The generation of ultrafast and intense light pulses is an underpinning technology across the electromagnetic spectrum enabling the study of fundamental light-matter interactions, as well as industrial exploitation in a plethora of applications across the physical, chemical and biological sciences. A benchmark system for such studies is the modelocked Ti:Sapphire laser, which operates in the visible/near-infrared spectral range, and has grown from being a laboratory curiosity to an essential tool in a broad range of application sectors from fundamental physics to medicine. Beyond classic solid state systems, there has been impressive progress in developing high power ultrafast modelocked III-V semiconductor lasers in the optical range. These benefit from inexpensive wafer scale production and low system costs and are an enabling technology in application domains ranging from high resolution optical sampling to ultra-high speed communications.
Yet in the terahertz (THz) frequency range ( 0.5-5 THz) a semiconductor based technology platform for intense pulse generation has yet to be realised. Ultrafast excitation of photoconductive switches or nonlinear crystals offer only low powers ( µW), low frequency modulation and broadband emission with little control of the spectral bandwidth. The latter is a particular disadvantage for metrology, fundamental material excitations or THz communications where select frequency bands are required. An alternative approach is based on free electron lasers or large table-top optical ultrafast laser systems. While ultrashort and intense pulses with controlled frequencies can then be realised, the size and costs involved inhibit commercial uptakes, and often render impossible, their use for many applications. This is significantly hindering the development of the THz field, not only in Europe, but also internationally.
In the ULTRAQCL project we will breakthrough this technological gap and use THz quantum cascade lasers (QCLs) as a foundational semiconductor device for generating intense and short THz pulses. These devices, first realised in 2002 for the THz range, are the only practical semiconductor system that offers gain in the THz range, hence making them suitable for pulse generation. The ‘bandstructure-by-design’ nature of QCLs allows the frequency, bandwidth and pulse width to be entirely engineered. But, to date, solutions to generate pulses from QCLs have been based on active modelocking – this inherently limits the performance since the pulse width is restricted by the applied active microwave modulation whilst only low output powers are possible owing to the need to operate the QCL close to threshold. We will address these issues through the ULTRAQCL project, enabling ultrafast QCLs to become a ubiquitous technology for the THz range.
The aims of the ULTRAQCL will be achieved through the following set of the interacting objectives, each based on the unique ultrafast dynamics of QCLs :
O1. We will demonstrate the first self-starting mode-locked THz QCL;
O2. We will demonstrate the new ‘active techniques’ to reduce the pulse width and increase the output field by an order of magnitude compared to the state-of-the-art
O3. We will integrate ultrafast materials with THz QCLs and demonstrate the first gain-switched modelocking.
O4. We will demonstrate the first QCL-based THz ultrafast pulse amplifier.
These objectives will allow the current bulky, expensive and constraining systems to be replaced by an inexpensive semiconductor-based technology, permitting the entire properties of the pulse to be controlled. Our long term vision for this enabling pulsed technology is that it will unlock THz applications ranging from fundamental science to quantum optics through to THz communications. To emphasise this potential and demonstrate proof-of-principle, we will evaluate our technology and take the first steps in a set of exemplar research areas with the final objective of ULTRAQCL focused on application domains in the fundamental and applied sciences:
O5. We will demonstrate the application of THz pulsed lasers in a) Metrology; b) Coherent control of quantum systems; and c) ‘Optical’ pumping of polaritonic devices, opening the way for an inversion-less THz gain medium.