The operation of a photon scanning tunneling microscope (PSTM) is analogous to the operation of an electron scanning tunneling microscope, with the primary distinction being that PSTM involves tunneling of photons instead of electrons from the sample surface to the probe tip. A beam of light is focused on a prism at an angle greater than the critical angle of the refractive medium in order to induce total internal reflection within the prism. Although the beam of light is not propagated through the surface of the refractive prism under total internal reflection, an evanescent field of light is still present at the surface. The evanescent field is a standing wave which propagates along the surface of the medium and decays exponentially with increasing distance from the surface. The surface wave is modified by the topography of the sample, which is placed on the surface of the prism. By placing a sharpened, optically conducting probe tip very close to the surface (at a distance <λ), photons are able to propagate through the space between the surface and the probe (a space which they would otherwise be unable to occupy) through tunneling, allowing detection of variations in the evanescent field and thus, variations in surface topography of the sample. In this manner, PSTM is able to map the surface topography of a sample in much the same way as in electron scanning tunneling microscope. One major advantage of PSTM is that an electrically conductive surface is no longer necessary. This makes imaging of biological samples much simpler and eliminates the need to coat samples in gold or another conductive metal. Furthermore, PSTM can be used to measure the optical properties of a sample and can be coupled with techniques such as photoluminescence, absorption, and Raman spectroscopy.
History Conventional optical microscopy utilizing far-field illumination achieves resolution that is restricted by the Abbe diffraction limit. Modern optical microscopes with diffraction limited resolution are therefore capable of resolving features as small as λ/2.3. Researchers have long sought to break the diffraction limit of conventional optical microscopy in order to achieve super-resolution microscopes. One of the first major advances toward this goal was the development of scanning optical microscopy (SOM) by Young and Roberts in 1951. SOM involves scanning individual regions of the sample with a very small field of light illuminated through a diffraction limited aperture. Individual features as small as λ/3 are observed at each scanned point, and the image collected at each point is then compiled together into one image of the sample. The resolution of these devices was extended beyond the diffraction limit in 1972 by Ash and Nicholls, who first demonstrated the concept of near-field scanning optical microscopy. In NSOM, the object is illuminated through a sub-wavelength sized aperture located at a distance <λ from the sample surface. The concept was first demonstrated using microwaves, however the technique was extended into the field of optical imaging in 1984 by Pohl, Denk, and Lanz, who developed a near-field scanning optical microscope capable of achieving a resolution of λ/20. Along with the development of electron scanning tunneling microscopy in 1982 by Binning et al., this led to the development of the photon scanning tunneling microscope by Reddick and Courjon (independently) in 1989. PSTM combines the techniques of STM and NSOM by creating an evanescent field using total internal reflection in a prism under the sample and detecting sample-induced variations in the evanescent field by tunneling photons into a sharpened optical fiber probe.
Theory
Total internal reflection A beam of light travelling through a medium of refractive index n1 incident on an interface with a second medium of refractive index n2 (with n1>n2) will be partially transmitted through the second medium and partially reflected back through the first medium if the angle of incidence is less than the critical angle. At the critical angle, the incident beam will be refracted tangent to the interface (i.e. it will travel along the boundary between the two media). At an angle greater than the critical angle (when the incident beam is nearly parallel to the interface) the light will be completely reflected within the first medium, a condition known as total internal reflection. In the case of PSTM, the first medium is a prism, typically made of glass, and the second medium is the air above the prism.
Evanescent field coupling Under total internal reflection, although no energy is propagated through the second medium, a non-zero electric field is still present in the second medium near the interface. This field exponentially decays with increasing distance from the interface and is known as the evanescent field. Figure 1 shows the optical component of the evanescent field is modulated by the presence of a dielectric sample placed on the interface (the surface of the prism), hence the field contains detailed optical information about the sample surface. Although this image is lost in the diffraction limited far field, a detailed optical image may be constructed by probing the near field region (at a distance <λ) and detecting sample induced modulation of the evanescent field. This is accomplished through frustrated total internal reflection, also known as evanescent field coupling. This occurs when a third medium (in this case the sharpened fiber probe) of refractive index n3 (with n3>n2) is brought near the interface at a distance <λ. At this distance the third medium overlaps the evanescent field, disrupting the total reflection of light in the first medium and allowing propagation of the wave in the third medium. This process is analogous to quantum tunneling; the photons confined within the first medium are able to tunnel through the second medium (where they cannot exist) into the third medium. In PSTM, the tunneled photons are conducted through the fiber probe into a detector where a detailed image of the evanescent field can then be reconstructed. The degree of coupling between the probe and surface is highly distance dependent, as the evanescent field is an exponentially decaying function of distance from the interface. Hence, the degree of coupling is used to measure the tip to surface distance in order to obtain topographical information about the sample placed on the surface.
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