Severing the Link between Modal Order and Group Index Using Hybrid Guided Space-Time Modes
Abbas Shiri, Ayman F. Abouraddy
Abstract
The structure of an optical waveguide determines the characteristics of its guided modes, such as their spatial profile and group index. General features are shared by modes regardless of the waveguiding structure; for example, modal dispersion is inevitable in multimode waveguides, every mode experiences group-velocity dispersion, and higher-order modes usually travel at lower group velocities than their lower-order counterparts. We show here that such trends can be fundamentally altered in a multimode planar waveguide by exploiting hybrid guided space-time modes, whereupon dispersion is eliminated and the link between modal order and group index is altogether severed. Hybrid space-time modes are confined in one-dimension by the planar waveguide, and in the other by the underlying spatiotemporal spectral structure of the field itself. Direct measurements of the modal group delays confirm that the group index for low-loss, dispersion-free, hybrid space-time modes can each be tuned away from the group index of the same-order conventional mode, and that the transverse size of these hybrid modes can be varied independently of the modal order and group index. These findings are verified in a few-mode planar waveguide consisting of a 25.5 mm-long, 4-μm-thick silica film deposited on a MgF2 substrate.