I use the angular spectrum representation to compute exactly the Gaussian beam close to the waist (w0) in the case of a highly nonparaxial field (w0<λ). The computation is done in the vectorial case for a polarized Gaussian beam. In the area of the waist, the contribution of the propagating and evanescent waves is discussed. Moreover, the Gaussian wave is developed in terms of a series, which permits one to get analytical expressions for both propagating and evanescent waves when the observation is close to the waist.
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Patrick C. Chaumet (2006) studied this question.
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