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Abstract Using visible‐range and infrared (3–5 µm) high‐speed video cameras, we observed collisions of adjacent branches in downward negative stepped leaders. Typically, a lagging (chasing) branch (CB) approached a leading branch (LB) from aside at about 90° angle and connected to the lateral surface of the LB within some tens of meters or less of its tip. We infer that collisions can be facilitated by the attracting force of upward moving positive‐charge wave associated with stepping at the leading branch tip. Outcomes of branch collisions differ. The chasing branch may be absorbed by the LB, rebound, or temporarily bridge two branches. It appears that a heavily branched negative stepped leader creates a highly structured and rapidly changing electric field pattern inside the volume it occupies. We observed abrupt changes in the direction of branch extension, suggesting that the direction of local electric field can differ significantly from the ambient.
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Slow light mediated by mode topological transitions in hyperbolic waveguides
We show that slow light in hyperbolic waveguides is linked to topological transitions in the dispersion diagram as the film thickness changes. The effect appears in symmetric planar structures with type II films, whose optical axis (OA) lies parallel to the waveguide interfaces. The transitions are mediated by elliptical mode branches that coalesce along the OA with anomalously ordered hyperbolic mode branches, resulting in a saddle point. When the thickness of the film increases further, the merged branch starts a transition to hyperbolic normally ordered modes propagating orthogonally to the OA. In this process, the saddle point transforms into a branch point featuring slow light for a broad range of thicknesses, and a new branch of ghost waves appears.
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- Award ID(s):
- 1749177
- PAR ID:
- 10206886
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Optics Letters
- Volume:
- 46
- Issue:
- 1
- ISSN:
- 0146-9592; OPLEDP
- Format(s):
- Medium: X Size: Article No. 58
- Size(s):
- Article No. 58
- Sponsoring Org:
- National Science Foundation
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