THE UNIVERSITY OF NEW SOUTH WALESAT THE AUSTRALIAN DEFENCE FORCE ACADEMY

TERAHERTZ TECHNOLOGY

ZITE1291 ENGINEERING RESEARCH 1BCDF LITERATURE REVIEW PROJECT

MIDN GERARD MARTIN, RAN

SUPERVISOR: DR. GREG MILFORD

23 OCTOBER 2009

Introduction

Terahertz Technology refers to the research and development of devices operat-ing at terahertz (THz) frequencies, generally defined as 100GHz - 10THz. Thisessentially unexplored region of the electromagnetic spectrum has received muchinterest in recent years from both the scientific and corporate world because ofthe unique properties of THz waves. These unique properties are proving tobe very beneficial for many scientific fields and very profitable for companiesdeveloping the technology for commercial use. The ’THz gap’ has been studiedextensively since the 1980’s and many applications have been proposed partic-ularly in recent years due to the rapid development of semiconductor materials,laser technology and photonics [1]. This report will cover the current state ofTHz technology based on available literature. Additionally, analysis of the be-haviour of the radiation and the technical aspects of the devices will be used toexamine limitations on proposed applications and areas of potential research.

Technical Characteristics and Applications

THz waves are capable of penetrating many dielectric (non-conducting) mate-rials opaque to visible light [2]. THz waves can pass through materials such ascardboard, plastics and fabrics. Accordingly THz waves can be used to detectconcealed weapons (see Figure 1) and could replace conventional metal detectorsat airports and other places requiring such security. These penetration charac-teristics have been exploited by many, including NASA which uses THz wavesto inspect the protective foam used in their space shuttles and identify the thick-ness and any micro-structural variations [3]. It was imperfections in the foamsurrounding the fuel tank which led to the Columbia disaster, meaning advance-ments in THz technology could save a lot of money and, most importantly, lives.

Figure 1: Hidden weapon detection with THz waves [4]

Another characteristic of THz waves is that they are non-ionising, meaningthat unlike X-rays they do no damage to human tissue or DNA. The company

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TeraView [5] was the first to commercially exploit THz radiation, they produceproducts that use spectroscopic imaging to characterise molecular structures [6]and enable 3D imaging of structures and materials. THz waves are particularlyuseful in diagnosis as they can pass through clothing and skin [7] and detectabnormalities such as cancers and tumours.

Figure 2: Detection of a tumor, an application of TeraView research into dis-tinguishing types of tissue. [8]

TeraView also produces an explosives detection system. THz spectroscopycan be used to find the spectral features of a wide range of molecules [9], oncethese features are known it is possible to use these features to identify moleculesremotely. Explosive detection systems which use this process could be placedat airports and, using spectroscopy, remotely detect explosives (or any othermaterial of interest, drugs for example). Due to the penetrability of THz waves,molecules can be detected through suitcases, boxes and other packing material.Currently being developed is a diffuse reflection technique which could elimi-nate the need for line of sight between the emitter and detector [10]. Also ofconsiderable importance is the safety of using this radiation around humans.THz waves (meV photon energy) are non-ionising and so much safer than X-rays (keV photon energy), however tests will need to be done before devices areinstalled in public places such as airports.

THz waves experience relatively high attenuation in the Earth’s atmosphere(particularly compared to microwaves which experience nearly no attenuation)as the water molecules in the air readily absorb radiation at THz frequencies.While it is not possible to use THz waves for long distance communicationon Earth (’last mile’ high-speed short-distance communications has been pro-posed however), at higher altitudes transmission of THz waves becomes almostlossless, making aircraft-to-aircraft, aircraft-to-satellite and satellite-to-satellitecommunications a very real possibility [1].

It may seem that the high attenuation THz waves experience in the atmosphereis a problem however the interaction of THz waves and the atmosphere can be

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exploited with the right technology. A cities air quality, humidity, cloud coverand atmospheric chemical make-up can all be monitored remotely, from theground or from a satellite [11]. The National Institute of Information and Com-munications Technology (NICT, Japan) is currently conducting research intoremote sensing and is working towards a THz wave propagation model whichincludes development of an atmospheric radiative transfer model [as above]. Anumber of radiative transfer models exist in the microwave and infrared regionshowever there are a number of discrepancies between these and laboratory ob-servations of THz waves. The current models cannot account for some of theabsorption that occurs in the atmosphere (indicated by spectral lines) [12] andit is currently unknown why this is the case. Anomalous far-wing absorption,absorption by water vapor dimers or larger cluster and absorption by collisionsbetween atmospheric molecules have been proposed to explain this ’continuumabsorption’ [13][14][15].

History has shown that many advancements in technology have been due towar. This may prove to be the case for THz technology as there are manypromising military applications of THz waves. Research projects funded by theUS Army National Ground Intelligence Center [16] have found that the radarabsorption coating on stealth aircraft is ineffective against THz waves and thatTHz radar can detect hidden military targets, for example dug-in tanks (seeFigure 3) and land mines. Many other research and development programs arecurrently underway. In June 2009, the US Office of Naval Research awardedRaytheon a contract to develop a 100kW experimental Free Electron Laser(FEL) for missile defence which will operate at THz frequencies. Additionally,in April 2009, the US Navy awarded Boeing a $163M contract to develop anFEL ’directed energy anti-missile weapon’. In May 2009, The Defense AdvancedResearch Projects Agency (DARPA) awarded Northrop Grumman Corporationphase 1 of the $37-million Terahertz Electronics contract [17] which will involvedeveloping technology for the high speed integrated circuits that will be used inTHz communications and radar systems.

Figure 3: THz radar imaging of military targets [18].

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Devices

Many devices exist for producing either continuous or pulsed THz waves. Sincethe THz gap exists between the regions of microwaves and visible light, in de-veloping devices for use in the THz range both electronic and photonic deviceshave been used and modified. In general, electronic devices operate at the lowend and photonic devices at the high end of the THz region.

Types of sources include: electron beam sources - gyrotrons [19], Free Elec-tron Lasers (FELs) [20] and Backward Wave Oscillators (BWOs) [21], Far In-frared (FIR) pumped gas lasers - optically or electrically pumped CO2 lasers[22], solid state sources - electrically or optically pumped solid state (ceram-ics, glasses or crystals) lasers [23], semiconductor lasers - Quantum CascadeLasers(QCLs) [24] are most promising, parametric sources [25], Photomixers[26] and frequency multipliers - typically a solid state laser driving a Planar-Schottky diode frequency multiplier circuit [27] however frequencies are limitedto 2THz [28], up to 2.5THz has been produced with BWOs driving a chainof frequency multipliers [29]. A number of these devices are shown in Figure 4comparing their output power to their operation frequency. These are typicalnumbers only and it should be noted that cooling plays a major role in theoutput power.

Figure 4: Comparison of a number of continuous wave THz sources in terms oftheir output power and operation frequency [30].

The most promising sources are FELs, pumped lasers and QCLs and detailsof their operation will be covered here. Of course there are many other devices,the list above is certainly not exhaustive. Unfortunately it is impossible to dojustice to all research being done in such a rapidly expanding field.

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Figure 5: Setup of an undulator, as used in a free electron laser. The periodicallyvarying magnetic field forces the electron beam on an oscillatory path, whichleads to emission of radiation [31].

Free Electron Lasers [20] work by accelerating a beam of electrons to rela-tivistic speed and sending them through a magnetic structure. The electronsexperience alternating magnetic fields causing them to oscillate and follow asinusoidal curve (see Figure 5). As the electrons oscillate they accelerate andhence produce an electromagnetic wave. At the start of the tube the EM wavesare out of phase as the electrons are all accelerating at different times. Howeverwhen the EM waves are emitted they constructively interfere with electrons fur-ther down the tube. This happens many times and causes electron ’bunching’,where electrons will form groups. By the time they reach the end of the tubethey are emitting photons in phase. This produces coherent radiation [31] upto kilowatt level power [20].

Perhaps the best feature of FELs is that they are widely tunable, from mi-crowaves to X-rays [32]. The frequency is easily adjusted by either changing thespeed of the electrons before they enter the undulator or changing the strengthof the magnetic field [33].

A more widely used method of producing radiation at any frequency is thepumped laser. Pumped lasers consist of a gain medium. The electrons in thegain medium are ’pumped’ to a higher energy level. A decay is triggered and thedevice emits radiation proportional to the energy gap that the electrons decayacross (see Figure 6).

Figure 6: Transitions of electrons in the gas cavity of a pumped laser. Ametastable state is used to create population inversion and hence a coherentemission of THz radiation [34].

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In the case of a CO2 pumped gas laser, a CO2 laser is fired into a cavityfilled with a gas. This gas (the gain medium) then lases at THz frequencies [35].The emitted frequency is dependent on the type of gas in the cavity. Becauseof this, the device is not very tunable although limited tunable sources havebeen demonstrated [36]. Output power is very limited and with a pump laser of20-100W, an output from the gas cavity of 1-20mW can be expected althougha 2.5THz 30mW source has been demonstrated [22].

The last THz source to be discussed is the Quantum Cascade Laser (QCL).QCLs, like FELs, are very tunable (typically 800GHz-100THz) however needto operate at temperatures as low as 4K [37] and have relatively low power out-put [38]. QCLs work on inter-sub-band transitions of a semiconductor structure,the structure is constructed so that adjacent materials have progressively lowervalence bands (as shown graphically in Figure 7). Their operation is relativelysimple: under the influence of an electric field an electron tunnels into a quan-tum well, transitions down a sub-level in the quantum well and emits a photon.The electron then tunnels into the adjacent well and the process continues. Theprocess is very efficient as each step produces more optical gain and multiplephotons are emitted per electron.

Figure 7: Gain region of a QCL, shows electron energy versus position in thestructure, the overall downward trend of energy towards the right-hand side iscaused by an applied electric field. [39]

Quantum Cascade Lasers are quite compact and have a very narrow linewidth[39], making them particularly suitable for applications in spectroscopy. QCLsare being developed that can operate at room temperature which will makethem even more commercially viable [40]. QCLs operating at room tempera-ture have reached milliwatt output levels while liquid nitrogen cooled deviceshave reached hundreds of milliwatts [41].

Current problems with sources include but are certainly not limited to:

1. High frequency roll off in traditional semiconductor sources due to reactiveparasitics and circuit transit times.

2. Domination of resistance at high frequencies produce a large amount ofsignal losses.

3. Physical scaling of tube sources.

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4. The need for large magnetic and electric fields and high current densitiesin tube sources.

5. Cooling, cooling, cooling. Very few devices can operate reliably or contin-uously at room temperature.

Sensors also play an important role in THz technology however they arefar more developed than sources at this time and so will not be discussed indepth. The main method of sensing THz radiation is via thermal absorption,which can be used to change resistivity of a device, or change the volume orpressure of a gas, both of which can be measured. A photomixer can also beused to detect THz waves by mixing them with a beam of known frequency, thedifferences can be detected and the source beam decoded. Currently there arenear-quantum-limited detectors that can measure both broadband or extremelynarrowband signals up to or exceeding 1 THz [42].

The main problem with sensors is that the photon energy of THz radiation(1.2-12.4meV) is much lower than Earth’s background radiation (≈ 26meV).Therefore, high sensitive devices must use cryogenic cooling for operation in theTHz range [42]. There are also many other devices that fit into THz technologysuch as high speed integrated circuits, for processing data before it gets to thesource and after it’s received by the sensor.

Conclusion

Terahertz Technology is already being marketed, mainly for medical and mili-tary applications, and continuing advancements in the field will see many newapplications being realised and many industries being revolutionised (particu-larly in terms of non-destructive testing). Technical difficulties are being over-come literally as we speak and it will not be long before there are as littleproblems with sources as there are with sensors. The most promising sourcescurrently are Free Electron Lasers, CO2 Lasers and Quantum Cascade Lasers,which are already being used in the commercial environment. Only time willtell what great opportunities lie in this underdeveloped region of the electro-magnetic spectrum, the THz gap.

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