ON THE EVANESCENT FIELDS AND THE CAUSALITY OF THE FOCUS WAVE MODES

The diverging and converging field components of the source-free focus wave modes are studied within the framework of both the Whittaker and Weyl plane wave expansions, It is shown that, in the Weyl picture, th e evanescent fields associated with the diverging and converging compo nents of the focus wave mode solution cancel each other identically, T he source-free focus wave modes are, hence, composed solely of backwar d and forward propagating components of the Whittaker type, It will al so be shown that no evanescent fields are associated with the causal e xcitation of an aperture by an initial focus wave mode field. The dive rging field, however, is composed solely of causal components that pro pagate away from the aperture. With a specific choice of parameters, t he field generated by the aperture is a very good approximation to the source-free solution. Under the same conditions, the original focus w ave mode solution is composed predominantly of causal forward propagat ing fields. (C) 1995 American Institute of Physics.