insight review articles Box 2 Surface plasmon length scales Surface plasmon bandgaps 10 nm 100 nm 1 um 10 um 100 um 1 mm luminium t o,5 umo There are three characteristic length scales that are important for Periodic texturing of the metal surface can lead to the formation of SP-based photonics in addition to that of the associated light. The an SP photonic bandgap when the period, a, is equal to half the propagation length of the SP mode, &sp, is usually dictated by loss in wavelength of the SP, as shown in the dispersion diagram (a). Just the metal. For a relatively absorbing metal such as aluminium the as for electron waves in crystalline solids, there are two SP standing propagation length 2 um at a wavelength of 500 nm For a low loss vave solutions, each with the same wavelength but, owing to th metal, for example, silver, at the same wavelength it is increased to different field and surface charge distributions, they are of different O um. By moving to a slightly longer wavelength, such as 1.55 um, the propagation length is further increases towards 1 mm. The because of the greater distance between the surface charges and propagation length sets the upper size limit for any photonic circuit the greater distortion of the field, as shown schematically in b. SP based on SPs. The decay length in the dielectric material, &c,is modes with frequencies between the two band edges, w, and o pically of the order of half the wavelength of light involved and cannot propagate, and so this frequency interval is known as a dictates the maximum height of any individual features, and thus gap. By providing periodic texture in two dimensions, SP propagation omponents, that might be used to control SPs. The ratio of 8sp 8 in all in-plane directions can be blocked, leading to the full bandgap thus gives one measure of the number of SP-based components for SPs. At the band edges the density of SP states is high, and that may be integrated together. The decay length in the metal, 8 there is a significant increase in the associated field enhancement determines the minimum feature size that can be used as shown in the diagram, this is between one and two orders of magnitude smaller than the wavelength involved, thus highlighting the need for good control of fabrication at the nanometre scale. The combinations chosen give an indication of range from poor (Al at 0.5 um) to good (Ag at 1.5 um)SP performance This is not as serious a limitation as one might imagine, because, though we would often like to measure the optical energy( for which ve need to know both the electric and magnetic field strengths), many opticalinteractions are dominated by just one of the fields, andsoma Ding just one of them on asubwavelengthscale can provide invaluable proportional to the mapping of the distribution of E in the near-field cting zone one". Surprisingly, SPs with circular symmetry, sustained by a suit able metal coating of the tip, played a crucial role in the recent demon- stration"that the distribution of H associated with the optical wave can also be detected. Not only does this provide new possibilities for SPs, butit also enhances theirrole in suchphenomena as the magneto- optic Kerr effectin extending the realm in which SPs have impor- tant photonicapplications. Near-field mapping techniques such as PSTM have been essential for the developmentofSPdevicessuchas waveguidesandothercompo- nents. Forexample, they were used to map the field distributionassoci- ated with an SP waveguide based on a metal stripe-that isillustrated in Fig. 1. Other strategies for waveguiding have also been explored, for Figure 3 Surface plasmon Bragg reflector. a, An example of integration involving a ample, using a well defined stripe defect on a periodically modulated Bragg reflector on a sP waveguide is provided by carving a series of slots with sultable photonic surface, with the defect acting as a guide". It was recently sizes and separations in a gold stripe supported on a glass substrate, as shown on the pointed out that SP waveguides can be obtained by exploiting rough- SEM image (a) of a stripe similar to the one introduced in Fig. 1 (courtesy of E Devaux ess-induced Andersonlocalization, whichinhibits SP propagation In Universite Louis Pasteur, France, and ). C. Weeber, Universite de Bourgogne, France). this case, waveguides are formed by channels flattened across an other- White ba, 30 um: black bar, 2.5 um. The incident SP, with three maxima (see Fig.) wise roughmetal film". Waveguides can also be made of aligned metal is excited by total intemal reflectionillumination as described in the legend of Fig.1. The (for example, gold)nanoparticles: a recent PSTM study has demon- SP propagates from the left to the right until it reaches the Bragg reflecting zone strated the feasibility of laterally squeezing the optical near field by cou- dicated by the blue squares). The mirror effect is clearty observed in the PSTM image pling localized SPs of an ensemble of linearly aligned gold b)as the interference between incident and reflected SPs. To the right of the mirror zone. anoparticles", whichsuggestsanother to SP guiding the intensity falls dramatically to avalue not detectable by the PSTM tip. Bar, 2.5 um By making use of advanced lithographic techniques to texture the NatuReVol412414AugUst2003www.nature.com/nature e 2003 Nature Publishing Group 827This is not as serious a limitation as one might imagine, because, although we would often like to measure the optical energy (for which we need to know both the electric and magnetic field strengths), many optical interactions are dominated by just one of the fields, and so map￾ping just one of them on a subwavelength scale can provide invaluable new insights. For example, most PSTM tips provide images that are proportional to the mapping of the distribution of |E| 2 in the near-field zone36. Surprisingly, SPs with circular symmetry, sustained by a suit￾able metal coating of the tip, played a crucial role in the recent demon￾stration37 that the distribution of |H| 2 associated with the optical wave can also be detected. Not only does this provide new possibilities for SPs, but it also enhances their role in such phenomena as the magneto￾optic Kerr effect38,39 in extending the realm in which SPs have impor￾tant photonic applications. Near-field mapping techniques such as PSTM have been essential for the development of SP devices such as waveguides and other compo￾nents.For example, they were used to map the field distribution associ￾ated with an SP waveguide based on a metal stripe40–42 that is illustrated in Fig. 1. Other strategies for waveguiding have also been explored, for example, using a well defined stripe defect on a periodically modulated photonic surface, with the defect acting as a guide43. It was recently pointed out that SP waveguides can be obtained by exploiting rough￾ness-induced Anderson localization, which inhibits SP propagation. In this case, waveguides are formed by channels flattened across an other￾wise rough metal film44. Waveguides can also be made of aligned metal (for example, gold) nanoparticles: a recent PSTM study has demon￾strated the feasibility of laterally squeezing the optical near field by cou￾pling localized SPs of an ensemble of linearly aligned gold nanoparticles45, which suggests another approach to SP guiding46. By making use of advanced lithographic techniques to texture the insight review articles NATURE | VOL 424 | 14 AUGUST 2003 | www.nature.com/nature 827 a b Bragg reflecting zone Figure 3 Surface plasmon Bragg reflector. a, An example of integration involving a Bragg reflector on a SP waveguide is provided by carving a series of slots with suitable sizes and separations in a gold stripe supported on a glass substrate, as shown on the SEM image (a) of a stripe similar to the one introduced in Fig. 1 (courtesy of E. Devaux, Université Louis Pasteur, France, and J. C. Weeber, Université de Bourgogne, France). White bar, 30 mm; black bar, 2.5 mm. The incident SP, with three maxima (see Fig. 1), is excited by total internal reflection illumination as described in the legend of Fig. 1. The SP propagates from the left to the right until it reaches the Bragg reflecting zone (indicated by the blue squares). The mirror effect is clearly observed in the PSTM image (b) as the interference between incident and reflected SPs. To the right of the mirror zone, the intensity falls dramatically to a value not detectable by the PSTM tip. Bar, 2.5 mm. Periodic texturing of the metal surface can lead to the formation of an SP photonic bandgap when the period, a, is equal to half the wavelength of the SP, as shown in the dispersion diagram (a). Just as for electron waves in crystalline solids, there are two SP standing wave solutions, each with the same wavelength but, owing to their different field and surface charge distributions, they are of different frequencies. The upper frequency solution, v&, is of higher energy because of the greater distance between the surface charges and the greater distortion of the field, as shown schematically in b. SP modes with frequencies between the two band edges, v& and v–, cannot propagate, and so this frequency interval is known as a stop gap. By providing periodic texture in two dimensions, SP propagation in all in-plane directions can be blocked, leading to the full bandgap for SPs. At the band edges the density of SP states is high, and there is a significant increase in the associated field enhancement. Box 3 Surface plasmon bandgaps ++ ++ ++ –– –– ++ ++ –– –– a ω_ ω_ ω+ ω+ a ω b π/a kx ++ There are three characteristic length scales that are important for SP-based photonics in addition to that of the associated light. The propagation length of the SP mode, dSP, is usually dictated by loss in the metal. For a relatively absorbing metal such as aluminium the propagation length 2 mm at a wavelength of 500 nm. For a low loss metal, for example, silver, at the same wavelength it is increased to 20 mm. By moving to a slightly longer wavelength, such as 1.55 mm, the propagation length is further increases towards 1 mm. The propagation length sets the upper size limit for any photonic circuit based on SPs. The decay length in the dielectric material, dd, is typically of the order of half the wavelength of light involved and dictates the maximum height of any individual features, and thus components, that might be used to control SPs. The ratio of dSP:dd thus gives one measure of the number of SP-based components that may be integrated together. The decay length in the metal, dm, determines the minimum feature size that can be used; as shown in the diagram, this is between one and two orders of magnitude smaller than the wavelength involved, thus highlighting the need for good control of fabrication at the nanometre scale. The combinations chosen give an indication of range from poor (Al at 0.5 mm) to good (Ag at 1.5 mm) SP performance. Box 2 Surface plasmon length scales Aluminium at 0.5 µm Silver at 1.5 µm 10 nm 100 nm 1 µm 10 µm 100 µm 1 mm δm δd λ δsp © 2003 Nature PublishingGroup