Fabrication of plasmonic structures with well-controlled nanometric features: a comparison between lift-off and ion beam etching.

After providing a detailed overview of nanofabrication techniques for plasmonics, we discuss in detail two different approaches for the fabrication of metallic nanostructures based on e-beam lithography. The first approach relies on a negative e-beam resist, followed by ion beam milling, while the second uses a positive e-beam resist and lift-off. Overall, ion beam etching provides smaller and more regular features including tiny gaps between sub-parts, that can be controlled down to about 10 nm. In the lift-off process, the metal atoms are deposited within the resist mask and can diffuse on the substrate, giving rise to the formation of nanoclusters that render the nanostructure outline slightly fuzzy. Scattering cross sections computed for both approaches highlight some spectral differences, which are especially visible for structures that support complex resonances, such as Fano resonances. Both techniques can produce useful nanostructures and the results reported therein should guide the researcher to choose the best suited approach for a given application, depending on the available technology.

Origin of enhancement in Raman scattering from Ag-dressed carbon-nanotube antennas: experiment and modelling.

The D- and G-band Raman signals from random arrays of vertically aligned, multi-walled carbon nanotubes are significantly enhanced (up to ∼14×) while the signal from the underlying Si substrate is simultaneously attenuated (up to ∼6×) when the nanotubes are dressed, either capped or coated, with Ag. These Ag-induced counter-changes originate with the difference in geometry of the nanotubes and planar Si substrate and contrast in the Ag depositions on the substrate (essentially thin film) and the nanotube (nano-particulate). The surface integral equation technique is used to perform detailed modelling of the electromagnetic response of the system in a computationally efficient manner. Within the modelling the overall antenna response of the Ag-dressed nanotubes is shown to underpin the main contribution to enhancement of the nanotube Raman signal with hot-spots between the Ag nanoparticles making a subsidiary contribution on account of their relatively weak penetration into the nanotube walls. Although additional hot-spot activity likely accounts for a shortfall in modelling relative to experiment it is nonetheless the case that the significant antenna-driven enhancement stands in marked contrast to the hot-spot dominated enhancement of the Raman spectra from molecules adsorbed on the same Ag-dressed structures. The Ag-dressing procedure for amplifying the nanotube Raman output not only allows for ready characterisation of individual nanotubes, but also evidences a small peak at ∼1150 cm-1 (not visible for the bare, undressed nanotube) which is suggested to be due to the presence of trans-polyacetylene in the structures.

Fano-resonant aluminum and gold nanostructures created with a tunable, up-scalable process.

An up-scalable approach for creating Fano-resonant nanostructures on large surfaces at visible wavelengths is demonstrated. The use of processes suitable for high throughput fabrication and the choice of aluminum as a cost-efficient plasmonic material ensure that the presented insights are valuable even in consideration of typical industrial constraints. In particular, wafer-scale fabrication and the process compatibility with roll-to-roll embossing are demonstrated. It is shown that through adjustment of readily accessible evaporation parameters, the shape and position of the optical resonance can be tuned within a spectral band of more than 70 nm. The experimental data are complemented with rigorous coupled wave analysis and surface integral equation simulations. Calculated electric fields as well as surface charges shed light onto the physics behind the present resonances. In particular, a surface plasmon polariton is found to couple to a localized plasmonic mode with a hexapolar charge distribution, leading to a Fano-like resonance. Further understanding of the interactions at hand is gained by considering both aluminum and gold nanostructures.

Optical forces in nanoplasmonic systems: how do they work, what can they be useful for?

In this article, we share our vision for a future nanofactory, where plasmonic trapping is used to control the different manufacturing steps associated with the transformation of initial nanostructures to produce complex compounds. All the different functions existing in a traditional factory can be translated at the nanoscale using the optical forces produced by plasmonic nanostructures. A detailed knowledge of optical forces in plasmonic nanostructures is however essential to design such a nanofactory. To this end, we review the numerical techniques for computing optical forces on nanostructures immersed in a strong optical field and show under which conditions approximate solutions, like the dipole approximation, can be used in a satisfactory manner. Internal optical forces on realistic plasmonic antennas are investigated and the reconfiguration of a Fano-resonant plasmonic system using such internal forces is also studied in detail.

Broadband wide-angle dispersion measurements: instrumental setup, alignment, and pitfalls.

The construction, alignment, and performance of a setup for broadband wide-angle dispersion measurements, with emphasis on surface plasmon resonance (SPR) measurements, are presented in comprehensive detail. In contrast with most SPR instruments working with a monochromatic source, this setup takes advantage of a broadband∕white light source and has full capability for automated angle vs. wavelength dispersion measurements for any arbitrary nanostructure array. A cylindrical prism is used rather than a triangular one in order to mitigate refraction induced effects and allow for such measurements. Although seemingly simple, this instrument requires use of many non-trivial methods in order to achieve proper alignment over all angles of incidence. Here we describe the alignment procedure for such a setup, the pitfalls introduced from the finite beam width incident onto the cylindrical prism, and deviations in the reflected∕transmitted beam resulting from the finite thickness of the sample substrate. We address every one of these issues and provide experimental evidences on the success of this instrument and the alignment procedure used.

Controllable coherent perfect absorption in a composite film.

We exploit the versatility provided by metal-dielectric composites to demonstrate controllable coherent perfect absorption (CPA) or anti-lasing in a slab of heterogeneous medium. The slab is illuminated by coherent light from both sides, at the same angle of incidence and the conditions required for CPA are investigated as a function of the different system parameters. Our calculations clearly elucidate the role of absorption as a necessary prerequisite for CPA. We further demonstrate the controllability of the CPA frequency to the extent of having the same at two distinct frequencies even in presence of dispersion, rendering the realization of anti-lasers more flexible.

Combined antenna and localized plasmon resonance in Raman scattering from random arrays of silver-coated, vertically aligned multiwalled carbon nanotubes.

The electric field enhancement associated with detailed structure within novel optical antenna nanostructures is modeled using the surface integral equation technique in the context of surface-enhanced Raman scattering (SERS). The antennae comprise random arrays of vertically aligned, multiwalled carbon nanotubes dressed with highly granular Ag. Different types of "hot-spot" underpinning the SERS are identified, but contrasting characteristics are revealed. Those at the outer edges of the Ag grains are antenna driven with field enhancement amplified in antenna antinodes while intergrain hotspots are largely independent of antenna activity. Hot-spots between the tops of antennae leaning towards each other also appear to benefit from antenna amplification.

Electric and magnetic resonances in arrays of coupled gold nanoparticle in-tandem pairs.

We present an experimental and theoretical study on the optical properties of arrays of gold nanoparticle in-tandem pairs (nanosandwiches). The well-ordered Au pairs with diameters down to 35 nm and separation distances down to 10 nm were fabricated using extreme ultraviolet (EUV) interference lithography. The strong near-field coupling of the nanoparticles leads to electric and magnetic resonances, which can be well reproduced by Finite-Difference Time-Domain (FDTD) calculations. The influence of the structural parameters, such as nanoparticle diameter and separation distance, on the hybridized modes is investigated. The energy and lifetimes of these modes are studied, providing valuable physical insight for the design of novel plasmonic structures and metamaterials.

Narrow-band multiresonant plasmon nanostructure for the coherent control of light: an optical analog of the xylophone.

We demonstrate that it is possible to combine several small metallic particles in a very compact geometry without loss of their individual modal properties by adding a gold metallic film underneath. This film essentially acts as a "ground plane" which channels the optical field of each particle and decreases the interparticle coupling. The localization of the electric field can then be controlled temporally by illuminating the chain with a chirped pulse. The sign of the chirp controls the excitation sequence of the particles with great flexibility.

Characterization of the polarization sensitivity anisotropy of a near-field probe using phase measurements.

Amplitude and phase measurements of the near-field generated by isolated subwavelength apertures in a gold film are presented. The near-field distribution of such a structure is complex and the measured signal strongly depends on the electric field components effectively detected by the experimental setup. By comparing this signal with 3D vectorial calculations we are able to determine which electric field components are effectively measured. The sensitivity of the phase distribution is key to this measurement. The proposed characterization technique should prove extremely useful to calibrate a Scanning near-field optical microscopy (SNOM) beforehand in order to retrieve quantitative information on the polarization of the field distribution under study.

Near-field-induced tunability of surface plasmon polaritons in composite metallic nanostructures.

We numerically study near-field-induced coupling effects in metal nanowire-based composite nanostructures. Our multi-layer system is composed of individual gold nanowires supporting localized particle plasmons at optical wavelengths, and a spatially separated homogeneous silver slab supporting delocalized surface plasmons. We show that the localized plasmon modes of the composite structure, forming so-called magnetic atoms, can be controlled over a large spectral range by changing the thickness of the nearby metal slab. The optical response of single-wire and array-based metallic structures are compared. Spectral shifts due to wire-mirror interaction as well as the coupling between localized and delocalized surface plasmon modes in a magnetic photonic crystal are demonstrated. The presented effects are important for the optimization of metal-based nanodevices and may lead to the realization of metamaterials with novel plasmonic functionalities.

Resonant optical antennas.

We have fabricated nanometer-scale gold dipole antennas designed to be resonant at optical frequencies. On resonance, strong field enhancement in the antenna feed gap leads to white-light supercontinuum generation. The antenna length at resonance is considerably shorter than one-half the wavelength of the incident light. This is in contradiction to classical antenna theory but in qualitative accordance with computer simulations that take into account the finite metallic conductivity at optical frequencies. Because optical antennas link propagating radiation and confined/enhanced optical fields, they should find applications in optical characterization, manipulation of nanostructures, and optical information processing.

Influence of metal roughness on the near-field generated by an aperture/apertureless probe.

We study the influence of metal roughness on the near-field distribution generated by an aperture or an apertureless (scattering) probe. Different experimental parameters are investigated: roughness magnitude, aperture form, distribution of the roughness. Our results show that aluminium roughness has a dramatic impact on the emission characteristics of a near-field probe and in particular on its polarization sensitivity. Apertureless or scattering probes appear to be less sensitive to roughness and to provide a well confined field even with a somewhat rough probe.