Photon Scanning Tunneling Microscopy

An optical microscope is described that operates by taking advantage of frustrated total internal reflection, or the tunneling of photons, from a sample to an optical fiber tip. The photon scanning tunneling microscope (PSTM) employs an optically conducting, sharpened probe tip to map spatial variations in the evanescent and scattered field intensity distributions adjacent to a sample surface. The surface forms or is placed on the surface of an optical device in which total internal reflection (TIR) nominally takes place as incident light is directed at an incident angle greater than the critical angle. The signal variations are due to the local topography, morphology, and optical activity of the surface and form the basis of imaging. Evanescent field theory is discussed and physical principles of PSTM operation are examined by: (1) using a geometrical model to calculate the effective transmittance of the surface -probe system as a function of surface-probe separation for several probe tip geometries; and (2) considering the scattering of an evanescent field by a dielectric sphere. After a description of PSTM construction and operation, evanescent field intensity measurements are compared with model calculations. PSTM images of various sample surfaces demonstrate subwavelength resolution exceeding that of conventional optical microscopy, especially in the direction normal to the surface. Limitations and interpretation of PSTM images are discussed along with the PSTM's applicability to other forms of surface analysis.