We use finite-difference time-domain calculations to show that aluminum nanoparticles are efficient substrates for metal-enhanced fluorescence (MEF) in the ultraviolet (UV) for the label-free detection of biomolecules. The radiated power enhancement of the fluorophores in proximity to aluminum nanoparticles is strongly dependent on the nanoparticle size, fluorophore-nanoparticle spacing, and fluorophore orientation. Additionally, the enhancement is dramatically increased when the fluorophore is between two aluminum nanoparticles of a dimer. Finally, we present experimental evidence that functionalized forms of amino acids tryptophan and tyrosine exhibit MEF when spincoated onto aluminum nanostructures.Fluorescence is widely used in biology and medicine. However, low radiative emission rates limit the use of the intrinsic fluorescence of biomolecules, resulting in the need for external chemical labeling. The use of external labels requires chemical modification and additional steps which can perturb the functionality of ligand-receptor interactions. In many cases, selective fluorescence labeling of a small number of molecules in a tiny volume like a single cell is cumbersome, adding expense and complexity to the analysis. [1][2][3][4][5] Because of the problems noted above, there is interest in label-free detection methods [6][7][8] including surface plasmon resonance 9,10 and Raman scattering. [11][12][13] Direct measurement of native fluorescence by proteins is also being pursued. [14][15][16][17][18] Proteins exhibit intrinsic absorption maxima in the ultraviolet (UV) around 280 nm. 19 We 20-23 and others [24][25][26] have been investigating metallic nanostructures for improved fluorescence detection. Metallic structures can substantially modify spontaneous emission rates and the directionality of the emission, leading to metal-enhanced fluorescence (MEF). In the case of a flat metal film on a glass substrate, emission from a fluorophore near the film can excite surface plasmon polaritons on the film, which radiate back into the glass in a highly directional manner, a process termed surface plasmon-coupled emission (SPCE).* To whom correspondence should be addressed. E-mail: lakowicz@cfs.umbi.umd.edu. † University of Maryland School of Medicine. ‡ Argonne National Laboratory. § Lumerical Solutions Inc. SUPPORTING INFORMATION AVAILABLEFurther details are given as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. At present MEF and SPCE are obtained with mostly silver structures, 20,22,23,[27][28][29] with occasional use of gold, 30,31 and with relatively little attention being given to other metals such as aluminum. However, aluminum has low absorption at wavelengths ≤400 nm, and a recent article demonstrated that aluminum nanodisks have distinct plasmon resonances that extend into the UV. 32 Aluminum nanostructures have also been used for surface enhanced Raman spectroscopy (SERS). 33 These facts suggest that aluminum can potentially be used as a subs...

We use the finite-difference time-domain method to predict how fluorescence is modified if the fluorophore is located between two silver nanoparticles of a dimer system. The fluorophore is modeled as a radiating point dipole with orientation defined by its polarization. When a fluorophore is oriented perpendicular to the metal surface, there is a large increase in total power radiated through a closed surface containing the dimer system, in comparison to the isolated fluorophore and the case of a fluorophore near a single nanoparticle. The increase in radiated power indicates increases in the relative radiative decay rates of the emission near the nanoparticles. The angle-resolved far-field distributions of the emission in a single plane are also computed. This is informative as many experimental conditions involve collection optics and detectors that collect the emission along a single plane. For fluorophores oriented perpendicular to the metal surfaces, the dimer systems lead to significant enhancements in the fluorescence emission intensity in the plane. In contrast, significant emission quenching occurs if the fluorophores are oriented parallel to the metal surfaces. We also examine the effect of the fluorophore on the near-field around the nanoparticles and correlate our results with surface plasmon excitations.

This work develops an enhanced Monte Carlo (MC) simulation methodology to predict the impacts of layout-dependent correlated manufacturing variations on the performance of photonics integrated circuits (PICs). First, to enable such performance prediction, we demonstrate a simple method with sub-nanometer accuracy to characterize photonics manufacturing variations, where the width and height for a fabricated waveguide can be extracted from the spectral response of a racetrack resonator. By measuring the spectral responses for a large number of identical resonators spread over a wafer, statistical results for the variations of waveguide width and height can be obtained. Second, we develop models for the layout-dependent enhanced MC simulation. Our models use netlist extraction to transfer physical layouts into circuit simulators. Spatially correlated physical variations across the PICs are simulated on a discrete grid and are mapped to each circuit component, so that the performance for each component can be updated according to its obtained variations, and therefore, circuit simulations take the correlated variations between components into account. The simulation flow and theoretical models for our layout-dependent enhanced MC simulation are detailed in this paper. As examples, several ring-resonator filter circuits are studied using the developed enhanced MC simulation, and statistical results from the simulations can predict both common-mode and differential-mode variations of the circuit performance.

We report the spatial maps of the localized surface plasmon resonances associated photon emission in a truncated tetrahedral gold nanoparticle on a silicon substrate. Site-specific cathodoluminescence spectroscopy and imaging in a scanning electron microscope shows stronger photon emission in the visible range near the tips of the particle in contact with the substrate compared to the edges of the particle. Strong local field variations on a length scale as short as 19 nm are resolved. We also perform FDTD simulations of both the spectra and, for the first time, the full cathodoluminescence images. Excellent agreement is obtained with the experimental results, and the detailed information available from the simulated results makes it possible to identify the signature of out-of-plane higher order modes in the truncated tetrahedral gold particle.

We study the nature of fluorescence scattering by a radiating fluorophore placed near a metal nanoparticle with the finite-difference time-domain method. Angle-resolved light-scattering distributions are contrasted with those that result when ordinary plane waves are scattered by the nanoparticle. For certain sized nanoparticles and fluorophore dipoles oriented parallel to the metal surface, we find that the highest scattered fluorescence emission is directed back toward the fluorophore, which is very different from plane-wave scattering. The largest enhancements of far-field radiation are found when the dipole is oriented normal to the surface. We also examined the effect of the fluorophore on the near field around the particle. The fields can be enhanced or quenched compared to the isolated fluorophore and exhibit strong dependence on fluorophore orientation, as well as interesting spatial variations around the nanoparticle.