensity for varying tip positions is collected in the far-field.b,Scanning electron micrograph of a 100µm×100µm grid consisting of Ag plasmon waveguides.c,d,Topography and fluorescent NSOM scan of a 6µm×6µm area consisting of single fluorescent nanospheres (c,area highlighted by red box in b) and an area containing isolated fluorescent nanospheres in the vicinity of plasmon waveguides (d,area highlighted by green box in b).The fluorescent intensity of two single nanospheres (grey circles and data cuts A and B) can be compared with the intensity distribution of nanospheres on top of plasmon waveguides (red circles,data cuts WG1 and WG2) by using cuts (grey and red lines) through the fluorescent intensity scan along the waveguide axis. © 2003 Nature Publishing Group
LETTERS plasmon waveguides. 0. Energy attenuation lengths of 6 dB per 30 nm were predicted for plasmon waveguides consisting of closely spaced pherical Au nanoparticles on an indium tin oxide(ITo)substrate O Control B so, it was shown that a geometry change to spheroidal Ag particles 3+23 should allow foranattenuationlengthincrease toabout 6dB per 250nm 329+14mm ref 10). This kind of attenuation length is sufficient to allow detection of energy transport by localexcitation. Plasmon waveguides were fabricated by using electron-beam 3 lithography with lift-off on ITO-coated quartz slides, which allowed for 0.6 a good control over particle size and spacing. The waveguide structures consist of rod-shaped Ag nanoparticles with dimensions of 90 nm x 30 nm x 30 nm and a surface-to-surface spacing of 50 nm between adjacent particles. The long axes of the individual nanoparticles were oriented perpendicular to the waveguide chain axis to allow for an increased near-field coupling between the particles. The inset of Fig. I shows a scanning electron micrograph of one of these plasmon waveguides. To allow for the determination of the plasmon 0o00o resonances of the fabricated structures with a high signal-to-noiseratio using far-field spectroscopy, many plasmon waveguides were arranged in a 100 umx 100 um grid with a grating constant of 1 um as depictedin Distance (um) Fig. 2b. It has previously been shown that cross talk between different waveguides is negligible for this grating constant".. Thus, far-field extinction spectra on these arrays reflect the properties of individual Figure 3 Evidence for energy transportin plasmon waveguides by the widthof the plasmon waveguides and provide a probe of the near-field coupling intensity of fluorescent nanospheres. Individual data sets represent averagesoffive between the nanoparticles composing each waveguide. parallel cuts along the plasmon waveguide direction through the fiuorescent spots Figure I shows the far-field extinction spectrum of the fabricated highlighted in Fig. 2c, d for isolated nanospheres(controlAand B, blackand redd plasmon waveguides taken under normal-incidence white-light es located on top of plasmon waveguides (WG 1 andwG2, green illumination witha spot sizeof 100 amand a polarizationalong thelong and blue data points) as depicted in the inset Gaussian line-shape fits to the data an of nanoparticles and thus perpendicular tothe waveguide-chain increased width for nanospheres located on plasmon waveguides kis(red triangles). The extinction spectrum of a grid of single Ag nanoparticles of the same geometry with an inter-particle spacing of I um is also shown, for which the inter-particle coupling is negligible (black squares). The single-particle extinction spectrum peaks at filtered out by using a band-pass filter, and the 610 nmdyeemission was 2. 18 eV for a polarization along the long particle axis, corresponding to detected in the far-field with an avalanche photo diode. This scheme a resonance wavelength of 570 nm. The extinction spectrum of the enables the observation of energy transport in the following way: first, plasmon waveguide shows a resonance shift of about 100 meV to higher energy is transferred from the illuminating tip to the plasmon agreement with theoretical and numerical studies of aggregated noble- nanoparticle structure and excites a luoresenlaepagates along the merges because of near-field coupling between the particles, in iveguide. The excitation subsequentl osphere placed on metal nanoparticles and plasmon waveguides. 2. According to top of a waveguide at a sufficient distance from the excitation source numerical simulations, this resonance shift of 100 meV translates into a Energy transport would result in dye emission even when the maximumenergy attenuation length of the order of 6dB per 200nm for microscope tip is located away from the dye, and thus would manifest anexcitation at the single-particle resonance(2.18 eV in these samples), itself in an increased spatial width of the fluorescence spot of a at which the energy transfer in a plasmon waveguide is predicted to be nanosphere attached to a plasmon waveguide compared with a single mostefficient, independent of inter-particle spacing. 2. free nanosphere To probe energy transport directly in the fabricated plasmor Figure 2c,d shows simultaneously obtained topography and waveguides, local excitation is necessary as opposed to far-field fluorescent NSOM scans of thecontrol area outside the grid( Fig 2b,red excitation of all particles in the arrays. To accomplish this, the tip of an box)with single nanospheres only and of the plasmon waveguide grid illumination mode near-field scanning optical microscope(NSOM)(Fig 2b, green box) together with fluorescent nanospheres, respectively (Nanonics NSOM-100)(ref. 17)is used as a local excitation source for The samples were illuminated at the single-particle resonance nanoparticles in plasmon waveguides. To excite the mode of least wavelength of 570 nm by usingamicroscopetip witha 100nmaperture damping, laser light from a dye laser at a wavelength of 570 nm, The scans were done in constant gap mode, ensuring a fixed vertical responding to the single-particle resonance, was coupled into a distance between the exciting tip and the fluorescent nanospheres both multi-mode optical fibre attached to the Al-coated NSOM tip used for for isolated spheres and for spheres located on plasmon waveguides. excitation Figure 2a shows a schematic of our approach to excitation The scan direction was perpendicular to the plasmon waveguides, and and energy transport detection Power transport away from the directly images were built up from left to right. Figure 2c shows a scan of a excited nanoparticles in the plasmon waveguide is probed by the control area consisting of single nanospheres only, which was taken placement of carboxyl-coated polystyrene nanospheres(Molecular immediately before scanning the plasmon waveguide area. Probes Fluospheres F-8801, diameter 110+8 nm)filled with fluorescent Single nanospheres are clearly resolved in both the topography and the electron-beam-fabricated plasmon e waveguide structure. For this, the fluorescent NSOM image. Intensity variations are observed between a thin polylysine layer, and the nanospheres were subsequently variationsin the nanosphere diameter; theelongation of the fluorescent randomly deposited from an aqueous solution. The fluorescent dyes spots is due to a tip artefact Figure 2d shows the subsequent scan of a used show a strong absorption, peaking at 580-590 nm near the sample area comprising four plasmon waveguides in the left part of the plasmon resonance wavelength of a single fabricated Ag particle and image and fluorescent nanospheres. Two nanosphe heres(highlighted by having their emission maximum at 610 nm. The excitation light was red circles) are located on top of pla naturematerialsVol2lApriL2003iwww.nature.com/naturematerials @2003 Nature Publishing Group
LETTERS nature materials | VOL 2 | APRIL 2003 | www.nature.com/naturematerials 231 plasmon waveguides9,10. Energy attenuation lengths of 6 dB per 30 nm were predicted for plasmon waveguides consisting of closely spaced spherical Au nanoparticles on an indium tin oxide (ITO) substrate. Also, it was shown that a geometry change to spheroidal Ag particles should allow for an attenuation length increase to about 6dB per 250nm (ref. 10). This kind of attenuation length is sufficient to allow detection of energy transport by local excitation. Plasmon waveguides were fabricated by using electron-beam lithography with lift-off on ITO-coated quartz slides,which allowed for a good control over particle size and spacing. The waveguide structures consist of rod-shaped Ag nanoparticles with dimensions of 90 nm × 30 nm × 30 nm and a surface-to-surface spacing of 50 nm between adjacent particles. The long axes of the individual nanoparticles were oriented perpendicular to the waveguide chain axis to allow for an increased near-field coupling between the particles10. The inset of Fig.1 shows a scanning electron micrograph of one of these plasmon waveguides. To allow for the determination of the plasmon resonances of the fabricated structures with a high signal-to-noise ratio using far-field spectroscopy, many plasmon waveguides were arranged in a 100µm×100µm grid with a grating constant of 1µm as depicted in Fig. 2b. It has previously been shown that cross talk between different waveguides is negligible for this grating constant9,15. Thus, far-field extinction spectra on these arrays reflect the properties of individual plasmon waveguides and provide a probe of the near-field coupling between the nanoparticles composing each waveguide. Figure 1 shows the far-field extinction spectrum of the fabricated plasmon waveguides taken under normal-incidence white-light illumination with a spot size of 100µm and a polarization along the long axis of the nanoparticles and thus perpendicular to the waveguide-chain axis (red triangles). The extinction spectrum of a grid of single Ag nanoparticles of the same geometry with an inter-particle spacing of 1 µm is also shown, for which the inter-particle coupling is negligible (black squares). The single-particle extinction spectrum peaks at 2.18 eV for a polarization along the long particle axis, corresponding to a resonance wavelength of 570 nm. The extinction spectrum of the plasmon waveguide shows a resonance shift of about 100meV to higher energies because of near-field coupling between the particles, in agreement with theoretical and numerical studies of aggregated noblemetal nanoparticles16 and plasmon waveguides4,12. According to numerical simulations,this resonance shift of 100 meV translates into a maximum energy attenuation length of the order of 6dB per 200nm for an excitation at the single-particle resonance (2.18 eV in these samples), at which the energy transfer in a plasmon waveguide is predicted to be most efficient,independent of inter-particle spacing4,12. To probe energy transport directly in the fabricated plasmon waveguides, local excitation is necessary as opposed to far-field excitation of all particles in the arrays. To accomplish this, the tip of an illumination mode near-field scanning optical microscope (NSOM) (Nanonics NSOM-100) (ref. 17) is used as a local excitation source for nanoparticles in plasmon waveguides. To excite the mode of least damping, laser light from a dye laser at a wavelength of 570 nm, corresponding to the single-particle resonance, was coupled into a multi-mode optical fibre attached to the Al-coated NSOM tip used for excitation. Figure 2a shows a schematic of our approach to excitation and energy transport detection.Power transport away from the directly excited nanoparticles in the plasmon waveguide is probed by the placement of carboxyl-coated polystyrene nanospheres (Molecular Probes Fluospheres F-8801,diameter 110±8nm) filled with fluorescent molecules18 in close proximity to the waveguide structure. For this, the electron-beam-fabricated plasmon waveguide sample was coated with a thin polylysine layer, and the nanospheres were subsequently randomly deposited from an aqueous solution. The fluorescent dyes used show a strong absorption, peaking at 580–590 nm near the plasmon resonance wavelength of a single fabricated Ag particle and having their emission maximum at 610 nm. The excitation light was filtered out by using a band-pass filter,and the 610 nm dye emission was detected in the far-field with an avalanche photo diode. This scheme enables the observation of energy transport in the following way: first, energy is transferred from the illuminating tip to the plasmon waveguide. The excitation subsequently propagates along the nanoparticle structure and excites a fluorescent nanosphere placed on top of a waveguide at a sufficient distance from the excitation source. Energy transport would result in dye emission even when the microscope tip is located away from the dye, and thus would manifest itself in an increased spatial width of the fluorescence spot of a nanosphere attached to a plasmon waveguide compared with a single free nanosphere. Figure 2c,d shows simultaneously obtained topography and fluorescent NSOM scans of the control area outside the grid (Fig.2b,red box) with single nanospheres only and of the plasmon waveguide grid (Fig.2b,green box) together with fluorescent nanospheres,respectively. The samples were illuminated at the single-particle resonance wavelength of 570nm by using a microscope tip with a 100nm aperture. The scans were done in constant gap mode, ensuring a fixed vertical distance between the exciting tip and the fluorescent nanospheres both for isolated spheres and for spheres located on plasmon waveguides. The scan direction was perpendicular to the plasmon waveguides, and images were built up from left to right. Figure 2c shows a scan of a control area consisting of single nanospheres only, which was taken immediately before scanning the plasmon waveguide area. Single nanospheres are clearly resolved in both the topography and the fluorescent NSOM image. Intensity variations are observed between different fluorescent nanospheres, which are attributed to slight variations in the nanosphere diameter;the elongation of the fluorescent spots is due to a tip artefact. Figure 2d shows the subsequent scan of a sample area comprising four plasmon waveguides in the left part of the image and fluorescent nanospheres. Two nanospheres (highlighted by red circles) are located on top of plasmon waveguides, and one Figure 3 Evidence for energy transport in plasmon waveguides by the width of the intensity of fluorescent nanospheres. Individual data sets represent averages of five parallel cuts along the plasmon waveguide direction through the fluorescent spots highlighted in Fig. 2c,d for isolated nanospheres (control A and B,black and red data points) and nanospheres located on top of plasmon waveguides (WG 1 and WG2,green and blue data points) as depicted in the inset.Gaussian line-shape fits to the data show an increased width for nanospheres located on plasmon waveguides. –0.2 0.2 0.6 1.0 1.4 174±17 nm 193±23 nm 329±14 nm 343±27 nm Fluorescence intensity (a.u.) Fit widths: 0.0 0.4 0.8 1.2 1.6 Distance (µm) Control A Control B WG1 WG2 © 2003 Nature Publishing Group
LETTERS hereat the right side of the scan area(grey circle)is positioned at away from the nanosphere centre. At this distance, excitation by the ance from a waveguide structure and can thus serve as a further plasmon waveguide dominates over direct dye excitation. These fits control toensure that tip characteristics do not change during the scans. yield a decay length of 6 dB per 195 28 nm, which is in excellent The widths of the fluorescence spots of several control nanospheres of agreement with the estimate of 6 dB per 200 nm obtained from our far Fig 2c, d and of the nanospheres attached to waveguides of Fig 2d were field measurements and theoretical predictions. examined by taking cuts through the microscope data parallel to the An energy attenuation length of several hundred nanometre plasmon waveguide direction, as highlighted in the fluorescent images suggests the use of plasmon waveguides as functionalend-structures in (red and grey lines) integrated optical devices. Further improvements to the inter-particle Figure 3 shows averages of five parallel cuts through fluorescent coupling and the use of low-loss substrates should allow for the nanosphere spots A, B, WGI and wG2 Gaussian fits to the data for both fabrication of plasmon waveguides with larger energy-attenuation the control spheres(black and red lines and data points)and the two lengths, and thus enable numerous applications for light-guiding and spheres attached to plasmon waveguides(green and blue lines and data light-focusing elements operating below the diffraction limit. points)are included. The control nanospheres A and B were scanned before and after scanning the waveguide structures, and in both cases Received 9 December 2002; accepted 29 January 2003: published 2 March 2003 show similar fluorescent full-widths at half maximum(FWHM) of References 74+ 17 and 193 23 nm. Analyses of additional control spheres E.A.& Teich, M. C. Fundamentals of Photonic(Wiley, New York, 1991). (indicated by dashed grey lines in Fig. 2c)yielded similar widths of 27. 37282-3790 956ransmision through thap bends in photonic crystal waveguides. Phips. 160+12,205+13 and 188+17nm, respectively. This confirms that the 3. Quinten, M, Leitner, A Krenn, L.R.& Aussenegg E R Electromagnetic energy transport vial fluorescent spots WGI and wG2 of the nanospheres attached to the 4 Brongersma M L. Hartman, Lw&Atwater,, H.AElectromagneticenergytransferandswitching plasmon waveguides show a FWHM of329±14and343±27mm,skmm以k邮知是果( respectively. The resulting average fluorescence spot FWHM of control 6.Maier, SAetal.Plasmonics nanoscale optical devices. A. Mater. 13, 1501-1505(2001). particle arrays formed on engineered chaperonin protein waveguides was 336+30nm. The broadening of the fluorescence spots s template atur mate 1267-2526 2002). caused by the presence of the plasmon waveguide was thus 151+ 48nm. ys. Rev. Lett82,2590-2593(199) If the observed broadening of the fluorescent width of waveguide 9. Maicr, S. A, Brongersma, M L, Kik, G.& Atwater, H. A Ob aveguide, no difference in FWHM between control and waveguide 10. Maier,S.A, Kik, P G& Atwater, H. A Observation of coupled plasmon-polariton modes inAu pheresis expected in thedirection perpendicularto the waveguide 1714-1716(2002) axis. Indeed, the fitted FWHMs in this direction are found to be equal 11. Maier, S. A, Brongersma, M L& Atwater, H.A. Electromagnetic energy transport along arrays of within the fit error, giving 364 t 36 nm for control A and B and these values are rather large because of the shape of the tip aperture(see (submited. Optical-pulse propagation in metal nanopartide chin waveguides. Phys.Rev. Let. 352+ 40 nm for the waveguide nanospheres WGl and WG2. Note that 12 Maier,SAetal ance. Phys. Rev. Lett. 84, 4721-1724(2000). The increase in the width of the nanosphere fluorescence of about 4.Salerno, M, Felid, N Krenn. L R, Leitner, A Aussenegg E. R Near-field optical response of 150 nm for spheres attached to a plasmon waveguide structure can be 15. Schider, G. c al Optical properties of Ag and Au nanowire gratings. L AppL Phys. 90,3825-3830 attributed to local excitation of the plasmon waveguide followed by energy transport along the waveguide towards the fluorescent dye 16Quinten, &x Kreibig, UAbsorption and elastic scattering oflight by particle aggregates.AppL Opt. particle. This type of excitation is known to dominate over excitation by 32,61736182(1993). direct scattering of the exciting radiation from individual ormal-force scanned probe microscopy. Rev. Sci Instrum. 67. 3567-3572(1996). nanoparticles. From Fig. 3itis clear thatforafree-standingnanosphere 18 Fujihira, M. ct al. Scanning near-field optical microscopy of fluorescent polystyrene spheres with a the fluorescence signal decreases below the dark noise level if the t fluorescent nanosphere attached to plasmon waveguide can be excited The authors are grateful to Richard Muller, Paul Maker, and Pierre Echternach of the Jet Propulsion from distances up to 500 nm by the plasmon waveguide. bwtbeairy Our results thus provide direct evidence for energy transport over this 71775 and DM1-02-09678 and the Center for Science and Engineering of Materiaks at Caltech -g. tly by the NSF grants ECSO103543, El distance. To obtain quantitative information on theenergy decay length Correspondence and requests for materials should be addressed to S.M. we have performed exponential fits on the tails of the fluorescence Competing financial Interests intensity of the nanospheres atdistances greater than 200nm The authors declare that they have no competing financial interests. naturematerialsvol21apRil2003iwww.naturE.com/naturematerials @2003 Nature Publishing Group
LETTERS 232 nature materials | VOL 2 | APRIL 2003 | www.nature.com/naturematerials nanosphere at the right side of the scan area (grey circle) is positioned at a distance from a waveguide structure and can thus serve as a further control to ensure that tip characteristics do not change during the scans. The widths of the fluorescence spots of several control nanospheres of Fig. 2c,d and of the nanospheres attached to waveguides of Fig. 2d were examined by taking cuts through the microscope data parallel to the plasmon waveguide direction, as highlighted in the fluorescent images (red and grey lines). Figure 3 shows averages of five parallel cuts through fluorescent nanosphere spots A,B,WG1 and WG2.Gaussian fits to the data for both the control spheres (black and red lines and data points) and the two spheres attached to plasmon waveguides (green and blue lines and data points) are included. The control nanospheres A and B were scanned before and after scanning the waveguide structures, and in both cases show similar fluorescent full-widths at half maximum (FWHM) of 174 ± 17 and 193 ± 23 nm. Analyses of additional control spheres (indicated by dashed grey lines in Fig. 2c) yielded similar widths of 160 ± 12,205 ± 13 and 188 ± 17 nm,respectively.This confirms that the tip profile did not change appreciably during the scans. By contrast, the fluorescent spots WG1 and WG2 of the nanospheres attached to the plasmon waveguides show a FWHM of 329 ± 14 and 343 ± 27 nm, respectively. The resulting average fluorescence spot FWHM of control spheres was 185 ± 38 nm,whereas the average width of nanospheres on waveguides was 336 ± 30 nm.The broadening of the fluorescence spots caused by the presence of the plasmon waveguide was thus 151±48nm. If the observed broadening of the fluorescent width of waveguide nanospheres is due to transport of electromagnetic energy along the waveguide, no difference in FWHM between control and waveguide nanospheres is expected in the direction perpendicular to the waveguide axis. Indeed, the fitted FWHMs in this direction are found to be equal within the fit error, giving 364 ± 36 nm for control A and B and 352 ± 40 nm for the waveguide nanospheres WG1 and WG2. Note that these values are rather large because of the shape of the tip aperture (see microscope scans in Fig. 2). The increase in the width of the nanosphere fluorescence of about 150 nm for spheres attached to a plasmon waveguide structure can be attributed to local excitation of the plasmon waveguide followed by energy transport along the waveguide towards the fluorescent dye particle.This type of excitation is known to dominate over excitation by direct scattering of the exciting radiation from individual nanoparticles12.From Fig.3 it is clear that for a free-standing nanosphere the fluorescence signal decreases below the dark noise level if the tip is located approximately 200 nm away from the sphere centre, whereas a fluorescent nanosphere attached to a plasmon waveguide can be excited from distances up to 500 nm by the plasmon waveguide. Our results thus provide direct evidence for energy transport over this distance.To obtain quantitative information on the energy decay length we have performed exponential fits on the tails of the fluorescence intensity of the waveguide nanospheres at distances greater than 200nm away from the nanosphere centre. At this distance, excitation by the plasmon waveguide dominates over direct dye excitation. These fits yield a decay length of 6 dB per 195 ± 28 nm, which is in excellent agreement with the estimate of 6 dB per 200 nm obtained from our farfield measurements and theoretical predictions. An energy attenuation length of several hundred nanometres suggests the use of plasmon waveguides as functional end-structures in integrated optical devices. 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