Metasurface coupler of thin waveguide combiner for Augmented Reality headsets

Optical see-through head mounted display (HMD) is a key device for AR applications attracting significant interest of major industrial players. Currently most of the AR-based HMDs utilize optical waveguide structures in order to reduce the overall size and weight of the devices.  Waveguide combiners are based on Total Internal Reflection (TIR) propagation of the entire field in an optical guide. The core of a waveguide combiner consists of in- and out-couplers surface relief gratings or holographic volume gratings. To couple light into the waveguide and provide good color uniformity, diffracted non-zero order light should have high intensity across a wide angular range. For AR applications, several full-color waveguide solutions have been developed for light in-coupling into the optical device. To enable the transmission of multiple wavelengths with high RGB field of view (FOV), we can fabricate a multi-waveguide solution representing a combination of the stacked waveguides optimized for a different color. Reducing the number of waveguides while keeping a high FOV is the key challenge since it miniaturizes and simplifies the system. However, there are challenges that arise from the use of a thin waveguide. If the in-coupling grating has relatively large dimensions compared to a thickness of the waveguide, then there is a risk that light will exit the waveguide through the same grating that was meant to in-couple the light. That is, the grating couples the light into the waveguide, and the light is reflected from the opposite surface of the waveguide by TIR, but if the light has not traveled far enough laterally, the light will strike the same grating again, and much of that light will exit the waveguide. As a result, the efficiency of the in-coupled light will be dramatically reduced. New solutions must be addressed this issue by creating a new transmissive metasurface on a top of the waveguide and by providing the polarization transformation of in-coupled light leading to an effective reflection of the deflected light by the in-coupling metasurface.

The student will analyze possible solutions as reported in literature in view of the fabrication requirements in the imec fab. Subsequently, a model will be elaborated for the selected  structures and the student will design the target structure using finite-difference time-domain electromagnetic simulations. The student will evaluate in detail the performance of designed structure based on the above-mentioned simulations. And finally, the student will draw the mask layout for the realization of these structures in the imec fab.