Topic
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.
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TL;DR: It is demonstrated that the use of a near-field probe to map the evanescent field of an optical standing wave in a fused-silica whispering-gallery mode microresonator results in only the lowest radial mode numbers' being strongly excited, as predicted.
Abstract: We demonstrated the use of a near-field probe to map the evanescent field of an optical standing wave in a fused-silica whispering-gallery mode microresonator. The periodicity of the observed standing wave allows us to estimate accurately the radial mode number of the whispering-gallery mode resonance that is being excited. We find that the use of a fiber half-coupler to excite these resonances in fused-silica microspheres results in only the lowest radial mode numbers’ being strongly excited, as predicted.
68 citations
TL;DR: In this paper, an electric field sensor for near field measurement using both piezoelectric and converse picolectric resonances is described. But the most distinguished feature of this probe is that a signal is transmitted outside neither electrically nor optically, but mechanically.
Abstract: This invention relates to an electric field sensor for near field measurement. The electric field sensor of the present invention uses both piezoelectric and converse piezoelectric resonances. Composed of no metallic parts, the probe of the sensor minimizes field disturbance. The most distinguished feature of this probe is that a signal is transmitted outside neither electrically nor optically, but mechanically.
68 citations
TL;DR: In this paper, three signal processing techniques are presented to provide the decomposition of the near-field pressure into hydrodynamic and acoustic components, based on the application of the wavelet transform to the measured pressure data and possess the distinctive property of requiring a very simple arrangement to obtain the desired results.
Abstract: An experimental investigation of pressure fluctuations generated by a single-stream compressible jet is carried out in an anechoic wind tunnel. Measurements are performed using a linear array of microphones installed in the near region of the jet and a polar arc of microphones in the far field. The main focus of the paper is on the analysis of the pressure fluctuations in the near field. Three novel signal processing techniques are presented to provide the decomposition of the near-field pressure into hydrodynamic and acoustic components. The procedures are all based on the application of the wavelet transform to the measured pressure data and possess the distinctive property of requiring a very simple arrangement to obtain the desired results (one or two microphones at most). The hydrodynamic and acoustic pressures are characterized separately in terms of their spectral and statistical quantities and a direct link between the acoustic pressure extracted from the near field and the actual noise in the far field is established. The analysis of the separated pressure components sheds light on the nearly Gaussian nature/intermittent behaviour of the acoustic/hydrodynamic pressure. The higher sensitivity of the acoustic component to the Mach number variation has been highlighted as well as the different propagation velocities of the two pressure components. The achieved outcomes are validated through the application to the same data of existing separation procedures evidencing the advantages and limitations of the new methods.
68 citations
TL;DR: In this article, the authors studied the near field and scattering resonances of a double array of subwavelength dielectric cylinders in TM polarization and proved the existence of bound states at specific distances between the arrays in the spectral region.
Abstract: Electromagnetic bound states in the radiation continuum are studied for periodic double arrays of subwavelength dielectric cylinders in TM polarization. They are similar to localized waveguide mode solutions of Maxwell’s equations for metal cavities or defects of photonic crystals, but, in contrast to the latter, their spectrum lies in the radiation continuum. The phenomenon is identical to the existence of bound states in the radiation continuum in quantum mechanics, discovered by von Neumann and Wigner. In the formal scattering theory, these states appear as resonances with the vanishing width. For the system studied, the bound states are shown to exist at specific distances between the arrays in the spectral region where one or two diffraction channels are open. Analytic solutions are obtained for all bound states (below the radiation continuum and in it) in the limit of thin cylinders (the cylinder radius is much smaller than the wavelength). The existence of bound states is also established in the spectral region where three and more diffraction channels are open, provided the dielectric constant and radius of the cylinders are fine-tuned. The near field and scattering resonances of the structure are investigated when the distance between the arrays varies in a neighborhood of its critical values at which the bound states are formed. In particular, it is shown that the near field in the scattering process becomes significantly amplified in specific regions of the array as the distance approaches its critical values. The effect may be used to control optical nonlinear effects by varying the distance between the arrays near its critical values.
68 citations
TL;DR: In this paper, an invariant S=Rhijk Fhi Fjk was defined for electromagnetic fields in curved space-time and further shown that a necessary and sufficient condition that a conformally flat space time may represent a null electromagnetic field is that S = 0.
67 citations