At the Brooklyn 5G Summit 2019, NYU Professor Ted Rappaport, whom I consider a communication visionary, gave a keynote talk about Terahertz Communication.
5G addresses mmWave technology at the high end which covers the 30 GHz to 300 GHz part of the spectrum. The next generation Terahertz region spans 300 GHz to 3 THz where the wavelengths go from 1 mm to as low as 100 μm. Rapport commented that all of the 5G work done for mmWave 5G will help carry developers over to the THz regime.
Click on the slideshow image below to see where NYU Professor Ted Rappaport and his team at NYU Tandon School of Engineering thinks 6G is going:
Some applications above 100 GHz (Image courtesy Ted Rappaport keynote at B5GS 2019)
Visible light, beginning with Red, the lowest frequency, at 430 THz and the high end at violet, just before the ultraviolet spectrum, ends at 790 THz. So, 6G frequencies will be only a two orders of magnitude below visible light. New applications at these high frequencies, become a sensor, which will make possible see-in-the-dark cameras, cameras that can look behind walls, a health body scan monitor, and a very accurate positioner—just to name a few possible applications.