Selective Mapping of Tip-Launched Near-Field Scattering of Surface Plasmon Polaritons for Retrieving Dispersion Relation in Silver Nanoflakes

Scattering-type near-field scanning optical microscopy (s-NSOM) has been beneficial for observing various nano-optical phenomena by overcoming the spatial resolution diffraction limit, creating optical dispersion, and investigating optoelectronic systems. We used s-NSOM on mono-crystalline silver nanoflakes to selectively decompose tip-launched surface plasmon polariton (SPP) signals and suppress the undesired geometrical effects arising from the silver nanoflakes and s-NSOM tips. Comparing the Fourier transform analysis data from all the edges of silver nanoflakes, we successfully identified the momentum of tip-launched SPPs. We have conducted energy spectral measurements on mono-crystalline silver nanoflakes and applied a geometry-independent factor: tip-launched SPP. We neglected the additional complex contribution from various shapes of the nanoflakes in the visible-near infrared regime. These experimentally obtained momentums of SPPs have a good agreement with analytical calculations. We observed that the propagation of the SPP increases with a decrease in excitation energy even without additional destructive fabrications on the surface, although the surface quality of silver nanoflakes is affected by dirt and oxidation. This study provides a precise interpretation of the experimental studies on the various polariton studies using s-NSOM.