We report on the resonant coupling between localized surface plasmon resonances (LSPRs) in nanostructured Ag films, and an adsorbed monolayer of Rhodamine 6G dye. Hybridization of the plasmons and molecular excitons creates new coupled polaritonic modes, which have been tuned by varying the LSPR wavelength. The resulting polariton dispersion curve shows an anticrossing behavior which is very well fit by a simple coupled-oscillator Hamiltonian, giving a giant Rabisplitting energy of ∼400 meV. The strength of this coupling is shown to be proportional to the square root of the molecular density. The Raman spectra of R6G on these films show an enhancement of many orders of magnitude due to surface enhanced scattering mechanisms; we find a maximum signal when a polariton mode lies in the middle of the Stokes shifted emission band.PACS numbers: 71.36.+c,73.20.Mf, There is currently considerable interest in the interaction between excitonic and photonic states, as a means of modifying the photophysical properties of a system. Potential novel applications include lasers, 1 optical switches, 2 and sensors.3 In microcavities, mixing of exciton and photon modes leads to the formation of new polaritonic states, and has been observed in both organic 4,5,6 and inorganic systems. 7 More recently, coupling has been observed between excitonic and plasmonic states for semiconductor heterostructures.8,9 Localized plasmons are the subject of many current investigations, as they can dramatically alter the optical properties of a locally situated molecule: enhancement and confinement of the excitation field has important consequences in surface enhanced Raman scattering (SERS).10 Furthermore, localized surface plasmon resonances (LSPRs) can be engineered to produce large modifications in fluorescence intensity and lifetime.11,12 Due to this, the interaction between localized plasmon modes and excitonic states has been studied recently for a variety of nanostructured systems: these include nanoparticles, 13,14 nanorods, 15 nanovoids, 16 and subwavelength hole arrays. 17 For all these systems, strong coupling is manifested as an anticrossing behavior in the dispersion curve of the plasmon mode at the energy of the uncoupled exciton mode, indicating the formation of a hybridized exciton-plasmon polariton state; the resulting mode splitting is determined by the coupling strength of the two systems.In this work, we report on the resonant coupling between LSPRs in nanostructured silver films (NSFs), and two different excitonic states in an adsorbed dye, and we demonstrate the importance of this mechanism for SERS. The coupling strength was tuned by varying the LSPR wavelength from 450 to 750 nm; the resulting excitonplasmon polariton peak positions are very well fit by a three-coupled-oscillator Hamiltonian, which gives a Rabisplitting energy comparable to the largest values reported to date. Raman spectra have been taken for each film at two different wavelengths, and in both cases we find a maximum signal enhancement when the middle of t...
We have investigated the effects of tuning the localized surface plasmon resonances (LSPRs) of silver nanoparticles on the fluorescence intensity, lifetime, and Raman signal from nearby fluorophores. The presence of a metallic structure can alter the optical properties of a molecule by increasing the excitation field, and by modifying radiative and nonradiative decay mechanisms. By careful choice of experimental parameters we have been able to decouple these effects. We observe a fourfold increase in fluorescence enhancement and an almost 30-fold increase in decay rate from arrays of Ag nanoparticles, when the LSPR is tuned to the emission wavelength of a locally situated fluorophore. This is consistent with a greatly increased efficiency for energy transfer from fluorophores to surface plasmons, resulting in a significant increase in quantum yield. Additionally, spatial mapping of the surface enhanced Raman scattering signal from a nanoparticle array reveals highly localized differences in the excitation field.
Highly ordered periodic arrays of silver nanoparticles have been fabricated which exhibit surface plasmon resonances in the visible spectrum. We demonstrate the ability of these structures to alter the fluorescence properties of vicinal dye molecules by providing an additional radiative decay channel. Using fluorescence lifetime imaging microscopy (FLIM), we have created high resolution spatial maps of the molecular lifetime components; these show an order of magnitude increase in decay rate from a localized volume around the nanoparticles, resulting in a commensurate enhancement in the fluorescence emission intensity.
We report modifications to the optical properties of fluorophores in the vicinity of noble metal nanotips. The fluorescence from small clusters of quantum dots has been imaged using an apertureless scanning near-field optical microscope. When a sharp gold tip is brought close to the sample surface, a strong distancedependent enhancement of the quantum dot fluorescence is observed, leading to a simultaneous increase in optical resolution. These results are consistent with simulations of the electric field and fluorescence enhancement near plasmonic nanostructures. Highly ordered periodic arrays of silver nanotips have been fabricated by nanosphere lithography. Using fluorescence lifetime imaging microscopy, we have created high-resolution spatial maps of the lifetime components of vicinal fluorophores; these show an order of magnitude increase in decay rate from a localized volume around the nanotips, resulting in a commensurate enhancement in the fluorescence emission intensity. Spatial maps of the Raman scattering signal from molecules on the nanotips shows an enhancement of more than five orders of magnitude.Keywords scanning near-field optical microscopy · ASNOM · fluorescence lifetime imaging · FLIM · plasmon · nanoparticle · Raman · SERS
We have investigated the effects of tuning the localized surface plasmon resonances (LSPRs) of silver nanoparticles on the fluorescence intensity, lifetime, and Raman signal from nearby fluorophores. The presence of a metallic structure can alter the optical properties of a molecule by increasing the excitation field, and by modifying radiative and non-radiative decay mechanisms. By careful choice of experimental parameters we have been able to decouple these effects. We observe a four-fold increase in fluorescence enhancement and an almost 30-fold increase in decay rate from arrays of Ag nanoparticles, when the LSPR is tuned to the emission wavelength of a locally situated fluorophore. This is consistent with a greatly increased efficiency for energy transfer from fluorescence to surface plasmons. Additionally, surface enhanced Raman scattering (SERS) measurements show a maximum enhancement occurs when both the incident laser light and the Raman signal are near resonance with the plasmon energy. Spatial mapping of the SERS signal from a nanoparticle array reveals highly localized differences in the excitation field resulting from small differences in the LSPR energy.
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