We combine confocal microscopy using supercontinuum laser illumination and an interferometric detection technique to identify single nanoparticles of diameter below 10 nm. Spectral analysis of the signal allows us to record the plasmon resonance of a single nanoparticle. Our results hold great promise for fundamental studies of the optical properties of single metal clusters and for their use in biophysical applications.

SummaryWe report on the fabrication, characterization and application of a probe consisting of a single gold nanoparticle for apertureless scanning near-field optical microscopy. Particles with diameters of 100 nm have been successfully and reproducibly mounted at the end of sharp glass fibre tips. We present the first optical images taken with such a probe. We have also recorded plasmon resonances of gold particles and discuss schemes for exploiting the wavelength dependence of their scattering cross-section for a novel form of apertureless scanning near-field optical microscopy.

Using scanning probe techniques, we show the controlled manipulation of the radiation from single dipoles. In one experiment we study the modification of the fluorescence lifetime of a single molecular dipole in front of a movable silver mirror. A second experiment demonstrates the changing plasmon spectrum of a gold nanoparticle in front of a dielectric mirror. Comparison of our data with theoretical models allows determination of the quantum efficiency of each radiating dipole.It is a well established matter that the radiation of an oscillating electric dipole can be manipulated if it is placed in front of a planar interface [1]. Experiments investigating this system date back to Drexhage [2] who looked at the influence of a metallic mirror on the fluorescence lifetime of ensembles of Eu 3+ ions. By preparing a large number of samples, each with a different spacing between the mirror and the emitter layer, two major effects were observed. Firstly, it was shown that the decay rate (Γ) oscillates at large distances due to the retarded interaction of the dipoles with their own reflected fields. Secondly, it was shown that Γ is strongly modified very close to the mirror due to the energy transfer to the metal [1,2]. Since that time, numerous works have investigated the modification of spontaneous emission from ensembles in thin dielectric layers [1]. Various key parameters such as the dipole's orientation, its distance to the interface and its quantum efficiency are, however, averaged in ensemble measurements.Due to challenges such as detection sensitivity, photostability and position control, experiments with single emitters have been scarce. Some researchers have nevertheless shown effects of the local dielectric environment by adding an index matching fluid to eliminate an interface [3,4] or by introducing the subwavelength boundary of a sharp tip [5,6]. In this work we study the fluorescence lifetime and intensity of a single molecule at a well-defined orientation and position, while moving an external silver mirror in its vicinity. We also examine the plasmon spectrum of a well-characterized single gold nanoparticle at various locations in front of a dielectric mirror. These experiments allow us to demonstrate, for the first time, both the far-field modulation and the nearfield modification of the total decay rate (Γ) for individual dipoles. Since the far-field modulations are only due to changes in the radiative decay rate (Γ r ) we can determine the quantum efficiency η = Γ r /Γ of each dipole.A theoretical description of dipole decay in multi-layer structures was first developed by Chance et al. [7] and has been expanded by many authors to cover numerous * Present address:FOM-Institute for Atomic and Molecular Physics (AMOLF), 1098 SJ Amsterdam, The Netherlands † Present address:Niels Bohr Institute, 2100 Copenhagen, Denmark ‡ Electronic address: vahid.sandoghdar@ethz.ch situations. In particular, Sullivan and Hall [8] present an elegant plane wave solution that can easily be adapted to our system. For a s...

An optomechanical sensor suitable for the study of quantum effects has been developed and characterized. The sensor reads out the vibrations of a microfabricated miniature silicon mechanical oscillator which forms one end mirror of a high finesse Fabry-Pérot cavity. The mechanical quality factor is up to Qϭ300 000 at 300 K and rises up to Qϭ4ϫ10 6 at 4 K. The thermal noise of the oscillator has been measured in the time and frequency domains at room temperature and at 4.5 K. The prospects for observing the standard quantum limit are discussed.

The existing optical microscopes form an image by collecting photons emitted from an object. Here we report on the experimental realization of microscopy without the need for direct optical communication with the sample. To achieve this, we have scanned a single gold nanoparticle acting as a nano-antenna in the near field of a sample and have studied the modification of its intrinsic radiative properties by monitoring its plasmon spectrum.PACS numbers: 07.79. Fc, 42.50.Lc, 78.67.Bf Over the years, several clever techniques such as darkfield, phase contrast, fluorescence, differential interference contrast, confocal, and scanning near-field microscopies have provided powerful ways of performing optical imaging. In all these methods, as in any other visual process, one "sees" an object when photons originating from it reach the detector. The details of the imaging mechanism depend sensitively on the intensity, phase and polarization of light both in the illumination and collection channels. The thought of recording optical images without receiving photons from the object, therefore, seems to be a contradiction in terms. In this Letter we show that this is indeed possible if one monitors the intrinsic spectral properties of a nanoscopic antenna scanned close to the sample.When an oscillating dipole is placed in confined geometries its radiative properties, such as eigenfrequency and linewidth, are modified [1,2]. In an intuitive picture these modifications are due to the interaction of the oscillating dipole with its image dipoles whereby the boundary materials and their distances to the oscillator determine the strength and phase of this interaction. In an alternative point of view the radiative changes are due to the modification of the density of photon states available for emission. In the context of recent developments in nano-optics, theoretical investigations have extended these concepts to subwavelength geometries and have shown that the linewidth [3,4,5,6,7] and the transition frequency [5,6] of a dipole also respond sensitively to the optical contrast of its nanoscopic environment.