http://www.ece.ucsb.edu/Faculty/rodwell/publications_and_pre...
...for examples of some 600 GHz circuits, and:
http://www.semiconductor-today.com/news_items/2014/OCT/NORTH...
...for a >1THz circuit using InP HEMTs.
I wonder how you even test a 1THz amplifier. Also what kind of natural noise might exist in those bands
Most likely using a mixer or some other non-linearity to down convert into the frequency range of your spectrum analyzer.
The usual schtick is to measure the S-parameters a slower speeds and extrapolate out to the cutoff. (see chart 21 from the link above: http://www.ece.ucsb.edu/Faculty/rodwell/publications_and_pre...)
For the digital people, building a ring oscillator with N stages will cause the output to divide by N, and just make sure that f/N is in the range of your oscilloscope.
To actually measure things up in the THz range directly, there are more exotic methods: Superconducting bolometer is one that I've been involved with. But those are a PITA for a bunch of reasons.
[As it turns out, having a decent native oxide is also a winner.)
How does this effect energy transfer? This is the metric I assume most people care about as it is typically much faster than an electron can move.