[Paper Review] Vortical Reflection and Spiraling Fermi Arcs with Weyl Metamaterials

Hua Cheng, Wenlong Gao+11 more|arXiv (Cornell University)|Aug 12, 2020

Orbital Angular Momentum in Optics4 citations

TL;DR

This paper experimentally demonstrates helical phase distributions in the momentum-resolved scattering matrix of electromagnetic waves in a photonic Weyl metamaterial, leading to vortical reflection and spiraling Fermi arcs in an air gap between the metamaterial and a metal plate. The alignment-free angular vortical reflection enables direct manipulation of optical angular momentum, establishing a new platform for photonic Weyl systems.

ABSTRACT

Scatterings and transport in Weyl semimetals have caught growing attention in condensed matter physics, with observables including chiral zero modes and the associated magnetoresistance and chiral magnetic effects. Measurement of electrical conductance is usually performed in these studies, which, however, cannot resolve the momentum of electrons, preventing direct observation of the phase singularities in scattering matrix associated with Weyl point. Here we experimentally demonstrate a helical phase distribution in the angle (momentum) resolved scattering matrix of electromagnetic waves in a photonic Weyl metamaterial. It further leads to spiraling Fermi arcs in an air gap sandwiched between a Weyl metamaterial and a metal plate. Benefiting from the alignment-free feature of angular vortical reflection, our findings establish a new platform in manipulating optical angular momenta with photonic Weyl systems.

Motivation & Objective

• To experimentally observe helical phase distributions in the scattering matrix of electromagnetic waves in a photonic Weyl metamaterial.
• To demonstrate vortical reflection in the angle-resolved scattering response, enabling control of optical angular momentum.
• To realize spiraling Fermi arcs in an air gap between the Weyl metamaterial and a metal plate.
• To establish a new platform for manipulating optical angular momentum using photonic Weyl systems without alignment constraints.

Proposed method

• Design and fabrication of a photonic Weyl metamaterial with engineered subwavelength structures to emulate Weyl points in the photonic band structure.
• Measurement of the angle-resolved scattering matrix of electromagnetic waves using near-field scanning techniques to extract phase and amplitude information.
• Observation of a helical phase distribution in the scattering matrix, indicating the presence of phase singularities analogous to chiral zero modes in electronic Weyl semimetals.
• Creation of a planar air gap between the Weyl metamaterial and a metal plate to support and visualize spiraling Fermi arcs in the evanescent field.
• Use of angular vortical reflection to directly map the momentum-space topology without requiring precise angular alignment.
• Validation of the topological nature of the observed phenomena through comparison with theoretical models of Weyl systems.

Experimental results

Research questions

• RQ1 Can helical phase distributions in the scattering matrix of electromagnetic waves be experimentally observed in a photonic Weyl metamaterial?
• RQ2 How does vortical reflection manifest in the angular response of a Weyl metamaterial, and what are its implications for optical angular momentum control?
• RQ3 Can spiraling Fermi arcs be formed and observed in a planar air gap between a Weyl metamaterial and a metal plate?
• RQ4 To what extent does the alignment-free nature of angular vortical reflection enhance the practicality of photonic Weyl systems for device applications?
• RQ5 What is the topological origin of the observed spiraling Fermi arcs in the context of photonic Weyl semimetals?

Key findings

• A helical phase distribution was experimentally observed in the angle-resolved scattering matrix of electromagnetic waves, confirming the presence of phase singularities associated with Weyl points.
• Vortical reflection was demonstrated in the angular response, enabling alignment-free manipulation of optical angular momentum.
• Spiraling Fermi arcs were visualized in the evanescent field of an air gap between the Weyl metamaterial and a metal plate, confirming topological surface states.
• The observed phenomena are robust against sample misalignment, highlighting the practical advantage of angular vortical reflection in photonic systems.
• The experimental results are consistent with theoretical predictions of topological surface states in Weyl semimetals, validated through comparison with photonic band structure simulations.
• The work establishes a new platform for topological photonics with potential applications in integrated optics and quantum information.