Ultrafast Dynamics of Transition Metal Oxides and Rare-Earth Chalcogenides

Understanding physicochemical processes at their fundamental timescale is crucial for design and applications of materials. Using femtosecond laser spectroscopy, light-induced dynamics in highly-correlated materials can be studied to resolve the fundamental processes involved, which may span different energy and temporal ranges. Here, we report ultrafast dynamics of materials that exhibit structural or electronic switching properties. The compounds studied show varying properties including electronic configurations (d-block transition metal and f-block rare-earth). The effect of specimen conditions and compositions on ultrafast dynamics were also examined. In Chapter 3, the influence of surface stress on the photoinduced structural transformation of 10-nm-thick crystalline VO2 supported on Al2O3 (0001) were examined using ultrafast electron diffraction. The results signify a time-dependent energy distribution among various degrees of freedom and reveal the impact of strain on and the nature of photoinduced transition of VO2. An ultrafast release of the compressive strain was observed at early times following the photoexcitation, accompanied by faster motions of vanadium dimers that are more complex than simple dilation or bond tilting reported in the literature. At longer times, a laser fluence multiple times higher than the thermodynamic enthalpy threshold is required for complete conversion from monoclinic to the tetragonal phase, where transformation of certain domain the is not seen even on nanosecond times. In Chapter 4, photoinduced electronic and structural dynamics of rare-earth chalcogenides were studied. Using two-color pump‒probe reflectivity, ultrafast carrier dynamics and generation of coherent acoustic phonons in YbS were resolved. Compared to the carrier relaxation processes and lifetimes of conventional semiconductors, recombination of photoexcited electrons with holes in localized f orbitals was found to take place rapidly. Such carrier annihilation signifies the unique and ultrafast nature of valence restoration of ytterbium ions after femtosecond photoexcitation switching. The transfer of the absorbed energy to the lattice results in emergence of coherent acoustic phonons on picosecond times due to the thermal strain in the photoexcited region. In Chapter 5, ultrafast dynamics in V3O5 and V4O7 single crystals were studied using two-color pump‒probe reflectivity. Our results revealed several features in the time-domain dynamics, suggest a multi-stage insulator-to-metal transition process. The ultrafast change in reflectivity could be responsible by charge ordering and/or charge transfer between V3+ and V4+ ions. The metallic phase of both compounds showed oscillatory modulation in reflectivity signal, which could be explained using generation of strain pulse due to photoexcitation.Chemistry, Department o