The deBroglie wavelength of the photon IS its wavelength. That’s why it can’t squeeze into nm features and optical waveguides are still not used on-chip, after 35 years of effort.

"Information carrier" means the actual medium the light is travelling through, doesn't it? Which has to be matter of some sort.

Last time I checked the sun transfers its light through the vacuum of space to us.

Sorry, I must've missed that these optical CPUs contain vacuums of space for the light to travel through.

Well there is no matter in an struct vacuum obviously, so they are hard to see.

Indeed, as orlp mentioned, light is self-propagating and does not require a medium. This is broadly true for all EM waves.

But it needs SPACE, on the order of a few um minimum, that cannot be occupied by other devices.

We're talking about a CPU, not light traveling in a straight line forever.

Why isn't the EM field the medium? Or even spacetime?

You are confusing geometry, and the excitation traveling through that geometry.