Abstract: With the ongoing miniaturization of novel photonic and optoelectronic devices, two-dimensional (2D) materials, and in particular transition metal dichalcogenides (TMDs), have emerged as powerful candidates, as they can be thinned down to the monolayer limit and seamlessly integrated on-chip. These ultrathin semiconductors, with a direct band gap in the visible to near-infrared spectral range, enable light–matter interactions of unprecedented strength at the atomic scale. As a result of this extreme 2D confinement, strong excitonic effects arise, dominating their optical response even at room temperature. Equally important, however, are their symmetry properties. On the one hand, they give rise to helicity-dependent selection rules at the ±K points at the corners of the Brillouin zone. On the other hand, the broken space inversion symmetry in monolayer TMDs allows for the use of second-order nonlinear effects, such as second-harmonic generation. In addition, their nonlinear coefficients are orders of magnitude larger than those of conventionally used nonlinear crystals, such as beta-barium borate and lithium niobate. Combined with their broadband applicability, enabled by relaxed phase-matching conditions originating from their negligible thickness, these properties make them exceptionally promising for on-chip nonlinear optical applications. More recently, they have also gained traction in quantum photonics, with rapid progress in entangled photon generation via spontaneous parametric down-conversion in van der Waals materials.

In this talk, I will explore how spatial and time-reversal symmetries in TMDs can be harnessed to tailor their nonlinear optical response. In particular, I will demonstrate symmetry-enabled polarization switching and show how intense circularly polarized excitation can induce valley-selective bandgap modulation. I will then turn to quantum light generation, discussing spontaneous parametric down-conversion in 2D TMDs and the central role of symmetry in shaping the emitted quantum states. Finally, I will outline strategies to enhance conversion and modulation efficiencies through nanostructuring.

Bio: Sebastian Klimmer is a postdoctoral researcher in the nonlinear and quantum photonics group of Prof. Andrey Sukhorukov at the Australian National University (ANU). He completed his Ph.D. through the dual-award program Meta-Active, working with Prof. Giancarlo Soavi at Friedrich Schiller University Jena and Prof. Dragomir Neshev at ANU. His doctoral research focused on ultrafast nonlinear optics in two-dimensional (2D) materials. During this time, he contributed to the demonstration of ultrafast optical control of nonlinear signals, particularly in 2D transition metal dichalcogenides (TMDs), advancing their potential for high-speed photonic technologies. He further investigated the integration of TMDs with metasurfaces, establishing new approaches for controlling and tailoring optical responses at the nanoscale. These results were published in leading journals, including Nature Photonics and Nature Communications. Sebastian’s current research expands into quantum optics, where he investigates the optical properties of van der Waals layered materials for novel quantum light sources, as well as their integration with metasurfaces to achieve advanced and scalable functionalities.