SiN photonics and metasurfaces to autotune AR glasses

February 16, 2018 // By Julien Happich
Engineers from Columbia University have just won a USD 4.7M four-year grant from DARPA to pursue the development of compact lightweight Augmented Reality (AR) glasses that would dynamically monitor the wearer’s vision and display vision-corrected contextual images.

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.