Introduction

Terahertz radiation consists of electromagnetic waves within the ITU-designated band of frequencies from 0.3 to3 terahertz (THz; 1 THz = 1012 Hz). Wave lengths of radiation in the terahertz band correspondingly range from 1 mm to 0.1 mm (or 100 μm).

Terahertz radiation falls in between infrared radiation and microwave radiation in the electromagnetic spectrum, andit shares some properties with each of these. Like infrared and microwaveradiation, terahertz radiation travels in a line of sight and is non-ionizing.Like microwave radiation, terahertz radiation can penetrate a wide variety of non-conducting materials. Terahertz radiation can pass through clothing, paper,cardboard, wood, masonry and plastic. The penetration depth is typically less than that of microwave radiation. Terahertz radiation has limited penetration through fog and clouds and cannot penetrate liquid water or metal. THz is notionizing yet can penetrate some distance through body tissue, so it is of interest as a replacement for medical X-rays.

The earth's atmosphere is a strong absorberof terahertz radiation, so the range of terahertz radiation in air is limitedto tens of meters, making it unsuitable for long-distance communications.However, at distances of ~10 meters the band may still allow many usefulapplications in imaging and construction of high bandwidth wireless networkingsystems, especially indoor systems.

Research

Security

Terahertz radiation can penetrate fabricsand plastics, so it can be used in surveillance, such as security screening, touncover concealed weapons on a person, remotely. This is of particular interestbecause many materials of interest have unique spectral"fingerprints" in the terahertz range. This offers the possibility tocombine spectral identification with imaging. In 2002 the European Space Agency(ESA) Star Tiger team, based at the Rutherford Appleton Laboratory(Oxfordshire, UK), produced the first passive terahertz image of a hand.Passive detection of terahertz signatures avoid the bodily privacy concerns ofother detection by being targeted to a very specific range of materials andobjects.

Medicalimaging

Unlike X-rays, terahertz radiation is notionizing radiation and its low photon energies in general do not damage tissuesand DNA. Some frequencies of terahertz radiation can penetrate severalmillimeters of tissue with low water content (e.g., fatty tissue) and reflectback. Terahertz radiation can also detect differences in water content anddensity of a tissue. Such methods could allow effective detection of epithelialcancer with an imaging system that is safe, non-invasive, and painless.

The first images generated using terahertzradiation date from the 1960s; however, in 1995 images generated usingterahertz time-domain spectroscopy generated a great deal of interest.

Some frequencies of terahertz radiation canbe used for 3D imaging of teeth and may be more accurate than conventionalX-ray imaging in dentistry.

Scientificuse and imaging

In addition to its current use in submillimeterastronomy, terahertz radiation spectroscopy could provide new sources ofinformation for chemistry and biochemistry.

Recently developed methods of THztime-domain spectroscopy (THz TDS) and THz tomography have been shown to beable to image samples that are opaque in the visible and near-infrared regionsof the spectrum. The utility of THz-TDS is limited when the sample is verythin, or has a low absorbance, since it is very difficult to distinguishchanges in the THz pulse caused by the sample from those caused by long-termfluctuations in the driving laser source or experiment. However, THz-TDSproduces radiation that is both coherent and spectrally broad, so such imagescan contain far more information than a conventional image formed with asingle-frequency source.

Submillimeter waves are used in physics tostudy materials in high magnetic fields, since at high fields (over about 11tesla), the electron spin Larmor frequencies are in the submillimeter band.Many high-magnetic field laboratories perform these high-frequency EPRexperiments, such as the National High Magnetic Field Laboratory (NHMFL) inFlorida.

Terahertz radiation could let arthistorians see murals hidden beneath coats of plaster or paint in centuries-oldbuildings, without harming the artwork.

Communication

In May 2012, a team of researchers from theTokyo Institute of Technology published in Electronics Letters that it had seta new record for wireless data transmission by using T-rays and proposed theybe used as bandwidth for data transmission in the future. The team's proof ofconcept device used a resonant tunneling diode (RTD) negative resistanceoscillator to produce waves in the terahertz band. With this RTD, theresearchers sent a signal at 542 GHz, resulting in a data transfer rate of 3Gigabits per second. It doubled the record for data transmission rate set theprevious November. The study suggested that Wi-Fi using the system would belimited to approximately 10 meters, but could allow data transmission at up to100 Gbit/s. In 2011, Japanese electronic parts maker Rohm and a research teamat Osaka University produced a chip capable of transmitting 1.5 Gbit/s usingterahertz radiation.

Potential uses exist in high-altitudetelecommunications, above altitudes where water vapor causes signal absorption:aircraft to satellite, or satellite to satellite

Manufacturing

Many possible uses of terahertz sensing andimaging are proposed in manufacturing, quality control, and process monitoring.These in general exploit the traits of plastics and cardboard being transparentto terahertz radiation, making it possible to inspect packaged goods. The firstimaging system based on optoelectronic terahertz time-domain spectroscopy weredeveloped in 1995 by researchers from AT&T Bell Laboratories and was usedfor producing a transmission image of a packaged electronic chip. This systemused pulsed laser beams with duration in range of picoseconds. Since thencommonly used commercial/ research terahertz imaging systems have used pulsedlasers to generate terahertz images. The image can be developed based on eitherthe attenuation or phase delay of the transmitted terahertz pulse. Since thebeam is scattered more at the edges and also different materials have differentabsorption coefficients, the images based on attenuation indicates edges anddifferent materials inside of objects. This approach is similar to X-raytransmission imaging, where images are developed based on attenuation of thetransmitted beam. In the second approach, terahertz images are developed basedon the time delay of the received pulse. In this approach, thicker parts of theobjects are well recognized as the thicker parts cause more time delay of thepulse. Energy of the laser spots are distributed by a Gaussian function. Thegeometry and behavior of Gaussian beam in the Fraunhofer region imply that theelectromagnetic beams diverge more as the frequencies of the beams decrease andthus the resolution decreases. This implies that terahertz imaging systems havehigher resolution than scanning acoustic microscope (SAM) but lower resolutionthan X-ray imaging systems. Although terahertz can be used for inspection ofpackaged objects, it suffers from low resolution for fine inspections. X-rayimage and terahertz images of an electronic chip are brought in the figure onthe right. Obviously the resolution of X-ray is higher than terahertz image,but X-ray is ionizing and can be impose harmful effects on certain objects suchas semiconductors and live tissues. To overcome low resolution of the terahertzsystems near-field terahertz imaging systems are under development. Innearfield imaging the detector needs to be located very close to the surface ofthe plane and thus imaging of the thick packaged objects may not be feasible.In another attempt to increase the resolution, laser beams with frequencieshigher than terahertz are used to excite the p-n junctions in semiconductorobjects, the excited junctions generate terahertz radiation as a result as longas their contacts are unbroken and in this way damaged devices can be detected.In this approach, since the absorption increases exponentially with thefrequency, again inspection of the thick packaged semiconductors may not bedoable. Consequently, a tradeoff between the achievable resolution and thethickness of the penetration of the beam in the packaging material should beconsidered.