Speakers
Description
The angular momentum of lattice vibrations — phonon angular momentum — is a largely unexplored degree of freedom in solid-state systems, playing a key role in the understanding of ultrafast demagnetization processes and offering new pathways for phonon-driven ultrafast material control, particularly relevant to spintronics and valleytronics. While this research area is rapidly growing, phonon angular momentum states have been previously inferred from secondary effects, such as transient magnetic fields or photoluminescence.
In this work [1], we experimentally demonstrate the possibility to directly prepare and monitor coherent states of phonon angular momentum. For this, we utilize intense, helicity-tailored, single-cycle THz pulses [2] to coherently drive a doubly-degenerate Raman-active phonon mode. We consecutively track its vectorial trajectory via the THz-induced Kerr effect. We find that the circularly polarized THz pulse drives a circular Eg phonon state with a precisely controlled helicity via a nonlinear sum-frequency excitation [3]. The identified excitation process corresponds to coherent phonon-phonon angular momentum transfer, where an IR-active circular Eu phonon anharmonically drives a Raman-active Eg phonon of opposite helicity. We confirm the conservation of pseudo phonon angular momentum by the observed helicity reversal between the Eu and Eg helicity states, which is based on the discrete rotational symmetry of the lattice. This fundamental finding provides a new handle for investigating and controlling angular momentum channels in solid-state systems.
References
[1] O. Minakova et al., In preparation (2024).
[2] M. Frenzel et al., Optica 11, 3, p. 362-370 (2024).
[3] D. M. Juraschek and S. F. Maehrlein, PRB 97, 17, p. 174302 (2018)
