Physical Essence of Propagable Fractional-Strength Optical Vortices in Free Space

Fractional-order vector vortex beams (VVBs) have recently been demonstrated as new carriers of fractional-strength optical vortices. However, the inherent mechanism of the new vortex beams, formed by the combination of both unstable states, to propagate stably in free space requires clarification. In this study, this scientific problem is solved by revealing the physical essence of propagable fractional-strength optical vortices in free space. Therefore, three new viewpoints regarding these unique vortex beams are proposed: Abbe's diffraction limit, the phase evolution of the vortex beam, and the binary time vector property of phase. First, due to Abbe's diffraction limit, the inherent polarization modes are intertwined, thereby maintaining the entire unique vortex beam in free space. In the second case, the phase evolution of the vortex beam is the physical cause for the polarization rotation of fractional-order VVBs. Third, the phase is not merely a scalar attribute of the light beam and demonstrates a binary time vector property. This study provides entirely different physical viewpoints on the phase of the vortex beam and Abbe's diffraction limit, which may deepen the knowledge on the behavior of light beams in classical optics.