Many-Body Ultrafast Light-Matter Interactions in Two-Dimensional Graphene Optoelectronics

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Non-technical description: The recent discovery of graphene, a material of single atomic thickness, has spurred remarkable advances in condensed matter physics as well as, nanometer-scale device applications covering electronic, thermal and mechanical domains. In chip-scale optoelectronics and optical physics, graphene has a unique optical absorption defined solely by the interaction between light and electrons. This project examines and advances the many-body light-matter interactions in graphene towards optoelectronics, particularly for ultrafast optics, for multi-electron current generation in photodetectors, and for next-generation optical switches and modulators. The research involves laser-material interactions, nonlinear optics, material characterization, and synchronized material-device physics. In parallel and leveraging the fundamental science advanced, the education activity involves outreach to East Los Angeles minority-heavy high-schools and teachers, partnership with the Center for Excellence in Engineering and Diversity for low-income underrepresented first-generation college-bound youth, and a new graduate/undergraduate course on solid-state optoelectronics. Technical description: The unique linear and massless band structure of graphene, in a purely two-dimensional Dirac fermionic structure, has enabled an optical sheet conductivity that is remarkably frequency-independent, with broadband optical character spanning from visible to mid-infrared wavelengths. The underlying interband optical transitions can be tuned significantly via electric gating near the Dirac point, with a tunable charge-density-based Fermi level due to the low density of sp2-hybridized two-dimensional states. In this project the principal investigator examines the many-body light-matter interactions in graphene optoelectronics covering the electron-electron, electron-phonon, electron-photon scattering mechanisms and dynamics. These interactions are targeted towards device physics and applications in ultrafast optics (first thrust), multi-carrier photocurrent dynamics (second thrust), and electro-optics (third thrust). The first thrust examines all-optical nonlinearities such as four-wave mixing and nonlinear dynamics in graphene: this involves continuous-wave and pulsed measurements, with the gate-tunable Fermi levels in graphene and carrier dynamics. The second thrust examines optoelectronic photocurrent generation in graphene-silicon structures: this involves photocurrent mapping and comparison with monolithic silicon structures, gated-bias carrier dynamics, and hot carrier multiplication in electronic transport. The third thrust examines high-speed electro-optic modulators through interband and intraband transitions: this involves working with carrier frequencies in excess of 100 GHz to even the THz level, supported by Raman characterization and surface phonon engineering. Fermi level tuning, nonlinear signal detection techniques, along with surface material control and processing, are implemented to enable the unique device physics. The many-body scattering dynamics - each at the sub-picosecond timecales - provide a fertile ground for fundamental material and atomic layer engineering studies in graphene-based next-generation optoelectronics.