Conversion from Terahertz to Gigahertz

Information about the Terahertz (THz)

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.

Information about the Gigahertz (GHz)

The gigahertz is a unit of frequency equal to one thousand million hertz. Its symbol is GHz. It is the range of modern wireless communication and of the clock inside every current computer, and almost nothing in it was in everyday use before the 1990s.

Wi-Fi occupies two main bands, at 2.4 and 5 gigahertz, with a newer band near 6. The 2.4 gigahertz band is crowded because it is licence-free almost everywhere and shared with Bluetooth, cordless phones and microwave ovens, which operate at 2.45 gigahertz because water molecules absorb energy efficiently there. The 5 gigahertz band offers more channels and higher rates but is absorbed more strongly by walls, so it covers a smaller area.

Mobile telephony spans the range. Earlier generations used bands below 1 gigahertz for their long reach into buildings and across countryside, while newer allocations at 1.8, 2.6 and 3.5 gigahertz carry more data over shorter distances. The higher a frequency, the more bandwidth is available and the less it penetrates, and every generation of network design is a negotiation between those two facts.

Computer clock speeds crossed into gigahertz around the year 2000 and stalled near four a few years later. The limit is thermal: power dissipation rises steeply with frequency, and beyond about four gigahertz a conventional silicon processor cannot shed the heat. Performance gains since then have come from adding cores, widening instruction issue and improving memory hierarchies rather than from raising the clock.

Satellite links, radar and radio astronomy all work here. Weather radar typically runs between 2.7 and 5.6 gigahertz, air traffic control radar in similar bands, and the microwave background radiation that fills the universe peaks near 160 gigahertz, well above the range of terrestrial communication.

Millimetre-wave systems at 24 gigahertz and above are used for automotive collision-avoidance radar and for the highest-capacity mobile network bands. Their very short wavelengths allow small antennas and enormous data rates but are blocked by almost anything, including rain and foliage.

One gigahertz equals one thousand million hertz, 1000 megahertz, or 0.001 terahertz.