Noble-metal nanoparticles exhibit unique plasmon resonances compared to bulk metal that depend on the nanoparticle size, [1] shape, [2,3] and local dielectric environment. [4] At resonance they may be regarded as antennas because they are able to increase the electrical field of an incident plane wave by orders of magnitude in a small volume. This leads, at the same time, to an increase of the emission rate of a radiating molecular dipole that is placed in the volume of enhanced coupling. [5] This antenna property is the basis for applications of subwavelength metal structures in surface-enhanced Raman spectroscopy (SERS), [6] plasmon-enhanced fluorescence spectroscopy, [7] chemical and biological sensing, [8,9] and nearfield microscopy. [10,11] Furthermore, the plasmon resonances of complex metal structures may exhibit far-field responses that cannot be realized by classical molecular resonators. For example, the formation of strong magnetic dipoles in irregularly shaped metal nanoparticles may lead to an especially intriguing application as a material with a negative refractive index [12,13] and highly unusual optical properties. An experimental signature for such an effect has been demonstrated recently at infrared wavelengths (k = 3 lm) for split-ring structures.[14]An essential component for all of these applications is the ability to tailor the particle plasmon resonances according to the desired application. A strong dependence of the strength and wavelength of the plasmon resonances on the geometrical shape of a nanometer-sized metal object is well established for spheroids.[15] More recent calculations [16,17] proved that for increasingly complex structures several distinct and strong resonances may exist and the resonance wavelengths can be tuned by varying the nanoparticle geometry. The same calculations have shown that at resonance the field enhancements may be highly localized and dramatically enhanced at features with small dimensions, such as thin gaps [17] or at the corners [16,18] of a nanoparticle structure. These results imply that for maximized enhancement effects in nano-optical experi-COMMUNICATIONS

Abstract.The geometry of crescent-shaped noble-metal nanoparticles is systematically varied in terms of shape and size. The resulting changes in the plasmonic resonances of these structures are investigated by extinction spectroscopy revealing a rich polarization-dependent response in the nearinfrared region of the electromagnetic spectrum. A first approach towards the understanding of this behaviour, in analogy to previous models on confined modes in nanometric metal slabs, is presented and discussed. Variations in several geometrical parameters lead to changes in the optical response that can be understood within this model. Qualitative changes in the response are seen at the transition of the structures from an open 'crescent' to a fully connected ring, pointing to a high field localization between the two tips of the structure.

Photoemission electron microscopy was used to image the electrons photoemitted from specially tailored Ag nanoparticles deposited on a Si substrate (with its native oxide SiO x ). Photoemission was induced by illumination with a Hg UV-lamp (photon energy cutoffhω U V = 5.0 eV, wavelength λ U V = 250 nm) and with a Ti:Sapphire femtosecond laser (hω l = 3.1 eV, λ l = 400 nm, pulse width below 200 fs), respectively. While homogeneous photoelectron emission from the metal is observed upon illumination at energies above the silver plasmon frequency, at lower photon energies the emission is localized at tips of the structure. This is interpreted as a signature of the local electrical field therefore providing a tool to map the optical near field with the resolution of emission electron microscopy.

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We summarize some of our efforts in synthesizing and characterizing nanoscopic objects fabricated from semiconducting materials and noble metals. The optical properties of colloidal semiconductors (quantum dots) are analyzed, in particular, with respect to their spectral photoluminescence properties (bandgap engineering) and the characteristic emission blinking. The statistical evaluation of the on- and off-states seen in the time-dependent recordings of the photoluminescence emitted from a single nanoparticle confirmed the reported power-law probability distribution, however, with a superimposed decay of the on-state density (which is illumination intensity dependent). This results in a loss of fluorescence intensity upon extended illumination when these particles are used in biosensor assays. Next, a colloid particle-based template protocol for the fabrication of non-trivial Au nanostructures is described. The resulting nano-crescents can be varied in terms of their size and shape. It is demonstrated how their plasmonic resonance characteristics can thus be tuned with respect to the spectral position of their (multipole) absorbance peaks, and their polarization properties.