Ti:Sapphire Regenerative Amplified Laser System for ultrafast, high-field terahertz photonics

Lead Research Organisation: University of Leeds
Department Name: Electronic and Electrical Engineering


The terahertz (THz) region of the electromagnetic spectrum spans the frequency range between microwaves and the mid-infrared. Over the past decade, THz frequency radiation has attracted much interest for the development of new imaging and spectroscopy technologies, owing to its ability to discriminate samples chemically, to identify changes in crystalline structure, and to penetrate dry materials enabling sub-surface or concealed sample investigation. In particular, techniques such as THz time-domain spectroscopy (TDS) have enabling access to numerous low-energy excitations including molecular rotations, vibrations of crystal lattices, precession of spins and excitations of electron-hole pairs.

Nevertheless, there is growing interest in the rich physics that can be investigated when intense THz transients are used to resonantly control and manipulate the electronic, spin, and ionic properties of matter, rather than to merely monitor it. These emerging non-linear techniques go far beyond the weak light-matter interactions of absorption and emission, as embodied by linear THz spectroscopy, and are becoming powerful tools enabling the study of a wide range of non-equilibrium systems, nonlinear phenomena and quantum systems. Yet, these avenues have only recently begun to be explored within the THz spectral range, despite the wealth of materials that possess fundamental transitions at these frequencies and their potential for applications in quantum technology, photonics and signal processing.

To unlock these opportunities requires the generation of high-field (~kV/cm-MV/cm), ultrafast THz pulses for both narrowband and broadband excitation, which must be controlled, manipulated and detected with femtosecond precision. Ultrafast regenerative amplified laser (URAL) systems represent the only viable bench-top technology capable of delivering ultra-stable optical laser pulses on timescales <100 fs and with pulse energies >mJ, which are required for the generation of these ultrafast, intense THz pulses.

Through this funding we will establish a dedicated URAL-based THz facility that will open-up wide-scale and lasting access to these emerging fields of non-linear THz science and coherent control of matter. The coherent manipulation of quantum states will be explored in a range of exemplar systems including active THz quantum cascade laser devices, shallow-impurity-in-semiconductors, rare earth ion-in-solid materials, and self-assembled InGaAs quantum rods. Although these measurements are of fundamental interest in their own right, the investigation of such systems promises to underpin a number of applications ranging from solid-state implementations of spin-based qubits for quantum information systems to the development of single photon sources, optical fibre amplifiers, room-temperature vertical-cavity surface-emitting THz lasers, and long-sought mode-locked lasers exploiting the phenomenon of self-induced transparency. But this is not all. We will also exploit this technology to explore the control of chemical reaction pathways, including those associated with the initiation of explosive reactions.

This underpinning technology for the generation of ultrafast, intense THz pulses will not only support a range future research directions within the University of Leeds, but will enable us to establish for the UK an international facility for non-linear THz science and coherent control of matter that will impact over many research fields across the physical, chemical and biological sciences.

Planned Impact

The potential impact of the requested equipment, which will open-up wide-scale and lasting access to non-linear THz spectroscopy and coherent control of matter, is far-reaching and would encompass academic, economic and societal aspects.

Academics, both in the UK and internationally, will benefit in the short-medium term (1-5 years) through the new opportunities, scientific advancements and technological developments enabled by this infrastructure. Specifically, these include: increased understanding of how intense THz transients interact with condensed matter systems and biological materials; the development of disruptive THz technologies including both room-temperature and modelocked THz lasers; and opportunities for studying the coherent and dynamic control of solid-state chemistry. In addition, these scientific opportunities will lead to longer-term economic impacts through the training of PhD researchers, post-doctoral and early-career researches, both from within the SEEE as well as from external institutions. Specifically, users will acquire and refine skills of importance to the future UK economy including in: cryogenic systems, ultrafast and high-field photonics, condensed matter devices, and THz technology and methods. The opportunities enabled by this facility will also enhance the reputation of the University of Leeds as an international research institute, thereby improving the UK's competitiveness.

In the medium to long term there is potential for significant economic and societal impact, for example through the development of: 1) New solid-state implementations of spin-based qubits with applicability to emerging commercial realizations of quantum information processing communications; 2) New classes of optoelectronic and photonic devices including single photon sources and optical fibre amplifiers; 3) New disruptive THz technologies, including room-temperature THz lasers and mode-locked THz QCLs, which will raise the technological potential of the THz range; and 4) New medical diagnostic techniques for cancers. The future development of these technologies will ultimately support the UK economy, in the short-medium term, through licensing for manufacture by UK firms. In the medium-long term (5-10 years) there is potential for further positive impact on the UK economy, through the creation and growth of high-technology engineering companies, and the associated creation of wealth, and also by attracting R&D investment into the UK. Notably, THz systems have many potential application areas outside of academia including pharmaceutical process monitoring, airport security screening, chemical sensing, industrial inspection and medical imaging. There is therefore potential for these new THz laser technologies to be translated directly to industry, for example via TeraView Ltd in the UK (who supply THz spectroscopy systems to the pharmaceutical sector, where they are used to characterise polymorphs of drugs during development and production cycles).

The technologies emerging from the research enabled by this equipment will also have potential long-term impact (>10 years) in the public sector and society as a whole. For example, the potential developments in quantum information systems, leading ultimately to quantum computers, could have enormous benefits to society. The development of THz-based systems across the application areas highlighted above would also have significant positive implications in areas including: improved quality of life/public well-being (through non-invasive medical imaging techniques and diagnostic tools for cancer, and also improved production cycles of pharmaceuticals); national security (through new airport security screening systems); and protection of the global environment (through new environmental monitoring systems).

Through examples such as these, where societal benefits are immediately tangible, the public awareness and appreciation of science will also benefit.
Description Terahertz (THz) technologies have undergone remarkable development over the past decades, motivated in part by the diverse range of potential applications presented by this region of the spectrum. Foremost among these technological developments has been the THz-frequency quantum cascade laser (QCL), a compact electrically-pumped source of THz radiation in which photons are generated through intersubband transitions in a quantum heterostructure.Of particular interest, these unipolar semiconductor sources are characterised by ultrafast carrier relaxation lifetimes on the ps timescale, which allows modulation of both the amplitude and frequency over extremely large modulation bandwidths . This property makes them ideally-suited for applications in THz metrology, high-resolution spectroscopy, and ultra-high speed wireless communications with the potential for transmission rates exceeding 100 Gbit/s. High-frequency modulation of the gain is also a fundamental requirement to accomplish mode-locking in THz QCLs for the generation of ultra-short pulses and the synthesis of terahertz frequency combs.

We have developed and demonstrated an entirely new approach to modulation of a THz QCL through ultrafast acoustic modulation of the electronic states and electron transport in the QCL heterostructure. Our scheme relies on dynamically altering the mechanical properties of the heterostructure, on length scales comparable to the layer thickness, using an optically generated strain pulse propagating along the growth direction. These acoustic pulses permit modulation of electronic processes in large-area QCL devices without the frequency limits imposed by parasitic electronic effects.
Exploitation Route This work provides an entirely new approach to modulation of a THz QCL through ultrafast acoustic modulation of the electronic states and electron transport in the QCL heterostructure. This could open up new opprotunities for the use of THz QCLs in THz metrology, high-resolution spectroscopy, and ultra-high speed wireless communications.
Sectors Aerospace, Defence and Marine,Electronics,Pharmaceuticals and Medical Biotechnology,Other

Description Research conducted under this grant has been publicized in numerous news outlets internationally, including The Independent and Daily Mail news papers.
First Year Of Impact 2020
Sector Education
Impact Types Societal

Description Acoustic control of quantum cascade heterostructures: the THz "S-LASER"
Amount £509,154 (GBP)
Funding ID EP/V004751/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 04/2021 
End 05/2024
Description Coherent pulse propagation and modelocking in terahertz quantum cascade lasers
Amount £1,127,384 (GBP)
Funding ID EP/T034246/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 07/2020 
End 02/2024
Description Dynamics, Control and Energy Transfer at Terahertz Frequencies.
Amount £1,025,292 (GBP)
Funding ID EP/P007449/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 01/2017 
End 12/2021
Description EPSRC Doctoral Prize Fellowship
Amount £150,000 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 07/2017 
End 07/2019
Description EPSRC Doctoral Prize Fellowship award - David Bacon
Amount £100,000 (GBP)
Organisation University of Leeds 
Sector Academic/University
Country United Kingdom
Start 06/2017 
End 06/2019
Title Data to Support Photoconductive Arrays on Insulating Substrates for High-Field Terahertz Generation 
Description The data included in this repository supports our work on the design, fabrication and characterisation of large-area photoconductive THz array structures, consisting of a thin LT-GaAs active region transferred to an insulating substrate using a wafer-scale bonding process. The electrically insulating, transparent substrate reduces the parasitic currents in the devices, allowing peak THz-fields as high as 140 kV cm-1 to be generated over a bandwidth >5 THz. These results are achieved using lower pulse energies than demanded by conventional photoconductive arrays and other popular methods of generating high-field THz radiation. Two device sizes are fully characterised and the emission properties are compared to generation by optical rectification in ZnTe. The device can be operated in an optically saturated regime in order to suppress laser noise. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
URL http://archive.researchdata.leeds.ac.uk/682/
Description Collaboration with Tianjin University, China 
Organisation Tianjin University
Country China 
Sector Academic/University 
PI Contribution This facility will be used to develop schemes for generation of high-power THz pulses, with peak fields >100 kV/cm, using PPLN crystals. This work will underpin future experimental investigations in the field of nonlinear THz science.
Collaborator Contribution Design, development and provision of PPLN crystals for THz generation.
Impact Ongoing.
Start Year 2017
Description Collaboration with University of Nottingham 
Organisation University of Nottingham
Country United Kingdom 
Sector Academic/University 
PI Contribution Expertise in pulsed terahertz generation and detection; Access to experimental facilities including terahertz spectroscopy systems, clean-room fabrication facilities, and MBE growth facilities; Development, characterisation and provision of high-performance quantum cascade laser sources;
Collaborator Contribution Expertise in acoustic wave generation
Impact EPSRC grant EP/M01598X/1
Start Year 2015
Description Research publiclised in International media 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Research outcomes publicized in multiple International news outlets including The Independent, The Daily Mail and The Times of India
Year(s) Of Engagement Activity 2020
URL https://www.independent.co.uk/life-style/gadgets-and-tech/news/internet-speed-fast-cable-wires-terah...