Repost authored by Dave in Ohio-why terahertz and not mm-wave, or radio waves?

The Southeastern Universities Research Association has put together an 
excellent chart that shows the electromagnetic spectrum together with 
typical uses of the various wavelengths.  This really helps to 
maintain perspective.  Please take a few moments and look over the 
chart. 

Why terahertz and not mm-wave, micro, or radio waves?  They all 
penetrate matter easily.  Radio waves penetrate too well so there is 
not a good reflected signal plus the typical wavelengths are too long 
to give any useful spatial resolution.  Mm-waves do reflect a good 
deal of the source radiation but the resolution is limited to about 1 
mm. at best and typically a little less.  Visible light doesn't 
penetrate much at all.  But terahertz has very good penetration (we're 
speaking of non-polar and non-metallic materials) and significantly 
better resolution than mm-waves.  In addition, many molecules have 
resonances in the terahertz frequency range which can be used to 
identify the molecular species (spectroscopy).