Asking in most general shape, why do signals from upper Ghz and above don't seem to travel in wires? Radio frequency waves can propagate in wires - you can have a signal like that being induced from the wave interacting with the wire, being generated, measured directly, and so on. But starting from low Thz you need special hardware - some sort of diode, bolometers, piroelectric, photoelectric, etc, that just measure energy level for specific bands instead of producing a thz level signal in a wire. Is there a fundamental limit that prevents receiving light like radio, or is it a lack of suitably high-frequency active devices? Is it even possible to transmit signals at high Thz, early Phz range in a metallic wire?