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.

Abstract-Terahertz ellipsometry study of the soft mode behavior in ultrathin SrTiO3 films

P. Marsik1,a), K. Sen1, J. Khmaladze1, M. Yazdi-Rizi1, B. P. P. Mallett1 and C. Bernhard1

1 Department of Physics and Fribourg Center for Nanomaterials, University of Fribourg, Chemin du Musée 3, CH-1700 Fribourg, Switzerland

a) Electronic mail: premysl.marsik@unifr.ch

Appl. Phys. Lett. 108, 052901 (2016); http://dx.doi.org/10.1063/1.4940976

We present a combined study with time-domain terahertz and conventional far-infrared ellipsometry of the temperature dependent optical response of SrTiO3thin films (82 and 8.5 nm) that are grown by pulsed-laser deposition on (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) substrates. We demonstrate that terahertz ellipsometry is very sensitive to the optical response of these thin films, in particular, to the soft mode of SrTiO3. We show that for the 82 nm film the eigenfrequency of the soft mode is strongly reduced by annealing at 1200 °C, whereas for the 8.5 nm film it is hardly affected. For the latter, after annealing the mode remains at 125 cm−1 at 300 K and exhibits only a weak softening to about 90 cm−1 at 10 K. This suggests that this ultrathin film undergoes hardly any relaxation of the compressive strain due to the LSAT substrate.