Terahertz Gas-Fiber Photonics

Lead Research Organisation: University of Bath
Department Name: Physics

Abstract

The terahertz (THz) part of the electromagnetic spectrum lies between the domains of microwaves and optics. In the last 20 years there has been enormous growth in research on exploiting this spectral window for materials research and practical applications like security and medical screening. Many applications rely on sources of extremely short pulses of THz radiation and are limited by their presently low peak and average power. The recently discovered phenomenon of terahertz supercontinuum generation during the formation of laser ionized plasmas in free space combines remarkably broad, continuous and controllable bandwidth, stretching from the far to the mid infrared, with pulse energies as high as a micro-Joule. It promises to be an important new source of high peak and average power terahertz radiation for scientific studies of materials in extreme THz fields and for imaging and sensing. Its potential exploitation is however constrained by the low optical to THz conversion efficency, which is limited by diffraction, and the cost and low pulse repetition rate of the high energy laser systems currently needed. Our primary aim is to overcome these limitations by spatially confining the THz generation to the hollow core of a gas filled waveguide based on a type recently developed in Bath known as photonic crystal fiber. The combination of small core area and long interaction length is expected to reduce the threshold pump energy for THz generation by orders of magnitude. The optical waveguide will be integrated with a terahertz guide. By engineering the velocities of the optical and teraherts waves by composite waveguide design to achive a condition called 'phase matching' we will at the same time increase the conversion efficiency and thus retain the high peak power of the THz radiation. A secondary aim is to perform sensitive detection in similar composite guides by exploiting an intinsic optical nonlinearitiy of gases, thus creating the essential building blocks of what could be, with the addition of powerful fiber amplifiers currently in commercial development, a cost effective and robust 'all-fiber' platform for THz science and technology with unprecedented power and flexibility. This is our long term vision and ambition.

Planned Impact

In the short term (3 or less years), the beneficiaries of the proposed research are intended to be entirely academic users of pulsed THz spectroscopy, sensing and imaging systems. The applications of these cover many disciplines including physics, chemistry, biophysics, engineering and archaeology. On a 3 or longer year timescales there could be economic and possibly societal benefits. This is because the aim of the project is to develop a powerful new THz instumentation platform which will encourage the development of new applications and and facilitate existing ones. Although applications of THz technology outside the academic sphere are in their infancy, a 2008 market survey by BCC Research estimates that the global market in THz systems will rise from $77 million in 2008 to $500 million by 2018. This estimate is based on an assumed expansion in the sales of spectroscopy and imaging systems, particularly in the pharmaceutical and security sectors, and the eventual emergence of new markets and applications such as manufacturing quality control, chemical or bio-sensing, medical screening and art restoration. The applications of THz technology are often over sold because many of the triumphs of THz measurements, particularly time domain ones, in the lab are difficult to transfer to the real world because of the lack of intense enough and cheap enough pulsed sources. For the most part, applications of pulsed THz sources still rely on micro-Watt average power technology that has not significantly changed in the last 20 years. This bottleneck is addressed by the proposed advances in peak and average power and (on a 5 year timescale) affordability.
The UK has a strong commercial presence in both the pulsed and continuous wave technology sectors of THz instrumentation via companies such as Teraview, ThruVision and Terahertz.co.uk. Start-ups and SMEs presently play the most significant commercial role. Thus, we expect that successful innovations in THz technology resulting from our research could be quickly exploited and play a part in increasing the competitiveness of the UK and fostering global economic performance. To a large extent this will depend on parallel developments in fiber laser technology. If the later can be made sufficiently cheap (there is no fundamental constraint on doing this) then the envisaged THz systems could be used for closer to real time quality control in the pharmaceutical, plastics and semiconductor industries. In the public sector, applications as varied as security screening, melanoma and burns assessment and art/archaeology conservation might also become more practical and competitive. Therapeutic treatments analogous to the microwave treatments for menorrhagia and liver cancer (both pioneered by the medical microwave group in the physics department in Bath) are other possibilities since focused micro-Joule energy THz pulses are intense enough to disrupt tissue.
The project will have a major impact on the careers of the postdoc and, assuming recruitment, PhD students who will be working at the cutting edge of THz and fibre photonics. It should therefore provide a source of highly skilled potential employees.

Publications

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Andrews S (2014) Microstructured terahertz waveguides in Journal of Physics D: Applied Physics

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Pan Y (2019) Interferometric Refractive Index Sensing with Terahertz Spoof Surface Plasmons in Journal of Infrared, Millimeter, and Terahertz Waves

 
Description We have developed an optically pumped source of intense terahertz radiation (and associated detection systems) based on laser ionization of gases which can be used to drive materials driven into non-equilibrium states and study their subsequent properties. It is supported by a variety of coherent THz detection systems and can be rapidly reconfigured for different types of experiment, including ones at low temperature using a recently installed closed cycle cooling system. I believe that this is a nationally leading facility. Currently I am applying it to study carrier dynamics in a family of layered ternary chalcogenide materials in a collaboration with the Rob Simpson's group in the Singapore University of Technology (SUTD) and Design. A seconded SUTD PhD student who spent 6 weeks in my lab pump-primed this project which has been continuing with an EPSRC CDY+T student who started in Oct 2018. Similar materials are widely used for optical recording and are being developed for non-volatile RAM because switching between phases with very different reflectivity and conductivity can be achieved with either short optical or electrical pulses. The objective of the work in Bath is to better understand the fundamental limits on optical switching speed and energy. This work has generated a large amount of data that is proving tricky to fully understand but will eventually lead =to some publications. In addition, experiments have been performed on THz induced transparency in graphene and THz induced impact ionization and intervalley transfer in indium antimonide, primarily to optimise details of the THz experiments by comparing results with similar work carried out using a different approach by 2 groups in the USA. The new PhD student is currently extended the applications of the system e.g. to studying two dimensional plasmons in strong THz fields and topological insulators and charge density waves exhibiting materials, all from the chalcogenide family.
Exploitation Route We hope to collaborate on exploiting the intense THz source already developed to study complex materials and have formed a collaboration with Rob Simpson at Singapore University of Technology and Design to take this further by exploring sputter deposited chalcogenide phase change materials, including superlattices. These materials are of significant technological importance, appearing in many consumer products but are surprising complicated and poorly understood at a fundamental level. The collaboration was kick started with secondment of a PhD student from SUTD for 2 months in 2017 and is continuing wi the help of a Bristol-Bath condensed matter CDT student who started work on the project in 2018. He has made good progress, making several conference presentations and an initial paper is approaching completion. To take the project further we plan to collaborate with MBE growers to obtain access to single crystal material and diversify to explore other chalcogenides such as topological insulators and ones supporting charge density waves.
Sectors Education,Electronics,Other

 
Description Basic Award
Amount $42,700 (USD)
Funding ID FA9550-15-1-0012 
Organisation European Office of Aerospace Research & Development (EOARD) 
Sector Public
Country United Kingdom
Start 11/2014 
End 12/2015
 
Title High intensity THz pump-THz probe spectroscopy tool 
Description We have set up a system for broadband high intensity THz pump-THz/optical probe time resolved spectroscopy as part of our investigation into high intensity plasma based THz source development. Two single-cycle-like THz sources are available; one with 200 nano Joule pulse energy centred at 8 THz (plasma source) and one with 100 nJ Joule energy centred at 1.5 THz (using an organic nonlinear crystal pumped at 1300 nm). The peak electric fields available are in the range 0.5 to 1 MV/cm. Contrary to a common belief the source amplitude noise is not significantly worse than can be obtained by optical rectification in nonlinear crystals. The system allows very high dynamic range (up to 100,000) optical pump -THz probe spectroscopy with detection bandwidths of 7 using electro-optic sampling in GaP or 20 THz using four wave mixing in butane. More modest dynamic range (up to 1000) THz pump-THz probe spectroscopy with 3 THz detection bandwidth has been demonstrated in experiments on graphene, InSb and 2D electron gases in transmission . A reflection geometry is also possible with either an optical or THz probe. The temperature range 20-300 K has been accessible using a closed cycle cooler since early 2020. 
Type Of Material Improvements to research infrastructure 
Year Produced 2017 
Provided To Others? Yes  
Impact Only recently perfected so few published results so far. Work on phase change materials as part of a PhD project is at an advanced stage. Experiments on 2D electron gases and charge density wave systems in the same project are at an early stage. It is clear that an increase in THz field strength would be of great benefit in THz pump experiment We believe that this can be achieved using light harvesting nanostructures fabricated on the surface of thin film samples, a possible area for future research. 
 
Title Data set for the paper 'High dynamic range hyper-terahertz detection with silicon photoconductors' 
Description Supporting raw data for the figures applied physics letters article. The data shown in figs 1 to 3 was created as described in the paper. The methodology is described in full. The different curves in each figure have separate file names which are self explanatory when referenced to the figures . Each column of data in the individual files has a header describing whether the data is frequency,delay or signal. Signal units are given in the figures. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
 
Description Terahertz driven phase change materials 
Organisation Singapore University of Technology and Design (SUTD)
Country Singapore 
Sector Academic/University 
PI Contribution We have hosted a PhD student (Mr Jitendra Behera) for 6 weeks (Feb/March 2017)to perform exploratory terahertz experiments on thin films of phase change alloys and supertlattices.
Collaborator Contribution Sample growth and PhD student time.
Impact none as yet
Start Year 2017
 
Description Terahertz polaritonic gain 
Organisation Imperial College London
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution Characterization of material using intense THz pulses
Collaborator Contribution Design and modelling of material structure and properties
Impact Proof of principle material growth at Sheffiled II-V facility
Start Year 2015
 
Description Invited seminar given at the Institute of Physics, Chinese Academy of Science, Beijing Nov 2016 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk given in the Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, Nov 2106, entitled 'High field Terahertz Science- current status and prospects'
Year(s) Of Engagement Activity 2016
 
Description Presentation at Light Matter Interaction Workshop, Tel-Aviv Nov 2016 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talks entitled 'Time domain terahertz photonics - overview of capabilities and areas of interest'. Workshop was established to foster new collaborations between Universities of Bath (physics) and Tel-Aviv (physics, engineering and chemistry). Reciprocal workshop will be held in Bath in 2017.
Year(s) Of Engagement Activity 2016
 
Description Progress in Electromagnetic Research Conference, Shanghai 2016 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Presentation entitled 'Comparison of detectors for nonlinear terahertz spectroscopy'. Follow up engagement with several audience members has lead to ideas for improvements in detector design - will be written up for publication when testing is completed.
Year(s) Of Engagement Activity 2016
URL http://www.piers.org/piers2016Shanghai/
 
Description Seminar J W Goethe Institute, University of Frankfurt 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Seminar entitled 'Time domain terahertz photonics' given during visit to terahertz research group of Prof Hartmut Roskos
Year(s) Of Engagement Activity 2013
 
Description Seminar at Max Born Institute for Nonlinear Optics and Ultrafast Spectroscopy, Berlin 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Seminar on 'Terahertz generation and detection in gases' given in research group of Michael Woerner at MBI. Visit was partly to discuss technical issues associated with modelling of terahertz generation in waveguides with Joachim Herrmann and his postdoc.
Year(s) Of Engagement Activity 2013
 
Description Terahertz Network Meeting, Warwick Dec 2018 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact Talk about the high THz field capability in Bath and applications. Subsequently have had request for advice on how to set up such systems from several university groups.
Year(s) Of Engagement Activity 2018
 
Description Terahertz network meeting, Corpus Christi College, Oxford Jan 2017 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Workshop/network meeting on space application of terahertz technology.
Year(s) Of Engagement Activity 2017
 
Description Workshop on High Field Terahertz Science, Pecs, Hungary 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Workshop that provided context and specific subject knowledge to the research programme in an area new to the PI. Some useful contacts made.
Year(s) Of Engagement Activity 2012