Digitized subwavelength surface structure on silicon platform for wavelength-/polarization-/charge-diverse optical...

Area of Science:

• Photonics and Optical Engineering
• Optical Communications
• Nanotechnology

Background:

• Optical vortices with orbital angular momentum (OAM) offer a degree of freedom for scaling optical communication capacity.
• Existing optical vortex generators face limitations in size, bandwidth, and the order of OAM modes they can produce.

Purpose of the Study:

• To design, fabricate, and demonstrate a novel, compact optical vortex generator.
• To achieve wavelength-, polarization-, and charge-diverse optical vortex generation on a silicon platform.

Main Methods:

• Utilized the direct-binary search (DBS) optimization algorithm to design a digitized subwavelength surface structure.
• Fabricated the proposed structure on a silicon platform.
• Experimentally characterized the device's performance, including OAM mode purity and crosstalk.

Main Results:

• Demonstrated an ultra-compact device (∼3.6 × 3.6 μm²) with an ultra-wide bandwidth (1480-1630 nm).
• Successfully generated high-order OAM modes (up to OAM±2) for both x- and y-polarizations with >84% purity.
• Achieved low mode crosstalk, with worst-case values below -14 dB for polarization-diverse modes and below -10 dB for charge-diverse modes.

Conclusions:

• The developed digitized subwavelength surface structure is an effective solution for generating diverse optical vortices.
• The device enables multi-dimensional multiplexing by accessing wavelength, polarization, and spatial (OAM) dimensions of light.
• This work paves the way for chip-scale solutions for advanced optical communication systems.