Applications of metallic nanoparticles are based on their strongly size-dependent optical properties. We present a method for combining optical tweezers with dark field microscopy that allows measurement of localised surface plasmon resonance (LSPR) spectra on single isolated nanoparticles without compromising the strength of the optical trap. Using this spectroscopic information in combination with measurements of trap stiffness and hydrodynamic drag, allows us to determine the dimensions of the trapped nanoparticles. A relationship is found between the measured diameters of the particles and the peak wavelengths of their spectra. Using this method we may also resolve complex spectra of particle aggregation and interactions within the tweezers.

The direct detection of the spatiotemporal dynamics of nanolocalized optical near-fields on nanostructured metal surfaces, for example, imaging of localized surface plasmons (cf. Chapter 1) on rough or nanostructured metal films or the imaging of propagating surface plasmon polaritons at a vacuum-metal or metal-dielectric interface is a prerequisite to further control and optimize surface-plasmon based ultrafast nanooptics for future device development and applications [1][2][3][4].While free electrons in metals collectively respond to excitation from a light pulse, which is resonant to the surface plasmon frequency of the system, and squeeze and amplify the field intensity of the incoming plane light field into a subwavelength spatial volume, the typically broad frequency bandwidth of surface plasmon resonances supports an ultrafast response of these fields with rapid field changes on sub-femtosecond time scales [5].The sub-wavelength nanoscaled localization of optical fields in the vicinity of metal nanostructures and the ultrafast temporal evolution of such fields on a 0.1-100 fs time scale require the invention and development of new experimental methodologies, which combine nanometer (sub-optical) spatial resolution, sub-femtosecond temporal resolution, and optionally further nanospectroscopic information.Resolving the spatial distribution of such fields requires a microscopic technique with sub-optical spatial resolution, for example, in the 10-100 nm range. Scanning near-field optical microscopy (SNOM) techniques have been successfully applied with spatial resolutions of about ∼100 nm; however, the combination with ultrashort light pulses is still very difficult. Photoemission electron microscopy (PEEM) is a technique capable of resolving the spatial emission distribution of photoelectrons with an ultimate resolution of ∼10 nm.

The excitation of surface plasmons in metallic nanostructures by resonant ultrashort femtosecond light pulses produces interesting phenomena such as optical field nanolocalization, nanoscale electric field enhancement and ultrafast subfemtosecond beating of the plasmon eigenmodes. Nonlinear two-photon photoemission electron microscopy has proven to be a powerful tool for spatiotemporal characterization of such effects on the nanoscale below the optical diffraction limit. As a step toward using intense, few-cycle 4 femtosecond laser pulses to excite and control surface plasmons, we report on the multiphoton-photoemission electron microscopy experiments on lithographically-fabricated gold nanostructures excited by these few-cycle laser pulses. In addition, the effects of the shape and size of silver plasmonic structures, as well as the polarization of the excitation source are examined in the two-photon photoemission induced by picosecond laser pulses. Potential approaches toward spatiotemporal control of lightfield nanolocalization are described.

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