Active Quasi-Optics for High-Power THz Science

Light is the most familiar manifestation of an electromagnetic wave. These waves extend continuously from radio and TV transmissions, through mobile communications and WiFi, to microwaves, infrared, light, an finally to ultraviolet and X-rays. For many of these waves, compact, high power, room temperature sources have been developed: the microwave oven and the laser are everyday examples. One part of this spectrum, the terahertz region, which lies between the microwave and infrared wavelengths, is technologically challenging as regards providing sources. Transistor devices, as used for radio, are not able to switch fast enough, and fundamental physics limits the power of sources that are bright in the infrared and optical regions. Our proposal aims to provide a source that will be compact, efficient, operate at room temperature in air, and which will be more powerful and cheaper than alternatives.

Realisation of the source will enable new science and facilitate technology developments. High power terahertz waves can be used in biochemistry, the new field of bio-electromagnetics, and in chemical synthesis where the application of the terahertz wave affects the way that a chemical reaction proceeds. Higher power is needed for pulsed radars, for example future ground and spaceborne cloud radars that will provide input to national Met Offices and climate modellers. There are also potential military, security and industrial applications, where the possibility to transmit power through an absorbing material may be critical. This is the basis of scanners found at airports, where terahertz radiation is not believed to be a risk to public health associated with the ionising radiation from x-ray alternatives.

How will the source be made? The novel approach depends on microscopic semiconductor devices, called Schottky diodes, which are designed specifically to generate harmonics of an input frequency. In other words, the output wave is a distortion of the input. We shall specifically design dual purpose antennas for use with these diodes. These novel antennas (which we have called "Multennas" - multiplying antennas) will receive an input signal from a lower frequency illuminating source antenna, couple it to the Schottky diode, and then preferentially retransmit the desired harmonic. As each diode can only handle a small amount of power, it will be necessary to combine the outputs of many diodes to create a powerful source. The proposed way is to pattern an array of flat antennas on a plate of terahertz dielectric material, and to solder the Schottky diodes in place. The driving terahertz waves will arrive through the plate, and the total emitted wave will be the sum of the contributions of tens or hundreds of elements. Design and fabrication of the "Multennas" is challenging precision work, and sophisticated software, dedicated apparatus and expertise is needed.

Scientists and engineers from two of the UK's leading research institutes, Queen Mary University of London (QMUL) and the STFC Rutherford Appleton Laboratory (RAL), have joined forces to tackle the terahertz source problem. A team of experienced personnel at QMUL, who possess the antenna design skills and test facilities, will undertake these aspects of the project. An initial challenge will be to improve existing software to be able to model novel multenna structures. At RAL, where the team specialises in the production of world class Schottky diode devices, bespoke diodes will be designed, fabricated and mounted to the antennas on the supporting plate. Other scientists at QMUL will add tiny light-activated tuning devices to the array, made of a novel plastic whose properties can be changed by light. These tuners are needed to improve the performance as a whole, and to compensate for inevitable variations between the individual Schottky devices. The same material will be used to introduce tuneability to other elements in the network of the novel source.

High power continuous wave (CW) THz sources are highly desirable for a wide range of applications ranging from spectroscopy, bio-sensing, medical and pharmaceutical applications to imaging/security applications where a large operational bandwidth is not of major importance. The CW sources can also be used in narrow band spectroscopy or frequency domain interferometry, especially in detecting biological or chemical processes. This research will provide a high power tuneable THz source suitable for application in many areas, including:
Biomedical - delivering information on the mechanisms of protein folding. Mis-folding and aggregation of an underlying protein can create various human disorders. Prion diseases, Ataxia, Alzheimer's, Parkinson's and some forms of cancer are believed to be linked to underlying protein misfolding. How proteins fold in order to perform a biological task is not well understood. THz spectroscopy is a method, which can give valuable information on protein folding that is not yet achievable through other methods. A major limitation to these and other THz applications is the attenuation of incident energy by bulk water, a problem that can often be overcome only by increasing source power. Higher power may enable more absorbing samples to be studied, perhaps with a higher equivalent water thickness.
Quality Control - enable more effective quality control in the pharmaceutical industry. THz Interferometry offers a valuable solution to Quality Control. Frequency domain based interferometers can be used as an alternative to existing time domain systems. Being based on principles that have been proven and accepted for optical interferometers, high power THz sources are able to offer higher penetration and improved resolution on these existing systems.
RF assisted chemistry - control of chemical/biochemical reactions. There is potential for the research proposed in this application to be used to control chemical/biochemical reactions. The energy of photons in the terahertz region allows access a region where rotational activity and vibrational activity of small molecule systems are both present (e.g., McIntosh, A.I. et al., "Terahertz Spectroscopy: A powerful new tool for the chemical sciences?" Chem. Soc. Rev., 41 (6), 2072 - 2082 (2012)) and so suggests the possibility of reaction path control by excitation at specific THz frequencies, offering potentially massive impact to the petrochemical industry in terms of higher yields.
Security - the interest in THz imaging for the detection of concealed weapons, explosives and chemical and biological agents has gathered momentum over the past few years. THz waves are readily transmitted through non-metallic media and many of the substances of interest have characteristic THz spectra. In addition, THz imaging incurs minimal health risks. The main advantage of THz over millimetre wave detection is much better spatial resolution. Higher power can increase range, scanning speed or resolution.
Organic Semiconductor Industry - the research should open new horizons for novel organic semiconductor applications as tuning elements at mm-wave and THz frequencies. Organic semiconductors sensitivity to light emission in addition to their extraordinary mechanical properties can be successfully utilized at an industrial scale.
Impacting the general public, especially the young generation, will be via communication and dissemination of research. Popularised results will be published in our ee4fn magazine and will show that electronic engineering can be interesting and connected to many other areas of everyday life.
QMUL will benefit by strengthening its on-going research and broadening the research capabilities within the antenna, physics and biochemistry groups through establishing a world-leading R&D project that addresses industrial priorities in sub-mm wave electromagnetic, the physics of polymers and organic semiconductors, bio-chemistry and THz science in general