Metagrating meets the geometry-based efficiency limit for AR waveguide in-couplers.

Area of Science:

• Optics and Photonics
• Display Technologies
• Materials Science

Background:

• Augmented reality (AR) displays offer immersive experiences but face challenges with field-of-view (FOV) and eyebox size limitations.
• The conservation of etendue dictates a fundamental trade-off between FOV and eyebox size in AR systems.
• Exit-pupil expansion (EPE) can increase eyebox size but reduces system brightness.

Purpose of the Study:

• To establish a fundamental geometric limit on the in-coupling efficiency of waveguide combiners for AR displays.
• To provide a benchmark for evaluating the efficiency of in-coupler gratings.
• To explore the potential of metasurface-based gratings (metagratings) as efficient in-couplers.

Main Methods:

• Derived a geometry-based theoretical efficiency limit for waveguide combiners.
• Designed and optimized a metasurface-based grating (metagrating) and a scanned-beam grating (SRG) as in-couplers.
• Compared the experimental diffractive efficiencies of the metagrating and SRG against the theoretical limit.

Main Results:

• The metagrating achieved 28% efficiency, surpassing the SRG's 20% efficiency for the specified waveguide geometry.
• The metagrating's efficiency closely approached the derived geometric limit of 29%.
• Metasurfaces demonstrated superior angular response control compared to SRGs, leading to higher efficiency.

Conclusions:

• The geometry of the waveguide and in-coupler fundamentally limits combiner efficiency.
• Metasurfaces offer a promising approach for developing highly efficient in-couplers in waveguide AR displays.
• This work provides critical insights for designing next-generation AR display systems.