And do please show how to build an 0.85 THz oscillator...

Connect a capacitor to be base of a .85Thz transistor. My favorite design for the simplest form of relaxation oscillator [1].

[1] http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/relaxo... can't draw the one from the Art of Electronics here but this site captures it pretty well. The 'trick' is having a .85Thz transistor.

AFAIK, an oscillator is necessary for building a receiver. In other words, this is apparently a solved problem.

Well, you do get vacuum tubes that operate up to a terahertz - they don't count as solid-state devices, though.