From Wikipedia:
Terahertz radiation occupies a middle ground between microwaves and infrared light waves, and technology for generating and manipulating it is in its infancy, and is the subject of research. This lack of technology is called the terahertz gap. It represents the region in the electromagnetic spectrum that the frequency of electromagnetic radiation becomes too high to be measured by digitally counting cycles using electronic counters, and must be measured by the proxy properties of wavelength and energy. Similarly, in this frequency range the generation and modulation of coherent electromagnetic signals ceases to be possible by the conventional electronic devices used to generate radio waves and microwaves, and requires new devices and techniques.
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