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Terahertz Radiation

Terahertz radiation, also referred to as far-infrared radiation, is loosely defined as the region of the electromagnetic spectrum which lies between 100 GHz and 10 THz. This equates to a wavelength range of 3 mm to 30 μm or more appropriately for band structure engineered devices, in terms of energy, is approximately 0.4-40 meV. Often referred to as the terahertz ‘gap’ due to the few available sources, as it is the region where the two approaches for generating electromagnetic radiation meet - optical devices and electronics, shown in Figure 1. Terahertz frequencies are considered extremely high for electronics, demanding modulation speeds that push current device technology that is typically limited to the GHz range by the circuit resistance-capacitance product. At the opposing end of the scale, the majority of optical sources operate above 10 THz. With band structure engineered semiconductors, it is the creation of very low energy radiative transitions that is particularly challenging.

Figure 1: The terahertz region of the electromagnetic spectrum lies between the approach of electronic and optical devices1.

Applications

As terahertz technology matures, new applications continue to be realised. This section contains an overview of the major applications driving the research in this field.

Biomedical Imaging

The non-ionising nature of the far-infrared lends itself to safe biomedical in vivo imaging. It is thought that the strong attenuation of the radiation by water is the mechanism which produces the required contrast to allow the identification of skin cancer (containing a higher density of cells and therefore also water) from healthy tissue2. Furthermore, different imaging regimes are available. A reflection measurement (time of flight) can yield quite different data to an absorption measurement3. Figure 2 depicts examples of such biomedical applications.

(a) Diagnosis of basal cell carcinoma2.
(b) A human tooth in different imaging regimes3.

Figure 2: Biomedical terahertz imaging. Images courtesy TeraView Ltd.

Industrial & Security Imaging

The time domain nature of Terahertz Pulsed Imaging (TPI) makes three dimensional imaging possible by recording depth information as a result of reflection from intermediate surfaces4. Figure 3 shows TPI images of slices through a packaged integrated chip5. Conventional analysis of this type could not be done on a production line and would render the chip inoperable. Additionally, this form of non-destructive evaluation applies equally to large scale integration of devices present on the wafer before packaging6.

Figure 3: Slices through a packaged integrated chip by terahertz pulsed imaging5. Image courtesy TeraView Ltd.

The same technology is also applicable to personnel imaging for security purposes (for example at airports). Where x-rays are unsuitable for routinely imaging people, TPI has the capability to detect not only concealed metals but ceramics7. Figure 4 shows a terahertz image of a metal blade and ceramic disc from two different aspects, concealed in a synthetic fleece pocket.

Figure 4: Terahertz imaging of a metal blade and ceramic disc in a synthetic fleece pocket7. Image courtesy TeraView Ltd.

Spectroscopy

Far infrared wavelengths cover the vibrational and rotational modes of numerous molecules, making terahertz radiation suitable for a number of spectroscopy and sensing applications in the environment. Possibly the most financially lucrative applications are those involving pharmaceuticals and security screening. Terahertz spectroscopy has been shown to distinguish between different drug polymorphs8, aiding pharmaceutical research and patent filings, as well as possible uses in quality control of tablet coatings9. The detection and identification of explosives10, as well as chemical and biological warfare agents such as sarin11 and anthrax12 have been uniquely identified by terahertz spectroscopy. Figure 5 depicts examples of such spectroscopy based applications.

(a) Explosives spectra10.
(b) Sarin and Soman nerve agents11.

Figure 5: Examples of terahertz spectroscopy.

Sources

An overview of methods for generating terahertz radiation is given below. The discussion is by no means exhaustive, but includes the dominant technologies with an outline of the principle of operation for each.

Backward Wave Oscillators

Backward Wave Oscillators (BWOs) produce coherent photon emission by using electrons bunched in cyclotron motion under a large magnetic field within a vacuum tube. These devices emit at the lower end of the terahertz range, up to around 1.5 THz13, with output powers up to 12 mW14. Tuning over a narrow range is possible by varying the electron energy. BWOs principally operate in CW and are most often employed in spectroscopy applications. BWO systems are custom built and non-portable due to power supply requirements and fragile vacuum tubes, hence they are confined to scientific research. Furthermore, they have a limited lifespan, typically below 500 hours of operation.

Direct Multiplied Sources

Direct Multiplied (DM) sources take a sub-millimetre wave input and performs frequency multiplication to terahertz levels by means of a biased GaAs crystal embedded within an appropriate waveguide. Commercially available DM sources are very compact, just inches in length. Moreover, the sources operate at around 100 K, further aiding portability by the possibility for closed-cycle or Peltier cooling. Like the BWO, performance increases at lower frequencies. At the limit of 2 THz, 1 μW is possible, but at half this frequency powers increase by an order of magnitude15. DM sources are most commonly used as local oscillators in heterodyne detector systems.

Non-Linear Optical Methods

The two main methods for generating terahertz radiation via non-linear optical crystals are optical rectification and difference frequency generation (also known as frequency mixing or photomixing).

Photoconductive antenna are a method of optical rectification. They are room temperature, broadband terahertz emitters, operating between approximately 0.3 and 30 THz16 with peak powers of 1 μW17. They are the most common device for terahertz emission, usually found in time domain spectroscopy applications such as those discussed above. The device consists of a pair of metallic stripe contacts patterned on the surface of a sample of low temperature grown gallium arsenide (LT-GaAs). This antenna is subjected to an above bandgap laser pulse (typically 1 μm in wavelength from a Ti:Sapphire laser) of less than 100 fs, generating an electron-hole pair. The bias applied to the antenna (typically tens of volts) rapidly accelerates the carriers to produce a pulse of terahertz radiation. Output powers are therefore low due to the short interaction length carriers have between antenna contacts.

The frequency mixing method has a similar physical arrangement to that of the photoconductive antenna, but quite different properties. Instead of an expensive femtosecond laser, two laser beams are mixed to create a beat frequency on a single crystal photomixer. By tuning one of the beams, tunability in the terahertz is possible. Using a gallium selenide crystal with a sufficiently powerful Nd:YAG laser, peak powers of almost 70 W have been demonstrated with wide tunability of 0.2-5.3 THz18. Unlike the photoconductive antenna, operation is narrow band and CW operation is possible.

Optically Pumped Terahertz Lasers

Recent years have seen Optically Pumped Terahertz Lasers (OPTLs) progress from unstable bespoke systems to highly reliable, commercially available bench-top products. Lasing occurs between molecular rotational transitions in a low pressure gas cell, usually pumped by a 9-11 μm CO2 laser. Whilst discrete tuning is possible, it is done via the inelegant process of changing the pressure of the cell or the gas itself. CW operation is possible, with output in the range 0.4-8 THz and peak powers up to 180 mW19. Typical efficiency is less than 0.1 % resulting in significant heat loading requiring liquid coolant. Highly tuned systems have however managed to increase efficiency marginally, to around 1%20.

p-Germanium Laser

The p-type germanium (p-Ge) laser offers tunable coherent output in the range 1-4 THz with relatively high powers up to 10 W21. Population inversion occurs between either two light-hole Landau levels or a light-hole and heavy-hole subband. A strong magnetic field (of the order of one Tesla) is required to lift the valence band degeneracy, crossed with an electric field (around 2 kV cm-1) for charge pumping. Such a system is easily perturbed by phonon scattering, thus operation is limited to 4.2 K or below, with performance rapidly decreasing at higher temperatures. The heat loading in the germanium crystal further compounds this problem and limits the laser to relatively short pulse lengths (tens of microseconds) and slow repetition rates (tens of kHz).

Developments on semi-portable systems with permanent magnets and requiring no liquid helium supply22,23 have proved promising but performance is lacking. Primarily the requirement for liquid helium (an expensive commodity) and the bulk of a pulsed magnet system confines this laser to institutions dedicated to terahertz research.

Silicon Impurity State Laser

The silicon impurity state laser consists of a n-type silicon crystal doped around 1015 cm-3 which is optically pumped by a CO2 laser. Depending on the dopant used, a number of discrete wavelengths are available: 5.4 THz for phosphorus, 5.2 THz for antimony, 5.9 and 6.6 THz for arsenic and 5.8, 6.2 and 6.4 THz for bismuth. A small degree of tuning (tens of GHz) is however possible by the application of a magnetic field or strain. The relatively long-lived impurity states facilitate population inversion, but the resonant phonon-assisted relaxation employed in these systems is quickly degraded by thermal energy. Operation is therefore limited to pulsed mode, below approximately 20 K, with powers up to tens of milliwatts24.

Free Electron Laser

First demonstrated in 197725, free electron lasers account for the most powerful and tunable terahertz sources. However, the size, cost and complexity of such lasers confines them to major research institutes. As of 2004, there were only 43 operational free electron lasers in the world, with a further 25 proposed or under construction over the next decade26.

The technology is described in detail in Chapter~\ref{chp:lifetimes}, however an outline of the principle is as follows. A relativistic beam of electrons is injected through a periodically varying magnetic field, within an optical resonant cavity. Photon emission is produced by the transverse deflection of the electron, similar to the principle of the dipole antenna. The properties of such lasers make possible very high output powers and a large wavelength range of operation. The highest CW power achieved in the far infrared to date is 10 kW, with similar systems providing high power lasing from 10 nm (x-rays)27 to 10 mm (microwaves)28. The free electron laser has been shown to be a very scalable system. At many facilities work is ongoing to push operation to extreme powers (terawatts), wavelengths (sub-nanometre) and pulse duration (sub-femtosecond)29, although not all in the same system.

Quantum Cascade Lasers

The Quantum Cascade Laser (QCL) is the most modern, compact and convenient terahertz source and has recently become commercially available30,31. QCLs based upon III-V alloys have shown lasing from 3.4 μm32 to 161 μm33 and show great promise in realising the applications discussed above. Being the subject of this work, a detailed discussion of the technology follows in a later section.