Andrea Cavalleri

Abstract:

We report on the generation of high-energy (1.9 μJ) far-infrared pulses tunable between 4 and 18 THz frequency. Emphasis is placed on tunability and on minimizing the bandwidth of these pulses to less than 1 THz, as achieved by difference-frequency mixing of two linearly chirped near-infrared pulses in the organic nonlinear crystal DSTMS. As the two near-infrared pulses are derived from amplification of the same white light continuum, their carrier envelope phase fluctuations are mutually correlated, and hence the difference-frequency THz field exhibits absolute phase stability. This source opens up new possibilities for the control of condensed matter and chemical systems by selective excitation of low-energy modes in a frequency range that has, to date, been difficult to access.

Abstract:

Selective optical excitation of a substrate lattice can drive phase changes across heterointerfaces. This phenomenon is a nonequilibrium analogue of static strain control in heterostructures and may lead to new applications in optically controlled phase change devices. Here, we make use of time-resolved nonresonant and resonant x-ray diffraction to clarify the underlying physics and to separate different microscopic degrees of freedom in space and time. We measure the dynamics of the lattice and that of the charge disproportionation in NdNiO_{3}, when an insulator-metal transition is driven by coherent lattice distortions in the LaAlO_{3} substrate. We find that charge redistribution propagates at supersonic speeds from the interface into the NdNiO_{3} film, followed by a sonic lattice wave. When combined with measurements of magnetic disordering and of the metal-insulator transition, these results establish a hierarchy of events for ultrafast control at complex-oxide heterointerfaces.

Abstract:

We report possible light-induced superconductivity in the organic molecular solid K3C60, a superconductor at equilibrium below Tc=20 K. In our experiment we excited this alkali-doped fulleride with strong femtosecond pulses, tuned to be resonant with local molecular vibrational modes. By means of THz time-domain spectroscopy, we detected the pump-induced changes in the conductivity spectrum as a function of pump-probe time delay. Strikingly, at temperatures up to 100 K, we measured a light-induced response with the same optical properties of the equilibrium superconductor. An interpretation in terms of non-linear coupling between different vibrational modes may give hints to explain this emergent physics away of equilibrium.