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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Vortices in the near field of optical dipole radiation

Li, Xin 10 December 2010 (has links)
An oscillating electric dipole emits radiation, and the flow of energy is represented by the field lines of the Poynting vector. In the most general state of oscillation the dipole moment vector traces out an ellipse. We have evaluated analytically and numerically the field lines of the Poynting vector for the emitted light, and it appears that each field line lies on a cone, which has its axis perpendicular to the plane of the ellipse. The field lines exhibit a vortex structure near the location of the dipole, and they approach a straight line in the far field. The spatial extent of the optical vortex is well below the wavelength of the emitted radiation. It is shown that the asymptotic limit of a field line is displaced as compared to a ray which would come directly out of the source. This nearield vortex pattern will also lead to a shift of the intensity distribution of the radiation in the far field. The emission of radiation by a linearly oscillating electric dipole is drastically altered when the dipole is close to a mirror. The energy is not emitted along optical rays, as for a free dipole, but as a set of four optical vortices. At a larger distance from the dipole singularities and isolated vortices appear. It is shown that most of these interference vortices are due to the vanishing of the magnetic field at their centers. In the plane of the mirror there is a singular circle with a diameter which is proportional to the distance between the dipole and the mirror. Inside this circle, all energy flows to a singularity on the mirror surface. We have also demonstrated a peculiar property of energy transport of optical dipole radiation in a negative index of refraction material (NIM). When the particle is embedded in a NIM and the dipole moment is rotating, the direction of rotation of the field lines of energy flow is reversed as compared to the rotation of the field lines for emission in a dielectric.
2

Near field phenomena in dipole radiation

Xu, Zhangjin 01 May 2020 (has links)
In this dissertation we have studied nearield phenomena in dipole radiation. We have studied first the energy flow patterns of the radiation emitted by an electric dipole located in between parallel mirrors. The field lines of the Poynting vector have intricate structures, including many singularities and vortices. For a dipole parallel to the mirror surfaces, vortices appear close to the dipole. Vortices are located where the magnetic field vanishes. Also, a radiating electric dipole near the joint of two orthogonal mirrors is considered, and also here we find numerous singularities and vortices in the energy flow patterns. We have also studied the current density in the mirrors. Next we have studied the reflection of radiation by and the transmission of radiation through an interface with an  -near-zero (ENZ) material. For p polarization, we find that the reflection coefficient is -1, and the transmission coefficient is zero for all angles of incidence. The transmitted electric field is evanescent and circularly polarized. The transmitted magnetic field is identically zero. For s polarization, the transmitted electric field is s polarized and the transmitted magnetic field is circularly polarized. The next topic was the study of the force exerted on the dipole by its own reflected field near an ENZ interface. We found that, under certain circumstances, it could be possible that the dipole would levitate in its reflected field. This levitation is brought about by evanescent reflected waves. Finally, power emission by an electric dipole near an interface was considered. We have derived expressions for the emitted power crossing an interface. The power splits in contributions from traveling and evanescent incident waves. We found that for an ENZ interface, only evanescent dipole waves penetrate the material, but there is no net power flow into the material.

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