Engineers and physicists have long referred to low THz RF as such and not with optical terms. Also, there are many low THz applications but nature doesn't make

Engineers and physicists have long referred to low THz RF as such and not with optical terms. Also, there are many low THz applications but nature doesn't make it easy to work with. https://en.wikipedia.org/wiki/Terahertz_gap https://en.wikipedia.org/wiki/Terahertz_gap