Rick, thanks for the link. I am caught up with emails from the holiday break and found the work of Prof. Joungho Kim of KAIST in Phil Garou's article:
The link above describes some work of KAIST on glass interposers vs. Silicon. Interesting thing is that it is also the first instance where I see optical wave guides in the context of 2.5D/3D stacking.
Rick, I am looking forward to your next five... I would be curious to know what KAIST is going to present on 3D Stacks (haven't seen much from them on this).
Regarding 400Gbits systems, it could herald the beginning of the end of Copper chassis-to-chassis interconnects as we know it; for sure it will restrict the application to within cabinet interconnects. But there are still challenges on motherboards -need better dielectric materials which will drive up the costs, not to mention power and signal routing and signal integrity.