The terahertz range (or “THz”) is the spectral region located in between infrared lights and microwaves, situated in the ~300-GHz ~10-THz range (frequencies: 3×1011 Hz – 1013 Hz ; wavelengths: 1 mm – 30 μm). “THz” is an unexplored frequency band in the sense that there are no commercially available microelectronic devices that can generate, detect, or manipulate electromagnetic waves over the entire terahertz frequency band at room temperature.
The lack of the practical device technology makes us call it as ‘’THz gap’’. Let us illustrate it on the case of sources. From the low frequency side, this region can be approached by active electronic sources like oscillators using transistors, Gunn diodes, as well as RTD’s (resonant-tunnelling diodes). However, all conventional carrier-transit-type electron devices are facing a speed limit below 1 THz; they expand their cut-off frequencies by the downscaling, sacrificing output power. From the high frequency side, photonic sources like QCL’s (quantum cascade lasers) have been emerging to reach a-few-THz region but they suffer from thermal noise (phonon scattering), operating at 77K and lower, and lack of from room temperature operation. In summary, one can tell that nowadays the real THz range is too high in frequency for electronics and too low for photonics. This common interest of researchers and engineers from both domains has lead to an increase of interest in the THz.