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Terahertz Ultra-Short Pulses from Self-Induced Transparency Modelocked Quantum Cascade Lasers

Periodic Reporting for period 1 - TERAULTRA (Terahertz Ultra-Short Pulses from Self-Induced Transparency Modelocked Quantum Cascade Lasers)

Reporting period: 2016-03-01 to 2018-02-28

The terahertz (THz) frequency range lies between microwaves and visible radiation in the electromagnetic spectrum. This relatively unexplored region of the spectrum is attracting significant interest internationally due to its ability to discriminate samples chemically, to identify differences in physical structure, and to penetrate non-polar materials. However, it lacks a compact semiconductor source of ultrashort THz pulses, suitable for applications including ultrafast spectroscopy for materials analysis, and THz frequency comb generation with applications for trace gas sensing and atmospheric science. Although the quantum cascade laser (QCL) is a promising compact semiconductor source of THz radiation, its success in creating ultrashort pulses is limited due to the inherent fast gain recovery time. The exciting possibility has been proposed of exploiting the phenomenon of self-induced transparency (SIT) for passive laser modelocking, in which the pulse duration is mediated through the process of Rabi-flopping. The aim of the TERAULTRA project was to explore, for the first time, the feasibility of generating ultrashort THz pulses from QCLs by exploiting SIT phenomenon. In working towards this goal, this project aimed to explore, both theoretically and experimentally, the underlying phenomenon of Rabi-flopping and SIT in quantum heterostructures and QCLs in the THz frequency range. The project also aimed to explore alternative approaches to achieving broadband emission in THz QCLs such as using cascaded two-photon emissions. The TERAULTRA project: demonstrated, for the first time, SIT effects in quantum heterostructures at THz frequencies; designed, experimentally investigated and optimised THz QCL heterostructures for SIT-modelocking and short pulse generation; and proposed and designed the first two-photon emitting THz QCLs.
The TERAULTRA project started with the development of a detailed and versatile software tool for the simulation of light-matter interactions in quantum devices. This valuable simulation tool will be readily exploitable for: future designs of a broad range of quantum structures including QCLs; simulating, predicting and understanding dynamic light-matter interactions in quantum structures; and for the analysis and interpretation of experimental measurements of ultrafast phenomena in quantum devices.
In this project the developed tool was used together with a Schrödinger-Poisson self-consistent solver to design and characterise quantum heterostructures tailored to the establishment of SIT effects and Rabi flopping at THz frequencies. Passive and absorbing semiconductor heterostructures were grown, fabricated, and experimentally investigated. Experimental results confirmed the onset of SIT and Rabi-flopping in quantum heterostructures at THz frequencies, for the first time. THz QCL heterostructures tailored for SIT-modelocking with ~ps pulse durations were also designed, and the time-evolving dynamics were simulated. These structures were successfully grown, fabricated, experimentally investigated and subsequently optimised. The physical insights gained from this theoretical and experimental work will be exploited by academics internationally for the future development of SIT-modelocked QCLs.
The TERAULTRA project also led to the first ever reported design of a two-photon laser based on a semiconductor platform, which has enormous potential for exploitation by European research institutes for the development of high-power THz QCLs and THz frequency combs with broad spectral coverage. Further opportunities presented by this new laser concept include the development of quantum gyroscopes, quantum communication and the generation of polarization-entangled twin beams.
The Fellow went through extensive training in theoretical and experimental techniques including: the use of design tools for QCL heterostructures; testing and measurement techniques used extensively in the field of THz photonics; nanofabrication techniques; and extensive personal and professional training. The Fellow was engaged in transfer of skills with researchers at UNIVLEEDS involved in the related field of THz photonics; and in fostering of networking opportunities through existing collaborations with international institutions and industrial partners. The Fellow has also been involved in Protection of Intellectual Property rights through filing of patent disclosure.
The research outcomes of the TERAULTRA project have been disseminated at international conferences and workshops, and through publication in high-impact journals. These include an Invited talk at SPIE Commercial + Scientific Sensing and Imaging (USA) and an invited seminar at University of California (USA).
The TERAULTRA project has developed a versatile simulation tool for light-matter interaction in a heterostructure using Maxwell-Bloch solver; demonstrated SIT effects and Rabi flopping in the THz frequency range for the first time in a semiconductor material; produced designs of two-segment SIT-modelocking THz QCLs with dynamic simulation results of modelocked pulses and subsequently grew, fabricated, tested, and optimised the THz QCLs; and proposed and presented designs for two-photon emitting THz QCLs. Immediate impacts arising from the project are:
- New computational tools developed for the dynamic simulation of light-matter interactions in quantum devices will assist future research undertaken both at UNIVLEEDS and by the MSCA Fellow. Specific research activities this will impact include: research at UNIVLEEDS in the field of coherent control of matter using THz radiation; the investigation of THz frequency comb generation using two-photon emitting lasers, and investigation of the control and coherence of entangled emitted photons in two-photon lasers.
- New understanding of light-matter interactions in quantum heterostructures, including the phenomena of Rabi-flopping and SIT will not only be crucial to the future development and optimisation of SIT-modelocked QCLs by European research institutes, but also has the potential to impact the development of a range of optoelectronic devices based on high-field light-matter interactions.
- The first reported designs of THz QCL heterostructures tailored to SIT-modelocking, as well as the first experimental realisation and characterisation of these devices will pave a clear way forward for subsequent optimisation of these devices, and will have immediate impact on the development of SIT-modelocked QCLs by the European academic community. Ultimately, the foundational work undertaken in this project could be exploited for the development of a new class of ultrafast semiconductor THz sources.
- Significant impact is anticipated to arise from the first reported design of a two-photon laser based on a semiconductor platform. This entirely new laser concept has the potential to deliver new disruptive THz sources that will raise the technological potential of the THz range. Licensing of this technology for manufacture by European firms will create new market opportunities in sectors where THz systems have proven application, including for pharmaceutical process monitoring, airport security screening, chemical sensing, and industrial inspection. These new opportunities will also strengthen the competitiveness and growth of companies in these sectors.
Simulation of pulse reshaping in a semiconductor heterostructure