We present a theoretical study of the modifications of the radiative and nonradiative decay rates of an optical emitter in close proximity to a noble-metal nanosphere, based on exact electrodynamical theory. We show that the optimal nanosphere diameter for luminescence quantum efficiency enhancement associated with resonant coupling to plasmon modes is in the range of 30-110 nm, depending on the material properties. The optimal diameter is found to be a trade-off between ͑1͒ emitter-plasmon coupling, which is most effective for small spheres, and ͑2͒ the outcoupling of plasmons into radiation, which is most efficient for large spheres. In addition, we show that the well-known Gersten and Nitzan model does not describe the existence of a finite optimal diameter unless the model is extended with the correction factor for radiation damping. With this correction and a correction for dynamic depolarization, the mathematically simpler Gersten and Nitzan model provides a reasonably accurate approximation of the decay rate modifications associated with coupling to the dipole plasmon mode. We anticipate that the Gersten and Nitzan model in the form that we validate in this paper for spheres will allow the analytical investigation of the influence of shape anisotropy on plasmonenhanced luminescence.

In the presence of nanoscale silver island arrays, silicon quantum dots exhibit up to sevenfold luminescence enhancements at emission frequencies that correspond to the collective dipole plasmon resonance frequency of the Ag island array. Using electron-beam lithography to alter the pitch and particle diameter, this wavelength-selective enhancement can be varied as the metal array resonance wavelength is tuned from 600 to 900 nm. The luminescence intensity enhancement upon coupling is attributed to an increase in the radiative decay rate of the silicon quantum dots. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2191411͔Since the observation in 1990 of strong roomtemperature photoluminescence from porous silicon, 1,2 significant worldwide interest has been directed toward siliconbased photonics for integrated optoelectronics. An integral part of such systems is a Si-based, power-efficient light emitter. From that perspective, silicon quantum dots ͑Si QDs͒ have been intensively investigated as light sources. Si QDs exhibit efficient room-temperature luminescence due to the radiative recombination of quantum-confined excitons, and their emission frequency can be tuned through part of the visible and near-infrared spectrum by varying the QD size 3 or surface termination. 4 However, Si QDs suffer from low radiative decay rates relative to those of direct band gap semiconductors and organic dyes. Coupling of radiative dipole emission to metal nanostructures provides an opportunity to enhance the Si QD radiative decay rate, as previous reports have shown for semiconductor QDs ͑Refs. 5-7͒ and quantum wells. 8 Enhanced radiative emission has been attributed to electromagnetic coupling between the semiconductor active dipole emitters and surface plasmons in the metal. This coupling, which is stronger at frequencies near the plasmon resonance, can result in enhancements of the emission intensity and of the decay rates. 9In this letter, we report an enhancement of photoluminescence ͑PL͒ in Si QDs coupled to Ag island arrays fabricated by electron-beam lithography. When the surface plasmon resonance of the Ag island arrays is tuned throughout the Si QD emission spectrum, we observe a strong correlation between the frequency at which the PL emission is enhanced and the surface plasmon resonance frequency. From this behavior, we conclude that the observed PL enhancement is caused by an enhancement of the radiative decay rate of the Si QDs.Si QDs were produced by ion implantation of 11-keV Si + ions to a fluence of 1.7ϫ 10 16 cm −2 into a 1.6-mm-thick fused quartz strip ͑Technical Glass Products͒. Monte Carlo simulations performed with SRIM ͑Ref. 10͒ indicate that such an implantation yields a Gaussian depth distribution of Si in the SiO 2 , with a peak excess Si concentration of 10% at a depth of ϳ20 nm. The implanted quartz was annealed in Ar for 20 min each at 200 and 450°C, and then for 30 min at 1000°C, to form Si QDs with typical diameters of 3-5 nm.11 The samples were subsequently heated in forming gas ͑10% H 2 ,...

The photoluminescence intensity of silicon quantum dots is enhanced in a polarization-selective way by coupling to elongated Ag nanoparticles. The observed polarization dependence provides direct proof that the PL enhancement is due to electromagnetic coupling of the silicon quantum-dot emission dipoles with dipolar plasmon modes of the Ag nanoparticles. The polarization selectivity demonstrates the potential of engineered plasmonic nanostructures to optimize and tune the performance of light sources in a way that goes beyond solely enhancing the emission and absorption rates.

It is demonstrated that the photoluminescence intensity of optically active erbium ions positioned in close proximity of anisotropic Ag nanoparticles is significantly enhanced if the nanoparticles support plasmon modes that are resonant with the erbium emission. In addition, the photoluminescence intensity enhancement is found to be polarized corresponding to polarization of these plasmon modes. Both observations demonstrate that the photoluminescence enhancement is due to coupling of the Er3+ I13∕24−I15∕24 transition dipoles with plasmon modes in the Ag nanoparticles. As this coupling mechanism is known to affect the emission rate, metal nanoparticles provide an opportunity to reduce the effect of temperature or concentration quench processes that are known to occur in a wide range of erbium-doped materials.