SiN photonics and metasurfaces to autotune AR glasses

SiN photonics and metasurfaces to autotune AR glasses

Technology News |
By eeNews Europe

Unlike AR glasses that rely on diffraction gratings and built-in light guides to shine an image at the wearer’s eyes (from an integrated light engine or microdisplay), the researchers plan to combine SiN photonics (for the control part) with flat optics (made from purposely nanostructured thin films also known as metasurfaces).

The flat optics, as reported some years ago in Nature Materials under the title “Flat optics with designer metasurfaces” consist of nanoscale anisotropic light scatterers able to shape optical wavefronts into arbitrary shapes, with subwavelength resolution, by introducing spatial variations in the optical response of the light scatterers.

For these flat optics, the researchers have engineered optical materials (EnMats) that include new phase-transition correlated oxides and 2D excitonic transition metal dichalcogenides (TMDs). These EnMats combine an extremely high electro-optic response with very low losses in the visible (VIS) and near-infrared (NIR). This is where Silicon nitride (SiN) integrated photonics come in, proven to operate both in the VIS and NIR spectral ranges.

A possible implementation of the AR glass based on
Silicon Nitride integrated photonics with EnMats flat optics.
The AR glass consists of a 2D array of pixels, and
waveguide-coupled RGB and NIR lasers, a NIR detector,
a NIR isolator, electronic circuits and control software.

The novel AR glass would rely on pixel-sized tunable metasurfaces to generate ultrafast arbitrary wavefronts both in VIS and NIR, using the EnMats with their highly tunable complex optical refractive indices combined with SiN optical resonators to further enhance the electro-optic effect of the EnMats.

In a possible implementation, the AR glass would consists of a 2D array of pixels based on VIS and NIR electrically tunable SiN resonators coated with thin-film EnMats. Each metasurface pixel would be receiving red-green-blue and NIR light from waveguide-coupled on-board RGB and NIR lasers. The electrically tunable SiN resonators would allow each pixel to be individually tuned for projecting the image) but would also double as an NIR optical phased arrays for characterizing ocular aberrations. The AR glass would also feature one optical isolator to distinguish between NIR light projected into the eye and the NIR light reflected from the retina, enabling simultaneous light projection and detection in the NIR.

“We will couple laser light into a bus waveguide, distribute it over a network of branch waveguides covering the entire surface of the AR glass, evanescently couple it into the SiN resonators, and then scatter it into the eye,” explains Michal Lipson, Eugene Higgins Professor of Electrical Engineering at Columbia, who leads the research.

In order to dynamically characterize ocular aberrations of the wearer, the AR glass projects NIR light into the eye and collects reflected light from the retina. Then it can project aberration-corrected RGB contextual images into the eye. Images Courtesy of Nanfang Yu/Columbia Engineering

The scattered NIR light would then be detected to dynamically characterize ocular aberrations of the wearer’s eye and then apply pixel-level corrections through the tunable EnMats, directly to the image being projected at the retina. This means the projected image would always appear crystal clear, regardless of the wearer.

“The multi-functionality of our nanostructured AR glass is enabled by extreme capabilities that cannot be achieved using traditional optical elements,” is quoted Lipson on the Columbia University web page.

“Our system incorporates the capabilities of wavefront sensing and correction for lower and higher order ocular aberrations in real time, capabilities that no other display technology provides and that have been shown to be critical for clear or even ‘supernormal’ vision of images.”

The team also plans to develop a scalable fabrication process based on standard CMOS techniques and dry transfer processes to integrate the EnMats into the SiN integrated photonics platform. Part of their research will consist in developing the analytical and computational tools for modeling large resonator arrays and device performance dynamics.

Columbia Engineering –

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