I think we will see some breakthrough after the CPU industry goes into an existential crisis about going below 5nm. Current pipelines are just so damn expensive to replace that we will see gradual innovations. For example TPU-s[1] might be a standard in a few years like GPU-s are now.
The current trends are clearly in reducing power usage and size. That alone will be a huge innovation when I can hold a "server farm" in my pocket.
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.
It's not a goal I guess to shrink these "photon CPU-s" to 5nm at start.
> but this is still far from the realm in which it makes sense to replace an electronic transistor with an optical transistor
The electromagnetic spectrum even at mid-near infrared wavelengths frequencies could help chips operate on the THz scale! You might list a mountain of reasons it can't work, but it's just fun to imagine that it might be possible to turn a cycle of light to an operation.
You can build interesting things at that scale, in this research they also refer to communication as you mentioned [1]
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