Terahertz Technology

THE UNIVERSITY OF NEW SOUTH WALES AT THE AUSTRALIAN DEFENCE FORCE ACADEMY

TERAHERTZ TECHNOLOGY ZITE1291 ENGINEERING RESEARCH 1B CDF 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 operating at terahertz (THz) frequencies, generally defined as 100GHz - 10THz. This essentially unexplored region of the electromagnetic spectrum has received much interest in recent years from both the scientific and corporate world because of  the unique properties properties of THz waves. waves. These unique unique properties are proving proving to be very beneficial for many scientific fields and very profitable for companies developing the technology for commercial use. The ’THz gap’ has been studied extensively since the 1980’s and many applications have been proposed particularly in recent years due to the rapid development of semiconductor materials, laser technolog technology y and photonics photonics [1]. This report will cover cover the current state state of  THz technology technology based on available available literature. literature. Additionally Additionally,, analysis analysis of the behaviour of the radiation and the technical aspects of the devices will be used to examine examine limitations limitations on proposed applications applications and areas of potential potential research.

Technical Characteristics and Applications THz waves are capable of penetrating many dielectric (non-conducting) materials opaque to visible light [2]. THz waves waves can pass through through materials such such as cardboard, cardboard, plastics and fabrics. Accordingly Accordingly THz waves waves can be used to detect concealed weapons (see Figure 1) and could replace conventional metal detectors at airports and other other places requiring requiring such security security. These penetration penetration characcharacteristics have been exploited by many, including NASA which uses THz waves to inspect the protective foam used in their space shuttles and identify the thickness and any micro-stru micro-structura cturall variations variations [3]. It was imperfections imperfections in the foam surrounding the fuel tank which led to the Columbia disaster, meaning advancements in THz technology could save a lot of money and, most importantly, lives.

Figure 1: Hidden weapon detection with THz waves [4] Another Another character characteristic istic of THz waves waves is that they are non-ionising, non-ionising, meaning meaning that unlike X-rays they do no damage to human tissue or DNA. The company 1

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

Figure 2: Detection Detection of a tumor, tumor, an application application of TeraView eraView research into into distinguishing types of tissue. [8] TeraView eraView also produces an explosives explosives detection detection system. system. THz spectroscopy spectroscopy can be used to find the spectral features of a wide range of molecules [9], once these features are known it is possible to use these features to identify molecules remotely remotely.. Explosive Explosive detection detection systems which which use this process could be placed at airports and, using spectroscopy, remotely detect explosives (or any other material 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 eliminate the need for line of sight sight between between the emitter and detector detector [10]. Also of  considerable importance is the safety of using this radiation around humans. THz waves (meV photon energy) are non-ionising and so much safer than Xrays (keV photon energy), however tests will need to be done before devices are installed 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 While it is not possible possible to use THz wave wavess for long distanc distancee com commu munic nicati ation on on Earth (’last mile’ high-speed short-distance communications has been proposed however), at higher altitudes transmission of THz waves becomes almost lossless, making aircraft-to-aircraft, aircraft-to-satellite and satellite-to-satellite communications a very real possibility [1]. It may seem that the high attenuation THz waves experience in the atmosphere is a problem however the interaction of THz waves and the atmosphere can be 2

exploited exploited with the right technology technology.. A cities air quality quality, humidit humidity y, cloud cover and atmospheric chemical make-up can all be monitored remotely, from the ground or from a satellite satellite [11]. The National Institute Institute of Information Information and Communications Technology (NICT, Japan) is currently conducting research into remote sensing and is working towards a THz wave propagation model which includes includes development development of an atmospheri atmosphericc radiative radiative transfer transfer model [as above]. above]. A number of radiative transfer models exist in the microwave and infrared regions however there are a number of discrepancies between these and laboratory observation servationss of THz waves. waves. The current current models cannot cannot account for some of the absorption that occurs in the atmosphere (indicated by spectral lines) [12] and it is currently currently unknown unknown why this is the case. Anomalous Anomalous far-wing far-wing absorption, absorption, absorption absorption by water vapor dimers or larger cluster cluster and absorption absorption by collisions between atmospheric molecules have been proposed to explain this ’continuum absorption’ [13][14][15]. History has shown that many advancements in technology have been due to war. war. This This ma may y prove prove to be the case for THz techno technolog logy y as there are many many promising military applications of THz waves. Research projects funded by the US Army National Ground Intelligence Center [16] have found that the radar absorption coating on stealth aircraft is ineffective against THz waves and that THz radar can detect hidden military targets, for example dug-in tanks (see Figure 3) and land mines. Many other research and development programs are currently currently underway underway.. In June 2009, the US Office of Nav Naval al Research awarded awarded Raythe Raytheon on a contra contract ct to develo develop p a 100kW experim experimen ental tal Free Electro Electron n Laser Laser (FEL) for missile missile defence defence which which will operate at THz frequencies. frequencies. Additionally Additionally,, in April 2009, the US Navy awarded Boeing a $163M contract to develop an FEL ’directed energy anti-missile weapon’. In May 2009, The Defense Advanced Research Projects Agency (DARPA) awarded Northrop Grumman Corporation phase 1 of the $37-million Terahertz Electronics contract [17] which will involve developing technology for the high speed integrated circuits that will be used in THz 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. Since the THz gap exists between the regions of microwaves and visible light, in developing devices for use in the THz range both electronic and photonic devices have have been used and modified. In general, electronic electronic devices devices operate at the low end and photonic devices at the high end of the THz region. Types Types of source sourcess includ include: e: electr electron on beam sources sources - gyrotro gyrotrons ns [19 [19], ], Free Electron Lasers (FELs) [20] and Backward Wave Oscillators (BWOs) [21], Far Infrared (FIR) pumped gas lasers - optically or electrically pumped CO 2 lasers [22], solid state sources - electrically or optically pumped solid state (ceramics, glasses or crystals) crystals) lasers [23], semiconduc semiconductor tor lasers - Quantum Quantum Cascade Lasers(QCLs) Lasers(QCLs) [24] are most promising, promising, parametric parametric sources [25], Photomixer Photomixerss [26] and frequency multipliers - typically a solid state laser driving a PlanarSchottky diode frequency multiplier circuit [27] however frequencies are limited to 2 THz [28], [28], up to 2.5 THz has been been produced produced with with BWOs BWOs driving driving a chain chain of frequency multipliers [29]. A number of these devices are shown in Figure 4 comparing comparing their output p ower ower to their operation frequency frequency.. These are typical typical numbers only and it should be noted that cooling plays a major role in the output power.

Figure 4: Comparison of a number of continuous wave THz sources in terms of  their output power and operation operation frequency [30]. The most promising sources sources are FELs, pumped lasers and QCLs and details details of their operation will be covered here. Of course there are many other devices, the list above is certainly not exhaustive. exhaustive. Unfortunat Unfortunately ely it is impossible to do  justice to all research being done in such a rapidly expanding field. 4

Figure 5: Setup of an undulator, as used in a free electron laser. The periodically varying magnetic field forces the electron beam on an oscillatory path, which leads to emission of radiation [31]. Free Electron Lasers [20] work by accelerating a beam of electrons to relativistic tivistic speed and sending sending them through a magnetic magnetic structure. structure. The electrons electrons experience alternating magnetic fields causing them to oscillate and follow a sinusoidal sinusoidal curve curve (see Figure 5). As the electrons electrons oscillate they accelerate accelerate and hence produce an electromagnet electromagnetic ic wave. wave. At the start of the tube the EM waves waves are out of phase as the electrons are all accelerating at different times. However when the EM waves are emitted they constructively interfere with electrons further down the tube. This happens many times and causes electron electron ’bunching’, ’bunching’, where electrons electrons will form groups. groups. By the time they reach the the end of the tube they are emitting emitting photons photons in phase. phase. This produces coherent coherent radiation radiation [31] up to kilowatt level power [20]. Perhap Perhapss the best featur featuree of FELs FELs is that that they they are widely widely tunabl tunable, e, from from microwaves to X-rays [32]. The frequency is easily adjusted by either changing the speed of the electrons electrons before they enter the undulator undulator or changing changing the strength strength of the magnetic field [33]. A more widely used method of producing radiation at any frequency is the pumped laser. laser. Pumped lasers lasers consist of a gain medium. medium. The electrons electrons in the gain medium are ’pumped’ to a higher energy level. A decay is triggered and the device emits radiation proportional to the energy gap that the electrons decay across (see Figure 6).

Figu Figure re 6: Trans ransit itio ions ns of elec electr tron onss in the gas cavi cavity ty of a pumpe pumped d laser laser.. A metastable state is used to create population inversion and hence a coherent emission of THz radiation [34].

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In the case of a CO 2 pumped gas laser, a CO 2 laser is fired into a cavity filled with a gas. This gas (the gain medium) then lases at THz frequencies frequencies [35]. The emitted emitted frequency frequency is dependent dependent on the type of gas in the cavity cavity.. Because Because of this, the device is not very tunable although limited tunable sources have been demonstrate demonstrated d [36]. Output Output power is very limited and with a pump laser of  20-100W, an output from the gas cavity cavity of 1-20 mW can be expected expected although a 2.5 THz 30 mW source has has been demonstrated demonstrated [22]. [22]. The last THz source to be discussed is the Quantum Cascade Laser (QCL). QCLs, like FELs, are very tunable (typically (typically 800 GHz-100 GHz-100 THz) however however need to operate at temperatur temperatures es as low as 4 K [37] and have relatively relatively low power power output [38]. QCLs work on inter-sub-band transitions of a semiconductor structure, the structure is constructed so that adjacent materials have progressively lower valence valence bands (as shown graphically graphically in Figure 7). Their operation operation is relatively relatively simple: under the influence of an electric field an electron tunnels into a quantum 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. The process is very efficient as each step produces more optical gain and multiple photons photons are emitted per electron. electron.

Figure 7: Gain region of a QCL, shows electron electron energy versus versus position in the structure, the overall downward trend of energy towards the right-hand side is caused by an applied electric field. [39] Quantum Quantum Cascade Cascade Lasers are quite quite compact compact and have have a very narrow narrow linewidth linewidth [39], making them particularly particularly suitable suitable for application applicationss in spectroscop spectroscopy y. QCLs are being developed that can operate at room temperature which will make them even more commerc commercially ially viable [40]. QCLs operating at room temperature have reached milliwatt output levels while liquid nitrogen cooled devices have 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 reactive parasitics and circuit transit times. 2. Domination of resistance at high frequencies produce a large amount of  signal losses. 3. Physical Physical scaling scaling of tube sources. sources. 6

4. The need for large magnetic and electric electric fields and high current current densities densities in tube sources. 5. Cooling, cooling, cooling. cooling. Very few devices can operate reliably reliably or continuously at room temperature. Sensors also play an important role in THz technology however they are far more developed than sources at this time and so will not be discussed in depth. depth. The main method of sensing THz radiation is via thermal absorption, absorption, which can be used to change resistivity of a device, or change the volume or pressu pressure re of a gas, both of which which can be measured measured.. A photom photomixe ixerr can also be used to detect THz waves by mixing them with a beam of known frequency, the differences differences can be detected detected and the source beam decoded. decoded. Currently Currently there there are near-quantum-limited detectors that can measure both broadband or extremely narrowband 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 ( ≈ 26 meV). meV). Therefore, high sensitive devices must use cryogenic cooling for operation in the THz range [42]. There are also many other devices devices that fit into THz technology technology such as high speed integrated circuits, for processing data before it gets to the source and after it’s received by the sensor.

Conclusion Terahertz Technology is already being marketed, mainly for medical and military applications, and continuing advancements in the field will see many new applications being realised and many industries being revolutionised (particularly in terms of non-destru non-destructiv ctivee testing). testing). Technical echnical difficulties difficulties are being overovercome literally as we speak and it will not be long before there are as little problems problems with sources as there are with sensors. The most promising promising sources currently are Free Electron Lasers, CO 2 Lasers and Quantum Cascade Lasers, which which are already being used in the commercial commercial environmen environment. t. Only time will tell what great opportunities lie in this underdeveloped region of the electromagnetic spectrum, the THz gap.

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