Alasdair W. Clark

Multiple plasmon resonances from gold nanostructures

Understanding and controlling plasmon resonances from metallic nanoscale structures have been the focus of much attention recently, with applications including local surface plasmon resonance sensing, surface enhanced Raman spectroscopy, and negative refractive index materials. In this letter the authors demonstrate the fabrication of uniform arrays of split rings from gold and show that such structures are capable of supporting multiple plasmon resonances. The authors show that up to five plasmon resonances can be identified and use finite difference time domain modeling and absorption spectroscopy to fully characterize and identify each resonance. The implications of higher order surface plasmon resonances for sensing are discussed.

Tuneable visible resonances in crescent shaped nano-split-ring resonators

Electron beam lithography was used to fabricate gold crescent shaped split-ring resonators with 30 nm minimum feature size. By varying the crescent’s arc length over a range of nanometer-scale dimensions the authors demonstrate the tuneability of visible resonances within such structures. Results, which correlate closely with those predicted using finite-difference time-domain modeling, open the way for these devices to be used in near-field biological sensing.

Nanogap Ring Antennae as Plasmonically Coupled SERRS Substrates

The fabrication, optical characterization, and application of a new generation of ultrasmall multiple-split nanoring antennae is reported. Using electron-beam lithography, splits of ≈6 nm are engineered into silver nanophotonic ring structures to create concentrated areas of localized field coupling, which can be exploited for enhanced plasmonic applications. The plasmonic properties of three devices, containing 3, 4, and 5 splits, which have been spectrally tuned to 532 nm, are compared. Using finite-element analysis, the distinct plasmonic characteristics of each structure are explored and a description is given of how variations in the surface charge distribution affect intersegmental coupling at different polarization angles. The impact these changes have on the sensory functionality of each device was determined by a competitive DNA-hybridization assay measured using surface-enhanced resonance Raman spectroscopy. The geometry of these novel, circular, multiple-split rings leads to unique plasmon hybridization between the numerous segments of a single structure. This phenomenon is demonstrated to be applicable to extreme Raman sensitivity and may also find use in metamaterial applications.

Optical Properties of Multiple‐Split Nanophotonic Ring Antennae

Simulated plasmonic activity is reported for a series of Au nanoring antennae with 1–5 splits in their geometry. The rings have a radius of 75 nm and splits of 3–6 nm. By controlling the size of the splits, their location and the radius of the ring structure, we can control the number, intensity and frequency of the plasmonic hotspots.

Nanophotonic split-ring resonators as dichroics for molecular spectroscopy

We describe the design and fabrication of nanostructured silver split-ring resonators and demonstrate their application as dichroic sensors for multipurpose visible wavelength molecular spectroscopy. By producing arrays of these identical nanostructures, with critical dimensions of 30–150nm, we demonstrate the controllable generation of plasmonic resonances at two common visible laser wavelengths, produced as a consequence of the polarization of the light with respect to structural geometry. We show the application of these devices in surface enhanced Raman spectroscopy, using their dichroic properties to perform molecular sensing of a self-assembled monolayer of 2-mercaptopyridine at both 532 and 633nm.

Sequence-selective detection of double-stranded DNA sequences using pyrrole-imidazole polyamide microarrays

We describe a microarray format that can detect double-stranded DNA sequences with a high degree of sequence selectivity. Cyclooctyne-derivatized pyrrole-imidazole polyamides were immobilized on azide-modified glass substrates using microcontact printing and a strain-promoted azide-alkyne cycloaddition (SPAAC) reaction. These polyamide-immobilized substrates selectively detected a seven-base-pair binding site incorporated within a double-stranded oligodeoxyribonucleotide sequence even in the presence of an excess of a sequence with a single-base-pair mismatch.

Fabrication and tuning of nanoscale metallic ring and split-ring arrays

Metallic structures with dimensions smaller than the wavelength of light demonstrate optical properties which depend strongly on the nanoparticle size, shape, and interparticle spacing. The optical properties are caused by the excitation of localized surface plasmon resonances that lead to strong enhancement and confinement of the optical field and can be exploited for many applications including surface-enhanced Raman spectroscopy, near-field scanning optical microscopy, and negative refractive index materials. In order to fully exploit the properties of these structures, both a highly reproducible and flexible fabrication technique and an in-depth understanding of the optical properties are needed. In this article, the authors demonstrate the fabrication of arrays of gold rings and split rings on glass using electron beam lithography. Electron beam lithography allows not only precise control of the size, shape, and spacing of the arrays but also the scope to design novel shapes at will. We characterize these arrays using polarization dependent spectroscopy. The structures can support multiple plasmon resonances, demonstrating that excellent uniformity across the array is achieved. These resonances are further characterized using a finite difference time domain method to model the electric field distribution around the ring structures.

Engineering DNA binding sites to assemble and tune plasmonic nanostructures

Single DNA-nanoparticle binding is used to couple the plasmonic fields of nanophotonic bowtie dimers. The binding event is engineered to tune the resonance peak of the bowtie to correspond with a 633 nm laser. Surface enhanced Raman spectroscopy is performed on an individual bowtie, showing that a single DNA-nanoparticle binding event can be recorded.

Annular nanoplasmonic void arrays as tunable surface enhanced Raman spectroscopy substrates

We report the use of annular nano-voids in a metallic thin-film as programmable molecular sensors for surface-enhanced Raman spectroscopy (SERS). To date, research into these structures has focused on the exploration of their extraordinary optical transmission attributes. We now show that by using advanced lithography and simulation tools, we can generate a porous SERS material for molecular interrogation. Using ultra-thin annular structures, rather than simple circular holes, allows us to reduce both the volume and cross-sectional area of the void, maximizing the electric-field confinement, while, importantly for SERS, producing resonant conditions in the visible region of the spectrum. By comparing our annular films with conventional circular films with the same resonant frequency, we show a significant improvement in the efficiency of Raman scatter, creating stronger signals that also contain more spectral information.

Bridging the gap: rewritable electronics using real-time light-induced dielectrophoresis on lithium niobate

In the context of micro-electronics, the real-time manipulation and placement of components using optics alone promises a route towards increasingly dynamic systems, where the geometry and function of the device is not fixed at the point of fabrication. Here, we demonstrate physically reconfigurable circuitry through light-induced dielectrophoresis on lithium niobate. Using virtual electrodes, patterned by light, to trap, move, and chain individual micro-solder-beads in real-time via dielectrophoresis, we demonstrate rewritable electrical contacts which can make electrical connections between surface-bound components. The completed micro-solder-bead bridges were found to have relatively low resistances that were not solely dominated by the number of interfaces, or the number of discrete beads, in the connection. Significantly, these connections are formed without any melting/fusing of the beads, a key feature of this technique that enables reconfigurability. Requiring only a low-power (~3.5 mW) laser source to activate, and without the need for external power supply or signal generation, the all-optical simplicity of virtual-electrodes may prove significant for the future development of reconfigurable electronic systems.