And in between you have the terahertz gap, where we have no effective technology to emit or receive these frequencies.

Surely we can at least emit using blackbody radiation?

"Effective" compared to the state of the art in the bands on either side: in RF/microwave we have very fast arbitrary waveform generators, very nice amplifiers, and well-characterized conductors, and on the optical side we have lasers, lights, fibers, and more. The terahertz gap is so named because it's too high in frequency for our usual RF devices to work well, and too low in frequency for our usual optical devices to work well; terahertz work ends up being a mix of both, taking from either column as needed for a specific application. (You might hear the word "quasi-optical" used in this sense, though I've never heard the dual word "quasi-microwave"!)

We do have terahertz devices - they're just very limited compared to devices in adjacent bands, usually stated in terms of power. But there are a lot of hardworking and talented people working on narrowing the THz gap from all sides. It's a very very hot research area at the moment.

I guess we can't effectively modulate it fast enough to make it practical for communication