Speaker
Description
Hard x-ray free-electron lasers have emerged as an invaluable tool for studying materials dynamics near and far from equilibrium. Advanced nonlinear x-ray methods will allow us to greatly expand on our ability to understand and control materials properties at the atomic-scale. In this talk I will present results of all time-domain x-ray scattering-based spectroscopies to study transient lattice dynamics in photo-excited materials spanning the Brillouin zone using both optical and x-ray excitation. Ultrafast optical excitation typically produces broad-band coherences in the mean-square displacements of the ions that we subsequently resolve in time and momentum via femtosecond diffuse scattering [1,2]. I will briefly discuss the mechanism of excitation [3] whereby we can measure the excited state phonon-dispersion [4] and momentum-resolved anharmonic decay channels [5] using photo-excited bismuth as an example. We have also shown that to some extent selective-excitation using optical pulses can be used to control which modes are excited [3,6]. Finally, I will discuss progress towards arbitrary x-ray selective excitation using atomic-scale transient gratings including preliminary results of x-ray pump, x-ray probe experiments on cubic perovskites.
This work was supported by the U.S. Department of Energy.
[1] M. Trigo, M. Fuchs, J. Chen, M. P. Jiang, M. Cammarata, S. Fahy, D. M. Fritz, K. Gaffney, S. Ghimire, A. Higginbotham, S. L. Johnson, M. E. Kozina, J. Larsson, H. Lemke, A. M. Lindenberg, G. Ndabashimiye, F. Quirin, K. Sokolowski-Tinten, C. Uher, G. Wang, J. S. Wark, D. Zhu, and D. A. Reis. Fourier-transform inelastic x-ray scattering from time- and momentum-dependent phonon-phonon correlations. Nature Physics, 9(12):790–794, 2013.
[2] D. Zhu, A. Robert, T. Henighan, H. T. Lemke, M. Chollet, J. M. Glownia, D. A. Reis, and M. Trigo. Phonon spectroscopy with sub-mev resolution by femtosecond x-ray diffuse scattering. Physical Review B, 92:054303, 2015.
[3] T. Henighan, M. Trigo, M. Chollet, J. N. Clark, S. Fahy, J. M. Glownia, M. P. Jiang, M. Kozina, H. Liu, S. Song, D. Zhu, and D. A. Reis. Control of two-phonon correlations and the mechanism of high-wavevector phonon generation by ultrafast light pulses. Physical Review B, 94:020302, 2016.
[4] S. W. Teitelbaum, T. C. Henighan, H. Liu, M. P. Jiang, D. Zhu, M. Chollet, T. Sato, E ́. D. Murray, S. Fahy, S. O’Mahony, T. P. Bailey, C. Uher, M. Trigo, and D. A. Reis. Measurements of nonequilibrium interatomic forces in photoexcited bismuth, arXiv 1908.07161 2019.
[5] S. W. Teitelbaum, T. Henighan, Y. Huang, H. Liu, M. P. Jiang, D. Zhu, M. Chollet, T. Sato, E. D. Murray, S. Fahy, S. O’Mahony, T. P. Bailey, C. Uher, M. Trigo, and D. A. Reis. Direct measurement of anharmonic decay channels of a coherent phonon. Phyical Review Letters, 121:125901, 2018.
[6] S. W. Teitelbaum, T. Henighan, H. Liu, M. P. Jiang, M. Kozina, D. Zhu, M. Chollet, T. Sato, J. M. Glownia, M. Trigo, and D. A. Reis. Frequency-selective excitation of high-wavevector phonons. Applied Physics Letters, 113(17):171901, 2018.
