Tuneable Nonlinear Phononics in Layered Quantum Materials

Phonons are quantum mechanical quasiparticles acting like tiny packets of energy that make atoms vibrate harmonically in solids. Pushing one in a row of dominoes will cause a chain reaction of falling dominoes. Similarly, phonons make atoms in solids vibrate collectively and pass this vibrational energy to their neighbours. These vibrations are crucial for heat conduction and sound propagation. Phonon nonlinearity involves the scattering of phonons among themselves and is essential for explaining thermal expansion and heat conduction in solids. When we shine super bright terahertz (THz) light on atoms, they start vibrating anharmonically (nonlinear), leading to the emergence of transient phases, such as light-induced superconductivity. Nonlinear phononics profoundly impacts THz-range nonlinear optics and selective excitation of phonons and is critical for advancing medical imaging, fibre optics, 6G communications, and quantum communication. Achieving highly tuneable phonon nonlinearity is essential for fabricating optimal thermal conductors and insulators, as well as efficient phononic diodes and transistors. Exploiting phonon nonlinearity at the quantum level will lead to the emergence of quantum phononic technologies running parallel to quantum photonics. Unlike the photonic case, phononic nonlinearity is not inherently weak, leading to significantly shorter phonon lifetimes compared to photons. Therefore, striking the right balance in phonon nonlinearity is critical for quantum phononic applications--strong enough to induce quantum entanglement and correlation effects, yet still weak enough to ensure an adequate phonon lifetime. Layered materials, like graphene and transition metal chalcogenides, possess atomic-scale thickness and boast exceptional tuneability, achievable through external gating potential and lattice deformation (strain). Thanks to their remarkable sensitivity to tiny changes in inter-layer stacking, they provide a fertile ground for nonlinear phononics. This sensitivity is achieved through variations in the relative twisting or sliding of atomic layers, held together by weak van der Waals inter-layer forces. Intriguingly, asymmetrical two-dimensional materials host topological phonons that surprisingly remain resilient in the presence of disorder and thus are promising for applications in quantum phononics. This project will merge quantum field theory with ab-initio computational tools and aim to achieve high tuneability of phonon nonlinearity in layered materials. Phonon nonlinearity has two sources: direct interactions between phonons themselves, and indirect interactions where electrons act as messengers. The latter mechanism offers a promising avenue for achieving the desired level of tuneability by altering the number of electrons or their motion by utilising electromagnetic means or strain. Computing both sources of phonon nonlinearity in bilayer systems, we will investigate how shining light can create a temporary crystalline structure, aiming for ultrafast (in just a tiny fraction of a second!) optical control of material properties. Subsequently, we will investigate the impact of phonon nonlinearity on THz/infrared nonlinear optics, aiming to achieve photocurrent enhancement by leveraging phonon resonance. This will impact THz-sensing and converting solar and thermal energy into a reusable form. We will explore the effects of strain and external gating potential to achieve a high degree of tuneability. We will advance nano-phononics, quantum phononics, and THz nonlinear optics by introducing broadband adjustment of phonon nonlinearity, bridging it to phonon topology, and developing quantum theories for sound waves mixing with topological phonons and photons on the sea of electrons. This will impact industries reliant on nano-scale thermal management, as in cell phones, phononic logic gates and waveguides for quantum communication, and THz detection for remote sensing and material characterisation.