Probing Strong Electronic Correlations in Ferroelectrics and Multiferroics Through High-Order Harmonic Spectroscopy and First-Principles Calculations

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Nontechnical Abstract: One of the goals of modern condensed matter physics is to control the properties of materials so that they can be modified 'on demand'. In relevant materials intense pulses of laser light can force the materials far from equilibrium, dramatically changing their electronic, magnetic, and optical properties. This work focuses on experimental and computational studies of the non-equilibrium properties of such materials which exhibit unique electronic and magnetic responses to external stimuli, induced by intense infrared laser fields. Using advanced laser techniques, the dynamics of interactions in such materials are reconstructed from measurements of light emitted through a process known as high-order harmonic generation. While most previous studies of ultrafast dynamics in solids are performed at the femtosecond and longer timescales (1 femtosecond = 0.000000000000001 seconds), this project seeks to uncover details of the sub-femtosecond dynamics and to establish the link between ultrafast interactions in non-equilibrium materials and the spectral- and time-domain structure of high-order harmonic emission. The experimental and theoretical tools developed through this project can also be used to describe processes relevant to technological applications such as solar energy and spintronics. Students trained as a result of the award will gain experience in advanced laser and computational techniques, as well as state-of-the-art materials synthesis, preparing them for high-tech careers.Technical Abstract: Understanding and controlling the quantum many-body dynamics of strongly-driven, strongly-correlated materials is one of the grand challenges of modern science. Although the nonequilibrium dynamics of strongly-correlated materials have been studied for decades, the few- to sub-femtosecond dynamics of strong electronic correlation have been hidden from experiments. Through measurements and first-principles calculations of the high-order harmonics generated from intense mid-infrared laser fields in bulk and thin-film solids, this project aims to resolve electronic correlations in ferroelectric barium titanate and multiferroic bismuth ferrite crystals, and the role of these correlations in the strong-field laser response. Through measurements of the spectrum, polarization, and time-domain structure of the emitted high-order harmonic radiation, the project reveals the impacts of crystal structure and ferroelectric polarization, Berry curvature, and correlated electron dynamics on the high-order harmonic generation process. Calculations, using time-dependent density functional theory with exchange-correlation functions obtained via dynamical mean-field theory, accurately capture the time-domain Coulomb interactions at the heart of strong electronic correlations. The experimental and theoretical tools developed through this project can also be used to describe processes relevant to technological applications such as solar energy and spintronics. Students trained as a result of the award will gain experience in advanced laser and computational techniques, as well as state-of-the-art materials synthesis, preparing them for high-tech careers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.