Coherent pulse propagation and modelocking in terahertz quantum cascade lasers

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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.