insight review articles Surface plasmon basics SPs at the interface between a metal and d=ck dielectric material ave a combined ectromagnetic wave and surface charg character as shown in a.They are in character(H is in the y direction), and the generation ot quires an electric Id normal to the surface. This combined character also leads to the field component perpendicular to the surface being enhanced near the surface and decaying exponentially with distance away from it(b). The field in this perpendicular direction is said to be evanescent, reflecting the ound,non-radiative nature of SPs, and prevents power from propagating away from the surface. In the dielectric medium above the metal, typically air or glass, the decay length of the field, &g is of the order of half the wavelength of light involved, whereas the decay length into the etal. 8 is determined by the skin depth. C, The dispersion curve for a SP mode shows the momentum mismatch problem that must vercome in order to couple light and SP modes together, with the SP mode always lying beyond the light line, that is, it has greater momentum (ksp) than a free space photon(k)of the same frequency w experience many periods of the textured surface and thus display the cles224. The frequency and width of these modes are determined by PBG phenomenon. We also note with interest that recent develop- the particle's shape, material, size and environment.2,and for this offer the prospect of easily producing SP PBG substrates to act as pho- strates for SERSand potentially as aerials for fluorophores".The tonicsubstrates on which todefine SP photoniccircuits interaction between two ormore nanoparticles can lead tostillfurther At frequencies within a bandgap, the density ofSP modes iszero- levels of fieldenhancement2-, withevenmore dramaticeffects asso- no SP modes can be supported. However, at the band edges, the SP ciated with hot spotsinrandom" mode dispersion is flat and the associated density ofSP modes is high, corresponding to a high field enhancement close to the metal surface. Mapping surface plasmons and developing components Further, the nature of this flat band means that such modes can be The properties of SP devices are intimately linked to the activity and the excited by light that is incident over a wide range of angles, making distribution of SPs on the metal surface. Much is still not known about them good candidates for frequency-selective surfaces. Flat bands are the relationship between surface topology and the nature of the SP also associated with the localized SP modes of metallic nanoparti- modes, and so a more detailed study of the details of this SP activity is vital. Because of the way SPs are confined to the surface and because of Figure 2 SP photonic bandgap. The the subwavelength nature of the structures and fields involved, one SP dispersion curve shown in Box 1 cannot rely on traditional far-field techniques. Instead near-field tech- as directly imaged using a modified niquessuch as photon scanning tunnelling microsco oy(PSTM)ar prism coupling technique. a, The typically employed to map the fields on the metal surface, for example dispersion curve(here shown as those of the SP waveguide in Fig. 1. A PSTM is basically a collection inverse wavelength versus angle) for mode scanning near-field optical microscope where thesample liesona a flat surface is shown in the upper glass prism, which enables one to shine light in total internal reflection. picture here dark regions correspond The nanometre size tip, mostly obtained by pulling an optical fibr to coupling of incident light to the SP which may eventually be coated with metal, frustrates the total reflec mode and the colours are produced tionwhen scanning close to the surface and thereby maps the near-field onaphotographic fim by the intensities wavelength of the light used. b, If the Irprisingly, as we enhance the capabilities of near-field techniques metal surface is textured with a two- further to map the SP fields into the subwavelengthregime we come up dimensional pattern of bumps on an against an interesting variant of Heisenbergs uncertainty principle appropriate length scale (oughly half plied to the optical field, this principle says that we can only measure the wavelength of light)as shown in theelectric(E or themagneticfield(H) withaccuracywhen the volume this SEM, a bandgap is introduced 8f in which they are contained is significantlysmaller than the wave- into the dispersion curve of the length of light in all three spatial dimensions. More precisely associated SP modes. Bar, 0.7 um. Heisenberg s uncertainty principle binds Eand Hof the optical wave to C, The bandgap is clearly seen in the 8lthrough the cyclicpermutation of theirvector components(i, j lower picture where there is a spectral egion in which no SP mode (as △E△H indicated by the dark regions) exists so note the distortion of the SP As volumes smaller than the wavelength are probed, measurements mode and the edges of the bandgap ofopticalenergy become uncertain, highlighting the wither forming measurementsinthis regime e2003NaturePublishingGroupNatUrevOl424114AuGusT2003www.nature.com/natureexperience many periods of the textured surface and thus display the PBG phenomenon. We also note with interest that recent develop￾ments in the fabrication of periodic nanostructures via self-assembly offer the prospect of easily producing SP PBG substrates to act as pho￾tonic substrates on which to define SP photonic circuits. At frequencies within a bandgap, the density of SP modes is zero— no SP modes can be supported. However, at the band edges, the SP mode dispersion is flat and the associated density of SP modes is high, corresponding to a high field enhancement close to the metal surface. Further, the nature of this flat band means that such modes can be excited by light that is incident over a wide range of angles, making them good candidates for frequency-selective surfaces. Flat bands are also associated with the localized SP modes of metallic nanoparti￾cles23,24. The frequency and width of these modes are determined by the particle’s shape, material, size and environment23,25,26, and for this reason they are being pursued as tags for biosensing27,28 and as sub￾strates for SERS29 and potentially as aerials for fluorophores30,31. The interaction between two or more nanoparticles can lead to still further levels of field enhancement32–34, with even more dramatic effects asso￾ciated with hot spots in random structures35. Mapping surface plasmons and developing components The properties of SP devices are intimately linked to the activity and the distribution of SPs on the metal surface. Much is still not known about the relationship between surface topology and the nature of the SP modes, and so a more detailed study of the details of this SP activity is vital. Because of the way SPs are confined to the surface and because of the subwavelength nature of the structures and fields involved, one cannot rely on traditional far-field techniques. Instead near-field tech￾niques3,36 such as photon scanning tunnelling microscopy (PSTM) are typically employed to map the fields on the metal surface, for example, those of the SP waveguide in Fig. 1. A PSTM is basically a collection mode scanning near-field optical microscope where the sample lies on a glass prism, which enables one to shine light in total internal reflection. The nanometre size tip, mostly obtained by pulling an optical fibre, which may eventually be coated with a metal, frustrates the total reflec￾tion when scanning close to the surface and thereby maps the near-field intensities. Surprisingly, as we enhance the capabilities of near-field techniques further to map the SP fields into the subwavelength regime we come up against an interesting variant of Heisenberg’s uncertainty principle. Applied to the optical field, this principle says that we can only measure the electric (E) or the magnetic field (H) with accuracy when the volume dl 3 in which they are contained is significantly smaller than the wave￾length of light in all three spatial dimensions. More precisely, Heisenberg’s uncertainty principle binds E and Hof the optical wave to dlthrough the cyclic permutation of their vector components (i,j), DEi DHj $ùc 2 /2dl 4 . (3) As volumes smaller than the wavelength are probed, measurements of optical energy become uncertain, highlighting the difficulty with per￾forming measurements in this regime. insight review articles 826 NATURE | VOL 424 | 14 AUGUST 2003 | www.nature.com/nature SPs at the interface between a metal and a dielectric material have a combined electromagnetic wave and surface charge character as shown in a. They are transverse magnetic in character (H is in the y direction), and the generation of surface charge requires an electric field normal to the surface. This combined character also leads to the field component perpendicular to the surface being enhanced near the surface and decaying exponentially with distance away from it (b). The field in this perpendicular direction is said to be evanescent, reflecting the bound, non-radiative nature of SPs, and prevents power from propagating away from the surface. In the dielectric medium above the metal, typically air or glass, the decay length of the field, dd, is of the order of half the wavelength of light involved, whereas the decay length into the metal, dm, is determined by the skin depth. c, The dispersion curve for a SP mode shows the momentum mismatch problem that must be overcome in order to couple light and SP modes together, with the SP mode always lying beyond the light line, that is, it has greater momentum (ùkSP) than a free space photon (ùk0) of the same frequency v. Box 1 Surface plasmon basics +++ ––– ––– +++ a c E Hy × δd δm z Ez b ω=ck ω ω ko kx kSP z Metal Dielectric a b c Figure 2 SP photonic bandgap. The SP dispersion curve shown in Box 1 was directly imaged using a modified prism coupling technique. a, The dispersion curve (here shown as inverse wavelength versus angle) for a flat surface is shown in the upper picture; here dark regions correspond to coupling of incident light to the SP mode, and the colours are produced on a photographic film by the wavelength of the light used. b, If the metal surface is textured with a two￾dimensional pattern of bumps on an appropriate length scale (roughly half the wavelength of light) as shown in this SEM, a bandgap is introduced into the dispersion curve of the associated SP modes. Bar, 0.7 mm. c, The bandgap is clearly seen in the lower picture where there is a spectral region in which no SP mode (as indicated by the dark regions) exists. Also note the distortion of the SP mode and the edges of the bandgap. © 2003 Nature PublishingGroup