Direct Observation of Strongly Tilted Dirac Points at General Positions in the Reciprocal Space

TL;DR

This paper demonstrates the experimental realization of strongly tilted Dirac points at arbitrary positions in the reciprocal space of a metacrystal, breaking the symmetry constraints and enabling new photonic phenomena like off-axis conical diffraction and negative refraction.

Contribution

It introduces a general method to induce type-II and strongly tilted type-I Dirac points without mirror symmetry constraints in photonic systems.

Findings

Type-II Dirac points can be realized at arbitrary positions inside the FBZ.

Off-axis conical diffraction was experimentally observed.

Negative refraction occurs during the transformation of Dirac points.

Abstract

Type-II Dirac points (DPs), which occur at the intersection of strongly tilted and touching energy bands, exhibit many intriguing physical phenomena fundamentally different from the non-tilted type-I counterparts. Over the past decade, their discovery has spurred extensive research into electronic systems and other Bloch-wave systems, such as photonic and phononic crystals. However, current studies typically focus on type-II DPs along high-symmetry directions in the first Brillouin zone (FBZ) under mirror symmetry conditions, which are highly restrictive and limit further investigations and applications. To overcome the stringent constraint, here we identify and demonstrate the emergence of type-II DPs at general positions inside the FBZ without requiring the mirror symmetry. The type-II DPs, being accidental degeneracies, are experimentally realized on a metacrystal slab with H-shaped metallic patterns. Our findings indicate that even in the absence of mirror symmetry, type-II DPs can emerge at designated locations inside the FBZ by simply rotating the H-shaped patterns and adjusting geometrical and physical parameters. Furthermore, based on the rotated type-II DPs, off-axis conical diffractions have been both realized and experimentally observed. Meanwhile, we discovered that during the rotation process, the type-II DPs transform into off-axis type-I DPs, but still strongly tilted, resulting in the emergence of negative refractions. Hence, the generic method we propose for inducing type-II or strongly tilted type-I DPs without the high-symmetry limitations opens potential avenues for related research. For example, the observed off-axis conical diffraction and negative refraction could inspire future development and applications in photonics and other Bloch-wave systems.