One-way optical modal transition based on causality in momentum space

Area of Science:

• Photonics
• Quantum Optics
• Non-Hermitian Physics

Background:

• Parity-time (PT) symmetry has guided unidirectional light dynamics in optical k-space.
• Previous PT-symmetric potentials required V(x) = V*(-x), a condition linked to Hamiltonian symmetry rather than direct unidirectionality.

Purpose of the Study:

• To establish an alternative route to unidirectionality in k-space using causality in light-matter interactions.
• To demonstrate the link between real and causal momentum spectra and unidirectional optical mode transitions.

Main Methods:

• Employing the concept of causality to guide optical mode dynamics.
• Analyzing potentials with real and causal momentum spectra.
• Connecting these properties to exceptional points in PT symmetry.

Main Results:

• Potentials with real and causal momentum spectra induce unidirectional transitions of optical modes within the k-continuum.
• This phenomenon corresponds to an exceptional point on the PT symmetry degree.
• A critical link between non-Hermitian physics and spectral theory is revealed.

Conclusions:

• Causality offers a novel pathway to achieve unidirectionality in optical k-space.
• This method enables multi-dimensional designer manipulation of optical modes.
• It contrasts with previous one-dimensional approaches in PT-symmetric optics.