Tech Briefs

Potential applications include lasers and LEDs that operate at visible or infrared frequencies.

Plasmons are collective oscillations of the free electrons in a metal or an ionized gas. Plasmons dominate the optical properties of noble-metal nanoparticles, which enables a variety of applications including electromagnetic energy transport at nanoscale dimensions, single-molecule Raman spectroscopy, and photothermal cancer therapy. Plasmons also affect the spontaneous emission dynamics of optical emitters positioned in the vicinity of metal nanoparticles. The luminescence intensity can either be enhanced or quenched, depending on the geometry. Since the associated enhancements can potentially be several orders of magnitude, plasmon-enhanced luminescence is the subject of intense research. This project focused on plasmon-enhanced luminescence of silicon quantum dots (Si QDs) and optically active erbium ions. Both these emitters are compatible with silicon processing technology, and are therefore of great technological interest.

Three fabrication methods of Ag nanoparticles were developed. First, electron beam lithography was developed to fabricate Ag nanoparticles with well-defined sizes and shapes on insulating substrates. This technique is later applied in the experiments on plasmon-enhanced luminescence. Subsequently, a method, based on a sequential Si/Ag/Si electron-beam evaporation process, was used to fabricate metal nanoparticles that exhibit plasmon resonances in the infrared. Finally, the fabrication of small Ag nanoparticles was achieved by a sequence of Na+ ↔ Ag+ ion exchange and ion irradiation of Na+-containing glass.

The photoluminescence intensity of Si QDs can be enhanced in a spectrally selective way by coupling to Ag nanoparticles. The observed luminescence enhancements range between a factor 2 and a factor 6. The luminescence enhancement is polarized for elongated Ag nanoparticles. Based on both the spectral selectivity and the polarization selectivity, it was determined that the observed luminescence enhancement is due to coupling of the Si QD emission dipoles to plasmon modes, rather than due to an enhanced excitation rate. As a consequence, the concept of plasmon-enhanced luminescence could also be applied to enhance the luminescence intensity of electrically driven light sources. This possibility is explored by integrating Ag nanoparticles in prototype Si QD light-emitting devices fabricated using processing facilities at Intel. The Si QD electroluminescence intensity of these devices has been enhanced by up to a factor 2.5.

A theoretical investigation of plasmon-enhanced luminescence was performed focusing on the modifications of the radiative and nonradiative decay rates of an optical emitter positioned in close proximity to a noble-metal

nanoparticle. The influence of a spherical nanoparticle by exact electrody-namical theory was analyzed. It was shown that the optimal sphere diameter for luminescence quantum efficiency enhancement associated with resonant coupling to plasmon modes is in the range from 30 to 110 nm, depending on the material properties. The optimal diameter is found to be a trade-off between (1) emitter-plasmon coupling, which is most effective for small spheres, and (2) the outcoupling of plasmons into radiation, which is most efficient for large spheres. The distance at which the nanoparticles induce a substantial effect on the radiative decay rate ranges to a few tens of nanometers.

Results indicate that metal nanostructures can provide even larger improvements to nanoscopic light sources, e.g. based on single nanowires or single quantum dots. A larger spectral separation was found between the radiative dipole plasmon mode and the dark higher-order plasmon modes of an Ag nanoparticle for larger anisotropy. In the vicinity of such an anisotropic Ag nanoparticle, the quantum efficiency of a low-quantum-efficiency emitter (0.1%) can be enhanced by almost a factor 200, instead of a factor 60 for a spherical nanoparticle.

These results show that nanoparticle anisotropy does not only influence the plasmon resonance wavelength, but also the ratio at which different plasmon modes are excited by an emitter at short distance.

This work was done by Albert Polman of the Center for Nanophotonics, FOM-Institute AMOLF, Amsterdam, The Netherlands, for the Air Force Office of Scientific Research. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Photonics category. AFRL-0179