Fabrication of ideal geometric-phase holograms with arbitrary wavefronts

TLDR

In optics, phase is usually controlled by optical path differences, but a geometric‑phase shift offers an alternative that can encode arbitrary wavefronts into spatially varying anisotropy, though practical, efficient GP holograms have been lacking. The authors aim to develop two fabrication methods—interferometric and direct‑write—to produce high‑fidelity geometric‑phase holograms capable of recording the wavefront of any physical or virtual object. They use photo‑aligned liquid crystals to imprint an inhomogeneous optical‑axis profile in thin films a few micrometers thick, enabling the two fabrication approaches. The resulting holograms achieve near‑perfect efficiency (e.g., a 99 % efficient F/2.3 lens at 633 nm), high‑purity vortex plates with sub‑micron defects, and complex Fourier holograms with minimal leakage, demonstrating practical, broadband, low‑loss GP holography with high phase gradients.

Abstract

Throughout optics and photonics, phase is normally controlled via an optical path difference. Although much less common, an alternative means for phase control exists: a geometric phase (GP) shift occurring when a light wave is transformed through one parameter space, e.g., polarization, in such a way as to create a change in a second parameter, e.g., phase. In thin films and surfaces where only the GP varies spatially—which may be called GP holograms (GPHs)—the phase profile of nearly any (physical or virtual) object can in principle be embodied as an inhomogeneous anisotropy manifesting exceptional diffraction and polarization behavior. Pure GP elements have had poor efficiency and utility up to now, except in isolated cases, due to the lack of fabrication techniques producing elements with an arbitrary spatially varying GP shift at visible and near-infrared wavelengths. Here, we describe two methods to create high-fidelity GPHs, one interferometric and another direct-write, capable of recording the wavefront of nearly any physical or virtual object. We employ photoaligned liquid crystals to record the patterns as an inhomogeneous optical axis profile in thin films with a few μm thickness. We report on eight representative examples, including a GP lens with F/2.3 (at 633 nm) and 99% diffraction efficiency across visible wavelengths, and several GP vortex phase plates with excellent modal purity and remarkably small central defect size (e.g., 0.7 and 7 μm for topological charges of 1 and 8, respectively). We also report on a GP Fourier hologram, a fan-out grid with dozens of far-field spots, and an elaborate phase profile, which showed excellent fidelity and very low leakage wave transmittance and haze. Together, these techniques are the first practical bases for arbitrary GPHs with essentially no loss, high phase gradients (∼rad/μm), novel polarization functionality, and broadband behavior.