> I wonder how you even test a 1THz amplifier.

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.

>I wonder how you even test a 1THz amplifier.

Most likely using a mixer or some other non-linearity to down convert into the frequency range of your spectrum analyzer.

And how do you get the 1THz signal in the 1st place? Maybe upconvert as well?