We report the first observation of third-harmonic signals from individual gold colloids down to 40 nm diameter. Excited with 1-ps pulses at 1500 nm, the colloids generate 500-nm light, close to the plasmon resonance. The third-harmonic intensity varies as the square of the colloid surface area. Although weak, the third-harmonic signals of gold labels as small as 15 nm in diameter are expected to be accessible with 100-fs pulses. They could be used in microscopy for single-biomolecule tracking.

We measure the transient absorption of single gold particles with a common-path interferometer. The prompt electronic part of the signal provides images for diameters as small as 10 nm. Mechanical vibrations of single particles appear on a longer time scale (period of 16 ps for 50 nm diameter). They reveal the full heterogeneity of the ensemble, and the intrinsic damping of the vibration. We also observe a lower-frequency mode involving shear. Ultrafast pump-probe spectroscopy of individual particles opens new insight into mechanical properties of nanometer-sized objects.

Several recently developed detection techniques opened studies of individual metal nanoparticles (1-100 nm in diameter) in the optical far field. Eliminating averaging over the broad size and shape distributions produced by even the best of current synthesis methods, these studies hold great promise for gaining a deeper insight into many of the properties of metal nanoparticles, notably electronic and vibrational relaxation. All methods are based on detection of a scattered wave emitted either by the particle itself, or by its close environment. Direct absorption and interference techniques rely on the particle's scattering and have similar limits in signal-to-noise ratio. The photothermal method uses a photo-induced change in the refractive index of the environment as an additional step to scatter a wave with a different wavelength. This leads to a considerable improvement in signal-to-background ratio, and thus to a much higher sensitivity. We briefly discuss and compare these various techniques, review the new results they generated so far, and conclude on their great potential for nanoscience and for single-molecule labelling in biological assays and live cells.

Individual noble-metal particles, with sizes ranging from a few tenths to some hundreds of nanometers, can now be detected by far-field optics. Single-particle microscopy gives access to inhomogeneity, distributions, and fluctuations, which were previously hidden in ensemble experiments. Scattering methods rely on dark-field illumination, spectral signatures of the metal particles, or both. More advanced techniques provide high sensitivity and improved selectivity with respect to other scatterers by isolating metal-specific signals, for example the refractive index change due to heating of the environment by a pump beam or the time-resolved optical response of the particle to a short pump pulse. We review and compare linear and nonlinear methods in far-field optical microscopy that have reached the single-particle regime by means of scattered light, thermal effects, photoluminescence, or nonlinear frequency generation.

Gold vibrations: A new elastic (stretching) mode, appearing in individual dumbbells of gold nanospheres at 5–7 GHz (see figure), is a function of the contact area. This can be used to estimate the contact area between the particles, which plays an important role in the local enhancement of electromagnetic fields in such nanoantenna structures.