The terahertz is a unit of frequency equal to one million million hertz. Its symbol is THz. It occupies the gap between the highest radio frequencies and the lowest infrared light, a region that for most of the twentieth century was so hard to generate or detect that it was known as the terahertz gap.
The difficulty was practical rather than theoretical. Electronic oscillators run out of speed above a few hundred gigahertz because charge carriers cannot respond fast enough, while optical sources such as lasers become inefficient below the infrared. The band between them had no convenient technology on either side, and for decades it was the least explored part of the electromagnetic spectrum.
That changed with ultrafast lasers and photoconductive antennas, which generate terahertz pulses by illuminating a semiconductor with a laser pulse lasting a few femtoseconds. Terahertz time-domain spectroscopy now measures the response of materials across the band, and the technique has become a standard tool in materials science.
Security screening is the best-known application. Terahertz radiation passes through clothing, paper, plastic and ceramics but is stopped by metal and absorbed by water, and its photon energy is far too low to ionise anything, so it can image concealed objects without the risks of X-rays. Body scanners at airports work on this principle.
Molecular spectroscopy uses the band because many large molecules have rotational and vibrational modes here. Explosives and drugs have characteristic terahertz signatures, and the technique is used in pharmaceutical quality control to identify crystal forms that look identical by other methods. Art conservation applies it to see beneath paint layers without touching a canvas.
Astronomy observes at these frequencies from high, dry sites or from orbit, because atmospheric water vapour absorbs terahertz radiation strongly. Cold dust and molecular gas in star-forming regions radiate here, which is why observatories are built at high altitude in Chile and at the South Pole.
Communications research has begun to move into the band. Data rates rise with available bandwidth, and there is far more room above 100 gigahertz than below it, so laboratories have demonstrated wireless links carrying hundreds of gigabits a second at frequencies approaching a terahertz. The obstacle is that water vapour absorbs these waves strongly, limiting a link to a few hundred metres in clear air and much less in rain. That confines the technology to short hops: connections between rooftop antennas, links inside a data centre, or the last stretch between a lamp-post and a building rather than anything resembling a broadcast.
One terahertz equals one million million hertz, 1000 gigahertz, or a wavelength of about 0.3 millimetres.
| Unit | Symbol | 1 Terahertz equals | 1 of these equals | Converter |
|---|---|---|---|---|
| Nanohertz | nHz | 1 × 1021 nHz | 1 × 10-21 THz | Terahertz to Nanohertz |
| Microhertz | µHz | 1 × 1018 µHz | 1 × 10-18 THz | Terahertz to Microhertz |
| Millihertz | mHz | 1 × 1015 mHz | 1 × 10-15 THz | Terahertz to Millihertz |
| Hertz | Hz | 1000000000000 Hz | 1 × 10-12 THz | Terahertz to Hertz |
| Kilohertz | kHz | 1000000000 kHz | 0.000000001 THz | Terahertz to Kilohertz |
| Megahertz | MHz | 1000000 MHz | 0.000001 THz | Terahertz to Megahertz |
| Gigahertz | GHz | 1000 GHz | 0.001 THz | Terahertz to Gigahertz |