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Details of Grant 

EPSRC Reference: EP/J007676/1
Title: Quantum Cascade amplifiers for high power Terahertz time domain spectrometry
Principal Investigator: Apostolopoulos, Dr V
Other Investigators:
Researcher Co-Investigators:
Project Partners:
Department: Sch of Physics and Astronomy
Organisation: University of Southampton
Scheme: Standard Research
Starts: 24 February 2012 Ends: 23 August 2015 Value (£): 324,787
EPSRC Research Topic Classifications:
Optical Devices & Subsystems Optoelect. Devices & Circuits
EPSRC Industrial Sector Classifications:
No relevance to Underpinning Sectors
Related Grants:
Panel History:
Panel DatePanel NameOutcome
06 Sep 2011 EPSRC ICT Responsive Mode - Sep 2011 Announced
Summary on Grant Application Form
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.

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Organisation Website: http://www.soton.ac.uk