Nonreciprocal topological mode conversion by encircling an exceptional point in dynamic waveguides

Area of Science:

• Topological photonics
• Nonlinear optics
• Waveguide physics

Background:

• Exceptional points (EPs) in topological photonics enable eigenstate exchange and chiral mode conversion.
• Current methods for encircling EPs typically result in reciprocal, restricted transmittance.
• Achieving nonreciprocal phenomena is crucial for advanced photonic devices.

Purpose of the Study:

• To propose and demonstrate nonreciprocal encircling of EPs in a dynamic waveguide.
• To achieve direction-dependent topological conversions with asymmetric transmittance.
• To explore the potential for creating robust optical isolators.

Main Methods:

• Designing a dynamic waveguide with complex modulation.
• Engineering direction-dependent EPs in the quasienergy spectra.
• Implementing a closed loop for nonreciprocal EP encirclement.
• Analyzing transmittance for forward and backward propagation.

Main Results:

• Demonstrated direction-dependent EPs due to asymmetric phase-matching conditions.
• Achieved nonreciprocal topological conversion with significantly higher forward transmittance.
• Observed distinct mode outputs for forward (equal strength) and backward (specific mode) propagation.
• Forward propagation: even to odd mode conversion with high transmittance.

Conclusions:

• Nonreciprocal encircling of EPs in dynamic waveguides is feasible.
• The proposed method enables robust, direction-dependent topological conversions.
• The developed structure shows significant promise for realizing compact and efficient optical isolators.