Terahertz Radiation Basics and Sources

This article describes Terahertz radiation basics and its sources. It covers terahertz (THz) electronic devices like terahertz spectroscopy, terahertz imaging, terahertz detectors, terahertz transistors, and terahertz antennas.

As shown in Figure 1, the terahertz (THz) frequency region exists above the gigahertz region, between 0.1 to 10 THz. It bridges the gap between electronics and photonics. Terahertz electronic devices find use in medical, aerospace, and many other applications. THz applications range from physical (security imaging), biological (cell formation), medical (cancerous tumor detection), to communication (radar).

Figure 1: Terahertz frequency in EM spectrum

Terahertz radiations are non-ionizing and, therefore, considered safe for human beings.

Here are some features of THz radiations:

• They can penetrate a wide variety of non-conducting materials, e.g., paper, clothes, plastics, ceramics, etc.
• They can also penetrate clouds and fog.
• They are strongly absorbed by metal and water.
• They have higher resolution compared to microwaves.

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

The water absorption property of terahertz radiations is used to distinguish varying water contents, such as differentiating 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).

Proposed THz imaging applications are being developed for screening passengers at airports for security reasons and detecting the presence of cancerous cells in human beings.

Many companies are exploring commercial applications of terahertz electronics. The success of this area depends on the fabrication of electronic devices that operate above 100GHz, where traditional electronic circuits no longer function.

Terahertz Sources

Terahertz sources of radiations include:

• 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

Another 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 sources.

Other terahertz source technologies that have been investigated are:

• 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 that generate more than 1THz with approximately 1µW of output power. This is used in radio astronomy.
• The development is quantum cascade semiconductor lasers operating at wavelengths that fall in the 4.4 THz region. These are derived from 1500 alternating layers of GaAs and Aluminum GaAs with 2mW of peak power.
• JILA instruments generate terahertz radiation using ultrafast pulses of near-IR laser light which will enter through a lens and strike a semiconductor wafer studded with electrodes. This was invented by the 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 the 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 sources of radiation is the inter-digitated photoconductive antenna, which can be tuned to a 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 the THz range. This concept is used in THz spectroscopy and THz imaging, as it requires access to particular parts of the THz frequency spectrum as needed.

The following terahertz electronic devices are available directly:

• FETs, HEMTs
• Magnetron, Carcinotron
• Schottky, varactor diodes
• Gunn, RTD, Impatt diodes

Figure 2: Terahertz components