New Bragg diffraction grating has very wide angular bandwidth
High efficiency at steep deflection angles makes the grating useful for AR displays.
|A 1-in.-diameter Bragg polarization grating diffracts white light from an LED flashlight onto a screen placed nearby. Even though the difference between the light's input and output direction is very large, the grating is highly efficient for a wide set of input angles. The extremely large color separation occurs because the grating structure has a nanoscale periodic structure smaller than the wavelength of visible light. (Image: NC State)|
A transmission diffraction grating invented by North Carolina State University (NC State; Raleigh, NC) researchers has an experimentally verified angular acceptance angle of 40°, which is twice that of previous state-of-the-art diffraction gratings configured to steer visible light to large angles.1 The new grating holds promise for creating more immersive augmented-reality (AR) display systems.
The new grating, with a 400 nm period and tested at a 532 nm wavelength, is also significantly more efficient than previous designs. "In previous gratings in a comparable configuration, an average of 30% of the light input is being diffracted in the desired direction," says Xiao Xiang, a Ph.D. student at NC State and lead author of the paper. "Our new grating diffracts about 75% of the light in the desired direction."
The new grating achieves the advance in angular bandwidth by integrating two layers, which are superimposed in a way that allows their optical responses to work together. One layer contains molecules that are arranged at a slant that allows it to capture 20° of angular bandwidth. The second layer is arranged at a different slant, which captures an adjacent 20° of angular bandwidth.
The higher efficiency stems from a smoothly varying pattern in the orientation of the liquid-crystal molecules in the grating. The pattern affects the phase and thus the redirection of the light.
"The next step for this work is to take the advantages of these gratings and make a new generation of augmented-reality hardware," says Michael Escuti, a professor of electrical and computer engineering at NC State and one of the researchers. Escuti is also the chief science officer of ImagineOptix (Cary, NC), which funded the work and has licensed the technology.
1. Xiao Xiang et al., Scientific Reports (2018); doi: 10.1038/s41598-018-25535-0