Abstract-Terahertz-driven phonon upconversion in SrTiO3

M. Kozina, M. Fechner, P. Marsik, T. van Driel, J. M. Glownia, C. Bernhard, M. Radovic, D. Zhu, S. Bonetti, U. Staub,  M. C. Hoffmann,

https://www.nature.com/articles/s41567-018-0408-1

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.

Abstract-Local terahertz field enhancement for time-resolved x-ray diffraction

M. Kozina,  M. Pancaldi, C. Bernhard, T. van Driel, J. M. Glownia,   P. Marsik, M. Radovic, C. A. F. Vaz, D. Zhu,  S. Bonetti, U. Staub, M. C. Hoffmann

http://aip.scitation.org/doi/abs/10.1063/1.4977088

We report local field strength enhancement of single-cycle terahertz (THz) pulses in an ultrafast time-resolved x-ray diffraction experiment. We show that patterning the sample with gold microstructures increases the THz field without changing the THz pulse shape or drastically affecting the quality of the x-ray diffraction pattern. We find a five-fold increase in THz-induced x-ray diffraction intensity change in the presence of microstructures on a SrTiO3thin-film sample.

Abstract-Anisotropy of the infrared-active phonon modes in the mono-domain state of tetragonal SrTiO3 (110)

M. Yazdi-Rizi, P. Marsik, B. P. P. Mallett, C. Bernhard

(Submitted on 8 Aug 2016)

http://arxiv.org/abs/1608.02329

With infrared and terahertz ellipsometry we investigated the anisotropy of the infrared active phonon modes in SrTiO3 (110) single crystals in the tetragonal state below the so-called antiferrodistortive transition at T* = 105 K. In particular, we show that the anisotropy of the oscillator strength of the so-called R-mode, which becomes weakly infrared active below T*, is a valuable indicator for the orientation of the structural domains. Our results reveal that a mono-domain state with the tetragonal axis (c-axis) parallel to the [001] direction can be created by applying a moderate uniaxial stress of about 2.3 MPa along the [1-10] direction (with a mechanical clamp). The resulting, intrinsic splitting of the infrared-active phonon modes is reported.