Eye tracking has been an indispensable analysis method in a wide range of research fields, including Psychology, Neurology, and ophthalmology. Recent developments in augmented reality are pushing for more compact, transparent eye trackers compatible with head-mounted display or heads-up display. Oblique half-mirror and holographic waveguide satisfy these criteria and now widely used in eye-controlled displays, auto-driving, and near-to-eye displays. However, these still require bulky supplementary optics, are poorly transparent, and produce rainbow images due to non-zero diffraction in the visible spectrum.
Here, we demonstrate ultra-thin, rainbow-free eye tracking diffractive optical elements based on guided mode resonance that exhibits near-unity transmission. It consists of a 200-nm-thick Si3N4 slab waveguide sandwiched between a quartz substrate and a 100-nm-thick SiO2 capping layer designed for high transmission (>90%) over the whole visible spectrum. The insertion of 3-nm-thick Si grating layer at the interface between the slab waveguide and capping layer launches high-quality (Q~2,000), leaky guided modes in the slab waveguide at specific wavelengths for a fixed incident angle and polarization, which enables us to efficiently (13%) characterize resonant light diffraction at 870 nm. In the visible, on the other hand, the guided mode resonance becomes weak due to Si absorption, resulting in strongly suppressed rainbow-producing diffractions below 0.1% efficiency. By locating a single webcam at near-grazing angle, corresponding to the output diffracted order at 870 nm, the full anterior images of an artificial eyes are obtained. Our device opens a promising route toward ultra-compact, transparent, and non-obtrusive imaging for displays and optical switching applications.