In June 2017, Hongkun Park and collaborators published a paper in Nature Nanotechnology on their new spectroscopy technique for two-dimensional materials based on near-field coupling to surface plasmon polaritons. According to a News and Views article in the same edition, these "2D materials can be integrated into a photonics platform to make optical circuitry in which dark excitons can be used for encoding and transporting information on a chip."
The Park Lab's numerous, multi-purpose tools have implications for all-optical computing and solid-state quantum information processing, illuminating the behavior of immune cells in health and disease as well as the inner workings of the brain, and condensed matter physics, molecular structural determination, and biological sensing.
Abstract
"Transition metal dichalcogenide (TMD) monolayers with a direct bandgap feature tightly bound excitons, strong spin–orbit coupling and spin–valley degrees of freedom. Depending on the spin configuration of the electron–hole pairs, intra-valley excitons of TMD monolayers can be either optically bright or dark. Dark excitons involve nominally spin-forbidden optical transitions with a zero in-plane transition dipole moment, making their detection with conventional far-field optical techniques challenging. Here, we introduce a method for probing the optical properties of two-dimensional materials via near-field coupling to surface plasmon polaritons (SPPs). This coupling selectively enhances optical transitions with dipole moments normal to the two-dimensional plane, enabling direct detection of dark excitons in TMD monolayers. When a WSe2monolayer is placed on top of a single-crystal silver film, its emission into near-field-coupled SPPs displays new spectral features whose energies and dipole orientations are consistent with dark neutral and charged excitons. The SPP-based near-field spectroscopy significantly improves experimental capabilities for probing and manipulating exciton dynamics of atomically thin materials, thus opening up new avenues for realizing active metasurfaces and robust optoelectronic systems, with potential applications in information processing and communication."
Paper: Philip Kim, Mikhail D. Lukin, Hongkun Park, "Probing dark excitons in atomically thin semiconductors via near-field coupling to surface plasmon polaritons," Nature Nanotechnology 12, 856-860 (2017).
News & Views: Brightening the dark excitons, by Kian Ping Loh
