Bragg polarization gratings for wide angular bandwidth and high efficiency at steep deflection angles.

Area of Science:

• Optics and Photonics
• Materials Science

Background:

• Geometric phase optical elements offer enhanced performance over traditional propagation phase elements.
• Existing diffraction gratings like metasurfaces, volume holographic gratings, and surface-relief gratings have limitations in angular bandwidth and efficiency for steep deflection angles.

Purpose of the Study:

• To demonstrate a liquid crystal (LC) polymer Bragg polarization grating (PG) with significantly improved angular bandwidth and first-order efficiency.
• To overcome the limitations of prior art in diffraction grating technology for applications requiring wide angular acceptance and high light throughput.

Main Methods:

• Utilized simulations to identify an optimal grating structure with two slanted layers for increased angular bandwidth.
• Fabricated a monolithic film using a photo-aligned chiral nematic LC, controlling grating slants with chiral dopants.
• Characterized the fabricated grating's angular bandwidth, efficiency, polarization sensitivity, and spectral bandwidth.

Main Results:

• Simulations predicted a structure with 48° angular bandwidth and 70% average first-order efficiency.
• Experimental results showed a 40° angular bandwidth, 76% average efficiency, and 96% peak efficiency.
• The grating exhibited strong polarization sensitivity (300:1 contrast) and a 200 nm spectral bandwidth, comparable to prior PGs.

Conclusions:

• The developed LC polymer Bragg PG achieves wider angular bandwidth and higher efficiency than previous technologies.
• This grating technology is particularly promising for augmented-reality systems and nonmechanical beam steering applications.
• The dual-slanted grating layer approach effectively enhances optical performance in a compact form factor.