Nonlinear Terahertz Phononics: A Novel Route to Controlling Matter

CIC nanoGUNE Seminars

[NANOOPTICS SEMINAR] Sebastian Mährlein, Fritz Haber Institute of the Max Planck Society, Berlin, Germany.
nanoGUNE seminar room, Tolosa Hiribidea 76, Donostia - San Sebastian
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Nonlinear Terahertz Phononics: A Novel Route to Controlling Matter Periodically arranged atoms are the fundamental building blocks of solids, and determine the mechanical, thermal, and electric properties of a material. Thus, it is no surprise that lattice vibrations (phonons) govern a number of exciting phenomena such as spin transport in thermal gradients, phase transitions and superconductivity. In this work, we take advantage of phonons as a novel and specific pathway to drive ultrafast processes in solids. By direct excitation with intense, ultrashort THz electric- field transients, high frequency phonons in insulating solids are accessed on their intrinsic time- and energy-scales, while avoiding parasitic electronic processes. We provide new insights into the coupling between the lattice and magnetic ordering, which is of central relevance for rapid data processing and information storage in future technological applications. Therefore, pure spin-lattice coupling is investigated by resonant excitation of infrared- active phonon modes of the textbook ferrimagnetic insulator Yttrium Iron Garnet. Remarkably, two distinctive time scales for phonon-magnon equilibration are revealed. A surprisingly rapid change of magnetic order with a time constant of ~1 ps is found to be driven by phonon-induced fluctuations of the exchange coupling, which leads to a sublattice demagnetization under the constraint of conserved total spin angular momentum. In contrast, phonon modes with vanishing electric dipole moments were so far excluded from such direct THz excitation. In this thesis, a novel type of light-matter interaction is presented that enables coherent-phonon excitation via non-resonant two-photon absorption of intense THz fields. Here, it is demonstrated by the coherent control of the 40 THz Raman-active optical phonon in diamond via the sum frequency of two intense terahertz field components. Remarkably, the CEP of the driving pulse is directly imprinted on the lattice vibration. **Host** : R. Hillenbrand