Near and far field

About: Near and far field is a research topic. Over the lifetime, 15922 publications have been published within this topic receiving 220571 citations.

2-D periodic leaky-wave Antennas-part II: slot design

Abstract: The far-field radiation patterns of a two-dimensional (2-D) periodic slot leaky-wave antenna (LWA) are studied. The antenna consists of a two-dimensional periodic array of slots in a conducting plane that is printed on top of a grounded dielectric slab. The antenna is excited by a simple source such as a dipole inside the slab. Reciprocity along with the spectral-domain method is used to calculate the far-field pattern, and the radiation characteristics of the structure are investigated. A comparison between the present periodic slot LWA and a 2-D periodic patch LWA discussed in Part I is given to show the advantages of the slot antenna for certain applications. The slot LWA can achieve high directivity patterns, and a circularly-polarized version of the antenna can achieve good circular-polarization at broadside.

An Electromagnetic Near-Field Sensor for Simultaneous Electric and Magnetic-Field Measurements

Abstract: This paper describes the theory of a single sensor to perform simultaneous electric and magnetic near-field measurements. The theory indicates that it is possible to measure the magnetic-loop and electricdipole currents using a loop antenna terminated with identical loads at two diametrically opposite points. The theory also indicates that it is possible to choose an ideal load impedance for achieving equal electric and magnetic-field responses of the loop. Preliminary experiments have been performed using a plane-wave field to verify these results.

Bow-tie optical antenna probes for single-emitter scanning near-field optical microscopy

Abstract: A method for the fabrication of bow-tie optical antennas at the apex of pyramidal Si3N4 atomic force microscopy tips is described. We demonstrate that these novel optical probes are capable of sub-wavelength imaging of single quantum dots at room temperature. The enhanced and confined optical near-field at the antenna feed gap leads to locally enhanced photoluminescence (PL) of single quantum dots. Photoluminescence quenching due to the proximity of metal is found to be insignificant. The method holds promise for single quantum emitter imaging and spectroscopy at spatial resolution limited by the engineered antenna gap width exclusively.

Blackbody spectrum revisited in the near field.

Abstract: We report local spectra of the near-field thermal emission recorded by a Fourier transform infrared spectrometer, using a tungsten tip as a local scatterer coupling the near-field thermal emission to the far field. Spectra recorded on silicon carbide and silicon dioxide exhibit temporal coherence due to thermally excited surface waves. Finally, we evaluate the ability of this spectroscopy to probe the frequency dependence of the electromagnetic local density of states.

Guided complex waves. Part 2: Relation to radiation patterns

Abstract: The presence of complex waves in the near field of a source-excited plane homogeneous interface is shown to play an important role in determining certain features of the radiation field. A Kirchhoff-Huygens integration over the near field reveals that, whenever these complex waves are strongly excited, they account for peaks in the radiation pattern. Under unidirectional excitation, each complex wave yields a single peak at some oblique angle; for bi-directional excitation, either two peaks appear symmetrically located about a normal to the interface, or a single peak is present in this normal (broadside-on) direction. It is also shown that, when the complex wave is slowly decaying at the interface, the peaks occur close to the angle of definition θ c for that wave. These angles of maximum radiation and the associated gain functions are expressed, in all these cases, in terms of the location in the steepest-descent plane of the pole corresponding to the complex wave. These features are also shown to be consistent with the radiation mechanism discussed in the companion paper, Part I.