About using light: I don't believe light will replace the transistors performing logic except in a few niche applications (e.g. [1]). Light is physically constrained by it's wavelength. It's difficult to build interacting structures smaller than a few hundred nm and it's difficult to build light-generating elements with even shorter wavelengths--you're approaching the Deep UV and X-rays. Maybe you can get to the tens to low hundreds of nm with plasmonics, but this is still far from the realm in which it makes sense to replace an electronic transistor with an optical transistor. Furthermore, it's also difficult to achieve strong nonlinearities in optical systems, especially silicon. You need some sort of nonlinear element for switching.

Light-based communication probably will replace certain I/O blocks on chip. These tend to be quite large in terms of area after considering power and ESD constraints.

[1] Discussion here: https://news.ycombinator.com/item?id=13051984