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Using the localized surface plasmon resonance (LSPR) of gold nanoparticles for sensing applications has attracted considerable interest, since it can be very sensitive, even down to a single molecule, and selective for a specific analyte molecule with a suitable surface modification. LSPR sensing is usually based on the wavelength shift of the LSPR or a Fano resonance. Here, we present a new experimental approach based on the phase of the light scattered by a single gold nanoparticle by equipping a confocal microscope with an additional interferometer arm similar to a Michelson interferometer. The detected phase depends on the shape of the nanoparticle and the refractive index of the surrounding medium and can even be detected for off-resonant excitation. This can be used as a new and sensitive detection method in LSPR sensing, allowing the detection of changes to the local refractive index or the binding of molecules to the nanoparticle surface.

In this work, we present a novel technique to directly measure the phase shift of the optical signal scattered by single plasmonic nanoparticles in a diffraction-limited laser focus. We accomplish this by equipping an inverted confocal microscope with a Michelson interferometer and scanning single nanoparticles through the focal volume while recording for each pixel interferograms of the scattered and a reference wave. For the experiments, lithographically prepared gold nanorods were used, since their plasmon resonances can be controlled via their aspect ratio. We have developed a theoretical model for image formation in confocal scattering microscopy for nanoparticles considerably smaller than the diffraction limited focus. We show that the phase shift observed for particles with different longitudinal particle plasmon resonances can be well explained by the harmonic oscillator model. The direct measurement of the phase shift can further improve the understanding of the elastic scattering of individual gold nanoparticles with respect to their plasmonic properties.

Extracting signals at low single-photon count rates from large backgrounds is a challenge in many optical experiments and technologies. Here, we demonstrate a single-photon lock-in detection scheme based on continuous photon timestamping to improve the SNR by more than two orders of magnitude. Through time-resolving the signal modulation induced by periodic perturbations, 98% of dark counts are filtered out and the < 1 c o u n t / s contributions from several different nonlinear processes identified. As a proof-of-concept, coherent anti-Stokes Raman measurements are used to determine the vibrational lifetime of few molecules in a plasmonic nanocavity. This detection scheme can be applied to all single-photon counting experiments with any number of simultaneous modulation frequencies, greatly increasing SNR and resolving physical processes with picosecond time resolution while keeping the photon dosage small. The open instrumentation package provided here enables low-cost implementation.