Exciton Spectroscopy and Engineering in Strongly Correlated Electron Systems

California Institute Of Technology, Pasadena CA

Investigators

Abstract

Non-technical: Upon irradiation with light, an electron in a solid is excited to a state with higher energy, leaving behind a hole – a missing electron – in its initial state. The oppositely charged electron and hole experience electrostatic attraction and, in electrically insulating materials, can form an atom-like bound object dubbed an “exciton”. Excitons not only serve as a platform for studying atomic physics phenomena in a solid-state environment but are also central to the development of photonic and opto-electronic technologies. To date, excitons have largely been studied in conventional non-magnetic insulators where interactions between electrons are weak. But in theory, excitons can also exist in a class of materials called Mott-Hubbard insulators, where strong interactions immobilize electrons and can cause their intrinsic magnetic moments to adopt myriad ordered arrangements. Such an environment can endow excitons with highly unconventional properties, potentially leading to a new generation of highly tunable and multi-functional excitonic devices. Despite these opportunities, Mott-Hubbard excitons remain experimentally elusive. This project directly addresses this problem by deploying novel high-speed stroboscopic methods to probe the energetic, dynamical, and spatial characteristics of Mott-Hubbard excitons in a range of Mott insulating compounds. Products of this research are being incorporated into a broad outreach plan that targets retention of underrepresented and underprivileged groups in science. This includes developing an extensive teaching and internship program for high school students in the Pasadena School District and holding an annual workshop on Caltech campus for U.S. college level students-of-color interested in pursuing graduate research in STEM fields. Technical: Excitons in conventional rigid-band semiconductors are well described as Coulomb bound hydrogen-like electron-hole pairs. However, much less is understood about excitons in Mott insulators, which consist of holon-doublon pairs bound by both Coulombic and spin exchange interactions. The properties of these so-called Hubbard excitons (HEs), and how they are influenced by the complex electronic and magnetic orders often found in Mott insulators, remain open questions. The goals of this project are to experimentally observe HEs and to explore their energetic, dynamical, and spatial properties using a suite of ultrafast optical spectroscopic techniques operating in the terahertz frequency range. Research activities are divided into two major thrusts. Thrust 1 focuses on directly resolving HEs through intra-excitonic transition spectroscopy, and on systematically characterizing their dynamical behavior across Mott systems hosting different electronic and magnetic orders. Thrust 2 focuses on using optical high-harmonic generation to perform tomographic imaging of the spatial structure of HE orbitals. Strong field manipulation of HEs through field-induced tunneling ionization experiments will also be carried out. Establishing this fundamental understanding of HEs will pave the way for future opto-electronics applications that leverage strongly correlated materials. 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.