Circular Phononics: Driving Matter by Phonon Angular Momentum

• Applicant: Professor Dr. Sebastian Maehrlein
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 469405347

The structural arrangement of atoms and its symmetries dictate the physical properties and equilibrium states of matter. If this spatial structure can be modulated or changed at will by an ultrafast laser pulse, we can realize new transient states of matter with material features on demand. Non-equilibrium states and their interconnecting transitions are fundamentally determined by exchange and conservation of energy, linear momentum, and angular momentum. Even though exchange of energy and linear momentum between lattice excitations (phonons) and other degrees of freedom is a cornerstone of solid-state physics, phonon angular momentum is commonly just assumed to account for angular momentum conservation and its active control remains elusive.In this project, my team and I will prepare and coherently control phonon states with nonzero angular momentum to study and actively manipulate coupled electronic and spin degrees of freedoms. Thereby, we will establish phonon angular momentum as a novel ultrafast tuning knob for comprehensive material control. We will begin with the first proof of principle of defined preparation and detection of circular polarized coherent phonons in prototypical materials. Then we will proceed to van der Waals layered semiconductors and Weyl semimetals with inherently chiral lattice modes. In these time reversal symmetry-broken systems, circularly polarized in-plane phonons will provide a new non-invasive handle for spin- and valleytronics, whereas circular inter-plane shear modes will create or modulate Moiré potentials, thus paving the way for “dynamic twistronics”. Eventually, our approach allows inverting the long-standing problem of angular momentum loss during ultrafast demagnetization: We will seek an inverse ultrafast Einstein – de Haas effect by actively transferring angular momentum from the lattice to the spin-ordered system of a magnetic insulator, thereby joining the fields of phononics and magnonics.The proposed project requires the tabletop design and implementation of a phase-stable, strong-field, THz and mid-infrared (MIR) light source with switchable helicity on a single-shot basis. This technological development will additionally provide tools for a wide range of spectroscopies and field-driven phenomena, such as THz molecular circular dichroism, helicity-resolved vibrational spectroscopy, and Hall effects in the THz to MIR regime. Overall, with this research project, we will introduce angular momentum to the arsenal of structural dynamics for ultrafast material control.

• DFG Programme: Independent Junior Research Groups