Quantum Cascade amplifiers for high power Terahertz time domain spectrometry

Lead Research Organisation: University of Southampton
Department Name: School of Physics and Astronomy

Abstract

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.

Planned Impact

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.

Publications

10 25 50
 
Description Our research has resulted in a novel type of emitter for THz, lateral photo-Dember emitters that can increase the power performance of present spectrometers. This new emitter design matches the performance of the current state of the art technology. We have published 6 journal publications on the subject (from 2012 a total of 70 citations), I have given 7 invited seminars the last two years including a seminar in MIT, Boston, and a further 5 conference presentations. For our work we have published one invited review article in the journal of Physics D of IOP.
The emitters can be used in future THz spectrometers, also further investigation into their operation and interface with other THz elements will give even better results. We have found out during the grant that the lateral photo-Dember emitters where much more interesting and important than what we thought at the beginning. so we have investigated this part of the proposal much more than initially intended and we have demonstrated multiplexing them etc. Although we have done the coupling measurements with QCLs we have not so much investigated this subject because there were already a lot of publications from Dhillon and Barbieri in Paris so we decided that from a scientific perspective it would be much better if we expanded the investigation of the emitters
Exploitation Route The LPD large multiple emitters are already marketed by a company after our publication and I believe that they are an excellent design that can compete with current technology.
Sectors Aerospace, Defence and Marine,Electronics

URL http://www.thz.soton.ac.uk
 
Description Already Laser Quantum, a UK company active on lasers and THz has asked us for our emitters to investigate their performance. Also Protemics GMBH markets emitters based on our 2014 publication in Applied Physics Letters about multiple double metal lateral Photo Dember emitters {D. McBryde, P. Gow, S. A. Berry, M.E. Barnes, A. Aghajani, and V. Apostolopoulos, Appl. Phys. Lett. 104, 201108 (2014).} see protemics website in http://www.protemics.com/index.php/products/thz-emitters
First Year Of Impact 2013
Sector Aerospace, Defence and Marine,Electronics,Manufacturing, including Industrial Biotechology
Impact Types Cultural,Economic

 
Description DSTL UK FRANCE PhD
Amount £200,000 (GBP)
Organisation Defence Science & Technology Laboratory (DSTL) 
Sector Public
Country United Kingdom
Start 09/2012 
End 12/2016
 
Description Cavendish Cambridge 
Organisation University of Cambridge
Department Cavendish Laboratory
Country United Kingdom 
Sector Academic/University 
PI Contribution The collaboration is based on the growth of samples in Cavendish and our use for fabrication of emitters, lasers etc
Collaborator Contribution Molecular beam epitaxy
Impact The publications that we got together
Start Year 2010
 
Description Chemical engineering cambridge 
Organisation University of Cambridge
Country United Kingdom 
Sector Academic/University 
PI Contribution We provide emitters to test their performance in THz spectroscopy usually in cryostats
Collaborator Contribution Expertise - apparatus in Cambridge in THz -TDS Axel Zeitler
Impact publications to come
Start Year 2010
 
Description Teraview 
Organisation Teraview Ltd
Country United Kingdom 
Sector Private 
PI Contribution We have worked with TeraView on two KTS secondments on using outputs of our research mainly on THz refractive index algorithms to problems encountered by TVIiew
Collaborator Contribution Advice, in kind contribution of a Tsunami spectra Physics Laser, with pump and pulse picker
Impact General collaboration, transfer of knowledge
Start Year 2010