  The subwavelength confinement of surface plasmon polaritons (SPPs) makes them
attractive for various applications such as sensing, light generation and solar
energy conversion. Near-field microscopy associated with interferometric
detection allows to visualize both the amplitude and phase of SPPs. However,
their full quantitative characterization in a reflection configuration is
challenging due to complex wave patterns arising from the interference between
several excitation channels. Here, we present near-field measurements of SPPs
on large monocrystalline gold platelets in the visible spectral range. We study
systematically the influence of the incident angle of the exciting light on the
SPPs launched by an atomic force microscope tip. We find that the amplitude and
phase signals of these SPPs are best disentangled from other signals at grazing
incident angle relative to the edge of the gold platelet. Furthermore, we
introduce a simple model to explain the phase shift observed between the SPP
amplitude and phase profiles. Using this model, the wavelength and propagation
length of the tip-launched plasmons are retrieved by isolating and fitting
their signals far from the platelets edges. Our experimental results are in
excellent agreement with theoretical models using gold refractive index values.
The presented method to fully characterize the SPP complex wavevector could
enable the quantitative analysis of polaritons occurring in different materials
at visible wavelengths.
