Frequency Shifts of an Electric-Dipole Resonance near a Conducting Surface

Abstract

The resonance frequency of an electric dipole placed near a conducting surface is shifted by the dipole-surface interaction. This Letter reports the observation and measurement of these shifts at optical frequencies for an experimental system that consists of a metal island film spaced a distance d from a continuous Ag film. The dependence of the shift in the frequency of the island resonance on d shows good agreement with that predicted by a classical theory of the dipole-surface interaction. PACS numbers: 71.36.+c, 73.60.Dt The interaction between electromagnetic radiation and an electric dipole is a fundamental process in physics. When the dipole is placed within a few wavelengths of a conducting surface, the major features of the interaction are changed from those observed for an isolated dipole. Experiments at optical frequencies have demonstrated (1) the strong dependence of the molecular fluorescent decay rate on the separation between a molecule and a metal surface, ' (2) the transfer of energy between molecular dipoles and surface plasmons on the metal surface, 2~a nd (3) the ability of the dipole-surface interaction to enhance the strength of such processes as molecular Raman scattering5 and molecular fluorescence. 6 In addition to these three effects, theoretical treatments of the problem also predict the existence of a shift in the dipole's resonance frequency.~' 2 This frequency shift is a universal feature of the dipole-surface interaction and, in simple terms, results from the coupling between the dipole and its own image in the metal surface.In spite of the fact that these frequency shifts should appear for both microscopic and macroscopic dipoles, there have been no reports of their observation, perhaps because their magnitudes are predicted to be quite small for the typical metalmolecule system as a result of the narrow linewidths associated with molecular resonances. 8 This Letter reports the results of opticalfrequency experiments on a dipole-metal system that exhibits resonance-frequency shifts that we attribute to the dipole-surface interaction. The observation of these shifts is significant because it confirms a previously untested feature of the theory, and because it reveals an important aspect of a class of sample geometries of widespread interest in the study of surface phenomena. Our sample geometry is shown in Fig. 1 and consists of a three-layer structure deposited on a glass substrate. First, a continuous silver film of thickness 50 nm was evap--substratẽ OOO~A g LiF Ag islond film FIG. 1. Cross section of three-layer sample geometry used in the present experiment. The continuous silverfilm has a thickness of 50 nm. The thickness of the LiF spacer layer, d, was varied between 5 and 80 nm. The silver island film had a mass thickness of 3 nm, orated onto a glass substrate. After partial masking with a movable shutter, a lithium fluoride (LiF) spacer layer of thickness d was deposited over a portion of the silver surface. Further masking preceded the slow (0.5 A/sec) deposition of e...

Plasmon-Enhanced Second-Harmonic Generation from Ionic Self-Assembled Multilayer Films

“…This is reminiscent of result recently reported by Murray et al 34 and earlier by Holland and Hall. 35 In these works, the LSPR resonance frequency of metallic nanoparticles exhibited a similar modulation as a function of the distance to a nearby dielectric or metallic interface. The phenomenon was explained with a model by Chance, Prock, and Silbey, 36 where the interference between the nanoparticles and their reflection off the interface leads to a periodic modulation in the resonance frequency as a function of the thickness of the dielectric spacer.…”

Adaptive silver films for surface‐enhanced Raman spectroscopy of biomolecules

“…The localized plasmon resonances supported by the constituents of metal island films experience a frequency shift when placed in close proximity to a conducting surface [62]. The strong dependence of the spectral position of the reflectivity minimum on the spacer thickness is caused by enhanced absorption [63].…”

“…The enhancements in absorption and elastic scattering [64] are likely associated with the existence of strongly enhanced local fields. These changes are attributed to an enhancement of the dipoledipole interaction between the different particles mediated by propagating modes that are supported by a layered sample [64], interaction between dipole and its own image [62], and interference effects between waves reflected from different interfaces in the sample [63].…”

“…A metal nanostructured film positioned near a mirror-like metal surface with a sandwiched dielectric layer can show dramatic change in the film's optical properties [62][63][64]. The localized plasmon resonances supported by the constituents of metal island films experience a frequency shift when placed in close proximity to a conducting surface [62].…”

Interaction between localized and delocalized surface plasmon polariton modes in a metallic photonic crystal

“…Shifts of the resonance frequencies as well as lifetime modifications have been observed in dependence on the dipole-surface separation. In particular, systems which are composed of a dipole-active layer (e.g., metal island films) placed in front of a metal mirror have been extensively investigated [2,3]. A very interesting alternative situation may arise when the studies are extended to more complex structures which incorporate a spatial ordering of the dipole ensemble, e.g., a periodic dipole arrangement [4].…”