Observation of Bulk Fermi Arc and Polarization Half Charge from Paired Exceptional Points (1709.03044v1)

Abstract: The ideas of topology have found tremendous success in Hermitian physical systems, but even richer properties exist in the more general non-Hermitian framework. Here, we theoretically propose and experimentally demonstrate a new topologically-protected bulk Fermi arc which---unlike the well-known surface Fermi arcs arising from Weyl points in Hermitian systems---develops from non-Hermitian radiative losses in photonic crystal slabs. Moreover, we discover half-integer topological charges in the polarization of far-field radiation around the Fermi arc. We show that both phenomena are direct consequences of the non-Hermitian topological properties of exceptional points, where resonances coincide in their frequencies and linewidths. Our work connects the fields of topological photonics, non-Hermitian physics and singular optics, and paves the way for future exploration of non-Hermitian topological systems.

• The paper demonstrates the existence of bulk Fermi arcs caused by non-Hermitian radiative losses in photonic crystal slabs.
• It reveals the emergence of half-integer topological charges in far-field polarization from paired exceptional points.
• The study combines analytical models, numerical simulations, and polarimetric measurements to validate its non-Hermitian topological framework.

Observation of Bulk Fermi Arc and Polarization Half Charge from Paired Exceptional Points: An Academic Summary

The paper entitled "Observation of Bulk Fermi Arc and Polarization Half Charge from Paired Exceptional Points" offers a comprehensive exploration of novel non-Hermitian topological phenomena, particularly focusing on bulk Fermi arcs and polarization half charges in photonic crystal slabs. This work stands at the intersection of topological photonics, non-Hermitian physics, and singular optics, expanding our understanding of non-Hermitian systems, which are inherently open and subject to radiative losses, unlike traditional Hermitian systems.

Key Contributions and Findings

The primary contributions of the paper are twofold: the theoretical proposition and experimental validation of bulk Fermi arcs that develop due to non-Hermitian radiative losses, and the discovery of half-integer topological charges in far-field polarization around these arcs. These outcomes are attributed to the non-Hermitian characteristics of exceptional points (EPs), points at which two or more resonances coalesce in their frequencies and line widths.

1. Bulk Fermi Arcs:

• The paper demonstrates the existence of a bulk Fermi arc, an open-ended isofrequency contour that connects paired EPs in photonic crystal slabs. Unlike the surface Fermi arcs in Hermitian systems arising from Weyl points, these arcs reside in the bulk dispersion of a two-dimensional system due to non-Hermitian effects.

2. Polarization Half Charges:

• Near the Fermi arc frequency, the polarization in the far-field exhibits half-integer winding numbers, representing robust topological features akin to orientation-reversing properties on a Möbius strip.
• The paper offers experimental verification of these polarization configurations, providing direct evidence of the $ν=\pm1/2$ topological index related to EPs.

Numerical and Experimental Validation

The paper provides a rigorous comparison between analytical models, numerical simulations, and experimental measurements. The simulations corroborate the analytical models by capturing the behavior of isofrequency contours and validating the double-Riemann-sheet topology near paired EPs. Experimentally, the authors utilize photonic crystal slabs fabricated with precise specifications to capture scattered isofrequency data, demonstrating bulk Fermi arcs and examining the polarization states through comprehensive polarimetric measurements.

Implications and Future Directions

The implications of this research are substantial for both theoretical and practical applications within the field of photonics and beyond.

• Practically, the findings suggest new methodologies for creating vector-vortex beams with half-integer topological charges, which have potential applications in advanced optical communication systems and photonics-based sensing technologies.
• Theoretically, this paper advances the understanding of non-Hermitian topological phases, opening avenues for further investigation into associated phenomena such as non-Hermitian band topology's impact on light-matter interactions, including Purcell factor modifications and nonlinear optics.

Future research could extend these concepts to other wave systems, including acoustics, electronics, and polaritonic systems, leveraging non-Hermitian topological properties to explore new functional materials and devices. The observed features of EPs promise broader applicability, potentially influencing areas like single-mode lasing and sensitive optical signal manipulation driven by geometric phases.

In conclusion, this paper presents significant advancements in the field of non-Hermitian topological systems, broadening both the scientific understanding and practical utility of photonic crystals in modern technological applications. The exploration of EPs and their topological implications continues to offer fertile ground for future research and breakthroughs in the field.