Excitation and Reemission of Molecules near Realistic Plasmonic Nanostructures

Abstract

The enhancement of excitation and reemission of molecules in close proximity to plasmonic nanostructures is studied with special focus on the comparison between idealized and realistically shaped nanostructures. Numerical experiments show that for certain applications choosing a realistic geometry closely resembling the actual nanostructure is imperative, an idealized simulation geometry yielding significantly different results. Finally, a link between excitation and reemission processes is formed via the theory of optical reciprocity, allowing a transparent view of the electromagnetic processes involved in plasmon-enhanced fluorescence and Raman-scattering.KEYWORDS: Surface integral equation, realistic nanoparticle, nanoantenna, near-field, SERS, fluorescence enhancement I n the past decade, one important application of plasmonic nanostructures has been the exploitation of their localized field enhancement for amplifying fluorescence and Raman scattering.1-5 On the experimental side, astonishing single-molecule sensitivity has been demonstrated 6-13 while large numbers of numerical investigations have been performed studying the effect of size and shape on the optical response, and thus the tunability of such structures. [14][15][16][17][18][19] While the performed simulations studied many aspects of the particles' geometric configuration, for the most part they were restricted to idealized geometric primitives such as cylinders and prisms, which often bear only limited resemblance to realistic, fabricated structures. 20 In addition, many reports on numerical studies show only the structures' farfield response, for example, the scattering or extinction cross sections. 21,22 Fluorescence or Raman enhancements, however, depend on the near-field properties of the structure, which can differ quite drastically from observations in the far-field. In this study, we investigate the effect of some such simplifications often made in numerical analyses on the validity of their results, our main focus set on the difference between realistic and idealized objects.When simulating nanophotonic structures, one can choose from a wide range of methods. Differential equation formulations such as the finite element method (FEM) 23 or the finite difference time-domain (FDTD) method 24 are flexible in their application but require absorbing boundary conditions for simulating isolated particles. Other specialized methods yield very accurate results, but only for particular geometries, for example, rigorous coupled-wave analysis (RCWA) for gratings and periodic geometries. 25 Integral equation (IE) methods have proved to be very useful in scattering analyses as their calculations fulfill the Sommerfeld radiation condition 26 and thus intrinsically include the complete far-field information. In particular, surface integral equation (SIE) methods, also referred to as boundary element methods (BEM), have grown in popularity in optics 27 as they unify a range of advantages otherwise not found in a single method. Unstructured m...

Mechanisms of Fano Resonances in Coupled Plasmonic Systems

“…30 Such nanorods are relatively easy to fabricate in a repeatable and reliable manner and less sensitive to fabrication tolerances. 31 Furthermore, the controlled interaction between nanorods forming dipole nanoantennas 32,33 produces a very high field enhancement in the gap, making these structures well suited as ultrasensitive biosensors, 34,35 plasmonic traps 36À38 and substrates for surface-enhanced Raman spectroscopy (SERS). 39 The assembly of nanorods into more complex plasmonic oligomer structures further improves these functionalities and has been pursued by several research groups who have demonstrated Fano interference between electric and magnetic multipolar modes.…”

Strong enhancement of forbidden atomic transitions using plasmonic nanostructures

“…Using the theory of electromagnetic reciprocity [20,21], the enhancement can be interpreted to apply to spontaneous emission as well [9]. A large increase in the local density of optical states in the vicinity of the nanoantenna causes a decrease in the excited state's lifetime by the same factor as derived for the excitation enhancement.…”

“…A quantitative expression for the enhancement in an arbitrary field is derived to then be applied to the numerically calculated field around a nanoantenna. The plasmonically enhanced field is shown to boost absorption and spontaneous emission of the quadrupole transition by 6 orders of magnitude, much more than dipole-allowed transitions, for which enhancement is in the hundreds [9].…”

Investigating the influences of the precise manufactured shape of dipole nanoantennas on their optical properties

“…However, they did not investigate the influence of deviations in geometric parameters as total length, width, curvatures etc. In another interesting work by Kern et al, the authors showed that realistic nanostructures deviating in all three dimensions from a perfectly rectangular block perform different than the ideal structures [17]. However, the authors did not relate the simulated structure quantitatively with real measured structures.…”