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Scott Eric Madaras , Physics - Final Oral Exam for the Ph.D., Title: Insulator to Metal Transition Dynamics of Vanadium Dioxide Thin Films

Zoom Link is available upon request. Please email Ellie at evwilk@wm.edu.

Abstract: Vanadium Dioxide(VO2) is a strongly correlated material which was been studied for many decades. VO2 has been proposed for uses in technologies such as optical and IR modulators, optical switches and Mott memory devices. These technologies are taking advantage of VO2’s insulator to metal transition (IMT) and the corresponding changes to the optical and material properties. VO2’s insulator to metal transition can be accessed by thermally heating, applied electric field, or ultra-fast photo-induced processes. Recently, thin film VO2 that has been grown on Titanium Dioxide doped with Niobium (TiO2:Nb) has shown promise as a possible UV photo detector with high quantum efficiency which utilizes a heterostructure between these two materials. In this work, the dynamics of the IMT on thin films of VO2 are explored. It will be shown that surface plasmons generated from an Au thin film can induce the IMT in a thin film of VO2 using the enhanced electric field generated by the surface plasmons as well as detecting the IMT by those plasmons and their corresponding resonance changes. Time resolved pump probe studies were also done on thin films of VO2 grown on TiO2 and TiO2:Nb. The pump used had a UV photon energy of 3.1eV (400nm wavelength) and was selected because these samples show photocurrent generation at that wavelength. The fluence threshold of the IMT at 3.1eV was significantly lower than published values that use 1.55eV photons as a pump. The time response of the IMT shows rich dynamics in the reflectivity of the sample. The response was partially attributed to internal interference of the reflected probe beam from the inhomogeneous layers formed inside the film by different phases of VO2, and which can be elucidated by a diffusion model with respect to its optical properties. Finally, the photocurrent generation time constants for the sample with highest quantum efficiency are given and compared to its ultrafast photoinduced IMT time constants. 

Bio: Scott Madaras was born and grew up in Yorktown VA. He attended the University of Denver as an undergraduate where he studied physics while swimming on the full scholarship. He graduated in 2011 with a B.S. in Physics with minors in mathematics and business statistics. He began at William & Mary in the Fall of 2014 there he joined Ale Lukaszew and Irina Novikova’s research group, where he studied IMT dynamics and process in vanadium dioxide thin films using surface plasmons and ultrafast time resolved pump probe measurements.

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