Terahertz radiation

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For other uses of "T-ray", see T-ray (disambiguation).

Electromagnetic waves sent at terahertz frequencies, known as terahertz radiation, terahertz waves, T-rays, T-light, T-lux and THz, are in the region of the electromagnetic spectrum between 300 gigahertz (3x10¹¹ Hz) and 3 terahertz (3x10¹² Hz), corresponding to the wavelength range starting at submillimeter (<1 millimeter) and 100 micrometres (ending edge of far-infrared light).

Plot of the zenith atmospheric transmission on the summit of Mauna Kea throughout the range of 1 to 3 THz of the electromagnetic spectrum at a precipitable water vapor level of 0.001 mm. (simulated)

[edit] Introduction

Like infrared radiation or microwaves, these waves usually travel in line of sight. Terahertz radiation is non-ionizing and shares with microwaves the capability to penetrate a wide variety of non-conducting materials. They can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. They can also penetrate fog and clouds but cannot penetrate metal or water.

The Earth's atmosphere is a strong absorber of terahertz radiation, so the range of terahertz radiation is quite short, limiting its usefulness. In addition, producing and detecting coherent terahertz radiation was technically challenging until the 1990s.

The Terahertz Photonics and Electronics research group at the Institute of Microwaves and Photonics at the University of Leeds in the United Kingdom is one of the main centres of research into uses of Terahertz radiation. [1]

[edit] Sources

While terahertz radiation is emitted as part of the black body radiation from anything with temperatures greater than about 10 kelvin, this thermal emission is very weak. As of 2004 the only effective stronger sources of terahertz radiation are the gyrotron, the backward wave oscillator ("BWO"), the far infrared laser ("FIR laser"), quantum cascade laser, the free electron laser (FEL), synchrotron light sources, and single-cycle sources used in Terahertz time domain spectroscopy. The first images generated using terahertz radiation date from the 1960's; however, in 1995, images generated using terahertz time-domain spectroscopy generated a great deal of interest, and sparked a rapid growth in the field of terahertz science and technology. This excitement, along with the associated coining of the term "T-rays," even showed up in a contemporary novel by Tom Clancy.

There have also been solid-state sources of millimeter and submillimeter waves for many years. AB Millimeter in Paris, for instance, produces a system that covers the entire range from 8 GHz to 1000 GHz with solid state sources and detectors. Nowadays, most time-domain work is done via ultrafast lasers. .

[edit] Theoretical and technological uses under development

• Medical imaging:
  • Terahertz radiation is non-ionizing, and thus is not expected to damage DNA, unlike X-rays. Some frequencies of terahertz radiation can penetrate several centimeters of tissue and reflect back. Terahertz radiation can also detect differences in water content and density of a tissue. Terahertz imaging could allow effective detection of epithelial cancer and replace the mammogram with a safer and less invasive or painful imaging system.
  • Some frequencies of terahertz radiation can be used for 3D imaging of teeth and could be more accurate and safer than conventional X-ray imaging in dentistry.
• Because of terahertz radiation's ability to penetrate fabrics and plastics it can be used in surveillance, such as security screening, to uncover concealed weapons on a person, remotely. This is of particular interest because many materials of interest, such as plastic explosives, exhibit unique spectral fingerprints in the terahertz range. This offers the possibility of combining spectral identification with imaging. Some controversy surrounds the use of terahertz scanners for routine security checks due to the potential capability to produce detailed images of a subject's body through clothing.
• Spectroscopy in terahertz radiation could provide novel information in chemistry and biochemistry.
• The recently developed techniques of THz time-domain spectroscopy (THz TDS) and THz tomography have been shown to be capable of performing measurements on, and obtaining images of, samples which are opaque in the visible and near-infrared regions of the spectrum. The utility of THz-TDS is limited when the sample is very thin, or has a low absorbance, since it is very difficult to distinguish changes in the THz pulse caused by the sample from those caused by long term fluctuations in the driving laser source or experiment. On the other hand, the fact that THz-TDS produces radiation that is both coherent and broadband means that such images can contain far more information than a conventional image formed with a single-frequency source.
• There are potential applications to satellite telecommunications, and high-altitude communications (aircraft to satellite or satellite to satellite).
• Many possible applications of terahertz imaging have been proposed in manufacturing, quality control, and process monitoring. These generally exploit the fact that plastics and cardboard are transparent to terahertz radiation, so that it is possible to inspect packaged objects.
• One of the main applications of submillimeter waves in physics is the study of condensed matter in high magnetic fields since at high fields (say above 15 T), the Larmor frequencies are in the submillimeter band. This work is carried out at many high-magnetic field laboratories around the world.
• Another important application is in millimeter/submillimeter wave astronomy

[edit] Terahertz versus millimeter and submillimeter waves

One terahertz is 10¹² Hz. Conventionally the microwave band extends to 30 GHz or so. While the far-IR is nominally reckoned to start at around 1 THz. So the terahertz band lies between micowaves and the far-IR. On the other hand, in this frequency range the wavelengths of electromagnetic waves (in vacuum) are millimeter or sub-millimeter. So, logically, terahertz waves are the same thing as millimeter or submillimeter waves. However, in practice people who use the term terahertz are generally speaking of signals generated by ultrafast optical techniques or far-IR lasers. Focusing a sub-picosecond pulse on a photoconductive antenna of suitable dimensions will produce EM waves in the THz band. On the other hand, people who use the term millimeter or submillimeter waves are invariably speaking of sources and detectors based on harmonic multiplication of microwave signals.

[edit] References

• "Revealing the Invisible". Ian S. Osborne, Science 16 August 2002; 297: 1097.
• Article in Nature 14 November 2002 (local copy from the Jefferson Lab)]
• News and Views in Nature 14 November 2002 (local copy from the Jefferson Lab)
• Instrumentation for millimeter-wave magnetoelectrodynamic investigations... Review of Scientific Instruments, 2000

[edit] Books on millimeter and submillimeter waves and RF optics

• Quasioptical systems: Gaussian beam quasioptical propagation and applications, Paul F. Goldsmith, IEEE Press

• Millimeter wave spectroscopy of solids, edited by G. Grüner, Springer

• Detection of light: from the ultraviolet to the submillimeter, George Rieke, Cambridge

• Modern millimeter-wave technologies, Tasuku Teshirogi and Tsukasa Yoneyama, eds, IOS press

• Optoelectronic techniques for microwave and millimeter-wave engineering William Robertson, Artech

[edit] See also

• Terahertz time domain spectroscopy
• Heterojunction bipolar transistor
• HEMT
• Picarin lens

[edit] External links

• The Terahertz Science and Technology Network
• Terahertz radiation: applications and sources by Eric Mueller
• Blinded By The Light At 20,000 THz
• Microtech Instruments, Inc. For THz components and systems
• Shadowy T-rays: Hunting Tumors and Exploring the Universe
• The Center for Terahertz Research at Rensselaer Polytechnic Institute
• The Terahertz Photonics and Electronics research group at the University of Leeds
• The Terahertz Technology group at Chalmers University of Technology