Coherent pulse propagation and modelocking in terahertz quantum cascade lasers

The generation of ultrafast and intense light pulses is an underpinning technology across the electromagnetic spectrum enabling time-resolved measurements, nonlinear photonics, coherent control of matter, and frequency comb synthesis for high-precision metrology and spectroscopy. Yet in the terahertz (THz) region of the electromagnetic spectrum (~0.5-5THz), which spans the frequency range between microwaves and the mid-infrared, a compact semiconductor-based technology platform for intense and ultrafast pulse generation has yet to be realised. Established pulse generation schemes, based on excitation of photoconductive emitters or nonlinear crystals using bulky and expensive near-infrared lasers systems, offer only low frequency modulation, or broadband emission with little control of the spectral bandwidth and pulse width. These limitations are significantly hindering the development of the THz field not only in the UK but internationally, with adverse consequences for both fundamental scientific research and the development of future applications in metrology, materials analysis and molecular spectroscopy, and ultra-high speed THz communications.
One promising solution to closing this technological gap is the THz frequency quantum cascade laser (QCL) - a compact and high-power semiconductor laser based on a quantum-engineered semiconductor superlattice. However, modelocking these sources is inherently difficult to achieve due to the very fast gain recovery time in these structures. Indeed, active modelocking approaches adopted to date have succeeded only in achieving pulse widths down to ~4ps, and only low output powers are possible.
In this programme we will explore a radically new approach to pulse generation in lasers, based on the phenomenon of self-induced transparency in which pulses of the correct energy and pulse duration propagate without loss in the laser cavity whilst the growth of continuous waves is supressed. Although this concept has been discussed since the 1960s, the observation of this effect in semiconductor devices has remained elusive owing to the typically short coherence times of inter-band laser transitions. QCLs, however, are the ideal tool to realize SIT-modelocking owing to their large dipole moments, relatively long inter-subband coherence times, and, importantly, the possibility of combining resonant gain and absorbing periods with engineered dipole moments.
We will explore the coherent interaction of intense, ultrafast THz pulses with intersubband semiconductor heterostructures and THz QCL devices for the first time. Although these measurements are of fundamental interest in their own right, the investigation of such systems will lead to the development of the first modelocked semiconductor laser exploiting self-induced transparency. Through this approach, we will bring about a step change in QCL modelocked technology and develop THz QCLs into a foundational, compact semiconductor technology for generating intense and ultrafast THz pulses, with inherent advantages of high powers, broad spectral coverage and the ability to electrically-control the emission properties. This will pave the way for the application of modelocked THz QCLs across a wide range of areas of academic and industrial relevance, including non-linear THz science, quantum optics, ultra-high-speed THz communications, and high-precision metrology and molecular spectroscopy.
But that is not all. We will also demonstrate proof-of-principle applications of these new QCL sources for molecular spectroscopy, leading to a compact, all-solid-state and electrically-controlled multi-heterodyne THz spectrometer offering >500 GHz spectral coverage and sub-millisecond acquisition times. Through this goal we will translate to the THz region the unequalled combination of broad spectral coverage, high detection sensitivity, narrow spectral resolution and fast acquisition enabled by laser frequency combs at mid- and near-infrared frequencies.

The potential impact of the development of a compact semiconductor platform for intense and ultrafast THz pulse generation, and its application for multiheterodyne spectroscopy, is far-reaching and would encompass academic, economic and societal aspects.

Academics will benefit in the short-medium term (1-5 years) through the new opportunities, scientific advancements and technological developments resulting from this programme. Foremost is the development of disruptive THz technologies for ultrafast, intense THz pulse generation, which will open up opportunities for time-resolved tomography and spectroscopy, nonlinear THz science, and coherent control (including the dynamic control of solid-state chemistry). Other impacts include the development of all-solid-state multiheterodyne spectroscopy systems, with applications in atmospheric science, and increased understanding of how intense THz transients interact with condensed matter systems including quantum cascade laser devices.

These scientific opportunities will lead to longer-term economic impacts through the training of PhD researchers, post-doctoral and early-career researches, both at the University of Leeds (UoL) as well as external institutions through collaborative and aligned research programmes. Specifically, researchers will acquire skills of importance to the future UK economy including in: ultrafast and high-field photonics, condensed matter devices, and THz technology. In addition, this programme will enhance the reputation of the UoL as an international research institute, thereby improving the UK's competitiveness. In particular, it will provide advocacy and publicity for the national facility for THz Nonlinear Science, hosted at UoL and funded through an EPSRC Strategic Equipment grant (EP/P001394/1).

In the medium-long term (5-10 years) there is potential for significant economic and societal impact through the development of: 1) New disruptive THz sources, which will raise the technological potential of the THz range; 2) the next generation of THz spectrometers based on these sources; 3) ultra-high speed communication systems based on ultrafast THz technology; and 4) new classes of optoelectronic and photonic devices. The development of these technologies will support the UK economy, in the short-medium term, through licensing for manufacture by UK firms. In the medium-long term there is potential for 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 including pharmaceutical process monitoring, airport security screening, industrial inspection, chemical sensing and atmospheric science. There is therefore potential for these new THz lasers to be translated directly to industry, for example via project partners TeraView Ltd (who supply THz spectroscopy systems to the pharmaceutical sector), Rutherford Appleton Laboratory (who develop THz airborne radiometers for the European Space Agency and UK Met Office), and Menlo Systems (who supply optical frequency comb and THz systems to customers from industry and academia worldwide) (see Letters of support).

The technologies emerging from this grant will also have long-term impact (>10 years) in the public sector and society as a whole. For example, the development of THz-based systems across the application areas highlighted above would have significant positive implications in areas including: protection of the global environment (through new environmental monitoring systems); improved quality of life/public well-being (through improved production cycles of pharmaceuticals); and national security (through new tomographic and spectroscopic systems for airport screening). Through examples such as these, where societal benefits are immediately tangible, the public awareness of science will also benefit.