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Ultrashort Pulse Generation from Terahertz Quantum Cascade Lasers

Periodic Reporting for period 2 - ULTRAQCL (Ultrashort Pulse Generation from Terahertz Quantum Cascade Lasers)

Reporting period: 2016-10-01 to 2019-03-31

The generation of ultrafast and intense light pulses is an underpinning technology throughout the electromagnetic spectrum enabling the study of fundamental light-matter interactions, as well as industrial exploitation in a plethora of applications across the sciences. A benchmark system for such studies is the modelocked Ti:Sapphire laser, which has grown from being a laboratory curiosity to an essential tool in a broad range of application sectors. Beyond Ti:Sapphire systems, there have been impressive developments in semiconductor based devices for pulse generation in the optical range. These benefit from low system costs and are an enabling technology in new application domains including frequency combs and high speed communications.

However, in the terahertz (THz) frequency range, with its proven applications in imaging, metrology, communications and non-destructive testing, a semiconductor based technology platform for intense and short pulse generation has yet to be realised. Ultrafast excitation of photoconductive switches or nonlinear crystals offer only low powers, low frequency modulation or broadband emission with little control of the spectral bandwidth.

In the ULTRAQCL project we have broken through this technological gap, using THz quantum cascade lasers (QCLs) as a foundational semiconductor device for generating ultrashort THz pulses. QCLs are the only practical semiconductor system that offer gain at THz frequencies, hence making them suitable for pulse generation, with the ‘bandstructure-by-design’ nature of QCLs allowing the frequency, bandwidth and pulse width to be entirely engineered. We have demonstrated: the first self-starting (passive) mode-locked THz QCL; the first active modelocked THz QCL with dispersion compensation; polariton based frequency combs; and, new concepts for modelocked laser action in ultrafast systems. The ULTRAQCL project has implemented these radical schemes for pulse generation and enabled ultrafast QCLs to become a ubiquitous technology for key applications in the THz range.
The results builds on solid foundations developed across the project where the consortium concentrated on QCLs based on low threshold QCLs, with large spectral bandwidths, that satisfy the majority of the objectives. This resulted in state-of-the-art spectrally broadband QCLs with milliwatts average powers and >2W peak powers and uniform mode spacing, vital for pulse and frequency comb generation. Importantly these samples also show low electrical power dissipation, resulting in CW operation, vital for the applications investigated. These advances in QCL technologies were supported with new fundamental studies in the dynamics of all intersubband systems. High field THz systems were optimised throughout the project and gave unprecedented direct access to the previously unknown dynamics. These insights were vital for optimising and understanding ultrafast QCLs.

These advances permitted a step-change in pulse generation in THz QCLs with stable sub-5 picosecond pulse trains that could be routinely shown within the consortium with active and self-starting modelocked QCLs. This was based on the introduction of integrated and external Gires-Tournois interferometers that permitted to compensate for dispersion of the QCLs. These concepts could be applied to a range of QCLs and showed shortest reported pulse durations of ~1ps.

Two new passive modelocking schemes were realised. i) ultrafast saturable absorbers based on intersubband polaritons, where strong coupling is used to reduce the power requirements for saturation. Compact integration with QCLs permitted to enhance the locking of the QCL modes, resulting in a frequency comb over the entire dynamic range of the QCL; ii) the first realisation of new types of QCLs based on self-induced transparency (SIT) for modelocking. The QCLs suitability to SIT stems from their rapid gain recovery times and relatively long coherence time. All these developments have been supported by theoretical and modelling permitting predictive tools to optimise ultrafast THz QCLs.

These important developments in pulse generation were fed into applications. A highlight has been a strong impact in metrology measurements. New techniques were realised to perform high precision spectroscopy for absolute spectral measurements of methanol. Other proof-of-principle applications have been realised impacting microwave technology i) the ultrafast response of THz QWIPs for communication systems and ii) modelocked QCLs for low noise microwave radiation generation.
Other ‘spin-offs’ from ULTRAQCL has been the development of high technology THz systems. These include i) new THz sources for time domain spectroscopy for high resolution spectroscopy; ii) realisation of high field THz systems based on highly nonlinear crystals and quartz based photoconductive switches adapted to high and low repetition rate laser systems.

These world class results has been disseminated extensively with many scientific publications in high impact journals. The results have been exploited through patent applications and engaging with industry throughout the project. The latter has been through an industrial advisory committee and several events including where the technology was presented to leaders in the domain of THz technology. These highlights have shown how modelocked QCLs have become an ambiguous technology for the THz range and provide the basis of further advances in their performances and applications beyond the ULTRAQCL programme.
A range of wide reaching objectives have been attained throughout ULTRAQCL. A highlight has been based on the realisation of quantum cascade structures that have permitted pulse generation down ~1ps, an order of magnitude better than the performances of lasers at the start of the project. Pre-ULTRAQCL, THz pulses from QCLs were limited to 10ps or higher. From 2016 onwards, we have realized a step-change in short pulse generation to sub-5ps, followed by a steady progression to shorter pulses (and an equivalent increase in spectral bandwidth). These advances have been linked to spectrally wide QCLs combined with dispersion compensation that have further permitted important impacts in frequency comb generation. Further highlights, described in this report, show novel realisations such as self-starting and controllable modelocking, new polaritonic absorbers that stabilise QCL emission, lasers based on new effects such as self-induced transparency, high field THz physics showing the new insights into the dynamics of the novel devices realised, and an in-depth theoretical models that provides an important predictive tool for the performances of the ULTRAQCL devices.

These advances have strongly impacted the field in terms of technical realisation and proof-of-principle applications of QCLs, and have been realised through a range of interacting partners and tasks not previously attempted. Particular highlights have been THz-QCL based metrology for absolute spectral measurements of gases and new proof-of-concepts for radio-frequency technologies that will impact ultrahigh bandwidth tele-communications. These advances will strongly impact academic and industrial communities operating in the THz range, as well as continual benefits to Europe that will maintain its world-leading technological and scientific knowledge in this important electromagnetic range.
Short Pulse generation from THz QCLs