Plasmons, i.e. collective oscillations of conduction electrons, have a strong influence on the optical properties of metal micro- and nanostructures and are of great interest for novel photonic devices. Here, plasmons on metal-dielectric interfaces are investigated using near-field optical microscopy and differential angular reflectance spectroscopy. Emphasis is placed on the study of plasmon interaction with individual nanostructures and on the nonlinear process of surface plasmon amplification. Specifically, plasmon transmission across single grooves in thin silver films is investigated with the help of a near-field optical microscope. It is found that plasmon transmittance as a function of groove with shows a non-monotonic behavior, exhibiting certain favorable groove widths with strongly decreased transmittance values. Additionally, evidence of groove mediated plasmon mode coupling is observed. Spatial beating due to different plasmon wave vectors produces distinct interference features in near-field optical images. A theoretical approach explains these observations and gives estimated coupling efficiencies deduced from visibility considerations. Furthermore, stimulated emission of surface plasmons induced by optical pumping using a organic dye solution is demonstrated for the first time. Therefore, a new setup named twin-attenuated-total-reflection scheme is introduced. The experiment is governed by a theoretical model which exhibits good agreement. Together they provide clear evidence of the claimed process.