RESUMO
A new extended x-ray-absorption fine structure spectroscopy study of local bonding identifies for the first time significant concentrations of Ge-Ge bonds in amorphous Ge2Sb2Te5. The study provides a new understanding of the local molecular structure of this phase-change material. Application of bond constraint theory indicates that the amorphous phase is an ideal network structure in which the average number of constraints per atom equals the network dimensionality. Analysis within this framework imparts new and significant insights concerning the nature of the reversible optically driven amorphous-crystalline phase transition of Ge2Sb2Te5.
RESUMO
Electric field polarization orientations and gradients close to near-field scanning optical microscope (NSOM) probes render nano-Raman fundamentally different from micro-Raman spectroscopy. With x-polarized light incident through an NSOM aperture, transmitted light has x, y and z components allowing nano-Raman investigators to probe a variety of polarization configurations. In addition, the strong field gradients in the near-field of a NSOM probe lead to a breakdown of the assumption of micro-Raman spectroscopy that the field is constant over molecular dimensions. Thus, for nano-Raman spectroscopy with an NSOM, selection rules allow for the detection of active modes with intensity dependent on the field gradient. These modes can have similar activity as infra-red absorption modes. The mechanism can also explain the origin and intensity of some Raman modes observed in surface enhanced Raman spectroscopy.
RESUMO
The Huygens-Fresnel principle provides a conceptual understanding for wave propagation and diffraction. Recently the principle has been reexamined to suggest that it is also valid in the near field. We reformulate the problem in terms of nonradiative optics, focusing particularly on the obliquity factor inherent in the forward-directed propagation of light. In the near field of matter no explicit obliquity factor exists.
