Near field plasmon and force microscopy

A scanning plasmon near field optical microscope (SPNM) is presented which combines a conventional far field surface plasmon microscope with a stand-alone atomic force microscope (AFM). Near field plasmon and force images are recorded simultaneously both with a lateral resolution limited by the probe size to about 20 nm. At variance to previous work, utilizing a scanning tunneling microscope (STM) with a metallic tip, a dielectric silicon-nitride tip is used in contact mode. This arrangement is more suitable for biological and chemical samples. Approaching the tip towards the surface the exponential behaviour of the evanescent field and interference between scattering at the tip and the surface are observed. SPNM and AFM images of gold grains show strong correlation induced by the topography. Images of a transition between two different dielectric areas show optical contrast in the SPNM image which cannot be obtained from the topographic force image. Contrast in the SPNM images can be adjusted, or even inverted, by choosing proper surface plasmon excitation conditions. Characteristic fringe patterns are observed caused by interference between the evanescent wave propagating along the surface and scattering of the surface plasmon at larger objects.