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Terahertz radiation basics and Terahertz sources

This article describes Terahertz radiation basics mentioning terahertz sources. It covers terahertz(THz) electronic devices viz. terahertz spectroscopy, terahertz imaging, terahertz detector, terahertz transistor, terahertz antenna etc.

As shown in the figure-1 below terahertz(THz) frequency region exists above gigahertz region. It is between 0.1 to 10 THz. It separates electronics from the photonics. Terahertz electronic devices are used in medical, aerospace and other wide variety of applications. THz applications are ranging from physical (for security imaging), biological (cell formation), medical (cancerous tumour detection), communication (radar) etc.



Terahertz frequency in EM spectrum

Terahertz radiations are non ionizing and therefore considered to be safe for human beings. Following are the features of THz radiations:
➤They can penetrate in a wide variety of non conducting materials e.g. paper, clothes, plastics, ceramics etc.
➤It can also penetrate in the clouds and fogs.
➤They are strongly absorbed by metal and water.
➤They have higher resolution compare to microwaves.

The property of cloud and fog penetration of THz radiation is used in radars. They can produce high resolution images of objects through fog, cloud and dust storms in order to support in aircraft landing in such harsh environments.

The property of water absorption of terahertz radiations is used to distinguish varying water contents such as to differentiate between fatty meat and lean meat. This application is also employed in process & quality control as well as in biomedical imaging(i.e. terahertz medical imaging).

The proposed THz imaging applications are to be developed for screening passengers at the airports for security reasons as well as for detecting presence of cancerous cells in the human beings.

Many companies are exploiting commercial applications of terahertz electronics. The success of this area depends on fabrication of electronic devices which operates above 100GHz where in traditional electronic circuits no longer function.

Terahertz sources of radiations are as follows:
• BWO-Backward wave oscillator
• The gyrotron
• Quantum cascade laser
• FIR laser-Far IR laser
• synchrotron light sources
• FEL-Free electron laser
• single cycle sources as used in THz spectroscopy
• photo-mixing sources

One more type of CW terahertz source is the optimally pumped THz laser. These lasers are used around the world mainly for environmental monitoring, astronomy and plasma diagnostics. The field of time domain spectroscopy also relies on broadband short pulse THz source.

Other terehertz source technologies which have been investigated are as follows:

• Tunable CW THz source using photo mixing of near IR lasers.

• Direct multiplied sources will use millimeter wave sources and multiply their output to obtain THz frequencies. These DM sources are used as local oscillators which generates more than 1THz with approx. 1µW as output power. This is used in radio astronomy.

• The development is quantum cascade semiconductor lasers operating at wavelengths which fall in 4.4 THz region. These are derived from 1500 alternating layers of GaAs and Aluminium GaAs with 2mW of peak power.

• JILA instruments which generates terahertz radiation using ultrafast pulses of near IR laser light which will enter through lens and strikes semiconductor wafer studded with electrodes. This is invented by Joint Institute for Laboratory Astrophysics.

Terahertz(THz) Electronic devices

Following are typical terahertz electronic devices widely used for various functions.

THz transistor:
THz transistor technology is emerging now. This leads to bipolar transistors and high electron mobility transistors based on short channel Si CMOS and InGaAs. These terahertz electronic devices have cutoff frequencies and maximum frequency of oscillations in THz frequency range.

Terahertz detector:
Si schottky diodes demonstrate millimeter wave detection. This is used in terahertz(THz) mixer design.

Terahertz antenna:
One of the terahertz source of radiation is inter-digitated photoconductive antenna which can be tuned to frequency in the THz range.

terahertz spectroscopy and terahertz imaging:
Changing the spacing between electrodes in the antenna structure will enable emission to be centered as desired in THz range. This concept is used in THz spectroscopy and THz imaging. As it requires access to particular parts of THz frequency spectrum as needed.

Following terahertz electronic devices are available directly.
• FETs, HEMTs
• Magnetron, Carcinotron
• Schottky, varactor diodes
• Gunn, RTD, Impatt diodes


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