Table of contents

We present an overview of 'aluminium plasmonics', i.e. the study of both fundamental and practical aspects of surface plasmon excitations in aluminium structures, in particular thin films and metal nanoparticles. After a brief introduction noting both some recent and historical contributions to aluminium plasmonics, we discuss the optical properties of aluminium and aluminium nanostructures and highlight a few selected studies in a host of areas ranging from fluorescence to data storage.

Metallic nanostructures are the building blocks for nanoplasmonics and for subsequent applications in nanooptics. For several decades, plasmonics have been almost exclusively studied in the visible region by using nanostructures made of noble metals exhibiting plasmonic properties in the near infrared to visible range. This notwithstanding, emerging applications will require the extension of nanoplasmonics toward higher energies, particularly in the UV range. Therefore, alternative metals, often described as poor metals are emerging to achieve that goal. Among all these metals, aluminium appears to be one of the most appealing for extending plasmonics towards ultraviolet energies. Aluminium is cheap, widely available, compatible with optoelectronic devices and exhibits plasmonic properties over a wide range of energies, from the infrared to the deep UV. Our aim is to present a review of current research centred on the fabrication of aluminium nanostructures. Mastering the geometry of aluminium nanostructures is extremely important in order to tune their plasmonic properties and target a given application. First we give an introduction to the nanofabrication of aluminium nanostructures within the context of plasmonics. The review then focuses on the possible geometries that such structures may take when fabricated with specific fabrication techniques. Each technique is detailed and the plasmonic properties of the aluminium nanostructures are briefly described. When possible, an example of an application is given. Finally, the future applications of aluminium plasmonics are highlighted and a conclusion with perspectives is given.

We investigated the plasmonic response of arrays of Al nanowires fabricated in high-vacuum and embedded within a transparent protective medium. The nanostructures exhibited a strongly-birefringent plasmonic response which, depending on the mutual orientation of the incident-field polarization and the nanowire axis, allowed the plasmon resonance to span the whole spectral range from the visible to the deep-ultraviolet regime. Comparing the experimental data with theoretical calculations allowed to rationalize the optical response in terms of non-ideal nanowire morphologies arising from the bottom-up character of the nanofabrication method. The broadband plasmonic response suggests the potential application of these systems in plasmon-enhanced photovoltaics, exploiting the great advantage of the low-cost of aluminium.

We explore localized surface plasmon resonances in small (5–30 nm radius) aluminum and silver nanoparticles using classical electrodynamics simulations, focusing on radiative (far-field scattering) effects and the unique characteristics of aluminum as a plasmonic material. In Al spheres, higher-order plasmon resonances (e.g. quadrupoles) are significant at smaller sizes (>15 nm) than in Ag spheres. Additionally, although the plasmon width is minimized at a radius of about 15 nm for both materials, the Al plasmon linewidth (~1.4 eV) for the dipole mode is much larger than that observed in Ag (~0.3 eV). The radiative contribution to damping dominates over non-radiative effects for small (5–20 nm) Al spheres (>95%) whereas for similar size Ag spheres damping is almost entirely attributed to the bulk dielectric function (non-radiative). For Al nanorods the linewidths can be narrowed by increasing aspect ratio such that for an aspect ratio of 4.5, the overall Al (0.75 eV) linewidth is reasonably close to that of the same size Ag rod (0.35 eV). This narrowing arises from frequency dispersion in the real part of the Al dielectric function, and is associated with a 65% (1.5 to 0.5 eV) decrease in the radiative contribution to the linewidth for Al. Concurrently, an increase in the non-radiative width occurs as the aspect ratio increases and the plasmon tunes to the red. This demonstrates that anisotropy can be used as a parameter for controlling Al plasmon dephasing where the composition of the plasmon linewidth (radiative or non-radiative) can be tailored with aspect ratio. Overall, these data suggest that localized surface plasmon resonance dephasing mechanisms in Al nanostructures are inherently different from those in the noble metals, which could allow for new applications of plasmonic materials, tunable plasmon lifetimes, and new physics to be observed.

We report photoelectron emission enhancement of aluminum thin films by surface plasmon excitation in the deep-ultraviolet region. Deep-ultraviolet light with a wavelength of 266 nm has enough energy to cause electron emission from aluminum and excite surface plasmons on aluminum. We applied the Kretschmann configuration to excite surface plasmons. The enhancement factor of the emission current is found to depend on the enhanced electric field intensity excited by surface plasmons. The maximum emission efficiency of photoelectrons is 2.9 nA mW⁻¹ for an aluminum thickness of 19 nm with an alumina thickness of 4 nm. The characteristics of the dependence of the photoelectron emission efficiency on the applied bias between the anode and cathode are investigated.

Aluminum nanoparticles fabricated by oblique angle deposition (OAD) successfully increased the yield and reaction rate of UV photocatalysis due to the localized surface plasmon resonance (LSPR) effect. Nanoparticles 20–60 nm in size were formed in an area larger than ~1 cm² when the film was highly tilted during the thermal deposition process. The size and density of these nanoparticles were readily controlled by the deposition thickness and speed. The yield of photocatalytic reactions increased by a factor of ~2, while the reaction rate increased by up to ~10 times. The aluminum nanostructures presented here are of tremendous advantage for future applications in photocatalysis through efficient coupling with UV light.

Aluminum and magnesium have a strong plasmonic response in the ultraviolet part of the spectrum. Due to the differences in their dielectric responses, however, there are qualitative differences in the fluorescence enhancement factors for Al and Mg nanoapertures. In order to make our results applicable to experimental studies, we further investigate fluorescence enhancement in these nanoapertures using UV emitters that excite near 270 nm and emit near 340 nm with both high and low quantum yields (QY). Our results suggest that Al nanoapertures may perform better for molecules with low QY, while Mg may perform better for molecules with high QY. We further consider the effects of bulk and surface oxidation of the two metals. In order to improve fluorescence enhancement, avoiding bulk oxidation during film deposition is important, although bulk oxidation could be used as an optimization parameter for some enhancement mechanisms.

Using the finite difference time domain method, this paper investigates electromagnetic nanofocusing of ultraviolet light incident upon Al V-groove structures. A parametric study of the field enhancement around the V-groove tip area has been conducted through the change of groove depth, width, and the tip angle. A short-wavelength threshold of about 180 nm, lying well above the plasma wavelength of Al, has been identified, below which the electric field enhancement at the tip rapidly decreases. Through simplification of the V-groove model into a metal-dielectric-metal waveguide with gap width corresponding to depth-dependent V-groove width, modal solutions were obtained for the complex propagation constant, from which the adiabatic parameter and mode propagation length were determined. These results suggest that the decrease in propagation length with decreasing wavelength is the underlying cause of the short-wavelength threshold.

We demonstrate that the inclusion of aluminum (Al) seed layers allows for the growth of finer grain, smoother magnesium (Mg) films, allowing for improved plasmonic response in the UV spectral range. We deposit Al seed layers and Mg films using DC/RF magnetron sputtering. For Al seed layer thicknesses of 2, 5, 10 and 15 nm, we measure the corresponding optical constants over the wavelength range of 250 to 600 nm using a variable angle spectroscopic ellipsometer and compare the results to a reference 100 nm thick Mg film deposited without a seed layer. The optical constants of Mg depend on the seed layer thicknesses and the surface morphology of Mg films. The surface morphology of the Mg films are characterized using scanning electron microscopy, atomic force microscopy and x-ray diffraction and the surface oxide layer on Mg is examined using x-ray photoelectron spectroscopy. We observe that the presence of the seed layer improves the LSP figure of merit in the UV spectral range of 250–400 nm. From the perspective of plasmonics applications, we find that the best localized surface plasmon resonance (LSP) figures of merit are observed for the 10 nm thick Al seed layer. In fact, the LSP figure of merit of the 100 nm Mg film with and without the Al seed layer is found to be greater than Al films in the spectral range of 250–400 nm, confirming earlier findings with thicker Mg films.