Optical element, image waveguide method, head-mounted display apparatus and diffractive waveguide display

An optical element includes a waveguide substrate, first and second optical film structures, and a sub-wavelength nanostructure. The waveguide substrate has opposite first and second sides, as well as a light entering surface and a light exiting surface, and is configured to allow an image beam to enter into the interior thereof through the light entering surface and propagate the image beam in a manner of total internal reflection. The image beam exits the waveguide substrate via the light exiting surface and after one or more reflections. The first and second optical film structures are respectively arranged on the first and second sides of the waveguide substrate. The sub-wavelength nanostructure is arranged on the first side of the waveguide substrate. The sub-wavelength nanostructure is configured to receive and diffract the image beam to couple the image beam to the waveguide substrate. The first and second optical film structures are configured to reflect a part of the image beam with an incident angle