Research Article
The Effect of Oxidation on the Far-Field
Scattering of Aluminium Patch Antennae from
Visible to UV
Duncan McArthur and Francesco Papoff
Department of Physics, SUPA, University of Strathclyde, 107 Rottenrow, Glasgow G4 0NG, UK
Correspondence should be addressed to Francesco Papof; f.papof@strath.ac.uk
Received 31 January 2019; Accepted 9 June 2019; Published 26 June 2019
Academic Editor: Giancarlo C. Righini
Copyright © 2019 Duncan McArthur and Francesco Papof. Tis is an open access article distributed under the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.
In this paper we evaluate the efect of oxidation on the total power scattered in the far feld by a 60nm radius Al sphere in the
presence of a substrate comprised of either Al or silica (SiO
2
). Using an efective medium approach to model the Al particle with an
outer layer of alumina (Al
2
O
3
), we fnd that the UV peak of total energy scattered in the far feld shifs towards longer wavelengths
for volume fractions of Al
2
O
3
up to 20%. When particles with these volume fractions are held above an Al substrate, enhancement
of two orders of magnitudes of the far-feld power radiated by a dipole in the gap can be observed. For larger volume fractions of
Al
2
O
3
, the total intensity of light scattered is signifcantly reduced.
1. Introduction
In recent years a large number of papers in nanophotonics
have been investigating the properties of aluminum because
it is an abundant and low cost metal with plasmon modes
in the visible and the ultraviolet [1–8]. Following earlier
experiments [2], we have found [9, 10] that aluminum
nanoparticles and substrates are ideally suited for label-free
detection of weakly fuorescent molecules in the ultraviolet,
because they have resonances with a much stronger far-feld
radiative enhancement than similar nanostructures of gold
or silver, for wavelengths shorter than 370 nm. In particular,
Al nanostructures can signifcantly enhance the detection of
fuorescence by emitters for which the emission rate is much
less than the internal nonradiative decay rate [11] when the
emitter is strongly coupled to electromagnetic modes that
efciently transport energy into the far feld. By changing the
size of the nanoparticle, resonances can be tuned between
150 nm and 650 nm and enhance, by orders of magnitude,
both the far-feld radiation and the decay rates of dipolar
emitters placed in the gap between the nanoparticle and the
substrate. Tis could have a profound impact on sensing of
many important molecules that have radiative decays in the
ultraviolet much weaker than nonradiative decays. Examples
of such molecules are alkanes [12], most amino acids [13] in
proteins and peptides, and DNA bases.
2. Methods
In order to evaluate the feasibility of these applications,
it is important to consider in what way oxidation of the
Al surfaces may afect the performance of this system. In
this paper we investigate how the ratio between the power
radiated above the substrate,
, and the total power emitted
by the dipole in vacuum,
0
, depends on the volume fraction
of alumina (Al
2
O
3
) in the presence of a substrate of either
silica (SiO
2
) or Al. We consider substrates with thickness such
that the light refected by the lower surface is negligible, so
that the substrates can be modeled as semi-infnite. Te top
layer of the Al substrate can also be oxidized, but for sake
of simplicity we ignore here this efect, which should not
change qualitatively the results presented when the oxidation
layers are thin and do not support guided modes. We consider
the following two confgurations. Te frst confguration is
similar to the standard set-up of a scanning near feld optical
microscope [14]: the particle lies on top of the substrate and
Hindawi
International Journal of Optics
Volume 2019, Article ID 9687803, 5 pages
https://doi.org/10.1155/2019/9687803