Surface plasmon scanning near-field optical microscopy

We have constructed a surface plasmon scanning near-field optical microscope. Surface plasmons (SP) are excited by laser light at the silver-air interface in the Kretschmann attenuated total reflection geometry. A scanning tunneling microscope or atomic force microscope tip penetrates the evanescent electromagnetic field and serves as a scatterer of the SP's which, in turn, radiate conical electromagnetic waves. The conical radiation is collected on a photodiode by an annular mirror or a fiber-optic bundle. It is shown that the SP's are predominantly scattered in the forward direction. The conical radiation intensity is recorded as a function of the tip position, creating an optical image of the silver surface. It is demonstrated that the technique's spatial resolution is determined by the tip size, rather than the wavelength of light, though the interference of the scattered SP's creates interference patterns that may present a problem.