The phase transition in thin films and single crystals probed optically

 Understanding phase transitions is one of the grand challenges in condensed matter physics.  This talk will cover my studies of the nanoscale mechanisms that underlie the phase transition in VO2, both in a thin film comprising a collection of close-packed nanocrystalline grains and in single crystals.

Understanding phase transitions is one of the grand challenges in condensed matter physics.  Even seemingly simple phase transitions, such as ice nucleating from nanoscale “seeds” in water and crystallizing into ice as the temperature is lowered, still harbor mysteries.  Vanadium dioxide (VO2), first studied six decades ago, undergoes a solid-solid phase transition in which a metallic phase nucleates and grows inside the insulating phase as the temperature is raised.  This unusual behavior has triggered widespread interest in applications ranging from ultrafast optical switches to smart thermal window coatings, cooling films for spacecraft surfaces, and even nano-mechanical grippers.  However, the dynamics of this phase transition remain puzzling even in single crystals of VO2.  I have been studying the nanoscale mechanisms that underlie the phase transition in VO2, both in a thin film comprising a collection of close-packed nanocrystalline grains and in single crystals.  One way of nondestructively accessing this information is using nanoantennas excited by light.  We fabricate dipole antennas on the surface of VO2 thin films and single crystals and use scattering-scanning near-field optical microscopy, in which the sharp metallic tip also functions as a nanoscale antenna.  Here I will contrast the behavior of the insulator to metal transition in thin films with single crystals that show completely different behavior depending on width and/or aspect ratio.

Christina McGahan

Christina McGahan graduated summa cum laude with a B.S. in Physics from Gettysburg College in 2011 and was elected to Phi Beta Kappa.  She earned a Ph.D. in Physics from Vanderbilt University in 2017 with a dissertation entitled “Interactions of Gold Plasmons and Vanadium Dioxide”.  That work focused on simulating, fabricating, and experimentally probing the interaction of plasmonic antennas and the phase transition in VO2 (vanadium dioxide). The antennas act as local reporters of this phase transition, and were used to examine phases of VO2 that coexist in a single crystal of VO2 and to track hydrogen doping dynamics in vanadium dioxide thin films.  Her research interests are in the nanoscale changes in materials that cause large changes in optical or electronic properties.  This entails optimizing these small changes in simulations, developing fabrication processes to make these nanoscale changes in materials, and determining methods to measure the resulting changes in optical or electronic properties with high spatial resolution.