A grating is a more precise version of the rough surface that typically spreads light across the light-guide behind the liquid crystal panel in a display. If we could switch gratings on and off, we could control where the panel is illuminated and who gets to see the picture. But how do you switch a grating on and off?

 

One way is to move the grating in and out of the evanescent field. This is present at the surface of any conventional light-guide and if the grating is inside the field, a guided ray sees it, otherwise not. The field is only ~ 3 microns thick so movement is tiny and several groups have made panels like this. But it's like trying to lift a carpet up and down: air doesn't have much time to rush in and out so it's not easily done.

 

Another approach is to use liquid crystal gratings. Liquid crystal is almost unique for giving a huge electro-optic change while using very little power: that's why it makes such great displays. But in order to put voltage across the liquid crystal, you need a transparent conductor and these are never all that transparent. For the display, that doesn't matter, but light bounces many times through a light-guide and will gradually be absorbed by the transparent conductor.

 

Instead, we are experimenting with deep gratings, often called Bragg gratings. These switch on in the sense of diffracting light only if you illuminate them at the right angle. No transparent conductor is needed but we do have to change things around somewhat…