Terahertz_An introduction

1. Terahertz frequency: A new
addition to electromagnetic
spectrum.
Prof .V.Krishnakumar
Professor and Head
Department of physics
Periyar University
Salem, India.

2. Aim of the talk
• Introduce the history of terahertz science.
• Properties.
• Application.

3. 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.

4. Terahertz E.M radiation / Rays /
Waves ?
1012 Hz = 300 μm = 33.3 cm-1 = 4 meV = 50 Kelvin
1. Unavailability of stable femtosecond
1. Unavailability of stable femtosecond
lasers.
lasers.
2. Unavailability of appropriate THz source
2. Unavailability of appropriate THz source
and detector
and detector

5. 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.

6.  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.

7.  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.

8. 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.

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

10.  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

11.  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.

12.  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.

13. Airport Scanners May Harm
DNA, Not Just Privacy and Dignity

16. "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.

17.  In medical
Terahertz 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

18.  In Manufacturing
Many 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.

19. 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.
 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
therefore
1000’s Gbit/s dates rates are on offer with terahertz carrier
waves.

20. 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.

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

22.  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

23. Safety of terahertz radiation
In 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.

25. End note
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.