Quantum Cascade amplifiers for high power Terahertz time domain spectrometry

Terahertz (THz) light forms part of the electromagnetic spectrum, between microwaves and infrared. It can penetrate a range of materials - including polymers, ceramics and semiconductors - and shows excellent contrast in their internal microstructures. In addition, intermolecular vibrational modes in solids and hydrogen bonding networks in liquids all have resonances at THz frequencies. These unique properties of THz radiation have in recent years permitted new methods to study the interaction of molecules. However, a major limitation of this technique is the lack of high power broadband sources that can be used for spectroscopic imaging and tomography applications. Our proposed plan addresses precisely this problem.

Currently, most commercial THz spectrometers are time-domain spectrometers (TDS), where THz pulses are generated on antennas by a photocurrent created from a pulsed laser. The detection scheme uses a similar antenna where carriers are generated by the same pulsed laser. The advantage of this apparatus is that the detection scheme is synchronous: the receiver is only "on" when the THz electric field is incident, and this results in a very high signal-to-noise ratio of approximately 50 dB. The disadvantage is that the THz pulses have only micro-Watts of output power, thus the apparatus will only penetrate thin or transparent materials. The major competing THz technology is that of quantum cascade (QC) lasers, which generate radiation with tens of mWatts of power. However, the power advantage of QC lasers is lost by the lack of sensitive detection techniques, and hence they are not used commercially. Until now researchers have tried to combine the technologies of THz-TDS and QCs but the two geometries have proven very difficult to integrate, with antenna emitters in particular proving incompatible with integration.

However, a new geometry emerged in 2010: the so-called the lateral photo-Dember effect that can be used to generate broadband THz pulses. The effect is quite simple, relying on the different mobilities of holes and electrons in a semiconductor which create a changing dipole under photoexcitation to generate THz pulses. We believe that this effect has great potential because it is flexible and its geometry is compatible with integration and quantum cascade lasers. Using the lateral photo-Dember effect will provide an elegant means of coupling a THz pulse into the QC structure, directly, with great efficiency. We intend to exploit this effect and generate THz pulses directly on the facet of a QC cavity and amplify them in the QC waveguide. Therefore we will combine the high output power of quantum cascade lasers with the detection sensitivity and broadband nature of state-of-the-art time-domain technology. It is a game-changing approach that is, according to all indications, absolutely feasible. It is very rare to propose such a potentially high impact research route, which is at the same time such low risk!

Detailed THz spectroscopic studies of samples in our groups have demonstrated an excellent potential to reveal the microstructure of the materials which is key to its industrial performance; but non-destructive studies of the entire specimen are currently impossible due to limited available power. We will use the high power pulses generated by the QC amplifier to explore non-destructive imaging of samples of key importance to sustainable energy and healthcare research. We will apply it to non-destructive THz imaging and tomography across a range of materials, which it is not possible to achieve with today's instruments due to their inherent lack of power. Our research will make a fundamental contribution to explore novel routes to high power broadband THz devices and we will demonstrate how such technology can advance understanding of materials and processes in the chemical and pharmaceutical industries.

Terahertz time domain spectroscopy (THz-TDS) is currently emerging as an important technology, making the Terahertz region of the electromagnetic spectrum available for academic research and industry. The potential of this technology for the pharmaceutical, catalysis and medical sectors has already been demonstrated, both for research and quality control applications. A major shortcoming of THz-TDS however is that it has low power; this limits the applications to very low loss materials or short optical paths. In this project we aim to combine THz-TDS technology with the THz quantum cascade gain regions to produce high power THz pulses which are also phase resolvable. This system will create a step-change in the power available in THz-TDS and therefore the applications that are available for this technology. The realisation of this system will benefit the UK economically through UK-based companies' exploitation of the technology and socially through the improvements in healthcare diagnostics and green technologies.

A primary route for the impact of our work will be through the companies that we have partnered with. Pfizer will not only provide samples and advice during the project, but in addition will be in an excellent position to exploit the technology upon the successful completion of the project. The specific application we have identified is in the use of THz-TDS pulses for research and quality control of advanced tablet coatings that control the release of a drug. TeraView will provide access to their commercial spectrometers and expertise. TeraView's close involvement will ensure that the technology developed can be rapidly tested and integrated into commercial systems. An example application that also TeraView has studied is quality control of solar panels. Silicon solar cells are prone to defect formation during production, which reduce efficiency and increase production costs. The defects have very low contrast using standard techniques, such as IR imaging, but are much more clearly visible using THz radiation.

The rapid take-up of the new technology by these companies will not only benefit the UK economically through the improved competitiveness of these industries, but will benefit the UK socially too. Healthcare is already an important market for terahertz applications and the increased power available with the proposed system will greatly expand the number of uses that THz-TDS could find. These include enabling research, such as the development of advanced tablet coatings (mentioned above) and medical diagnostic imaging. Another potential benefit for the UK society is through research in green technologies and quality control in their production. During the project we hope to demonstrate the potential of the system for research into catalytic systems that are increasingly used to remove combustion products that are harmful to the environment. Johnson Matthey will provide samples and expertise that will allow us to demonstrate the applicability of this technology to the catalysis industry. This will enable new insights into the function of catalysis and improve production techniques. Therefore, over longer time scales we expect to see the technology being used for quality control of products. We expect the systems developed in this project will lead to the use of THz in online process monitoring in the pharmaceutical and energy industry. Our technology will develop an instrument with increased penetration depth of THz radiation, which can find applications in the security, healthcare and energy sectors.