T-RAYS

INTRODUCTION

In physics terahertz radiation refers to electromagnetic waves sent at frequencies in the terahertz range. It is also referred to as sub millimeter radiation, terahertz waves, terahertz light, T-rays, T-light, T- lux and THz. The term is normally used for the region of the electromagnetic spectrum between 300 gigahertz(3x10¹¹ Hz) and 3 terahertz (3x10¹² Hz), corresponding to the submillimeter wavelength range between 1 millimeter (high-frequency edge of the microwave band) and 100 micrometer (long-wavelength edge of far-infrared light).

Fig:1.1 Range of THz ray

Like infrared radiation or microwaves, these waves usually travel in line of sight. Terahertz radiation is non-ionizing submillimeter microwave radiation and shares with microwaves the capability to penetrate a wide variety of non-conducting materials. Terahertz radiation can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics.

Fig 1.2 Frequency Spectrum of Tera Rays

It 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 for communications. In addition, producing and detecting coherent terahertz radiation was technically challenging until the 1990s.

1.1 Terahertz versus submillimeter waves

The terahertz band, covering the wavelength range between 0.1 and 1 mm, is identical to the submillimeter wavelength band. However, typically, the term "terahertz" is used more often in marketing in relation to generation and detection with pulsed lasers, as in terahertz time domain spectroscopy, while the term "submillimeter" is used for generation and detection with microwave technology, such as harmonic multiplication.

1.2 Why should anyone care about THz wave

Energy: Objects at room temperature (300 K) emit thermal energy in this range (6 THz). Half the luminosity and 98% of the photons released since the Big Bang fall into the Submillimeter and Far IR regions. Signature: From GHz to THz frequencies, numerous organic molecules exhibit strong absorption and dispersion due to rotational and vibration transitions. These transitions are specific to the targets and enable T-rays fingerprinting Safer: T-rays have low photon energies (4 m eV 1 THz, one millions times weaker than a x-ray photon) and will not cause harmful photo ionization in biological tissues.

1.3 Where would THz Have the Most Impact

Defense: homeland security, chemical and biological agents detection, explosives detection, see-through-the- wall, imaging in space using satellites.

Commercial: biomedical, such as skin imaging for cancer detection, forgery, mail inspection, luggage inspection, gas spectroscopy, non-contact and non-destructive method.

Research: physics, plasma fusion diagnostics, electron bunch diagnostics, THz wave microscope, zero resistivity under THz radiation, Left Hand Materials (LHM) at THz range, THz .

1.4 SCIENCE BEAT:

The nation's premier source of low-energy or "soft" x-rays for scientific and technological purposes may also become a premier source of T-rays— far-infrared or terahertz frequency (trillion-cycle-per-second) radiation.

Plans to build a new synchrotron ring dedicated to generating high-powered T-ray beams at Berkeley Lab's Advanced Light Source (ALS) are moving forward thanks in part to a recent collaborative experiment at the Thomas Jefferson Accelerator Facility. In that experiment, a linear accelerator or linac was used to produce beams of terahertz frequency radiation thousands of times more powerful than beams obtained from tabletop and even free electron lasers.

"With this experiment, we've shown that the basic physics predictions behind the production of high-powered terahertz radiation in a synchrotron are correct," says Michael Martin, a Berkeley Lab physicist who, along with colleague Wayne McKinney, was a key participant in the Jefferson Lab experiment. "We're now confident we can design a terahertz ring for the ALS and make

Unless they're at a temperature of absolute zero, all objects, animate and inanimate, give off terahertz radiation, the heat from molecular vibrations. This "black-body" radiation is emitted at such low intensities — typically less than a millionth of a watt per square centimeter — that we're unaware of it. However, T-rays have immense scientific and technological importance because their spectral range embraces the vital interface between electronics and photonics. Using ultra-fast lasers and nonlinear crystals to generate coherent light beams, scientists have already put T-rays to good use for a variety of purposes that include the nondestructive imaging of biological and other materials and the manipulation of the electronic properties of semiconductors. It is widely believed that T-rays could be put to even better use if the power of T-rays beams could be substantially boosted

"With high-powered coherent terahertz beams we could make full-field, real-time, video-rate movies, which could be very useful in medical imaging," says Martin. "They should also be useful for security inspections, because terahertz radiation goes through most everything except metal and water, and you don't have the shielding issues you do for X-rays." In recent years, scientists have used fem to second lasers and semiconductors or nonlinear crystals to generate coherent pico second pulses of T-rays at about one-ten-thousandth of a watt of power. In the experiment at Jefferson Lab, using that facility's Energy Recovery Linac (ERL), researchers were able to sustain coherent T-ray beams at an average power of 20 watts over a broad bandwidth of far-IR frequencies.

"Coherent synchrotron radiation has been measured at linacs before," says Martin, "but this is the first time it's been measured at a linac that runs its electron beams at a repetition rate and current comparable to what you’d get in a synchrotron ring” Jefferson Lab's ERL is a superconducting radio-frequency electron accelerator that recovers the energy of spent electron bunches. The combination of superconducting accelerator cavities and energy recovery enable it to operate at a repetition rate as high as 75 megahertz, which means the average current of its electron beams, up to 5 milliamps, is much higher than the current of conventional electron linacs. In the T-ray experiment at Jefferson Lab, led by Gwyn Williams and reported in the November 14 issue of the journal Nature, very short bunches of electrons — about 500 femto seconds in pulse length — were accelerated to energies of about 40 million electron volts (MeV). These 40 MeV femto second pulses were then passed through the powerful magnetic field of a bending magnet with a one meter radius. This produced a sideways shove in their trajectory which caused the electrons to shed energy in the form of high-powered T-rays "Because this was a linac rather than a synchrotron, each pass of the electron beam through the bend magnet field carried new electrons, which meant a lot of timing jitter and current fluctuations that resulted in a low signal to noise ratio," says Martin. "With a synchrotron, where you have the same electrons passing through the bend magnets over and over again, the signal-to-noise ratio will be much higher. Nonetheless, this experiment was a proof of principle for our proposed coherent terahertz synchrotron ring, because it enabled us to make the optical measurements that needed to be made."

Berkeley Lab's ALS is an electron synchrotron facility consisting of three main components — a linac that can lift the energies of electrons to 50 MeV; a booster synchrotron ring for raising those electrons to nearly two billion electron volts; and a storage ring approximately 200 meters in circumference (660 feet) that can circulate the energized electrons for hours in a tightly constrained, ribbon-shaped beam no thicker than a human hair.

Beams of light are extracted from this stored electron beam through the use of bending, wiggler, or un dilator magnetic devices. While the ALS is optimized for the extraction of x-ray and ultraviolet light — its best X-ray beams are a hundred million times brighter than the light from X-ray tubes — its bend magnets also generate intense beams of photons at mid-infrared frequencies, between 20 and 300 terahertz.

Martin is the manager and McKinney is the spokesperson for the infrared beam lines at the ALS. They are also leaders in proposing that a dedicated T-ray synchrotron ring be built atop the ALS booster ring. This ring would measure about 66 meters in circumference, be stocked with 30 quadruple and a dozen dipole or bend magnets, and would make use of the same electron linac and booster ring used to fill the ALS storage ring. It would be optimized to operate as a coherent.
"Our terahertz ring would generate electron beams at much higher repetition rate (1.5 gigahertz) and higher current than the ERL at Jefferson Lab," says Martin. "It would produce terahertz light beams at an average of 50 watts of power to serve many beam lines simultaneously."The projected cost for constructing this new synchrotron ring runs between $10 and $20 million, which is about the cost of one to two new premier X-ray beam lines at the ALS."We've never had such powerful terahertz beams before, so it's difficult to say what the most important applications for it will be," Martin says. However, he adds, the scientific interest in high-powered T-ray beams is intense. He and his colleagues expect to submit to the U.S. Department of Energy a formal proposal for a dedicated T-ray ring at the ALS later this year.

SOURCES OF TERA-RAYS

Terahertz radiation is emitted as part of the black body radiation from anything with temperatures greater than about 10 Kelvin. While this thermal emission is very weak, observations at these frequencies are important for characterizing the cold 10-20K dust in the interstellar medium in the Milky Way galaxy, and in distant starburst galaxies. Telescopes operating in this band include the James Clerk Maxwell Telescope, the Caltech Submillimeter Observatory and the Submillimeter Array at the Mauna Kea Observatory in Hawaii, the BLAST balloon borne telescope, and the Heinrich Hertz Submillimeter Telescope at the Mount Graham International Observatory in Arizona. Planned telescopes operating in the submillimeter include the Atacama Large Millimeter submillimeter radiation restricts these observatories to very high altitude sites, or to space.

As of 2004^[update] the only viable sources of terahertz radiation were the gyrotron, the backward wave oscillator ("BWO"), the far infrared laser ("FIR laser"), quantum cascade laser, the free electron laser (FEL), synchrotron light sources, photo mixing sources, and single-cycle sources used in Terahertz time domain spectroscopy such as photoconductive, surface field, Photo-dember and optical rectification emitters. The first images generated using terahertz radiation date from the 1960s; 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.

In mid-2007, scientists at the U.S. Department of Energy's Argonne National Laboratory, along with collaborators in Turkey and Japan, announced the creation of a compact device that can lead to a portable, battery-operated sources of T-rays, or terahertz radiation. The group was led by Ulrich Whelp of Argonne's Materials Science Division. This new T-ray source uses high-temperature superconducting crystals grown at the University of Tsukuba, Japan. These crystals comprise stacks of Josephson junctions that exhibit a unique electrical property: when an external voltage is applied, an alternating current will flow back and forth across the junctions at a frequency proportional to the strength of the voltage; this phenomenon is known as the Josephson effect. These alternating currents then produce electromagnetic fields whose frequency is tuned by the applied voltage. Even a small voltage – around two milli volts per junction – can induce frequencies in the terahertz range, according to Welp.

In 2008 engineers at Harvard University announced they had built a room temperature semiconductor source of coherent Terahertz radiation. Until then sources had required cryogenic cooling, greatly limiting their use in everyday applications.

2.1 Far- Infrared Sources and Sensors

· Far-Infrared Sources:

ü Free electron laser

ü Gunn oscillators

ü Photoconductive antenna (more power)

ü Electro-optic crystal (optical rectification)

· Far-Infrared Detectors:

ü Bolometer

ü Pyro electric detector

ü Photoconductive dipole antenna (greater sensitivity)

ü Electro-optic crystal (more bandwidth)

2.2 First room-temperature semiconductor source of coherent Terahertz radiation:

Fig 2.2: Semiconductor source of coherent Terahertz radiation

A photograph a bar with 10 terahertz laser sources developed by the Harvard University engineers. One of the lasers is connected to the contact pad (seen on the left) by two thin gold wires. A 2mm-diameter Silicon hyper-hemispherical lens is attached to the facet of the device to collimate the terahertz output. The emission frequency is 5 THz, corresponding to a wavelength of 60 microns. Credit: Courtesy of the Capasso Lab, Harvard School of Engineering and Applied Sciences.

2.3 High-power terahertz radiation from relativistic electrons:

Terahertz (THz) radiation, which lies in the far-infrared region, is at the interface of electronics and photonics. Narrow-band THz radiation can be produced by free-electron lasers1 and fast diodes2,3. Broadband THz radiation can be produced by thermal sources and, more recently, by table-top laser-driven sources4–6 and by short electron bunches in accelerators7, but so far only with low power. Here we report calculations and measurements that confirm the production of high-power broadband THz radiation from sub pico second electron bunches in an accelerator. The average power is nearly 20 watts, several orders of magnitude higher than any existing source, which could enable various new applications. In particular, many materials have distinct absorptive and dispersive properties in this spectral range, so that THz imaging could reveal interesting features. For example, it would be possible to image the distribution of specific proteins or water in tissue, or buried metal layers in semiconductors8,9; the present source would allow full-field, real-time capture of such images.

High peak and average power THz sources are also critical in driving new nonlinear phenomena and for pump–probe studies of dynamical properties of materials10,11. The THz region (1 THz < style="mso-spacerun:yes"> For example, a blackbody source at 2,000 K provides less than 1mWper cm21 of spectral power density for a typical spectroscopy application. Whereas narrowband sources have been available using free-electron laser (FEL) technology1,12, a significant advance in broadband THz sources has occurred over the past decade with the advent of coherent THz radiation emission from photo carriers in biased semiconductors. Table-top systems using optical rectification of femto second lasers either at high repetition rates5 or high peak power6 are routinely available. The present work describes a different process for producing coherent THz radiation by accelerated electrons. Like the method described in ref. 5, the process begins with pulsed laser excitation in GaAs, but makes use of photo emission to produce bunches of free electrons in space. Using the energy-recovered linac (ERL) at the Jefferson Laboratory FEL13, very short electron bunches (,500 fs) are brought to relativistic energies (,40MeV) in a linac and then transversely accelerated by a magnetic field to produce the desired THz emission as synchrotron radiation. This unique accelerator is capable of a running with a relatively high average beam current (up to 5 mA). Like the THz emitter described in ref. 5, the electrons experience a common acceleration. If the electron bunch dimensions are small (in particular, the bunch length is less than the wavelength of observation), we again obtain multi particle coherent enhancement14,15. Such coherent synchrotron radiation has been observed from electrons accelerated in linacs7,16–18, from compact waveguide FELs19 and from magnetic undulators19–21. Coherent THz radiation has been discussed, and observed from electron bunches in storage rings22–25, but not yet stably enough for use as a light source. Active programmes to study THz radiation from linacs or storage rings are underway at BESSY II (Berliner Elektronen speicherring- Gesells chaft fu¨r Synchrotrons trahlung;essy.de) and DESY (Deutsches Elektronen Synchrotron) in Germany, and at Brookhaven National Laboratory and Lawrence Berkeley National Laboratory in the USA. In addition, programmes are underway at ENEA-Frascati to generate broadband THz radiation by exploiting the distinctive properties of waveguide FELs which arise when the electron velocity is close to the group velocity of the wave packet26. Some linacs can create very short bunches (,1 ps) and produce coherent radiation up to a few THz, but most are limited to repetition rates of a few Hz, so the average power is quite low. The repetition rate for storage rings is of the order of 100 MHz, but the electron bunches are significantly longer (100 ps) owing to longitudinal damping through synchrotron radiation emission. Thus the emission is limited to the very low frequency regime (far-infrared), or arises from instabilities that briefly modify the bunch shape.

Our ERL accelerator system overcomes some of the limitations of conventional linacs and storage rings. Electron bunches as short as ,500 fs are produced by the standard technique of energy modulation (chirping) followed by compression in the dispersive region of a magnetic chicane27. The time taken for an electron bunch to pass through the accelerator is less than 1ms; thus longitudinal damping is negligible. But unlike most linacs, our system operates at a very high repetition rate (up to 75 MHz) by using superconducting radio frequency cavities and recovering the energy of the spent electron bunches13 so that the average current is orders of magnitude higher than in conventional linacs.

Figure 2.3 Comparison between coherent THz radiation generated by an 80-MHz conventional laser-driven THz source (a) and the relativistic source described here (b).

charge, a is the acceleration, c the speed of light and g is the ratio of the mass of the electron to its rest mass. With all factors except g the same, we see from equation (1) that for our case the power radiated by a relativistic electron exceeds that from a conventional THz emitter by a factor of g4 à 214 à 2 £ 105: In practice, the electron energy can be significantly larger, but this simply adds intensity at higher frequencies and leaves the low frequency (THz) intensity essentially unchanged. We mention again that both cases benefit from multi particle coherent emission, as many radiating charges are contained physically within one period of the emitted THz radiation. Theoretically, the more general expression for the power emitted by an electron bunch, as a function of frequency (q) and solid angle (Q), is derived by extending the classical theory of electrodynamics28 for a single electron, to a system of N electrons, thus14,15:

where b is the ratio of the velocity of the particle bunch to the velocity of light, nˆ is a unit vector along the direction of propagation (to the observer), r(t) is the location of the electron bunch centre, and N is the number of particles in the bunch. The term N2f (q) represents the coherent enhancement, and includes the form factor f (q) which is the Fourier transform of the normalized longitudinal particle distribution within the bunch; that is, where S(z) is the distribution

(3)

function for particles in the bunch, measured relative to the bunch centre. A practical solution to the second major term in equation (2) has been presented in ref. 29. For the present calculations (Fig. 2), we assume 40-MeV electron bunches each carrying 100 pC of charge, passing through a 1-m-radius bend at a 37.4-MHz repetition rate. For simplicity, we assume that each bunch has a Gaussian particle distribution of width j, yielding a Gaussian form factor: where lis the wavelength

(4)

of the light at frequency q. The bunches are not strictly gaussian in practice, but this approximation is useful for estimating the overall power and spectral content. The electron beam had an r.m.s. width and height of 1 mm. Given a natural emission angle, 1.66(l/r)1/3 of 0.11 rad at 1 THz, where r is the bending radius, the diffraction-defined source size was of the order of the electron beam size, and the emitted radiation had an extremely high degree of transverse spatial coherence. This allows for interferometer by a simple form of wave front division30. In our experiments using the ERLTHz source, the electrons were generated using the frequency-doubled output of a Nd:YLF laser (model Antares, made by Coherent) operating at, or a sub-multiple of, 74.8 MHz, and with an average power of a few watts. Light of wavelength 530 nm was incident on a negative electron affinity Cscoated GaAs cathode. The resulting photoelectrons were accelerated using a d.c. voltage of 300 kV into a superconducting linac , and accelerated to an energy of 40MeV. Although the electrons are initially emitted from the cathode with a pulse length of,40 ps full width at half-maximum, they become tightly bunched in the accelerator to pulse lengths less than 1 ps. After passing through the accelerator system, the electrons are decelerated in the same linac to an energy of 10MeV before reaching the beam dump, thus recovering most of the beam energy. This energy recovery allows average current of up to 5mA and electron bunches containing up to 135 pC, using an r.f. system nominally capable of accelerating only 1.1mA beam current. The ERL THz radiation was extracted from a dipole magnet of 1m bending radius immediately before the FEL cavity, the latter being unimportant for these experiments. For the total power measurements, the radiation left the accelerator vacuum chamber through a 10-mm-aperture diamond window subtending an angle of 20 £ 20 mrad relative to the source point. The emerging beam was focused onto a calibrated LiTaO3 pyroelectric detector, calibrated with equipment traceable to NIST. This detector had a nearly flat response (J25, Molectron) out to THz wavelengths owing to a black organic coating, and a nominal responsivity of 8.83V J21 (^2%). The spectral content of the ERLTHz radiation was analysed using a rapid-scan Michelson interferometer (Nexus 670, Nicolet) with a silicon beam splitter. The light was detected using a 4.2 K bolometer (Infrared Laboratories) with a 2mm £ 2mm boron-doped Si composite element, fed from a 12-mm-diameter f/4 Winston cone. It was fitted with a black polyethylene filter to ensure no radiation above 600 cm21 was detected. The diamond window on the accelerator was replaced by a larger crystal-quartz window to

Figure 2.4 Calculations of the average power emitted by a 10-mm2 thermal source at 2,000 K

Increase the energy collection to 60 £ 60 mrad. A spherical mirror of 80 cm focal length produced a 48-mm-diameter collimated beam compatible with the interferometer optics. A switching mirror allowed a remote choice of source, namely the THz energy from the accelerator, or a T à 1,300 K thermal reference source (see below). For the spectroscopy experiments, the analysis and detection system did not have sufficient dynamic range to cover the seven decades in power difference between the two sources. But as mentioned earlier, the ERL THz source could be run at a precisely defined lower repetition rate. In this way we could reduce the average power without changing the spectral content. We chose to make measurements at 584 kHz, instead of 37.4 MHz, and at a charge per bunch of 34 pC instead of 100 pC, thereby reducing the ERLTHz power by a factor of {Ö34 £ 106Ü=Ö584 £ 103Ü} £ Ö100=34Ü2; or approximately 550. We have another reference point for determining the absolute power, as we were able to switch sources from the ERLTHz emission port to a 1,300 K thermal source (the spectrometer’s standard ‘globar’ source). This allowed us to measure the relative power using the same spectrometer and detection system. At a frequency of 12 cm21 we obtained a ratio of intensity fromthe ERLTHz source to that of the globar of 2 £ 104. To compare with the calculation, we multiply the results for the THz source by the reduction factor of 550, as discussed earlier. This implies a measured advantage of the ERL THz source over the globar of 107. The calculation predicts an enhancement of 0:6=Ö6 £ 1028Ü à 107: The data are shown in Fig. 3, and the result affirms the large ERL THz power. The level of agreement is somewhat surprising, as our simple arguments have ignored diffraction and detection efficiency. In fact, owing to uncertainties in the absorbance of the detector coating in the THz region, the reported data place a lower bound on the power measurement. One additional property of super-radiant emission from electrons is the dependence of the intensity on the square of the number of particles per bunch from equation (2). In Fig. 4 we plot the integrated intensity as a function of bunch charge, which shows good agreement with the theoretical N2 curve. Finally, we measured the polarization of the emitted THz radiation. The intensity ratio for the horizontal to vertical polarization components is 3 for synchrotron radiation in the long-wavelength limit. This assumes full collection of the emitted radiation.We note that the dominant intensity is near 30 cm21, which has a natural opening angle of 86 mrad. As the emission pattern is ‘clipped’ by the 60-mrad collection optics, the calculated ratio is expected to be higher, approaching a value of 6. Using a wire-grid polarizer placed between the Michelson modulator and the detector, we measured a ratio of 5 and consider this to be good agreement. We have produced broadband, high-brightness, THz radiation with close to 1W per cm21 of average spectral power density into the diffraction limit, and peak spectral power densities about 104 times higher than this. However, one area of concern remains—the question of timing jitter and current fluctuations, and their effect on coherent detection and signal to noise ratio. As energy-recovery linac THz sources are being considered for other applications, some of which involve timing, it is important to evaluate timing jitter and current fluctuations. We have made preliminary measurements using a frequency analyser, and plan further detailed studies.

Figure 2.5 Comparison between measured (solid line) and calculated (dashed line) THz spectral intensity.

Figure 2.6 Measured THz intensity as a function of beam current (square symbols), showing the quadratic dependence expected for coherent emission (solid line).

T-RAYS IN SECURITY

A lot of movies show a typical feature which aids the detectives to see through enemies’ camps. This is in the form of a goggle, which would not raise the concern of the adversary, and at the same time, the hero is able to see where the weapons are hidden and in several cases, disable the bombs. X-rays are used to provide this sort of a facility, which is purely fictional. The use of X-ray is limited and the machine cannot be contained to a small goggle.

But scientists are now developing a new technology which might help to an extent in this matter. By the use of terahertz rays, the scientists are hoping to detect all the security threats, like weapons and bombs which might go undetected in the present days. Terahertz rays extend from the upper limit of microwaves to the lower limit of infrared rays.

Fig 3.1 Vision of hidden arms

Now a days, X-rays are used to detect any fracture in the human body, and also in the security checks to check for any weapons. The use of X-rays are limited as they are harmful to the human body causing disintegration of DNA and hence cancers may be precipitated. Terahertz rays or T-rays are considered harmless, that is, they do not predispose the human body to cancers and hence can be used freely. This benefit also applies at the security checks at any airports or public places where T-rays would reflect on any weapons and can be detected easily. For that matter, T-rays have an additional benefit of detecting even plastic objects which often go undetected in X-ray. Sciensists also claim that T-rays can tell the material of the item which is reflected, as it uses a technique called spectroscopy. In this new technology, every item would be reflected in a different color, so making out a particular item as harmful is easy.

T- rays are not required to be used close to the scanned item, as in the case of X-rays. It can be safely kept at a distance and scanning is done. A lot of research is going on to enhance the capacity of T-rays to use the spectroscopy at a distance.

Fig 3.2

The possibilities of T-rays are several. They can be used in the pharmacology industry to aid in detection of the contents of the medicines, used to detect tumors in a human body, aid in detecting any defect in the machines. These are just a few of the benefits.With a lot of benefits under its collar, one wonders what these T-rays are?

They are part of the electromagnetic spectrum, and one area which has been less studied till now. The band width of the T-rays can be between 500gigahertz and 10 terahertz.

Studies have been going on to analyze T-rays, recently particle accelerators have been used to break up the rays and study them.

New technology are being developed to contain the T-rays and direct them accordingly. Also a cost effective method of production need to be developed. The use of synchtron to bend the electrons and produce T-rays is under research. There are still concerns of cost and cooling of the rays, and the size which is important. In addition to these, creation of a continuous wave of T-rays is a challenge the scientists are facing as when T-rays were generated in pulses, it was seen So, how can this be done? Experiments are going on to see whether using two infra red laser machines can be used. The rays produced by the two machines combine to produce a T-ray. A new machine with indium-gallium-arsenide is under consideration.

Also a new technology called Quantum Cascade laser was developed in 1994, which produced rays even in the terahertz range. The wavelengths produced were directly related to the thickness of the semiconductor, which was the reason for success of these lasers. The working of the laser is as follows. An electron is put into the machine which goes to a level and shake up the protons, then fall into another level, emitting phonons. This cycle is repeated and a ray of phonons are produced.

Once the item is detected, a reflection image has to be produced as well. Experiments are going on to produce a detector to create images which can be easily captured by a video camera and reproduced.

There are defects in this new technology as well. But the positive factors are more than the negative factors, so this new technology would be of real help in detecting a lot of security threats.

3.1 T Rays vs. Terrorists: Widening the Security Spectrum:

The 10 August 2006 arrest in Britain of 24 terrorists bent on smuggling bomb components aboard airplanes and combining them en route is just the latest salvo in the Darwinian battle between developers of terrorist weaponry and those seeking to defeat them. The array of diabolical methods available to terrorists is truly terrifying, ranging from nuclear weapons and “dirty bombs” to biological and chemical weapons and explosives.

Detection and assessment of terrorist threats is generally possible today with enough time, money, and laboratory equipment, but the ideal technology would be fast, accurate, cheap, easy to use, and portable or able to remotely detect threats, with an emphasis on prevention. No technology now exhibits all these virtues, but under the pressure of terrorists’ inventiveness, researchers are working steadily to develop and apply improved systems.

Now researchers at Argonne National Laboratory are getting promising results from experiments using “T rays,” the terahertz (THz) part of the electromagnetic spectrum. In March 2006 Argonne announced that a research team there had shown for the first time that T rays can be used to identify explosives and poison gas precursors. The Argonne team also successfully used millimeter-wave radar to remotely detect airborne chemicals and the effects of radiation in the air. These results are currently being written up for publication.

T rays and millimeter waves are at the low-energy end of the electromagnetic spectrum, between microwaves and infrared frequencies. According to Nachappa “Sami” Gopalsami, a senior electrical engineer at Argonne and a lead researcher on the THz sensor project, the general characteristics of T rays and millimeter waves are the same. “But,” he says, “new physics and phenomena are beginning to be explored as we move up in frequencies.”

Although many detection techniques currently in use are based on electromagnetic radiation and mass spectrometry, T rays and millimeter waves have not previously been used in this context, mainly due to an inability to generate broadband pulses in these frequencies. In the Argonne experiments, however, THz spectrometry sensors provided unambiguous identification of explosive chemicals, including TNT and plastic explosives. Gopalsami says this method is “highly specific” and will eliminate interference from confounding elements.

The Argonne team has been collaborating with researchers at Dartmouth College, Sandia National Laboratory, Sarnoff Corporation, and AOZT Finn-Trade of St. Petersburg, Russia. Funding has come from the U.S. Air Force, the Department of Energy, and the Department of Defense. But although national security imperatives are the driving force behind current research, many of the resulting technologies could also prove useful in environmental health applications.

Being able to remotely detect and identify chemicals will be helpful in monitoring gas pipeline leaks, chemical plants, vehicle emissions, and the like. Gopalsami says the T ray technology can detect some of the most important environmental hazards including ozone, volatile organics, and cyanide compounds. Medical applications, particularly imaging techniques for body tissues and teeth, are also in the offing, especially because the THz zone is on the opposite end of the electromagnetic spectrum from X rays, and thus of lower energy and far less damaging to living tissue.

3.2 Detection difficulties:

Technical problems plague many existing detection methods. For example, X rays can penetrate almost anything but can harm the object being studied, and in living organisms they may damage DNA and cause cancer. Laser and other optical instruments are less harmful, but their performance can be affected by wind, humidity, fog, and smoke.

Just tracking terrorists’ movements is a nightmare. In a paper presented at the March 2002 Conference on Technology for Preventing Terrorism, David Dye of Lawrence Livermore National Laboratory noted that the United States has 7,606 miles of land border and some 12,452 miles of coastline. Further, Dye reported, 633.7 million people entered the United States at the nation’s 361 ports of entry. Even in the months just after September 11, the Coast Guard boarded only about 35% of the 5,112 vessels entering U.S. ports. Wrote Dye, “The government simply cannot perform 633.7 million hand searches every year, no matter how great the threat.”

“Our biggest concern is explosives,” says Nico Melendez, a spokesman for the Transportation Security Administration (TSA). Melendez says the TSA started airport screening for explosives using what’s called an “air shower” system in the summer of 2004. In this system, passengers step into a booth-like portal that releases puffs of air aimed at their clothing and skin. An air sample is then collected and analyzed by an ion mobility spectrometer, which compares the air’s components against a database containing spectrographic profiles of target chemicals such as TNT, C-4, and Semtex. According to a 24 May 2006 press release from the Port of Portland (Oregon), 28 airports in the United States are now using air shower portals.

THz waves are also useful for passenger screening because they can penetrate beneath clothing to detect hidden weapons. Peter Adrian, a senior analyst with business consultancy Frost & Sullivan, says, “One of the historical problems with gas sensors [including ion mobility spectrometers] is that they can be affected by extraneous environmental factors.” Conventional mobility spectrometers searching for explosives and trace levels of chemical warfare agents can’t always pick the target signal out of the “noise” of the many other chemicals in the environment, such as perfumes, and may be susceptible to false positives, causing delays and passenger frustration.

Faster and more accurate identification of questionable materials is crucial to effective protection from terrorism. With too many false positives, people will become desensitized to the danger. At the same time, a false negative means the system has failed, with potentially devastating consequences. The TSA is currently funding Argonne research into replacing the ion mobility spectrometer with THz spectrometry, says Gopalsami, who adds that with proper funding the device could be taken into the field in two years.

3.3 Putting T rays to the task:

Argonne’s THz spectrometry technology measures the rotation of a molecule in the vapor or gas phase. Every molecule’s rotational pattern is unique, and exciting a molecule with T ray frequencies reveals the “fingerprint” for that molecule. A spectral identification algorithm uses the information to determine the specific compound being examined by matching it with a spectral library. One disadvantage of THz spectrometry, says Gopalsami, is that to be detected a molecule must be polar, or asymmetrical; methane, for example, cannot be detected this way because it is nonpolar, or symmetrical.

Quick and accurate identification of a molecule is easiest when the molecules are rotating unimpeded in gas or vapor form under pressures well below normal atmospheric pressure, so that collisions between molecules are decreased. This is easy to establish in a laboratory, but difficult in field conditions. However, the Argonne researchers were able to overcome this handicap with millimeter-wave frequencies, which are less sensitive to atmospheric conditions; their longer wavelengths (relative to cloud particles) cause less reflection and scattering of the millimeter waves. “Additionally,” says Gopalsami, “there are gaps or windows in the millimeter-wave spectrum in which common molecules in air are mostly transparent to the millimeter waves.” Using millimeter-wave frequencies, the team identified airborne poison gas chemicals from 60 meters away and chemicals related to nuclear weapons from 600 meters.

A major issue for counterterrorist sensor development is whether a sensor must have a physical sample or whether it can detect and analyze a substance at a distance. The former are called “point sensors,” and the latter are “remote” or “standoff” detectors. Chemical, biological, and explosive materials generally require a point sensor. However, in an experiment with AOZT Finn-Trade, the Argonne team was able to tell when a nuclear power plant 9 kilometers away was in operation or idle by measuring radiation-induced changes in the air around the plant. Those changes were observable using microwave radar, but the team is also experimenting with millimeter-wave radar to achieve higher sensitivity of detection.

Bioweapons also pose serious risks, and the development of sensors capable of rapid remote detection has been slow. The litany of known and possible biological agents is frightening, among them the viruses that cause smallpox, anthrax, plague, and Ebola hemorrhagic fever. Further, in an article in the 2006 special issue of EMBO Reports, authors Jonathan Tucker and Craig Hooper described how advances in protein engineering could make so-called fusion toxins another front-runner for terrorists. These custom-made “designer” poisons unite two or more naturally occurring toxins, such as ricin and botulinum, to create a toxin significantly more toxic than either parent. Not only that, but unless counterterrorist researchers can stay abreast of possible combinations, a fusion toxin could be invisible to a sensor looking for a match in a preexisting library.

For bioweapon detection, Argonne researchers are working on a sensor based on dielectric properties of molecules. Dielectric materials are nonconducting and exhibit a complex property called a dielectric constant that can be measured by resonator techniques. Furthermore, they resonate at particular frequencies. DNA appears to resonate strongly in the THz region; therefore, the dielectric approach may eventually enable early detection of biological molecules without the use of more complex and much slower biochips that rely on analytical tools such as polymerase chain reaction.

As new technologies are developed, they will not necessarily eliminate older methods. “It’s hard to make a categorical statement that one approach is better than the others,” says Dye. Because the range of terrorist weapons is so broad, he adds, “You’ll end up with niche applications.” In the swirl of national security challenges, however, using a new part of the electromagnetic spectrum offers rich promise for thwarting the terrorist arsenal—and likely will produce benefits for environmental health as well.

T-RAYS OVER X-RAYS:

Like x-rays, t-rays or terahertz waves see through most materials. They're also more versatile, safer and can even detect invisible tumors, concealed weapons and biological agents: While the 1890s saw the advent of x-ray technology, the past 12 months have witnessed substantial progress in another powerful imaging innovation—t-ray technology. T-rays are terahertz frequencies, which represent a little-used portion of the electromagnetic spectrum—and like x-rays, they can penetrate most materials. But t-rays are thought to be safer than x-rays. And because compounds react to terahertz radiation in distinctive ways, a t-ray-based imaging system can identify a concealed object's chemical composition. Because of its capabilities, "terahertz imaging is getting hotter and hotter," says Xi-Cheng Zhang, a terahertz pioneer at Rensselaer Polytechnic Institute. And the technology has potential for a more varied list of applications than that of x-rays, from detecting tumors, to identifying biological warfare agents, to spotting plastic explosives. T-rays can even see through paper and clothing, which means that a terahertz camera And t-ray-producing devices have improved substantially over the past year, thanks to several research projects. These new devices are better at emitting t-rays within a confined frequency band—a prerequisite for accurate chemical sensing and medical imaging—in sharp contrast to the terahertz sources currently on the market, which generally release many frequencies at once. Indeed, it's no easy feat to produce and detect terahertz frequencies, which are higher than microwaves but lower than infrared light. "You're never sure whether to use electronics-based or optics-based" technology, says Martyn Chamberlain of the University of Leeds in England, a prominent terahertz researcher. But with the past year's innovations, terahertz technology is zeroing in on its most near-term application—medical imaging. In fact, in one advanced project, TeraView, an England-based startup, employed t-rays to detect skin cancers that other imaging systems could not discern. In particular, the t-rays were able to spot tumors that develop invisibly.

Terahertz imaging can also be used to recognize unfamiliar biological materials, since biomolecules can't help but vibrate at terahertz frequencies, with each doing so in a characteristic manner. This happens because specific proteins soak up certain t-ray frequencies, which alter their molecular arrangement, thus giving them revealing terahertz "fingerprints." Sensors can pick up these fingerprints and display the protein's identity, making the technology useful in the automated detection of biological warfare agents, such as anthrax. Another possible application for t-rays is in chemical sensing, since other large molecules, such as polymers, also react to terahertz waves distinctively. In fact, QinetiQ of Farnborough, England, has made a terahertz camera that captures highly revealing images of people through their clothing. But the fact that proteins respond to t-rays may have implications for human exposure. To determine their effect on people, the European Union is funding a program, called Terahertz Bridge, which has already released reassuring preliminary results. Testing t-ray quantities that would be needed for bodily imaging, researchers have found no indication of permanent, x-ray-like tissue damage. "So far, it's safe," says Gian Piero Gallerano, coordinator of Terahertz Bridge.

And in other good news for terahertz advocates, t-ray instruments continue to get more precise. For example, Vermont-based Vermont Photonics has built a device that produces t-rays by transmitting an electron beam across the rippled surface of a conductor, with the beam causing electrons in the conductor to go up and down the surface's grooves and thus shake loose t-rays. The terahertz frequency generated can even be manipulated by changing the energy of the electron beam, says Vermont Photonics cofounder Michael Mross. The company says the device can be used to examine interactions between biomolecules for applications such as drug discovery.

Another innovative t-ray-producing device is the quantum cascade laser, which last year, Qin Hu, an MIT electrical engineer, used to generate a continuous terahertz beam at a narrow frequency. Such lasers are normally used to produce infrared light, and transplanting the technology into the terahertz region demands extremely precise control over materials. Indeed, while scientists must labor painstakingly to produce t-rays, nature does not have to try so hard. In fact, terahertz radiation is still proliferating throughout space from its starting point in the Big Bang. Notes Chamberlain, "The universe is full of this stuff." And in short order, humans may be able to find uses for it.

Frequencies around 1 THz have been difficult to achieve without a host of expensive components. This has prohibited their adoption in wide-use applications like airport screening stations. A research team at the U.S. Department of Energy's Argonne National Laboratory has now changed that. They've constructed a small, even battery powered device, which generates T-Rays have similar, but distinctly different, properties to X-Rays. They cannot penetrate through metal or water, but they can penetrate through common materials like leather, fabric, cardboard, paper and even up to half a centimeter into human flesh, allowing for some interesting medical applications. Each of these materials or substances give off a specific signal which can be identified by the scanning device. Whenever something is detected which is questionable, then a normal scan through the X-Ray machine, or a pat-down, would be desirable. However, for the majority of users in an airport screening station, for example, the intrusion would be far less than it is today.

4.1 How it works?

T-Rays propagate like radio waves or visible light. People who are exposed to THz radiation will suffer no ill effects, according to Argonne National Labs, because their emission strength is insufficient to ionize atoms. In X-Rays, it's this ionizing phenomena, which knocks electrons loose,

Fig 4.1

Scientists at Argonne National Labs were able create the new form of T-Rays using high-temperature superconducting crystals grown at the University of Tsukuba (Japan). The crystals comprise stacks of what are called Josephson junctions. These exhibit unique electrical properties, ultimately creating the Josephson effect. When voltage is applied, an alternating current begins to flow back and forth across the junctions at a frequency proportional to the strength of the voltage. The alternating currents produce EM fields. When they tune the voltage to around 2millivolts per junction, they emit frequencies in the THz range.
Each junction is about 1/10,000th the thickness of a human hair. It takes layer upon layer of these tiny junctions to produce a signal strong enough to be even remotely useful. And, while all of the junctions are alternating at the same frequency, during the initial experiments they were not in phase. The researchers had to figure out a way to get them all in sync, otherwise they'd all be canceling each other out, making the signal useless.

Fig 4.2

They turned to a natural side-effect of a resonance cup. By constructing cavities of a particular shape formed specifically for the frequency targets, when voltage was applied they all emit signals which are in phase. However, the researchers have since encountered another significant hurdle, one which remains even now. The energy being generated in the resonance cavities cannot be completely extracted. Much of it is lost, resulting in a low yield of usable signal. Current data shows usable frequencies between 0.4 and 0.85 THz, and even then at only a very low power of 0.5 microwatts. The researchers are hoping to get it up to 1 milliwatt (2,000x more powerful). If they can do that, they claim that a wide-ranging host of low-cost, easily adaptable applications will be possible. Theoretically, this may be possible with today's design, provided they can extract more of the signal generated in the resonance cavities.

APPLICATION OF TERA RAYS

5.1 OPTICAL DISK OF TERA BYTE:

Current optical disks, such as CD (700MB), DVD (7GB) and Blue Ray-DVD (25GB), employ writing of spots by focused laser beam, where each spot has one data bit.This technology is limited by the diffraction limited minimum spot size achievable. A digital holographic disk (DHD), employing holographic principles for the storage and retrieval of huge amounts of data, is envisaged as the next generation optical storage device after Blue Ray-DVD. Here, each spot has one page of data with >1 Mbits of data per each page. It offers storage capacities of the order of many Terabytes on a disk and can support transfer rates in Gbps in comparison to currenttechnology's few Mbps. Unique correlation capability of holographic storage enables data search at tens of Gbps.

5.2 MEDICAL IMAGING:

[1] Terahertz radiation is non-ionizing, and thus is not expected to damage tissues and DNA, unlike X-Ray Some frequencies of terahertz radiation can penetrate several millimeters of tissue with low water content (e.g. fatty tissue) and reflect back. 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.

[2] 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.

5.3SECURITY: Terahertz radiation can penetrate fabrics and plastics,so it can be used in surveillance, such as security screening, to uncover concealed weaponson a person, remotely. This is of particular interest because many materials of interest have unique spectral "fingerprints" in the terahertz range. This offers the possibility to combine spectral identification with imaging. Passive detection of Terahertz signatures avoid the bodily privacy concerns of other detection by being targeted to a very specific range of materials and objects.

5.4 SCIENTIFIC USE AND IMAGING:

[1] Spectroscopy in terahertz radiation could provide novel information in chemistry and biochemistry.

[2] Recently developed methods of THz time-domain spectroscopy (THz TDS) and THz tomography have been shown to be able to perform measurements on, and obtain 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. However, THz-TDS produces radiation that is both coherent and broadband, so such images can contain far more information than a conventional image formed with a single-frequency source.

[3] A primary use of submillimeter waves in physics is the study of condensed matter in high magnetic fields, since at high fields (over about 15 teslas), the Larmor frequencies are in the submillimeter band. This work is performed at many high-magnetic field laboratories around the world.

[4] Submillimetre astronomy.

[5] Terahertz radiation could let art historians see murals hidden beneath coats of plaster or paint in centuries-old building, without harming the artwork.

5.3 COMMUNICATION:

Potential uses exist in high-altitude telecommunications, above altitudes where water vapor causes signal absorption: aircraft to satellite, or satellite to satellite.

5.4MANUFACTURING:

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.

Way-out applications:

With the help of a T-ray imaging system made by a Michigan-based company called Picometrix, NASA can discover small defects of foam in the infamous space shuttle tiles. T-rays have astronomical applications as well. The Herschel Space Observatory a satellite due to launch in 2008 is the terahertz version of the Hubble telescope. In Chile, one of the world's largest telescope arrays, the Atacama Large Millimeter Array (ALMA), is being constructed; it will monitor terahertz wavelengths in hopes objects in the very early universe. However, T-ray technology is still in its infancy, and Mittleman warns against the danger of overselling its capabilities.

CONCLUSION

Since T-rays have low photon energies (4 meV @ 1 THz, one millions times weaker than a x-ray photon) and will not cause harmful photoionization in biological tissues.

Next a few years could be a Golden Year of THz Photonics.

THz have the most impact in homeland security, chemical and biological agents detection, explosives detection, see-through-the- wall, imaging in space using satellites. 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. Recently developed methods of THz time-domain spectroscopy (THz TDS) and THz tomography have been shown to be able to perform measurements on, and obtain images of, samples which are opaque in the visible and near infrared regions of the spectrum.

Further, there is the potential of increasing the terahertz output power to mili watt levels by optimizing the semiconductor nanostructure of the active region and by improving the extraction efficiency of the terahertz radiation.