Demonstration of gate-tunable 2DEG surface polaritons in LaAlO3/SrTiO3 bilayers

• Applicants: Dr. Felix Gunkel; Professor Dr. Thomas Taubner
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 522417350

In this follow-up proposal, we will address novel surface polaritons at atomically controlled oxide interfaces. Such surface polaritons enable a strong light-matter interaction on the nanoscale, which is not accessible in classical systems, and yield a wide range of nanooptical and polaritonic applications. In the first project phase of this proposal, we achieved a fundamental understanding of the optical near-field interaction of light and 2-dimensional electron gases generated at buried oxide interfaces. Moreover, we were able to correlate the detailed ionic structure of the interface to resulting electronic transport properties and magnetic signatures. Based on this gained knowledge, we are now able to probe the polaritonic behavior of the interface in the follow-up project phase. Major objective of this second project phase is the unambiguous demonstration of surface polaritons in these interface systems as well as the characterization and modelling of the full dispersion relation of these novel polaritonic states. In addition to that, we aim to systematically control the carrier density and vertical distribution of carriers at the interface via electric field effect, making it likely that also the polaritonic properties of the interface can be manipulated via electric-field-control. The demonstration of gate-tunable 2DEG polaritons will hence be subject to this follow-up proposal. For this, we will investigate LaAlO3/SrTiO3- bilayers, which will be gated from the thin films’ back-side. In this bilayer geometry, it is expected that the required gate voltage, typically in the range of hundreds of volts, can be reduced to a few volts or even toward the mV-range, drastically facilitating the field control. At the same time, the bilayer structure yields new scientific challenges toward understanding the interaction of electronic charge carriers and growth-induced crystal defects, which will be addressed within the scope of the follow-up project. In dedicated preliminary experiments, we were able to achieve a fundamental understanding of the optical properties of the electron gas and moreover found first experimental indication of surface polaritons at the LaAlO3/SrTiO3- interface. In addition, we were able to demonstrate that LaAlO3/SrTiO3-bilayers can be achieved with sufficient quality to obtain enhanced gating-behavior. Based on these results, we now aim to realize bilayer field-effect devices and unravel their polaritonic properties via nanooptical scanning probe microscopy and modelling. The intended demonstration of gate-tunable 2DEG polaritons will reflect a significant step toward realizing oxide-based nanooptical and polaritonic devices and will generate a major leap in fundamental understanding of matter-light-interactions on the nanoscale.

• DFG Programme: Research Grants