Terahertz frequency: A new addition to electromagnetic spectrum.

Prof .V.KrishnakumarProfessor and Head Department of physicsPeriyar UniversitySalem, India.

Aim of the talk

• Introduce the history of terahertz science.

• Properties.

• Application.

Terahertz spectrum

Terahertz mean trillion cycle per second. Terahertz radiation is sometimes considered a subset of infrared radiation. i.e., terahertz waves lies between long-wavelength of infrared and short-wavelength of microwave radiation.

Terahertz E.M radiation / Rays / Waves ?

1012 Hz = 300 m = 33.3 cmμ -1 = 4 meV = 50 Kelvin

1. Unavailability of stable femtosecond lasers.

2. Unavailability of appropriate THz source and detector

1. Unavailability of stable femtosecond lasers.

2. Unavailability of appropriate THz source and detector

Properties

Terahertz waves can penetrate through materials opaque to other parts of the EM spectrum. Packaging materials - including paper, cardboard, textiles, plastics, wood, ceramics, semiconductors, and dried and frozen materials are transparent to some degree.

Due to its comparatively low photon energy (4 meV at 1 THz), THz radiation does not initiate any changes in chemical structure, as opposed to UV radiation or X-rays for example. ... do not subject a biological tissue to harmful radiation.

In the same way that visible light can create a photograph, radio waves can transmit sound and x-rays can view within the human body, terahertz waves can create images and transmit information.

Characteristic properties of THz radiation are high penetration depth and low scattering combined with good spatial resolution. Resolution of THz wave is 1 mm.

The unique rotational, vibrational and translational responses of materials (molecular, radicals and ions) within the THz range provide information that is generally absent in optical, x-ray and NMR images.

Frequency of the THz wave is fall in the region of rotational frequencies of many gas molecules as well as the vibrational transition of weak bond.

Water has many strong absorption lines in the THz range.

Applications

Terahertz imaging - new steps toward real-life applications Radiation penetrates many common barrier materials enabling concealed objects to be seen. Wavelengths are short enough to give adequate spatial resolution for imaging or localisation of threat objects. Radiation at these frequencies is non-ionising and, at modest intensities, safe to use on people.

Nondestructive detection of illicit drugs using spectral fingerprints - The identification of the drugs is done by spectral fingerprinting.

Laser-terahertz emission microscope for semiconductor device under test. For inspecting electrical failures in large scale integration circuits (LSIs). The difference in pattern of laser and LTEM images shows the electrical failures in chips.

visible image

terahertz image

Water content measurement in plants and seeds,

Monitoring the leaf moisture level by measuring its transmission in the millimeter wave region is used in plant factory for monitoring the growing process.

Monitoring of water/ice state in food stuffs

Using difference in absorption coefficients for water and ice (ice absorbs three order of magnitude less than water) one can monitor the water/ice state in foodstuffs.

In security It is used for airport security to detect illegal substances hidden in people’s clothes. This would be a great alternative to the X-Ray scanners that some airports have started using as the terahertz waves are much lower in energy and much safer.

Airport Scanners May Harm DNA, Not Just Privacy and Dignity

"It's passenger imaging technology, so it allows us to see the entire image of the passenger's body and anything that might be hidden on the person”

The new technology includes new privacy protection also. The screener in the viewing room can't see the passenger's face and the images from the machine are deleted, once the traveler is cleared to fly.

In medicalTerahertz radiation can also detect differences in

water content and density of a tissue. Such methods could allow effective detection of epithelial cancer with a safer and less invasive or painful system using imaging. Some frequencies of terahertz radiation can be used for 3D imaging of teeth and may be more accurate and safer than conventional X-ray imaging in dentistry.

A mouse prostate section with tumor tissue (circle) as imaged with terahertz, optical, and staining techniques. The terahertz image shows significantly reduced absorption of terahertz radiation in this region compared to normal tissue, suggesting its usefulness for detecting tumors

In ManufacturingMany possible uses of terahertz sensing and imaging are proposed in manufacturing, quality control, and process monitoring. These generally exploit the traits of plastics and cardboard being transparent to terahertz radiation, making it possible to inspect packaged goods.

THz image of a chocolate bar contaminated with a buried glass, stone and metal splinter. Existing inspection systems such as ultrasound or X-ray may fail to detect such contaminants.

In Communication Current wireless systems utilise carrier waves less than 5 GHz which restrict their maximum data rate, 100’s Mbit/s typically.

Higher frequency carriers enable high data rates and therefore1000’s Gbit/s dates rates are on offer with terahertz carrier waves.

Moisture content in food is also of question, as the weight of food is increased if there is a higher moisture content. This is of particular concern with large quantities of dried foods, which could lead to significant changes to a company’s profitability and reputation. Thus new methods are needed in this case to increase safe food quality and production.

Terahertz spectroscopy

Explosive material may uniquely absorb at 1.15 THz and at 1.85 THz. Hence it is used to detect explosive material by sing its fingerprint images.

Organic molecules exhibit strong absorption from GHz to THz through rotational and vibration transitions providing fingerprints in the THz band.

Sub millimeter astronomy

THz detects cold matter (140 K or less), such as clouds of gas and dust in our and nearby galaxies. New stars beginning to form radiate heat as they contract and are clearly seen in the THz range.

Image shows the beginning of new star formation

Safety of terahertz radiationIn contrast to X-ray radiation, terahertz radiation is non-

ionizing and therefore safe for humans. X-rays are ionizing and therefore poses significant health risks.

THz exhibits an extremely low photon energy so that there is no danger that chemical bonds are broken up and that the examined material is changed.

The emitted power is very low leading to insignificant heating.

THz research will become one of the most promising research areas in the 21st century for transformational advances in imaging, as well as in other interdisciplinary fields. However, terahertz wave (T-ray) imaging is still in its infancy.

Compared to the relatively well-developed science and technology at microwave, optical and x-ray frequencies, basic research, new initiatives and advanced technology developments in the THz band are very limited and remain relatively unexplored.

Much of the equipment used in terahertz research is large and heavy, and requires special operating conditions— such as controlled temperature and humidity, and use of liquefied gasses— which make it hard to implement these systems in real-life applications. Research is under way to decrease the size and weight of sources and detectors, as well as to make terahertz systems less bulky and easier to manipulate and less demanding of special operating conditions.

End note