Type of microscopic technique used for imaging surfaces by tunneling of photons
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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.
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