Evanescent and propagating fields of a strongly focused beam in the near-field and far-field regions

摘要

Evanescent and propagating field distributions from a strongly focused wave beam with subwavelength waist w_a « λ as a function of polar angle and distance are investigated. Exact amplitudes and intensities of evanescent E_(ev) and propagating E_p fields, including interference terms, are presented both in near and far field regions. It is shown that the amplitude of E_(ev) decays as exp(-r/w_a) in near-field region and evanescent waves do not contribute to the far field in the forward direction as well as in the transverse directions θ= π/2, even though the oscillating evanescent field of the same strength, but opposite in sign to the propagating field, exists in the transverse sheet. A far-field method for characterizing apertures based on the relationship between the relative intensity of propagating modes and a subwavelength aperture diameter is proposed.