Giant REsonant non-LINearities in Quantum cascade lasers – RE-LINQ

Quantum Cascade Lasers (QCLs) are powerful intersubband laser sources operating throughout the mid-infrared (MIR) and terahertz (THz) regions. As well as high output powers, they also possess giant optical nonlinearities based on resonant excitations that are orders of magnitude greater than those of the bulk. These arise when an incident photon has exactly the same energy as a specific material transition. A wide range of efficient resonant nonlinear effects within the QCL cavity have been shown, such as second harmonic generation, sum frequency and difference frequency generation to access difficult to reach electromagnetic ranges. These effects are all based on intersubband nonlinearities.

Recently the members of this consortium have demonstrated a new interaction using the enhanced resonant interband nonlinearities of THz QCLs. We have shown that a nonlinear interaction between the THz beam of the QCL, E(QCL) and a resonant NIR excitation, E(NIR) of the effective bandgap can occur, permitting the generation of THz optical sidebands i.e. E(NIR)+E(QCL) and E(NIR)-E(QCL). Importantly, efficiencies comparable to equivalent experiments using entire facilities such as free electron lasers (FELs) were obtained. This opens up applications in THz detection via upconversion, wavelength shifting and the possibility of studying high THz-optical field interactions using compact and powerful QCLs. This projects aims to take these significant results as a base and to build a fundamental understanding of the nonlinear effect, to optimise the interaction and to demonstrate the resonant frequency mixing at room temperature. In particular, we will show how both resonant interband and intersubband properties of a quantum well system can be coherently combined and optimised within the cavity of a QCL. The gain of the QCL will be used to negate the effect of the intersubband losses, and the interband nonlinearity of the external pump will be maximised through a combination of bandstructure engineering and the integration of quantum dots within the QCL. Through these advances, efficient nonlinear optical mixing will be realised between the injected near-infrared interband excitation and the QCL intersubband emission.

The project is separated into three distinct parts. The first will provide the base of the project with THz optical sideband generation with THz QCLs. Here the process will be optimised, and will include the investigation of higher order nonlinear effects and the feasibility of using the process to upconvert the THz emission to the NIR for efficient THz detection. Secondly as the efficiency of process is expected to be independent of phase matching, MIR QCLs will be investigated for the first time providing the significant advantage of higher power densities and room temperature operation. The final exploratory part of the project will attempt the combination of the first two sections with the use of quantum dots providing an “atomic-like” nonlinearity with interband nonlinear susceptibilities theoretically approaching those of intersubband nonlinearies (~10^6 pm/V).

This project gathers a consortium of world-class expertise along all the required research chain from device fabrication up to spectroscopy applications: the field of material growth (LPN), QCL design and processing (MPQ), and ultrafast optical/THz and nonlinear spectroscopy (LPA). Successful realisation and optimisation of such a nonlinear interaction will have a fundamental impact on our understanding of the engineering of enhanced susceptibilities and intense THz light-matter interactions. Furthermore, high efficiency frequency conversion provides a stepping stone to a broad range of new functionalities for QCLs. These include an integrated solution for ultrafast and large bandwidth all-optical wavelength shifting with MIR QCLs and up-conversion of the QCL emission to the technologically mature near infrared range for sensitive THz detection.