Direct manipulation of the atomic lattice using intense long-wavelength laser pulses has become a viable approach to create new states of matter in complex materials. Conventionally, a high-frequency vibrational mode is driven resonantly by a mid-infrared laser pulse and the lattice structure is modified through indirect coupling of this infrared-active phonon to other, lower-frequency lattice modulations. Here, we drive the lowest-frequency optical phonon in the prototypical transition metal oxide SrTiO₃ well into the anharmonic regime with an intense terahertz field. We show that it is possible to transfer energy to higher-frequency phonon modes through nonlinear coupling. Our observations are carried out by directly mapping the lattice response to the coherent drive field with femtosecond X-ray pulses, enabling direct visualization of the atomic displacements.


Figure: a, Strong terahertz radiation (red) interacts with the STO soft-mode phonon (yellow). The degree of resonant overlap can be tuned by temperature. Energy is exchanged with higher-frequency phonon modes (turquoise, purple) through nonlinear coupling. The STO unit cell and two lowest-frequency zone-centre TO eigenmodes are indicated at the top of the figure. b, Phonon motion is probed in the time domain with ultrafast XRD in reflection geometry.

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Reference: Kozina, M., M. Fechner, P. Marsik, T. van Driel, J. M. Glownia, C. Bernhard, M. Radovic, D. Zhu, S. Bonetti, U. Staub and M. C. Hoffmann (2019). Terahertz-driven phonon upconversion in SrTiO₃. Nat. Phys. (10.1038/s41567-018-0408-1) Kozina-2019