Periodic Reporting for period 2 - ULTRAQCL (Ultrashort Pulse Generation from Terahertz Quantum Cascade Lasers)
Reporting period: 2016-10-01 to 2019-03-31
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.
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.
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.