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Photoinduced Ultrafast Insulator-metal Phase Transition In VO2Films

Posted on:2015-06-05Degree:MasterType:Thesis
Country:ChinaCandidate:X XueFull Text:PDF
GTID:2298330422489335Subject:Optics
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The insulator-metal phase transition is an important phenomenon in solidmaterials that is expected to be utilized for applications in electrical and opticaldevices. Vanadium dioxide (VO2) undergoes a first order phase transition from thelow temperature insulator phase to the high temperature metal phase at the criticaltemperature Tc=68℃. Along with the phase transition, there is a drastic change inthe optical and electrical properties of VO2which makes VO2a material of great value.However, the physical mechanism behind the phase transition is complicated, becauseof the simultaneous change in the electronic structure and the crystal lattice in thematerial. There is also still no widely accepted uniform theory to interpret the drivingmechanism of the phase transition.In this thesis, by employing the transmission-type THz time domainspectroscopy (TDS) and optical pump and terahertz probe (OPTP) technique, wesystematically investigated the photoinduced phase transition in VO2films withvarious particle sizes and film thicknesses.First, we investigated a series films with different thicknesses (22,75,150nm) bytransmission-type THz-TDS and Raman spectrum at the temperature range from40to100℃. And these two groups of data that characteristic insulator-metal phase changewere compared.Furthermore, the particle size and film thickness dependence of the photoinducedmetal-insulator phase transition in VO2films has been studied systematically bytime-resolved terahertz spectroscopy at room temperature. It is found that thedynamical photoinduced phase transition from insulator to metal consists of twoprocesses: a1.7ps fast process and a slow process with a typical time constant of40ps. Both of the two processes show particle size independence. The40ps slowprocess is revealed to arise from the longitudinal propagation of the metallic phasefrom the photoexcited surface to the interior of the VO2film. A phase boundarypropagation speed with a magnitude of~2400m/s is obtained, which is close to the velocity of sound in solid materials and coincides with the prediction of diffusionlessphase transformation. Our experimental results clearly establish the entire procedureof photoinduced phase change in the VO2film.
Keywords/Search Tags:vanadium dioxide, optical pump terahertz probe, insulator-metal phasetransition
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