Andrea Cavalleri

Abstract:

Photo-excitation can drive strongly correlated electron insulators into competing conducting phases, resulting in giant and ultrafast changes of their electronic and magnetic properties. The underlying non-equilibrium dynamics involve many degrees of freedom at once, whereby sufficiently short optical pulses can trigger the corresponding collective modes of the solid along temporally coherent pathways. The characteristic frequencies of these modes range between the few GHz of acoustic vibrations to the tens or even hundreds of THz for purely electronic excitations. Virtually all experiments so far have used 100 fs or longer pulses, detecting only comparatively slow lattice dynamics. Here, we use sub-10-fs optical pulses to study the photo-induced insulator-metal transition in the magnetoresistive manganite Pr(0.7)Ca(0.3)MnO(3). At room temperature, we find that the time-dependent pathway towards the metallic phase is accompanied by coherent 31 THz oscillations of the optical reflectivity, significantly faster than all lattice vibrations. These high-frequency oscillations are suggestive of coherent orbital waves, crystal-field excitations triggered here by impulsive stimulated Raman scattering. Orbital waves are likely to be initially localized to the small polarons of this room-temperature manganite, coupling to other degrees of freedom at longer times, as photo-domains coalesce into a metallic phase.

Abstract:

The magnetoresistive manganite Pr0.7Ca0.3MnO3 becomes metallic when THz pulses are used to resonantly drive a stretching Mn-O vibration. A five-order-of-magnitude drop of the sample resistivity and ultrafast, nanosecond-lived reflectivity changes are observed. © 2007 Optical Society of America.

Abstract:

We use optical-pump terahertz-probe spectroscopy to measure the photoinduced phase transition in vanadium dioxide thin films. Our measurements reveal a fluence threshold needed to drive this transition that decreases with increasing temperature.

Abstract:

Selective vibrational excitation of insulating Pr0.7Ca0.3MnO3 at 17 mu m triggers a transition to a metallic state. A four order of magnitude drop of the sample resistivity and ultrafast, nanosecond-lived reflectivity changes are observed.