Nathan Ahmad

Analysis and design of a cross dipole nanoantenna for fluorescence-sensing applications

This paper introduces the novel concept of a cross dipole nanoantenna for use in fluorescence based sensing applications. The dual-arm nature of the cross nanoantenna allows a dual resonant structure to be designed such that the shorter arm resonates with the pump wavelength and the longer arm with the emission wavelength. This is expected to further enhance emission from any fluorescent molecule that can couple to both nanoantenna arms when compared with a singly resonant structure. The paper uses the finite-difference time-domain method to first analyze the two-arm nanoantenna case and then shows how intensity enhancement depends on the antenna geometry and tapering of arms in the antenna gap. The results show that smaller gap sizes always produce larger enhancement compared with lightning rod effects due to tapering. A four-arm cross nanoantenna is then studied, highlighting differences from the two-arm case. Finally, the effect of a diagonally aligned molecule transiting the central gap region is studied. The results show that two hotspots occur on either side of the central gap region when the molecule is aligned perpendicular to the transit direction and only a single central hotspot occurs when the alignment is parallel to the transit direction.

Solar absorbers using 1D and 2D periodic nanostructured nickel films

This paper presents results obtained from finite difference time domain and transfer matrix method modelling of nickel thin films. It is shown that by adding a periodic 1D grating to the surface of the nickel its optical absorption over the 300 nm–1000 nm spectral range can be dramatically increased. 2D gratings show excellent polarization independence making them ideally suited to high temperature solar thermal applications.

Deep-groove nickel gratings for solar thermal absorbers

This paper presents measured and modelled optical absorptance and reflectance for deep-groove nickel nano-gratings in the 450–950 nm wavelength range. The structures have been fabricated using focused ion beam etching and characterised using Fourier spectroscopy and the field distributions on the gratings have been studied using finite difference time domain modelling. Realistic grating structures have been modelled based on focused ion beam cross sections and these results are in good agreement between measured and modelled results. The roles of surface plasmon polaritons and slot modes are highlighted in the strong broadband absorbance that can be achieved with these structures.

Purcell enhancement and focusing effects in plasmonic nanoantenna arrays

This paper presents measured fluorescence results for PMMA dye-coated 5×5 gold plasmonic nanoantenna arrays. We use numerical electromagnetic modeling to show how array size and element spacing can be used to control emitted beam shape and compare this with experimental data. The Friis formula from RF antenna theory is used to calculate the intensity enhancement produced by the array. A figure of merit is then developed, which accounts for the very small mode volume from which the array emission is occurring.