现代传播：中国传媒大学学报

Discussions of the Evanescent States

Huang Zhi

Published Jan 1, 2008• 0 citations

Abstract

The evanescent states are common in the electro-magnetic environments,it's basic feature is the field strength decrease according to the exponential law from orginal position to far away.The electro-magnetic waves propagate through the reactive medium are common situations,corresponding wave propagation in the guided wave modes,dispersion mediums,and the ionic gases.In 1897,Lord Rayleigh's calculation predicated the evanescent states appear to propagate in rectangular metal waveguides.In order to analyze the phenomenon of waveguide below cut-off(WBCO),it seems that the evanescent states have a imaginary wave vector(a imaginary wave number).And so,we can say it is like a standing wave,the variation of field occurs in every position simultaneously,this means that they do not experience a phase shift in propagate direction.But the behaviour of WBCO was a bit strange,sometimes the phase constant β is negative,then in THz experiments the pulse delay is negative.Although the phase velocity of waveguide to be superluminal is well-known,the theoretical discussion does not explain why the phase velocity and the phase refractive index becomes negative.In quantum mechanics(QM),in barrier the evanescent states also have a imaginary wave parame-ter.This state can be explained by the Schrdinger equation,it is analogy with the Helmholtz equation in classical electro-magnetic theory.In practice,quantum tunnelling is the most popular physical process,it can be observed in many cases: the propagation of the graded index optical fiber,the two glass prisms of Bose experiment,etc.In scientist's opinion,to defy the light-speed limit is possible;for example,photons tunnel across a gap between two prisms yet arrive at same time as reflected photons that travelled a shorter distance,that means an ultra-fast transit between the two prisms:so much faster than the speed of light.As well-known,we are used the virtual particles in microscopic interaction processes since R.Feynman,but now we perhaps demonstrated here an example in the macroscopic range of a meter since G.Nimtz's experiment.

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