Emission quenching is analysed at nanometer distances from the surface of an absorbing nanoparticle. It is demonstrated that emission quenching at small distances to the surface is much weaker for magnetic-dipole (MD) than for electric-dipole (ED) transitions. This difference is explained by the fact that the electric field induced by a magnetic dipole has a weaker distance dependence than the electric field of an electric dipole. It is also demonstrated that in the extreme near-field regime the non-locality of the optical response of the metal results in additional emission quenching for both ED and MD transitions.
Upconversion (UC) of sub-band-gap photons can increase solar cell efficiencies. Up to now, the achieved efficiencies are
too low, to make UC relevant for photovoltaics. Therefore, additional means of increasing UC efficiency are necessary.
In this paper, we investigate both metal and dielectric photonic nanostructures for this purpose. The theoretical analysis
is based on a rate equation model that describes the UC dynamics in β-NaYF4 : 20% Er3+. The model considers ground
state and excited state absorption, spontaneous and stimulated emission, energy transfer, and multi phonon relaxation.
For one, this model is coupled with results of Mie theory and exact electrodynamic theory calculations of plasmon
resonance in gold nanoparticles. The effects of a 200 nm gold nanoparticle on the local field density and on the transition
rates within in the upconverter are considered. Calculations are performed in high resolution for a three dimensional
simulation volume. Furthermore, the effect of changed local fields in the proximity of grating waveguide dielectric
nanostructure is investigated. For this purpose FDTD simulation models of such structures are coupled with the rate
equation model of the upconverter. The results suggest that both metal nanoparticles and dielectric nanostructures can
increase UC efficiency.
Investigations of optical losses induced by localized plasmons in protrusions on silver back contacts of thin-film silicon
solar cells are presented. The interaction of electromagnetic waves with nanoprotrusions on flat silver layers is simulated
with a three-dimensional numerical solver of Maxwell's equations. Spatial absorption profiles and spatial electric field
profiles as well as the absorption inside the protrusions are calculated. The results presented here show that the
absorption of irradiated light at nanorough silver layers can be strongly enhanced by localized plasmonic resonances in
Ag nanoprotrusions. Especially, localized plasmons in protrusions with a radius below 60 nm induce strong absorption,
which can be several times the energy irradiated on the protrusion's cross section. The localized plasmonic resonances in
single protrusions on Ag layers are observed to shift to longer wavelengths with increasing refractive index of the
surrounding material. At wavelengths above 500 nm localized plasmonic resonances will increase the absorption of
nanorough μc-Si:H/Ag interfaces. The localized plasmon induced absorption at nanorough ZnO/Ag interfaces lies at
shorter wavelengths due to the lower refractive index of ZnO. For wavelengths above 500 nm, a high reflectivity of the
silver back contacts is essential for the light-trapping of thin-film silicon solar cells. Localized-plasmon induced losses at
silver back contacts can explain the experimentally observed increase of the solar cell performance when applying a
ZnO/Ag back contact in comparison to a μc-Si:H/Ag back contact.
In conventional silicon solar cells, photons with energies lower than the silicon band gap (1.12 eV) are not absorbed in
the silicon layer. However, the near-infrared portion of the solar spectrum may still be able to contribute to photocurrent
generation if use can be made of up-conversion processes that transform two or more infrared photons into a photon of
sufficient energy to be absorbed in silicon. One possible material in which up-conversion processes occur are rare-earth
ions such as Er3+. It has recently been shown that up-conversion in such ions could be enhanced by optical near-field
coupling to metal nanoparticles in a highly controlled geometry. However, potential photovoltaic applications of the upconversion
enhancement will certainly be characterized by different geometric arrangements, with random distances
between ions and nanoparticles. Whether or not an overall enhancement of the up-conversion efficiency may be expected
under such realistic conditions is an open question. In this work, we address an important aspect of this question, namely
the particle-induced enhancement of the optical excitation rate in the rare-earth ions. Our model calculations show that
the excitation rate in Er3+ ions can be enhanced using spherical gold nanoparticles. The model includes random distances
between ions and nanoparticles, as well as random polarizations of the exciting light. The enhancement of the rate of
excitation of the fundamental transition results in increases of the up-conversion rate by up to 20% for an excitation
wavelength of 1523 nm, provided that photoluminescence-quenching effects due to nonradiative relaxation in the metal
can be neglected.