Speaker: Dr. Shao-Yu Chen
Title: Harnessing Dark Excitons for Light Upconversion through Plasmons and Strain
Abstract:
Momentum-dark excitons provide a reservoir of long-lived excitations in two-dimensional semiconductors, yet controlling their population and converting their energy into light remain key challenges. Here, we investigate plasmonic excitation and strain engineering as complementary routes to promote dark-exciton-mediated exciton–exciton annihilation (EEA) and upconversion photoluminescence in transition metal dichalcogenides (TMDs). In few-layer WSe2 coupled to gold nanorod arrays, excitation near the localized surface plasmon resonance produces more than an order-of-magnitude enhancement of upconverted emission around 450 nm under 785 nm excitation. In strained monolayer MoS₂ supported on microspheres, pronounced upconverted emission near 2.8 eV emerges in strained regions and exhibits a superlinear excitation-power dependence. We interpret these observations within a common framework in which the accumulation of momentum-dark excitons enables EEA to populate higher-energy radiatively bright states. Plasmonic structures enhance local excitation and may modify access to finite-momentum excitons and subsequent relaxation into dark reservoirs, whereas strain reshapes the excitonic energy landscape, potentially favoring dark-state occupation and tuning the energetic alignment of annihilation pathways. The resulting upconversion depends jointly on dark-exciton density, annihilation efficiency, and the availability of bright final states. Together, these complementary platforms support a strategy for harnessing dark-exciton interactions as a source of nonlinear light emission and motivate combined optical and mechanical control of excitonic energy conversion in atomically thin semiconductors.
Speaker: Dr. Ryo Noguchi
Title:Towards the Exploration and Control of Quantum Materials: Static and Time-Resolved Spectroscopy
Abstract:
Understanding and controlling the properties of exotic quantum materials are major challenges in condensed matter physics. Spectroscopic techniques provide complementary insights into these properties. In particular, angle-resolved photoemission spectroscopy (ARPES) directly maps electronic states in momentum space. However, ARPES alone cannot fully address many physical phenomena, and establishing and operating an ARPES system requires substantial investment. Our laboratory therefore combines access to existing ARPES facilities with in-house optical and time-resolved photoemission spectroscopy to advance the exploration and control of quantum materials. This talk will present our research strategy, near-term projects, and long-term goals.