TY - INPR A1 - Cavalleri, Andrea A1 - Siders, Craig A1 - Toth, Csaba A1 - Squier, Jeff A. A1 - Barty, C. P. J. A1 - Wilson, K. R. A1 - Sokolowski-Tinten, Klaus A1 - von der Linde, Dietrich A1 - Kammler, Martin A1 - Horn- von Hoegen, Michael T1 - Measurement of harmonic and anharmonic semi-conductor-lattice dynamics with ultrafast x-ray diffraction N2 - Many fundamental processes in solids, such as heat transport and phase transitions involve atomic movement on ultrafast time-scales, but cannot be directly measured with visible light. Using ultrafast x-ray diffraction, atomic motion can be measured on its natural time-scale. Photo-excitation of bulk and layered semiconductors was achieved with 800-nm wavelength femtosecond laser pulses. Optical excitation triggers a chain of relaxation processes, causing rapid lattice heating, generation and propagation of stress/strain waves into the bulk and eventual anharmonic decay of coherent acoustic phonons into the thermal bath of the solid. While the first steps of carrier relaxation, excitation and decay of Raman-active optical phonons have been in the past characterized optically at surfaces, the bulk acoustic dynamics at longer times has not. We measure length scales of lattice excitation, thereby detecting ultrafast heat diffusion via fast electrons, coherent acoustic pulse propagation into the bulk or across buried interfaces and anharmonic damping of non-equilibrium acoustic phonons. The measurement is performed with sensitivity on lattice spacing of 10 femtometers (i.e., 3 nuclear diameters). Y1 - 2000 UR - https://www.researchgate.net/publication/252175863_Measurement_of_harmonic_and_anharmonic_semi-conductor-lattice_dynamics_with_ultrafast_x-ray_diffraction ER - TY - INPR A1 - Siders, Craig A1 - Cavalleri, Andrea A1 - Toth, Csaba A1 - Squier Jeff A., A1 - Barty, C. P. J. A1 - Wilson, K. R. A1 - Sokolowski-Tinten, Klaus A1 - von der Linde, Dietrich A1 - Kammler, Martin A1 - Horn- von Hoegen, Michael T1 - Ultrafast Nonthermal Melting of Femtosecond-laser-irradiated Germanium Measured by Ultrafast X-ray Diffraction N2 - Many fundamental processes in nature, such as chemical reactions and phase transitions involve rearrangement of the constituent atoms. Such changes cannot be directly measured with visible light and occur transiently on time-scales comparable with the natural oscillation periods of atoms (femtoseconds to picoseconds). We present experiments on Ge-111 films irradiated with 100-fs 800-nm light pulses and probed by a time-delayed sub-ps x-ray pulse at 8 keV. After excitation of a dense electron-hole plasma within the 160-nm film, the ultrafast pathway of lattice disordering is measured as a function laser fluence. For close-to-threshold irradiation, inhomogeneous thermal melting, occurring via surface nucleation of the liquid phase and growth into the bulk determines the dynamics of the solid-to-liquid phase transition. For excitation at about twice the melting threshold, a nonthermal pathway, starting from a highly excited cold solid and relaxing into a hot liquid causes ultrafast melting. At long time delays, nearly perfect recrystallization of a (111) crystal is observed. Y1 - 2000 UR - https://www.researchgate.net/publication/252159566_Ultrafast_Nonthermal_Melting_of_Femtosecond-laser-irradiated_Germanium_Measured_by_Ultrafast_X-ray_Diffraction ER - TY - JOUR A1 - Cavalleri, Andrea A1 - Siders, C. W. A1 - Brown, F. L. H. A1 - Leitner, D. M. A1 - Tóth, Csaba A1 - Squier, Jeff A. A1 - Barty, C. P. J. A1 - Wilson, K. R. A1 - Sokolowski-Tinten, Klaus A1 - Horn-von Hoegen, Michael A1 - von der Linde, D. A1 - Kammler, Martin T1 - Anharmonic lattice dynamics in germanium measured with ultrafast x-ray diffraction JF - Physical review letters N2 - Damping of impulsively generated coherent acoustic oscillations in a femtosecond laser-heated thin germanium film is measured as a function of fluence by means of ultrafast x-ray diffraction. By simultaneously measuring picosecond strain dynamics in the film and in the unexcited silicon substrate, we separate anharmonic damping from acoustic transmission through the buried interface. The measured damping rate and its dependence on the calculated temperature of the thermal bath is consistent with estimated four-body, elastic dephasing times (T2) for 7-GHz longitudinal acoustic phonons in germanium. Y1 - 2000 U6 - https://doi.org/10.1103/PhysRevLett.85.586 VL - 85 IS - 3 SP - 586 EP - 589 PB - APS ER - TY - JOUR A1 - Sokolowski-Tinten, Klaus A1 - Horn-von Hoegen, Michael A1 - von der Linde, D. A1 - Cavalleri, Andrea A1 - Siders, C. W. A1 - Brown, F. L. H. A1 - Tóth, Csaba A1 - Squier, Jeff A. A1 - Barty, C. P. J. A1 - Wilson, K. R. A1 - Kammler, Martin T1 - Time-resolved x-ray diffraction study of ultrafast acoustic phonon dynamics in Ge/Si-heterostructures JF - International Conference on Ultrafast Phenomena , 9 July 2000, Charleston N2 - Using time-resolved x-ray diffraction the ultafast strain dynamics in fs-laserexcited Ge/Si-heterostructures has been studied. A fluence dependent, anharmonic damping of the impulsively generated acoustic phonons and vibrational transport across the buried Ge/Si-interface are observed. KW - Acoustic Phonon KW - Acoustic Transmission KW - Carrier Diffusion KW - Homogenous Expansion KW - Lattice Anharmonicity Y1 - 2000 UR - https://opg.optica.org/abstract.cfm?uri=UP-2000-MF43 PB - Optica ER - TY - JOUR A1 - Siders, C. W. A1 - Cavalleri, Andrea A1 - Sokolowski-Tinten, Klaus A1 - Tóth, Csaba A1 - Guo, T. A1 - Kammler, Martin A1 - Horn-von Hoegen, Michael A1 - Wilson, K. R. A1 - von der Linde, D. A1 - Barty, C. P. J. T1 - Direct Observation of Ultrafast Non-thermal Melting by Ultrafast X-ray Diffraction JF - Ultrafast Phenomena XII N2 - Ultrafast nonthermal melting of femtosecond laser-irradiated germanium was observed directly for the first time using sub-picosecond, 1.54-angstrom x-ray diffraction. Such measurements allow the study of transient atomic motion fundamental to many processes in physics, chemistry, and biology. Y1 - 2000 U6 - https://doi.org/10.1007/978-3-642-56546-5_80 VL - 66 SP - 276 EP - 280 PB - Springer ER -