No abstract

Metal cladded dielectric cylinders with submicron diameters may serve to model coated tips used in near-field scanning optical microscopy. The signal measured may be greatly influenced by resonance effects due to eigenmodes of the probe. Especially, using a photon scanning tunneling microscope setup, gold coated tips have been found to detect a signal proportional to the magnetic field distributions [E.Devaux et al., Phys. Rev. B 62, 10504 (2000)]. This effect is attributed to cylindrical surface plasmons. We present here fully retarded calculations of the dispersion and field patterns of the non-radiative plasmon modes in cylindrical geometry. We study the effect of varying the cylinder radius on the surface plasmon dispersion, thus justifying that the cylinder is a useful model for near field probes in spite of their slightly conical shape.

We investigate the optical properties of periodically modulated thin metal films in the Kretschmann-Raether configuration. We find that excitation of surface plasmons via the grating coupler effect at the metal surface not facing the incoming light is not possible in the case of a conformal modulation of the two metal surfaces, but only if the thickness of the metal film is varied over the grating period. In this case there is strong interaction between the surface plasmons on both surfaces and photonic band gaps open up in the dispersion. Whereas in reflection spectra the surface plasmons are seen as minima, they can appear as maxima, minima, or Fano-type resonances in transmission spectra. We show how thin film grating couplers can be tailored to achieve a field strength enhancement at a particular interface or pronounced photonic band gaps. ͓S0163-1829͑99͒00931-5͔

We present a qualitative model for current transport in the superconducting state through a series of two quantum-point contacts with a mesoscopic island between them. A Green's functions technique is merged with a rate-equation method in order to account for phase as well as charging effects. Multiple Andreev reflections are included in a nonperturbative manner and therefore our Ansatz despite some underlying assumptions is in principle not restricted to the low or high transmission regime. We find that in our system, multiple Andreev reflection is not totally suppressed by Coulomb blockade, but that the step pattern of current-voltage characteristics is determined by an interplay of multiple thresholds.

We construct a model describing current transport through a superconducting or normal conducting circuit consisting of two point contacts in series by extending a Green's functions technique. In between the contacts is a mesoscopically large and bulklike island. The model can, in principle, handle contacts in all transmission regimes. Coherent interaction throughout the whole system is included in the form of multiple and multiple Andreev reflections extending over both contacts while accounting for charging effects by a changing electrostatic potential of the island. Our calculations show that even though the onsets of certain current contributions are independent of the island charging energy, Coulomb blockade persists, especially in the normal state. Coulomb staircases can still be visible but get smeared out for particular ratios of the charging energy and the gap in the superconducting state. However, as a general trend, we find that including coherent coupling across the island does not very significantly change the shape of the current-voltage curves compared to the incoherent results.