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

在这个后续的建议中，我们将在原子控制的氧化物界面上解决新的表面极化。这种表面极化子能够在纳米尺度上实现强烈的光-物质相互作用，这在经典系统中是无法实现的，并且产生了广泛的纳米光学和极化应用。在本提案的第一个项目阶段，我们对埋藏氧化物界面产生的光和二维电子气体的光学近场相互作用有了基本的了解。此外，我们能够将界面的详细离子结构与产生的电子输运性质和磁特征联系起来。基于这些获得的知识，我们现在能够在后续项目阶段探测界面的极化行为。第二个项目阶段的主要目标是明确地展示这些界面系统中的表面极化，以及这些新型极化态的全色散关系的表征和建模。此外，我们的目标是通过电场效应系统地控制载流子密度和载流子在界面处的垂直分布，从而有可能通过电场控制来操纵界面的极化性质。因此，栅极可调谐的2DEG极化的演示将受到本后续提案的影响。为此，我们将研究从薄膜背面门控的LaAlO3/SrTiO3-双分子层。在这种双层几何结构中，预期所需的栅极电压，通常在数百伏特的范围内，可以降低到几伏特甚至接近毫伏范围，从而大大促进了场控制。同时，双层结构对理解电子载流子和生长诱导的晶体缺陷的相互作用提出了新的科学挑战，这将在后续项目的范围内得到解决。在专门的初步实验中，我们能够对电子气体的光学性质有一个基本的了解，并且在LaAlO3/SrTiO3-界面上发现了第一个表面极化的实验指示。此外，我们能够证明，LaAlO3/ srtio3 -双分子层可以获得足够的质量，以获得增强的门控行为。基于这些结果，我们现在的目标是实现双层场效应器件，并通过纳米光学扫描探针显微镜和建模揭示其极化特性。门可调谐2DEG极化子的演示将反映出实现基于氧化物的纳米光学和极化子器件的重要一步，并将在纳米尺度上对物质-光相互作用的基本理解上产生重大飞跃。

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.