HyperTerahertz - High precision terahertz spectroscopy and microscopy

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


The last 20 years have witnessed a remarkable growth in the field of THz frequency science and engineering, which has matured into a vibrant international research area. The modern THz field arguably began with the development of a pulsed (single-cycle) THz emitter - the semiconductor photoconductive switch - and the subsequent development of THz time-domain spectroscopy (TDS). Since then, considerable success has been achieved in the further development of this and other THz sources, including the uni-travelling carrier (UTC) photodiode and the quantum cascade laser (QCL). However, notwithstanding this, it is only the THz-TDS technology that has been developed sufficiently for commercialization as a complete system, leaving other THz devices, components and techniques still restricted to the academic laboratory. This is unfortunate, since despite the success of THz-TDS, the technique has a number of shortcomings including its high fs-laser dominated cost, low power, and limited frequency and spatial resolution, which could be addressed by QCL and UTC technologies if they were to be engineered into appropriate instruments.

In fact, a cursory comparison with the neighbouring microwave and optical regions of the spectrum reveals that THz frequency science and technology is still in its infancy, and not just in the context of commercial uptake. For example, the THz region significantly lags in the availability of precision spectroscopy instrumentation required to address sharp spectral features inherent to gases, for example, in atmospheric analysis, or in materials with long excited state lifetimes. THz technology also significantly lags in the fields of non-linear spectroscopy and coherent control, where powerful and controlled pulses of electromagnetic radiation interact with matter and manipulate its properties. In the optical and microwave regions, fascinating phenomena including electron-spin resonance and nuclear magnetic resonance were major breakthroughs, revealing a wealth of new science and engineering applications. These techniques, now standard across many disciplines, support much contemporary research and technology activity.

A further example of how THz technology compares unfavourably with other spectral ranges is in the context of THz microscopy and analysis below the diffraction limit, which intrinsically restricts such measurements to ensemble sampling of physical properties averaged over the size, structure, orientation and density of, for example, nanoparticles, nanocrystals or nanodomains. Although near-field imaging approaches have been adapted from the visible/infrared regions enabling THz measurements on the micro/nano-scale, no THz instrument currently provides the required spatial resolution and sensitivity, nor can address the enormous range of length-scales (spanning five orders of magnitude from electron confinement lengths (<10 nm) to the THz wavelength (~300 um)), nor can operate at cryogenic temperatures. In fact, on this point, the THz field is deficient even in the provision of basic technologies such as waveguides and coupling optics required to deliver THz signals with low loss into cryostats or industrial apparatus.

In this programme we will create the first comprehensive instrumentation for precise THz frequency spectroscopy, microscopy, and coherent control. This will be based upon our unique and proprietary capabilities to generate, and manipulate photonically, THz signals of unprecedentedly narrow (Hz) linewidth and with sub-wavelength spatial resolution. The instrumentation will then be exploited to create new challenge-led applications in non-destructive testing and spectroscopic analysis for electronics and atmospheric sensing, inter alia, as well as discovery-led opportunities within physics, quantum technologies, materials science, atmospheric chemistry and astronomy.

Planned Impact

This programme will develop new instruments for precise THz frequency spectroscopy, microscopy, and coherent control.

Economic Impact: Recent market studies predict strong growth in THz-based technologies over the next decade: e.g. Prescouter in 2015 predict that the market for current THz imaging and spectroscopy applications will grow from $127 million in 2016 to $570 million in 2021, and importantly, find that new microscopy and spectroscopy instrumentation and methodologies will accelerate this further. We will pursue a number of approaches to develop economic impact, as appropriate for the TRL level of the technology, including collaboration with existing companies to (i) transfer our technology into current commercial products, (ii) develop new commercial products, and (iii) create new companies where this will accelerate product development. Our new spectroscopy and microscopy instrumentation will provide versatile instrumentation for the scientific and R&D markets, with immediate application in non-destructive testing and spectroscopic analysis, inter alia. These include high-throughput microscopy requirements identified by Project Partner TeraView and other end-users in the testing of integrated circuits with international foundries, as well as other high-throughput THz imaging and microscopy applications in the medical, food, and pharmaceutical sectors. Longer-term economic impacts will be delivered through, for example, the applicability of our instrumentation to manipulate coherently quantum dots, and other mesoscopic electronic systems, in a compact and portable footprint. This adds capability to Project Partner Toshiba's development of single-photon and entangled-photon pair sources for applications to quantum key distribution and quantum computing, and the 'THz-to-Telecoms' bridge will lead to on-chip integrated quantum photon converters and hybrid single-photon detectors.

Knowledge Impact: We anticipate very considerable new knowledge generation in the fields of electronic engineering, condensed matter and optical physics, quantum technologies, materials science, atmospheric chemistry, inter alia, with new knowledge not only generated during the course of the Programme, but also created in the future though application of the instrumentation that we will develop. To give one example, our robust frequency-locked QCL spectroscopy systems will find application as local oscillators in satellite instrumentation for Earth-observation and planetary science, and in high-precision gas spectroscopy, and will create knowledge impact when exploited by chemists, atmospheric chemists, astronomers, and electronic engineers. Such instrumentation is necessary for future airborne and satellite-based Earth-observation missions above 2 THz, and with Project Partner DLR, we will integrate our technology onto SOFIA, a joint NASA/DLR project on a modified Boeing 747-SP aircraft to enable e.g. investigations and new knowledge of narrow-frequency outflows in shock regions of the interstellar medium. With RAL, we will address the more stringent requirements of satellite instrumentation enabling e.g. the analysis of OH (3.5 THz) and O (4.7 THz) species in the Earth's mesosphere/lower thermosphere, informing studies and new understanding of climate change, inter alia.

Societal Impact and Impact on People: THz science and technology is of national and global importance, and Society will benefit economically and through provision of new measurement and sensing instrumentation that will support high technology industry, including in the communications, non-destructive testing, and electronics sectors. We will also produce skilled People at doctoral and post-doctoral level having both technology and applications perspective, as well as developing current and future research leaders and role models.
Description The vision of the HyperTerahertz programme is to open up new opportunities for the terahertz (THz) frequency range through the development of the first comprehensive suite of instrumentation for precise THz spectroscopy, microscopy, and coherent control. These instruments will be based on our unique and proprietary capabilities to generate, and manipulate photonically, THz signals of unprecedentedly narrow (<10 Hz) linewidth and with sub-wavelength spatial resolution. The ambition is to create instruments with wide-ranging application for industrial and academic end-users, which will be compact and inexpensive, democratizing the field. We will translate prototype systems and capability to end-users through collaboration from the outset and exploit the instruments developed to create new challenge-led applications in non-destructive testing and spectroscopic analysis for electronics and atmospheric sensing, inter alia, as well as discovery-led opportunities within physics, quantum technologies, materials science, atmospheric chemistry and astronomy. The Programme comprises three interrelated projects, with a technological core comprising the development of high-precision spectroscopy (Project P1), coherent source control (Project P1), and near-field microscopy techniques and instrumentation (Project P2), directed towards application-driven instrumentation (Project P3). The programme commenced 1 June 2017, and significant progress has been made in all projects and against all milestones due.

Selected highlights include:

1. We have demonstrated the first continuous-wave injection locking of a QCL, with the QCL emission locked to a fibre-based near-infrared telecommunications frequency comb, itself referenced to a GPS-locked microwave frequency synthesiser, providing traceability of the QCL frequency to primary standards. This not only stabilizes the QCL frequency, and enables independent control of the frequency and phase for the first time, but also significantly reduces the QCL linewidth to <1 Hz, and allows the phase-locked continuous-wave QCL emission to be detected coherently. This brings the same frequency precision and accuracy that are available at microwave frequencies to the terahertz region of the spectrum for the first time, and as all components are semiconductor-based, compact integration is possible benefitting scientific application. On-going work is investigating extending the frequency comb span further, with 5 THz bandwidth now demonstrated.

2. The arbitrary waveform generator has been successfully used to program arbitrary QCL pulse emission with variable pulse lengths, and work is now underway to extend the range of the pulse lengths and delays available.

3. The NeaSpec-based self-mixing room temperature THz s-SNOM system has been established, demonstrating successful integration of self-mixing in a commercial platform. This technique enables compact absorption-coefficient-sensitive 3D imaging without the need for an external interferometer or for synchronization of the source to an ultrafast laser, and is intrinsically fast since it is fundamentally limited only by electron transport in the QCL. Furthermore, by combining this approach with a scattering probe, a near-field enhancement of the scattered field amplitude has been demonstrated for small tip-sample separations enabling coherent 2D imaging with a resolution well below the diffraction limit, now ~30 nm. Good progress has also been made in developing low temperature THz s-SNOM and a-SNOM instruments, and this work is on-going. We have demonstrated the tuning fork approach to THz-s-SNOM in free-space, and have developed a-SNOM probes with integrated nanowire detectors, and 5, 10 and 20 µm apertures have been explored. These instruments open the way for further miniaturization to enable discrete nanometre-scale structured materials and devices to be investigated at terahertz frequencies, and at cryogenic temperatures, for the first time.

4. A metal waveguide delivery system has been fabricated, and is functional at, and below, 4 K. A dedicated 4 K copper waveguided delivery system has been designed for a continuous flow cryostat has been built and is allowing simultaneous photoluminescence and waveguided THz measurements.

5. Studies of 2D GaAs-AlGaAs systems at 1.5 K under THz illumination using a THz waveguide probe in a 'wet' 1.5 K cryostat have shown a clear THz response in the magneto-transport characteristics demonstrating the experimental method and apparatus. On-going work will now focus on investigation of a range of nanostructured electronic systems.

6. Initial 4 K free-space illumination measurements on 2DHGs under QCL illumination have been performed by UCL/Leeds to investigate Floquet state formation, with dilution refrigerator measurements being performed in parallel with collaborators in South Africa using GHz waveguides exploiting UCL/Leeds designs.

7. Initial high-power THz studies of Rydberg systems and Topological Insulators have shown promising non-linear responses at THz frequencies.
Exploitation Route We have a series of formal project partners who will help translate the research, as well as an External Advisory Board, who have a role to help disseminate the results. We actively publish our work in order to disseminate the work to others, in journals and conference, as well as through industrial/KTN meetings, and in public events such as the Royal Society Summer Exhibition and the Cheltenham Science Festival.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics,Healthcare,Other

URL http://www.hyperterahertz.org/?page_id=9
Title Data associated with 'Gas spectroscopy through multimode self-mixing in a double-metal terahertz quantum cascade laser' 
Description We demonstrate a broadband terahertz gas spectroscopy through multimode self-mixing in a quantum cascade laser. The transmission spectra of methanol were simultaneously obtained within two distinct frequency ranges from 3.359 THz to 3.369 THz and from 3.426 THz to 3.433 THz. This approach has increased the total measurement bandwidth of self-mixing spectroscopy to 17GHz, by a factor of ~7, and underpins future work to develop the first broadband "detector-free" multi-gas QCL spectroscopy systems. Figure 1 shows schematic illustration of the configuration of the gas spectroscopy system. Figure 2 and figure 3 are the characterisation of emission spectra and the frequency tuning range of the QCL. Figures 4 and figure 5 shows the spectra of the methanol, obtained simultaneously from the two modes of the QCL. Figure 6 shows the absorption coefficients of the methanol at different pressure. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Title Dataset associated with "Tunable broadband terahertz polarizer using graphene-metal hybrid metasurface" 
Description An electrically tunable polarizer for terahertz frequency electromagnetic waves formed from a hybrid graphene-metal metasurface is demonstrated. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Title Dataset associated with 'Ultrafast switch-on dynamics of frequency tuneable semiconductor lasers' 
Description This dataset contains experimental data presented in the paper titled 'Ultrafast switch-on dynamics of frequency tuneable semiconductor lasers'. Single-mode frequency-tuneable semiconductor lasers based on monolithic integration of multiple cavity sections are important components, widely used in optical communications, photonic integrated circuits, and other optical technologies. To date, investigations of the ultrafast switching processes in such lasers, essential to reduce frequency cross-talk, have been restricted to the observation of intensity switching over nanosecond-timescales. Here, we report coherent measurements of the ultrafast switch-on dynamics, mode competition and frequency selection in a monolithic frequency-tuneable laser using coherent time-domain sampling of the laser emission. This approach allows us to observe hopping between lasing modes on picosecond-timescales and the temporal evolution of transient multi-mode emission into steady-state single mode emission. The underlying physics is explained through a full multi-mode, temperature dependent carrier and photon transport model. Our results show that the fundamental limit on the timescales of frequency-switching between competing modes varies with the underlying Vernier alignment of the laser cavity. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Title Dataset for Gas spectroscopy with integrated frequency monitoring through self-mixing in a terahertz quantum-cascade laser. 
Description This archive contains the dataset associated with the publication entitled 'Gas spectroscopy with integrated frequency-monitoring through self-mixing in a terahertz quantum-cascade laser';. In this work, we demonstrate a gas spectroscopy technique based on self-mixing interferometry in a 3.4-THz quantum-cascade laser (QCL). All previous QCL spectroscopy techniques have required additional THz instrumentation (detectors, mixers or spectrometers) for system-precalibration or in situ spectral analysis. By contrast, our system self-calibrates the laser frequency to a precision of 630 MHz (0.02%) by analyzing voltage perturbations across the QCL, in response to optical feedback from a mirror mounted on a 0-800-mm round-trip delayline. We demonstrate spectroscopy of methanol by introducing a gas cell into the feedback path, and show that a limiting absorption coefficient of ~0.0001 /cm is resolvable. This dataset includes the raw data used to generate all figures within the publication. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
Title Research data supporting "Amplitude stabilization and active control of a terahertz quantum cascade laser with a graphene loaded split-ring-resonator array" 
Description This directory contains the data relating to the figures in "Amplitude stabilization and active control of a terahertz quantum cascade laser with a graphene loaded split-ring-resonator array". Each opj file name describes which figure it is referring to. For any clarification or support, contact Binbin Wei at bw373@cam.ac.uk or weibb@hotmail.com. 
Type Of Material Database/Collection of data 
Provided To Others? Yes  
Description Industrial and end-user engagement 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact We attended and presented at the Innovate UK Showcasing Emerging Technologies 2018 - Photonics and Imaging KTN event in London on Tuesday 1 May. We presented a working THz QCL imaging system and discussed the application and potential of THz imaging and spectroscopy, and the HyperTerahertz programme.
Year(s) Of Engagement Activity 2018
URL https://ktn-uk.co.uk/events/showcasing-emerging-technologies-2018-photonics-and-imaging