P. Apell

Photoluminescence of noble metals

We have developed an explicit model to explain the radiative recombination in noble metals, arising from transitions between electrons in the spconduction band and holes in the d-band generated by optical excitation. We find that the observed photon distribution has its shape from two competing factors. The first is due to the optics and the final density of states for the exiting photons. This is off-set by a d-band density of states factor increasing as the number of available d-states increase. We give a satisfactory account of the observed spectrum, using constant matrix elements, and find that luminescence can be used as a complementary tool to the ordinary elastic light scattering, giving detailed information about occupied and unoccupied states, provided the elastic optical constants are measured on the same sample.

Maximized Optical Absorption in Ultrathin Films and Its Application to Plasmon-Based Two-Dimensional Photovoltaics

For ultrathin films of a given material, light absorption is proportional to the film thickness. However, if the optical constants of the film are chosen in an optimal way, light absorption can be high even for extremely thin films and optical path length. We derive the optimal conditions and show how the maximized absorptance depends on film thickness. It is then shown that the optimal situation can be emulated by tuning of the geometric parameters in feasible nanocomposites combining plasmonic materials with semiconductors. Useful design criteria and estimates for the spatial absorption-distribution over the composite materials are provided. On the basis of efficient exchange of oscillator strength between the plasmonic and semiconductor constituents, a high quantum yield for semiconductor absorption can be achieved. The results are far-reaching with particularly promising opportunities for plasmonic solar cells.

A Simple Derivation of the Surface Contribution to the Reflectivity of a Metal, and its Use in the Van der Waals Interaction

The reflection coefficients for light, polarized parallel and perpendicular to the plane of incidence, incident on a translationally invariant jellium surface are derived. The derivation is based on a form of pillbox integration of the two Maxwell equations which normally give the continuity of the parallel field components. In this manner we derive the corrections to the usual Fresnel result due to a varying electron density profile in the surface region. To illustrate the usefulness of these reflection coefficients explicit examples are given for the Van der Waals interaction between an adatom and a solid. The local field at the adatom, the lifetime of an excited adatom and the general form for the Van der Waals interaction incorporating the effects of non-locality in the solid.

Red shift of surface plasmons in small metal particles

Abstract It is shown that the experimentally found red shift of the surface plasmon resonance in a small metal particle is a direct measure of the first moment of the induced surface charge.

A General Non-Local Theory for the Electromagnetic Response of a Small Metal Particle

We derive, in the quasistatic limit, general formulas for the electro magnetic response of a small metal sphere. The jellium model is assumed but we show how corrections, to the classical situation due to the non-local response with a smoothly varying surface profile can be included in a simple manner. These corrections enter as two lengths, which are measures of the non-locality range. As an example we study the interaction between a dipole and the metal sphere, in the context of image field enhancement and damping because of electron-hole pair excitations in the particle.

Theory for Collective Resonances of the C60 Molecule

We discuss the collective resonances of the C60 molecule described by a spherical shell. Detailed results are given for the π and π + σ plasmons of C60, the polarisability and the dielectric function. We present results in good quantitative agreement with recent experiments. Some new features in the spectrum, like a monopole mode of oscillation, are predicted for a doped C60 molecule.

Interparticle coupling effects in surface-enhanced Raman scattering

We report experimental and theoretical results on the effect of electromagnetic coupling between metal particles in surface-enhanced Raman scattering (SERS). Model calculations of the near-field optical properties of Ag and Au nanoparticle-aggregates show that the electromagnetic surface-enhancement factor can reach 11 orders-of-magnitude in gaps between nearly touching particles. Single particles exhibit a much weaker enhancement, unless the particles contain extremely sharp surface protrusions. Data on spectral fluctuations in single-molecule SERS and measurements on the efficiency of nanofabricated SERS substrates give experimental support for the idea that an efficient interparticle coupling is a necessary requirement for an ultra-high surface-enhancement. We suggest a route for biorecognition induced coupling of metal particles for use in biosensing applications.

Surface plasmon de-excitation of multiply charged ions

Electrons in excited states of a multiply charged ion have traditionally been viewed as decaying through Auger processes. Competing with this is photon emission and decay through excitation of the surface plasmon of a solid. The latter is very effective in the energy range 10-30 eV. Utilising part of the available neutralisation energy it leads to a reduction in the number of ejected Auger electrons. For larger energies the photon emission dominates all the other processes.

Non-local optical effects at metal surfaces

The non-local response of an irradiated metal surface is discussed in terms of the induced density in response to an external field. The field and the induced charge near the surface can be determined by solving Maxwell equations in the surface region and match the solution to the incoming and reflected wave in vacuum and to the asymptotic solution in the bulk of the solid. For incoming fields of wave lengths long in comparison with the extension of the surface region a key quantity is the center of gravity of the screening charge. This complex function of the frequency is important for a number of properties such as the reflectivity, the surface plasmon dispersion, image fields, van der Waals interactions between a molecule and a metal surface, etc. Considerations similar to the ones for a planar surface are also applied to the surface properties of a small metallic sphere.

Density-functional account of van der Waals forces between parallel surfaces

The applicability of density-functional theory is extended to the area of van der Waals interactions between macroscopic bodies, in particular between two parallel surfaces. It is shown how the strength and the asymptotic form, including the van der Waals planes for the surfaces can be calculated with the electronic densities and the static image planes of the interacting objects as the only input. The calculation is carried out easily in a simple density-functional scheme, suggesting the possibility of extensions to the description of forces between more generally shaped macroscopic bodies, including the sample and tip in scanning-force microscopy.