Unidirectional exceptional point of reflectionless states in a magnonic mirror array.

Area of Science:

• Non-Hermitian physics
• Condensed matter physics
• Photonics and wave phenomena

Background:

• Exceptional points (EPs) are singularities in non-Hermitian systems where eigenvalues and eigenvectors merge.
• Reflectionless (RL) behavior in scattering systems occurs at EPs due to coalescing scattering states.
• Unidirectional RL EPs are theoretically proposed but experimentally challenging to realize.

Purpose of the Study:

• To experimentally demonstrate a unidirectional reflectionless exceptional point (RL EP).
• To engineer collective states in an anti-Bragg magnonic mirror array for unidirectional RL EP.
• To explore anomalous spectral responses and dark state behaviors at the unidirectional RL EP.

Main Methods:

• Fabrication of an anti-Bragg magnonic mirror array.
• Engineering collective states using a giant spin ensemble to break inversion symmetry.
• Coupling the spin ensemble to a waveguide at three spatially separated points.
• Experimental characterization of the reflection spectrum at the RL EP.

Main Results:

• Successful demonstration of a unidirectional RL EP in a magnonic system.
• Observation of a flattened and broadened reflection spectrum beyond the Lorentzian profile at the RL EP.
• Identification of dark state behaviors in spectral valleys, inaccessible via conventional measurements.
• Validation of broken inversion symmetry for achieving unidirectional RL.

Conclusions:

• The study provides the first experimental realization of a unidirectional RL EP.
• The engineered magnonic system offers a platform for controlling collective coherence in open systems.
• The findings pave the way for developing novel broadband unidirectional devices and exploring fundamental physics in non-Hermitian systems.