Nonlinear Optical Dynamics During Phase Transition in Vanadium Dioxide
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Nonlinear Optical Dynamics During Phase Transition in Vanadium Dioxide S. Lysenko, G. Zhang, V. Vikhnin, A. Rua, F. Fernandez and H. Liu Department of Physics, University of Puerto Rico, Mayaguez, PR 00681, USA
ABSTRACT Insulator-to-metal phase transition (PT) and relaxation dynamics in VO2 were studied with ultrafast transient reflection spectroscopy and degenerate four wave mixing technique. Optical pumping of free carriers in VO2 initiates PT on a femtosecond time scale. The relaxation dynamics is strongly dependent on pump energy. Spectral reflectivity behaviour at PT due to thermal heating and upon laser excitation of VO2 demonstrates close proximity in the relative change. PT induced by light was interpreted by exciton-controlled mechanism. Excited state dynamics of metallic VO2 is assigned to formations of electronic and hole polaronic states. INTRODUCTION Design and development of light-controlled solid-state optical switchers, storage devices, infrared laser cavity mirrors based on metal-oxide films is upcoming branch of modern optoelectronic technology. Vanadium dioxide (VO2) exhibits a wide range of physical properties including first order insulator-to-metal (I-M) phase transition (PT) upon thermal heating (transition point is Tt=340 K) or laser radiation. Light-induced transition is not associated with heating of the VO2 lattice [1]. During PT a structural deformation of the lattice symmetry from a monoclinic to a tetragonal (rutile) structure leads to considerable variations in the conductivity and VO2 optical properties. Recent experiments [1] show bandlike character of PT in VO2. It is expected that light- and thermally-induced PTs may have different mechanisms. The thermal mechanism of metal-insulator transition can be considered as spontaneous charge transfer O2→ V5+ with final recharged state O- − V4+ which is a driving force of PT [2]. In the case of ultrashort laser pumping the strong optical radiation causes electron transition from d|| - state of valence band to unoccupied excited mixed d||-π* - state of conduction band, and results in resonant ultrafast PT. It is very likely the light-induced PT has relations to the photogeneration of excitons [3]. In this paper the ultrafast pump-probe spectroscopy was employed to obtain the time-resolved information about light-induced structural transformation and transient electronic properties of VO2 in the metallic phase. EXPERIMENTAL Film growth The pulsed laser deposition (PLD) technique has been applied to create thin VO2 films on amorphous SiO2 substrate. To fabricate VO2 films the excimer KrF laser (“Lambda Physic 110”, wavelength λ=248 nm, 10-ns pulse duration, 14 J/cm2 fluence) has been used. Temperature regimes near substrate have produced a strong influence on the VO2 film morphology. To form thin VO2 films the substrate temperature was electrically fixed at 820 K. The VO2 films with
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thicknesses d=25 nm and d=50 nm, have been formed from straight flux of torch particles in the oxygen and argon atmosphere under 3
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