Evolution of Gaussian Vortex Beams in the Source Region
Y-Z. Ni and G-Q. Zhou
Based on the method of the vector angular spectrum, a Gaussian vortex beam is decomposed into the transverse electric (TE) and the transverse magnetic (TM) terms. The TE and the TM terms are referred to be the vectorial structure and offer an alternative approach to investigate the vectorial properties of Gaussian vortex beams. Though the dark hollow region is absent from the Gaussian vortex beam in the source plane, it appears upon propagation. Therefore, the behaviour of the Gaussian vortex beam in the source region is worthy of investigation. By means of mathematical techniques, the analytical expressions for the TE and the TM terms in the source region have been derived without any approximation, in which the propagation and the evanescent parts are separated from each other. The effects of the propagation and the evanescent parts on the vectorial structure are analysed with numerical examples. The evolutions of the TE term, the TM term, and the Gaussian vortex beam are graphically demonstrated in the source region. This research is beneficial to the optical trapping and the optical manipulation using Gaussian vortex beams.
Keywords: Laser beam, Gaussian vortex beam, vectorial structure, source region, beam propagation