Photonic spectral horizons for spin-orbit conversion.

Area of Science:

• Optics and Photonics
• Materials Science
• Nanotechnology

Background:

• Metasurfaces offer precise control over light properties.
• Wavelength-selective manipulation of optical angular momentum is crucial for advanced photonic applications.
• Existing methods often lack sharp spectral transitions or are difficult to fabricate.

Purpose of the Study:

• To develop a theoretical and computational framework for metasurface mirrors that switch optical angular momentum (OAM) with wavelength.
• To introduce and define a 'spectral horizon' as a sharp wavelength threshold for OAM reversal.
• To validate the framework using analytical modeling, Monte Carlo simulations, and full-field simulations.

Main Methods:

• Theoretical modeling of anisotropic meta-atoms to control phase retardance and amplitude balance.
• Development of an operational definition for the spectral horizon.
• Large-scale Monte Carlo validation and full-field simulations incorporating fabrication variability.

Main Results:

• A reference metasurface design using TiO2 nanopillars demonstrates a sharp spectral transition near 1550 nm.
• The design exhibits narrow switching bandwidth, high efficiency, and stability under realistic fabrication tolerances.
• The framework successfully connects metasurface dispersion, performance metrics, and fabrication tolerances.

Conclusions:

• A passive and reproducible method for wavelength-selective angular momentum control using metasurfaces has been established.
• The 'spectral horizon' concept provides a unified framework for designing and validating such devices.
• Potential applications include wavelength-division multiplexing, quantum state routing, and photonic logic.