New diffraction grating technology enables wearable displays to project 3D images using only one panel
UCF researchers have invented an optical device that can replace existing technologies in applications such as electronic beam steering for free-space communications, optical switching, and wearable (head-mounted) displays. The Polarization Volume Grating (PVG) provides wider bandwidths, polarization sensitivity and electrical control. It is thinner than conventional holographic gratings and has nearly 100 percent diffraction efficiency with large diffraction angles.
Current technologies only offer one or two of the advantages that PVGs offer. For example, holographic volume grating (HVG) devices provide large diffraction angles, but narrow diffraction bandwidths. Cycloidal diffractive waveplate (CDW) devices offer polarization sensitivity and high diffraction efficiency, but small diffraction angles. PVGs, however, provide all of the advantages of these technologies: high diffraction efficiency, large diffraction angles, polarization selectivity and wide diffraction bandwidths. In addition, PVGs can either reflect or transmit light beams, based on a manufacturer’s configuration needs. Thus, PVGs are an excellent, low-cost replacement for current technologies in many applications.
The invention comprises a PVG and methods for fabricating it. A PVG operates by rotating the optical axis of a birefringent medium (a reactive liquid crystal) in space to generate a periodically and continuously changing refractive index along two orthogonal (perpendicular) directions. The birefringent medium of the PVG has top and bottom surfaces and a middle bulk region with a tilted, gradient, periodic refractive index distribution. The fabrication process can consist of linear photo-polymerizable polymers (LPPs) or photo-alignment materials for the surfaces and reactive mesogens or liquid crystals (LCs) for the bulk region. Figure 1 illustrates the PVG principles.
As shown in Figure 1, the optical axis in the surface of the PVG rotates along the xz-plane. The rotating angle (α) changes continuously and periodically along the x and y axes at intervals of Λx and Λy, respectively. A chiral dopant added to the birefringent medium causes the bulk region (such as a liquid crystal) to exhibit helical (spiral) structures. The structures provide another periodicity (Λy) that is perpendicular to the surface and generates periodically slanted refractive index planes. When the number of index planes reaches a sufficient value or thickness (eight to 10 pitches), Bragg diffraction occurs. The helical structure (left- or right-handed helices) makes the Bragg diffraction polarization-selective. Thus, the invention can reflect or transmit light beams, and it can split an incident unpolarized beam into two well-separated yet polarized beams.
- Provides functionality for display projection that is currently not possible
- Low cost, simple and effective
- Electronic beam steering and directed energy laser systems
- Optical switching
- Augmented/virtual reality systems