A deeper understanding of light-matter interactions, with a focus on material dynamics, will lead to advances in new technologies. Strongly correlated materials are particularly interesting in this regard because any optical excitation that modifies the interplay between lattice, charge, orbital, or spin degrees of freedom can result in dramatic and potentially useful transformations (e.g., Basov et al., Nat. Mater. 16, 1077 (2017)). For example, understanding the electron dynamics in photovoltaics could lead to more efficient solar technology and clean energy. Phase transition dynamics in complex materials, including topological and strongly correlated materials have significant applications in electronic components, data storage devices, and even catalysts. Other materials of interest that can be studied include 2D materials (e.g., graphene), and ferromagnetic materials, which have applications in computing, solar power generation, and sensing. One such material is vanadium dioxide.

Vanadium dioxide (VO₂) is a strongly correlated material that undergoes an insulator-to-metal phase transition when either photoexcited or heated above 340 K. At equilibrium, VO₂ is in the M₁ phase, which is characterized by semiconducting electronic behavior, with a 0.67 eV band gap, and a monoclinic insulating structure. The most interesting aspect of VO₂ is that through photodoping, two distinct metallic phases can be reached depending on the pump fluence.

At high pump fluence, the photoinduced IMT in VO₂ is dominated by a transition from a monoclinic insulator (M₁) phase to rutile metal (R) phase, a well-known phase transition in literature and analogue of the equilibrium phase transition accessed via heating. This transition is associated with a lattice-structural transition between the monoclinic insulator and rutile metallic crystallography during which the system undergoes rearrangement of the atomic structure leading to a displacement of the V atoms along the c-axis and the collapse of the band gap. This leads to an increase of conductivity by about 5 orders of magnitude.

At low and intermediate fluence, however, the IMT is dominated by a transition to a monoclinic metallic (ℳ) phase that does not appear on the equilibrium phase diagram and was unknown until 2014 (Morrison et al., Science 346, 455 (2014)) when ultrafast electron diffraction (UED) measurements revealed its presence. In this case, the system retains the crystallographic symmetry of the parent, M₁ equilibrium phase, but exhibits a novel 1D antiferroelectric charge order that is not present at equilibrium presenting as a metastable monoclinic metallic phase. The electronic bands reorder yielding a partial overlap at the Fermi level and leads to a 1/3 the conductivity of the metallic R phase.

I used various spectroscopic techniques to study this phase transition including traditional pump-probe spectroscopy to investigate the quickness of the M₁ → R phase transition at high fluence, and the development of a time-resolved high harmonic spectroscopy (tr-HHG) technique to investigate the electronic band structure of the system during these phase transitions.

• M. R. Bionta, V. Wanie, V. Gruson, et al., “Probing the phase transition in VO₂ using few-cycle, 1.8 μm pulses,” Phys. Rev. B 97(12), 125126 (2018).

• Mina R. Bionta, Elissa Haddad, Adrien Leblanc, et al., “Tracking ultrafast solid-state dynamics using high harmonic spectroscopy,” Phys. Rev. Research 3, 023250.

• Mina R. Bionta “Tracking Ultrafast Solid-State Dynamics in VO₂ using High Harmonic Spectroscopy,” International Conference on Ultrafast Phenomena 2020, Shanghai, China (Virtual Presentation).