Quantum Cascade amplifiers for high power Terahertz time domain spectrometry

Lead Research Organisation: University of Cambridge
Department Name: Physics


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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Description Thz quantum cascade lasers have been developed along with other devices such as THz modulators and quantum cascade amplifiers.
Exploitation Route In academic and industrial research
Sectors Aerospace, Defence and Marine,Agriculture, Food and Drink,Digital/Communication/Information Technologies (including Software),Pharmaceuticals and Medical Biotechnology

Description In the development and applications of quantum cascade lasers at THz frequencies. The development of active THz optical components such as modulators.
Impact Types Economic

Title Research data supporting "Examination of L-Glutamic Acid Polymorphs by Solid-State Density Functional Theory and Terahertz Spectroscopy" 
Description Experimental crystallographic information files (.CIF), Experimental terahertz absorption spectra, theoretical absorption spectra, Gibbs free energy curves. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
Title Research data supporting "Terahertz response of organic amorphous systems: experimental concerns and perspectives" 
Description Terahertz spectra for several amorphous materials over a range of temperatures, from below to above the glass transition: Figure 2 (fig2_data.csv): Absorption coefficient and refractive index of polyvinylpyrrolidone/vinyl acetate at 1 THz prepared as tablet and thin film from melt. Figure 5 (fig5_data.csv): Terahertz absorption of amorphous sorbitol, trehalose and BSA between 80 - 340 K at 0.8 THz. Figure 6 (fig6_data_THz-TDS.csv): Dielectric losses of amorphous sorbitol observed by terahertz spectroscopy. Figure 6 (fig6_data_synchrotron.csv): Dielectric losses of amorphous sorbitol observed at Diamond synchrotron facility. Figure 7a (fig7a_data.csv): Rescaled absorption coefficient of amorphous sorbitol between 80 - 420 K measured by terahertz spectroscopy Figure 7b (fig7b_data.csv): Refractive index of amorphous sorbitol between 80-420 K measured by terahertz spectroscopy. 
Type Of Material Database/Collection of data 
Year Produced 2015 
Provided To Others? Yes  
URL https://www.repository.cam.ac.uk/handle/1810/252491
Description BMS 
Organisation Bristol-Myers Squibb
Country United States 
Sector Private 
PI Contribution Measurement of model drug compounds for stability prediction
Collaborator Contribution Advice through regular teleconferences Supply of sample polymer material Complementary characterisation data
Impact The discussions with BMS have been extremely helpful for our team to learn the important parameters when characterising materials in an industrial context; this will help with subsequent impact when translating the measurement technique into industrial applications.
Start Year 2014
Description Copenhagen (collaboration with TR) 
Organisation University of Copenhagen
Country Denmark 
Sector Academic/University 
PI Contribution Measurement of amorphous drug material using THz-TDS
Collaborator Contribution Complementary characterisation using thermal analysis methods (DSC, TGA, etc.) Sample preparation by scary drying
Impact A number of joint international conference presentations have resulted (4 posters and 2 talks)
Start Year 2013
Description MedImmune (funding of 9 month PDRA) 
Organisation AstraZeneca
Department MedImmune
Country United Kingdom 
Sector Private 
PI Contribution We have investigated the applicability of terahertz spectroscopy to accurately determine the collapse temperature in freeze-dried product.
Collaborator Contribution MedImmune provided funding for a 9 month postdoc extension for JS. They supplied materials and training and we were able to use their facilities.
Impact Ongoing, one conference presentation (invited talk for JS) at the 40th IRMMW-THz conference in Hong Kong in August 2015.
Start Year 2015
Description Physics at work 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Physics at work takes place every year at the Cavendish Laboratory. A total of 2000 school students visit to listern to talks and demonstrations. My research group gives around 20 presentations to 25 students each year about semiconductor physics.

Heightened interest in science and particular physics amongst local school students. Physics undergraduates are currently at record numbers in Cambridge.
Year(s) Of Engagement Activity Pre-2006,2006,2007,2008,2009,2010,2011,2012,2013,2014
URL http://www-outreach.phy.cam.ac.uk/physics_at_work/