The generation of light across the mid-infrared (MIR) and terahertz (THz) frequency spectral regions has become an enabling technology, opening up a plethora of sensing applications across the physical, chemical and biological sciences, as well as enabling the study of fundamental light-matter interactions. The key disruptor in this domain is the quantum cascade laser (QCL), which has grown from being a laboratory curiosity to an essential, practical and compact optoelectronic source for a broad range of application sectors.
This expansion of applications has, however, highlighted a significant gap in technological capability between the MIR and THz regions, i.e. 25 µm – 60 µm (12 – 5 THz), which is termed the far-infrared (FIR) region. Unlike the neighbouring MIR and THz regions, the FIR lacks a practical solid-state laser technology, owing to parasitic optical phonon absorption – the Restrahlenband – in the constituent III-V semiconductors used to fabricate QCLs. This is unfortunate, since there is a large number of applications that could be addressed by a compact FIR source. These range from sensing complex hydrocarbons in the petroleum-industry and astronomy, to understanding the intricacies of protein function in amino acids such as dipeptides and tripeptides. FIR sources are also an enabling technology for near-field microscopy in the solid state (phononics, plasmonics), leading ultimately to their use in quantum optics, for example in the manipulation of Rydberg atoms for quantum computation architectures.
In the EXTREME-IR project, we will deliver a technological breakthrough to realize functionalized, compact and coherent, solid state-based FIR sources, leading to an entirely new and disruptive platform of instrumentation, based on giant optical nonlinearities in 2D materials. 2D materials offer unique optical and electronic properties at the single atomic layer level, which are completely distinct from those found in bulk materials, including conventional semiconductors. This has led to the demonstration of quantum optics at room temperature, and the design of the next generation of ultrafast electronics. However, these materials have yet to be exploited in the FIR. In this project, we will exploit the distinct phonon spectra and nonlinearities in 2D transition metal dichalcogenides (TMDs) and Dirac matter (DM) to deliver new optoelectronic sources for the FIR spectral range. We will exploit new phenomena,distinct to TMDs and DM, such as giant intra-excitonic nonlinearities at room temperature and efficient high harmonic generation through utilization of plasmonics and resonators. By integrating these materials with optical pumping sources based on compact, state-of-the-art MIR and THz QCLs, we will enable the FIR electromagnetic region to be explored and exploited fully for the first time.
EXTREME-IR will overcome the limits of current technology by integrating 2D nonlinear materials in innovative cavities with established, compact and powerful pumping sources. The project targets the generation of narrowband tuneable radiation across the 25 µm to 60 µm (12 – 5 THz) range. This will be achieved through the following set of interacting objectives, each based on the unique optical phonon, mode confinement and extreme nonlinear properties of 2D materials, and exploiting QCL pump sources. We have also identified key applications where proof-of-concept demonstrations will be performed to show the transformative potential of our new technologies :
O1 : Demonstrate the first up-converted FIR photonic source using plasmonic confinement in DMs to enhance the optical nonlinearities by several orders of magnitude compared to typical semiconductor materials ;
O2 : Demonstrate the first down-converted FIR photonic source using TMDs with intra-excitonic transitions to provide giant optical nonlinearities (x² 10 nm/V, two orders of magnitude greater than the bulk properties) ;
O3 : Integrate DMs with THz QCLs to realize the first up-converted FIR source, both in on-chip and external cavity geometries, taking advantage of the inherently TM polarized emission of QCLs ;
O4 : Integrate TMDs with MIR QCLs to realise the first down-converted FIR source and improving the efficiency tenfold by overcoming the ‘quantum defect’ that plagues other nonlinear sources ;
O5 : Demonstrate the application of the hybrid 2D-QCL sources in metrology and near-field spectroscopy, opening a pathway for new sensing technologies in the FIR range.
These far-reaching objectives, combined with a theoretical understanding of the material bandstructures and nonlinearities, will enable the current bulky and impractical systems that are currently used to access the FIR spectral range to be replaced by inexpensive and compact semiconductor-based technologies. Our vision for this enabling 2D photonic technology is to unlock new applications in the FIR ranging from fundamental science and quantum optics through to metrology, spectroscopy and imaging across a breadth of industrial sectors.