Spectroscopic properties of a two-level atom interacting with a complex spherical nanoshell

Abstract

Frequency shifts, radiative decay rates, the Ohmic loss contribution to the nonradiative decay rates, fluorescence yield, and photobleaching of a two-level atom radiating anywhere inside or outside a complex spherical nanoshell, i.e. a stratified sphere consisting of alternating silica and gold concentric spherical shells, are studied. The changes in the spectroscopic properties of an atom interacting with complex nanoshells are significantly enhanced, often more than two orders of magnitude, compared to the same atom interacting with a homogeneous dielectric sphere. The detected fluorescence intensity can be enhanced by 5 or more orders of magnitude. The changes strongly depend on the nanoshell parameters and the atom position. When an atom approaches a metal shell, decay rates are strongly enhanced yet fluorescence yield exhibits a well-known quenching. Rather contra-intuitively, the Ohmic loss contribution to the nonradiative decay rates for an atomic dipole within the silica core of larger nanoshells may be decreasing when the silica core -inner gold shell interface is approached. The quasistatic result that the radial frequency shift in a close proximity of a spherical shell interface is approximately twice as large as the tangential frequency shift appears to apply also for complex nanoshells. Significantly modified spectroscopic properties (see computer program freely available at http://www.wave-scattering.com) can be observed in a broad band comprising all (nonresonant) optical and near-infrared wavelengths.

Non-radiative decay of a dipole emitter close to a metallic nanoparticle: Importance of higher-order multipole contributions

“…Metallic surfaces are known to increase the local electromagnetic fields and modify both excitation and emission rates of proximate fluorophores, chromophores, and quantum dots [1,2,3,4,5,6,7,8,9,10,11,12]. Especially promising is modifying fluorescence rates by means of metallic nanoparticles (MNPs) for addressing various issues of interest to biology [2,3,4,5].…”

Electromagnetic energy in multilayered spherical particles

“…The solution of this problem implies the definition of the electromagnetic field within or outside of a sphere which allows to get its absorption, scattering, extinction or other important characteristics. Among these properties, the cycle-and orientation-averaged electric |E| 2 and magnetic |H| 2 fields (in general, electromagnetic energy) within a particular layer (shell) or in the vicinity of a multilayered sphere is of great importance since it defines performance and suitability of a multilayered sphere for a large number of intriguing applications: nonlinear optics [17][18][19], lasing [20][21][22], heating [23][24][25], photocatalysis [26], fluorescence enhancement [27][28][29][30], plasmon-enhanced upconversion [31,32], energy harvesting and storing [33][34][35][36], surface-enhanced Raman spectroscopy [37][38][39], biology and medicine [40][41][42][43][44].…”

Optical cavity modes in gold shell colloids

“…A further consideration in fluorescence enhancement experiments is that very close to the metal surface ͑Ͻ10 nm͒, where the field enhancements are highest, coupling to higher-order dark plasmonic modes introduces quenching paths, limiting the effect of plasmon-enhanced luminescence. 6,[20][21][22][23] While the collective scattering and absorption resonances due to the free electrons in the metal shell are now reasonably well studied, there is an additional resonance that has not received much attention. This is the geometric cavity mode that results from confinement of light inside the dielectric core, with the cavity boundaries determined by the metal shell.…”

“…The enhanced local fields can be used to enhance the fluorescence emission of dyes close to the metal surface, 1,2 to give increased signals in Raman spectroscopy, 3,4 or to increase the photostability of luminescent dyes by shortening the excited-state lifetime. [5][6][7][8][9] An interesting class of metal nanoparticles are core-shell colloids, composed of a dielectric core surrounded by a metallic shell. The plasmon frequency of these particles can be tuned throughout the visible and near-infrared part of the spectrum by varying core diameter and shell thickness.…”