The Magnetic Manipulation of Surface Plasmons --- Consideration of Possible Technologies

Abstract

An as yet unexploited mechanism for producing controlled shifts in the frequency of lightwaves via their temporary conversion to surface plasmons propagating on a ferromagnetic surface or under the action of applied magnetic flelds is introduced. Indirect evidence of the observation of this phenomena is presented and the technological possibilities it might ofier are explored and discussed. Interfaces between dielectric and metallic media possessing negative permittivity () can support electromagnetic waves propagating as longitudinal density oscillations in the free-electron plasma at the metallic surface. Like photons, the quanta of these collective excitations remain bosons and are known as surface plasmons. Moreover, under conditions satisfying the relevant conservation laws, photons and plasmons are mutually transformable. Photons incident on a metallic surface may for example be temporarily induced to convert to plasmons that propagate on the surface for a while before their subsequent recovery as photons emitted back into the incident medium. The most exciting and complete conflrmation of the nature of this process is evidenced in the work of Altewischer etal. (1) which demonstrated that a photon, having followed a path that includes its conversion to a plasmon and back to a photon, retains entanglement with the twin with which it was originally created in a down-conversion process. The objective here is to draw attention to the potential to exploit this two way conversion process to operate on photons in ways not previously considered. More than 30 years ago Chiu and Quinn (2) and Nakamura and Paranjape (3) flrst described theoretically how propagating plasmons undergo frequency shifts when subject to in∞uence by ap- propriately orientated dc magnetic flelds (H) applied orthogonal to the plasmon ∞ux. In more recent times Smolyaninov etal. (4) have interpreted the equations of Chiu and Quinn (2) and Naka- mura and Paranjape (3) as describing a second-order mixing process between the ac plasmon fleld (Ep) and any applied dc magnetic fleld (H). If ´ (2) is a generalised susceptibility then such mixing processes generate terms of the form ´ (2) E 2 p H in the plasmon fleld energy density and the break- ing of inversion symmetry at the interfaces requisite for the very existence of surface plasmons determines that terms such as ´ (2) E 2 p will always be present. The plasmon energy and hence frequency consequently acquire a contribution linear in the applied fleld H which is real and in light of the results of Altewischer etal. (1) transposable to a photon emission fleld when the plasmon is intercepted by a grating out-coupler. We have obtained strong indirect evidence of the reality of this position by studying plasmon propagation on a nickel surface on which a linear grating with a period of 1.13m and depth of 0.7m is modulated, as shown schematically in Figure 1, by a shallower structure with an order of magnitude greater periodicity of about 12.5m (5). When this structured surface is illuminated at an angle of incidence (µi) of exactly 19.85 - by optical radiation with a wavelength of 800nm and conservation of wavevector is satisfled by the addition of that associated with the 1.13m structural periodicity to that of the incident photons, a deep absorption trough is observed as shown on the left of Figure 1 indicating the resonant gen- eration of a ∞ux of forward propagating surface plasmons. Because of the flnite spectral width of the incident optical beam, its residual divergence and the inherent error in the periodicity of the 1.13m coupling structure this ∞ux consists of plasmons with a narrow spectrum of wavevectors rather than a single identical wavevector and the population of this spectrum is primarily deter- mined by the Gaussian intensity proflle of the incident beam. The width of the plasmon spectrum at full width half maximum ¢Ksp(FWHM) may be estimated by examination and analysis of the form of the re∞ectivity trough in Figure 1 to be of the order of 0:35£10 i3 nm i1 . This plasmon ∞ux subsequently interacts in a much weaker fashion with the longer and shallower periodic structure which re-couples it back to an emissive optical fleld emerging from the surface at an angle of 77 -