The most left image shows the topography of a sample consisting of an array of 60 nm high gold discs with a diameter of approximately 200 nm on a glass substrate. These discs have a plasmonic resonance at a wavelength of 860 nm (FWHM 100 nm). These structures are illuminated with a 850 nm laser via the bottom objective. A gold tip scanning the surface acts as an antenna to the dipolar plasmon. The scattered intensity of the tip is collected via the parabolic mirror at the AFM head. The signal is evaluated via the integrated lock-in amplifiers at higher harmonics of the cantilever osciallation frequency. The higher the order of the reference frequency the lower is the influence of background scattering signal.
The images on the right show the tip scattering intensity at different reference frequencies. The dipolar plasmonic resonance is nicely resolved.
The images represent a sample scanning size of 2 µm x 2 µm.