David F. P. Pile

Channel plasmon–polariton in a triangular groove on a metal surface

One-dimensional localized plasmons (channel polaritons) guided by a triangular groove on a metal substrate are investigated numerically by means of a finite-difference time-domain algorithm. Dispersion, existence conditions, and dissipation of these waves are analyzed. In particular, it is demonstrated that the localization of the predicted plasmons in acute grooves may be substantially stronger than what is allowed by the diffraction limit. As a result, the predicted waves may be significant for the development of new subwavelength waveguides and interconnectors for nano-optics and photonics.

Two-dimensionally localized modes of a nanoscale gap plasmon waveguide

We report numerical analysis and experimental observation of two dimensionally localized plasmonic modes guided by a nanogap in a thin metal film. Dispersion, dissipation, and field structure of these modes are analyzed using the finite-difference time-domain algorithm. The experimental observation is conducted by the end-fire excitation of the proposed gap plasmon waveguides and detection of the generated modes using their edge scattering and charge coupled device camera imaging. Physical interpretation of the obtained results is presented and origins of the described modes are discussed.

Theoretical and experimental investigation of strongly localized plasmons on triangular metal wedges for subwavelength waveguiding

We report numerical analysis and experimental observation of strongly localized plasmons guided by a triangular metal wedge. Dispersion and dissipation of such wedge plasmons are analyzed using the finite-difference time-domain algorithm. Experimental observation is conducted by the end-fire excitation and near-field detection of the predicted plasmons on a 40° silver nanowedge. Good agreement with the theoretically predicted propagation distances is demonstrated. Differences between the theoretical and experimental field distribution are explained by insufficient resolution of the near-field optical probe.

Confinement and propagation characteristics of subwavelength plasmonic modes

We have studied subwavelength confinement of the surface plasmon polariton modes of various plasmonic waveguides and examined their relative merits using a graphical parametric representation of their confinement and propagation characteristics. While the same plasmonic phenomenon governs mode confinement in all these waveguides, the various architectures can exhibit distinctive behavior in terms of effective mode area and propagation distance. We found that the waveguides based on metal and one dielectric material show a similar trade-off between energy confinement and propagation distance. However, a hybrid plasmon waveguide, incorporating metal, low index and high index dielectric materials, exhibits longer propagation distances for the same degree of confinement. We also point out that plasmonic waveguides with sharp features can provide an extremely strong local field enhancement, which is not necessarily accompanied by strong confinement of the total electromagnetic energy. In these waveguides, a mode may couple strongly to nearby atoms, but suffer relatively low propagation losses due to weak confinement.

Single-mode subwavelength waveguide with channel plasmon-polaritons in triangular grooves on a metal surface

We demonstrate that single-mode operation of a subwavelength plasmonic waveguide in the form of a V-groove on a metal surface can be achieved by adjusting the depth of the groove. Strongly localized channel plasmon-polaritons (CPPs) are shown to propagate in such waveguides. If the groove depth is close to the penetration depth of the fundamental CPP mode, then all higher modes are not supported by the structure, leaving only the fundamental mode propagating in the groove. In this case, propagation distances of fundamental mode ∼10μm can easily be achieved together with strong subwavelength localization.

Plasmonic subwavelength waveguides: next to zero losses at sharp bends.

We demonstrate that approximately 100% transmission of a strongly localized channel plasmon polariton can be achieved through a sharp 90 degrees bend in a subwavelength waveguide in the form of a triangular groove on a metal surface--a feature that has previously been demonstrated only for photonic crystal waveguides, which do not provide subwavelength localization. Conditions for minimum reflection and radiative losses at the bend are investigated numerically by the finite-difference time-domain algorithm. Dissipation in the structure is demonstrated to be sufficiently low to ensure significant propagation distances (a number of wavelengths) of the localized plasmon in each of the arms of the bend.

Adiabatic and nonadiabatic nanofocusing of plasmons by tapered gap plasmon waveguides

Adiabatic and nonadiabatic nanofocusing of plasmons in tapered gap plasmon waveguides is analyzed using the finite-difference time-domain algorithm. Optimal adaptors between two different subwavelength waveguides and conditions for maximal local field enhancement are determined, investigated, and explained on the basis of dissipative and reflective losses in the taper. Nanofocusing of plasmons into a gap of ∼1nm width with more than 20 times increase in the plasmon energy density is demonstrated in a silver-vacuum taper of ∼1μm long. Comparison with the approximate theory based on the geometrical optics approximation is conducted.

Compressing surface plasmons for nano-scale optical focusing

A major challenge in optics is how to deliver and concentrate light from the micron-scale into the nano-scale. Light can not be guided, by conventional mechanisms, with optical beam sizes significantly smaller than its wavelength due to the diffraction limit. On the other hand, focusing of light into very small volumes beyond the diffraction limit can be achieved by exploiting the wavelength scalability of surface plasmon polaritons. By slowing down an optical wave and shrinking its wavelength during its propagation, optical energy can be compressed and concentrated down to nanometer scale, namely, nanofocusing. Here, we experimentally demonstrate and quantitatively measure the nanofocusing of surface plasmon polaritons in tapered metallic V-grooves down to the deep subwavelength scale - approximately lambda/40 at wavelength of 1.5 micron - with almost 50% power efficiency.

Controlling the phase and amplitude of plasmon sources at a subwavelength scale

We present a new class of nanoscale plasmonic sources based on subwavelength dielectric cavities embedded in a metal slab. Exploiting the strong dispersion near the Fabry-Perot resonance in such a resonator, we control the phase and the amplitude of the generated plasmons at the subwavelength scale. As an example, we present a subwavelength unidirectional plasmonic antenna utilizing interference between two plasmonic cavity sources with matched phase and amplitude.

Adiabatic nanofocusing of plasmons by a sharp metal wedge on a dielectric substrate

We demonstrate that efficient adiabatic nanofocusing of plasmons can be achieved using a sharp metal wedge (thin tapered film) on a dielectric substrate. It is shown that the quasisymmetric (with respect to the charge distribution across the wedge) plasmon mode can experience infinite adiabatic slowing down with both its phase and group velocities reducing to zero as the plasmon propagates towards the tip of the wedge. Conditions for strong local field enhancement near the tip are determined and analyzed. In particular, it is demonstrated that the electric field in the plasmon experiences much stronger local enhancement than the magnetic field. Two distinct asymptotic regimes with the electric field amplitude approaching either zero or infinity at the tip of the wedge (tapered film) are described. The results are compared to adiabatic nanofocusing of plasmons by metallic V grooves and sharp metal wedges in a uniform dielectric.