reported in resonantly excited metallic nanoparticle waveguides.
tures guide light via the refractive index difference between the core and cladding. However, unlike dielectric slot waveguides, both plasmonic and conventional waveguiding modes can be accessed, depending on trans- verse core dimensions. MIM waveguides may thus allow optical waveguide mode volumes to be reduced to subwavelength scales, even for frequencies far from the plasmon resonance. Several theoretical studies have already investigated surface plasmon propagation and confinement in MIM structures [36, 147]. How- ever, few studies have investigated wavelength-dependent MIM properties arising from realistic models for the complex dielectric function of metals. In this chapter, we discuss the surface plasmon and conventional waveguiding modes of MIM structures, characterizing the metal by the empirical optical constants of John- son and Christy [58] and numerically determining the dispersion, propagation, and localization for both field symmetric and antisymmetric modes.
When a plasmon is excited at a metallodielectric interface, electrons in the metal create a surface polar- ization that gives rise to a localized electric field. In insulator-metal-insulator (IMI) structures, electrons of the metallic core screen the charge configuration at each interface and maintain a near-zero (or minimal) field within the waveguide. As a result, the surface polarizations on either side of the metal film remain in phase and a cutoff frequency is not observed for any transverse waveguide dimension. In contrast, screening does not occur within the dielectric core of MIM waveguides. At each metal-dielectric interface, surface polariza- tions arise and evolve independently of the other interface, and plasma oscillations need not be energy- or wavevector-matched to each other.
Therefore, for certain MIM dielectric core thicknesses, interface SPs may not remain in phase but will exhibit a beating frequency; as transverse core dimensions are increased, “bands” of allowed energies or wave vectors and “gaps” of forbidden energies will be observed. This behavior is illustrated in Figure 8.1, which plots the TM dispersion relations for an MIM waveguide with core thicknesses of 250 nm (Figure 8.1(a)) and 100 nm (Figure 8.1(b)). The waveguide consists of a three-layer metallodielectric stack with an SiO2core and an Ag cladding. The metal is defined by the empirical optical constants of Johnson and Christy [58] and the dielectric constant for the oxide is adopted from Palik’s handbook [75].
Solution of the dispersion relations was achieved via application of a Nelder-Mead minimization routine in complex wave-vector space; details of implementation and convergence properties are described elsewhere
ventional waveguiding modes are found only at higher ener- gies共over a range of⬃1 eV兲, where photon wavelengths are small enough to be guided by the structure. The inset shows snapshots of the tangential electric field for both modes at a free-space wavelength = 410 nm 共⬃3 eV兲. As seen, thesb field is concentrated in the waveguide core with minimal
field is highly localized at the surface, with field penetration approximately symmetric on each side of the metal-dielectric interface. The presence of both conventional and SP waveguiding modes represents a transition to subwavelength-scale photonics. Provided momentum can be matched between the photon and the SP, and energy will be guided in a polariton mode along the metal-dielectric inter- face. Otherwise, the structure will support a conventional waveguide mode, but propagation will only occur over a narrow frequency band.
As MIM core thickness is reduced below 100 nm, the structure can no longer serve as a conventional waveguide.
Light impinging the structure will diffract and decay evanes- cently, unless it is coupled into a SP mode. Figure 3 illus- trates the TM dispersion of such subwavelength structures as core thickness is varied from 50 nm down to 12 nm. As in Fig. 2, the dispersion curve in the limit of infinite core thick- ness is also included 共black curve兲.
Figure 3共a兲plots the TM dispersion relation for the sym- metric electric-field bound modes with d= 50, 35, 20, and 12 nm. Insets plot the tangential electric field profiles for d
= 50 nm and d= 12 nm at free-space wavelengths of
= 1.55m 共⬃0.8 eV兲 and = 350 nm 共⬃3.5 eV兲, respec- tively. Functionally, the dispersion behaves like the thin-film sb modes of the IMI guide, with larger wave vectors achieved at lower energies for thinner films. As free-space energies approach SP resonance, the wave vector reaches its maximum value before cycling through the higher energy
“quasibound” modes共not shown; see Ref. 4兲. A 12-nm-thick oxide can reach wave vectors as high as kx= 0.2 nm−1 共sp
= 31 nm兲, comparable to the resonant wave vectors observed in 12 nm Ag IMI waveguides. However, unlike IMI struc- tures, the dispersion curve does not lie entirely below the single-interface共thick-film兲limit. Over a finite energy band- width, SP momentum exceeds photon momentum both in SiO2 and in vacuum. The 50-nm-thick oxide provides the most striking example of this behavior: dispersion lies com- pletely to the left of the decoupled SP mode. In addition, the low-energy asymptotic behavior follows a light line corre- sponding to a refractive index ofn= 0.15. This low effective index suggests that polariton modes of MIM structures more highly sample the imaginary component of the metal dielec- tric function than the core dielectric function. In the low- energy limit, the sb SP truly represents a photon trapped on the metal surface.
Figure 3共b兲plots the TM dispersion relation for the anti- symmetric electric-field bound modes, again withd= 50, 35, 20, and 12 nm. The tangential electric field profile共see inset, plotted for d= 50 nm at = 1.55m兲 confirms the purely plasmonic nature of the mode. In contrast with IMI struc- tures, whereabdispersion approaches the light line for thin- ner films, wave vectors of the MIM structure achieve larger values at lower energies. For energies well below SP reso- nance共sp兲, the observed behavior is not unlike thesbmodes of IMI guides. However, at higher energies, bound mode dispersion does not always terminate at sp. In fact, cutoff frequencies occur at least 0.25 eV below SP resonance for core thicknesses of less than 20 nm. And, while the maxi- mum wave vector always exceeds the decoupled SP reso-
艋 FIG. 2. 共Color online兲Transverse magnetic dispersion relations
and tangential electric field 共Ex兲 profiles for MIM planar waveguides with a SiO2core and a Ag cladding. Dispersion of an infinitely thick core is plotted in black and is in exact agreement with results for a single Ag/ SiO2interface plasmon.共a兲For oxide thicknesses of 250 nm, the structure supports conventional waveguiding modes with cutoff wave vectors observed for both the symmetric共sb, dark gray兲 and antisymmetric共ab, light gray兲 field configurations. 共b兲As oxide thickness is reduced to 100 nm, both conventional and plasmon waveguiding modes are supported. Ac- cordingly, tangential electric fields共Ex兲are localized within the core for conventional modes but propagate along the metal-dielectric interface for plasmon modes 关inset, plotted in 共a兲 at free-space wavelengths of = 410 nm共⬃3 eV兲 共top two panels兲, = 650 nm 共⬃1.9 eV兲, and= 1.7m 共⬃0.73 eV兲, and in共b兲 at = 410 nm 共⬃3 eV兲兴.
DIONNEet al. PHYSICAL REVIEW B73, 035407共2006兲
Figure 8.1: Dispersion relations for MIM planar waveguides with an SiO2core and Ag cladding
96
[31]. For reference, the figures include the waveguide TM dispersion curve in the limit of infinite core thickness, plotted in black. Allowed wave vectors are seen to exist for all freespace wavelengths (energies) and exhibit exact agreement with the dispersion relation for a single Ag/SiO2interface SP.
Figure 8.1(a) plots the “bound” modes (i.e., modes occurring at frequencies below the SP resonance) of a Ag/SiO2/Ag waveguide with core thicknessd=250 nm. The asymmetric bound (ab) modes are plotted in light gray; the symmetric bound (sb) modes are plotted in dark gray. As seen, multiple bands of allowed and forbidden frequencies are observed. The allowedabmodes follow the light line for energies below∼1 eV and resemble conventional dielectric core, conducting cladding waveguide modes for energies above∼2.8 eV. Tangential electric fields in eachabregime are plotted in the inset and highlight the distinction in mode localization: At 410 nm, the mode is localized within the waveguide core and resembles a conventional TM waveguide profile. At 1.7µm, dispersion is more plasmonlike and field maxima of the mode occur at each metal-dielectric interface. In contrast, thesbmodes are only observed for energies between 1.5 and 3.2 eV.
Dispersion for this mode is reminiscent of conventional dielectric core, dielectric cladding waveguides; with end-point asymptotes corresponding to tangent slopes (i.e., effective optical indices) ofn=8.33 at 1.5 eV andn=4.29 at 3.2 eV. For energies exceeding∼2.8 eV, wave vectors of thesbmode are matched with those of the SP and the tangential electric field transits from a core mode to an interface mode (cf. the 1st (∼3 eV) and 3rd (∼1.9 eV) panels of the inset). As the core layer thickness is increased through 1µm (data not shown), the number ofabandsbbands increases, with theabmodes generally lying at higher energies. In analogy with conventional waveguides, larger (but bounded) core dimensions increase the number of modes supported by the structure.
Figure 8.1(b) plots the bound mode dispersion curves for an MIM waveguide with SiO2core thickness d=100 nm. Again, the allowed abmodes are plotted in light gray while the allowed sb modes are dark gray. Although thesb mode resembles conventional waveguide dispersion, theabmode is seen to exhibit plasmonlike behavior. Accordingly, the conventional waveguiding modes are found only at higher energies (over a range of about 1 eV), where photon wavelengths are small enough to be guided by the structure.
The inset shows snapshots of the tangential electric field for both modes at a free-space wavelengthλ=410 nm (∼3 eV). As seen, the sb field is concentrated in the waveguide core with minimal penetration into
sentially unchanged. An explanation is provided by the Goos-Hanchen effect. As a signal propagates along the guide, it undergoes total internal reflection with slight phase shifts induced by field penetration into and out of the metal cladding. For thinner films, waveguide dimensions are com- parable to the field skin depth共兲in the metal. Moreover, the abExfield distribution exhibits a node at the waveguide me-
the metal surface. As waveguide dimensions are decreased, this enhanced field in the metal magnifies Goos-Hanchen contributions significantly. In the limit ofd艋, complete SP dephasing could result.
III. MODE PROPAGATION AND SKIN DEPTH