Brightening of Dark Trions in Monolayer WS₂ Via 83, 115 K Localized Surface Plasmons Enables New Technologies

Dark trions, quasi-particles found in two-dimensional semiconductors, hold considerable promise for future technologies due to their exceptionally long lifetimes, far exceeding those of conventional bright trions. However, activating these dark states, particularly at higher temperatures, presents a significant challenge. Sreyan Raha, Tara Shankar Bhattacharya, and Indrani Bose, all from Bose Institute, alongside Achintya Singha, now demonstrate a method for brightening dark trions in monolayer tungsten disulphide.

The team achieves this brightening by localizing surface plasmons within a disordered gold substrate, revealing a distinct spectral signature and providing evidence for semi-dark and bright trion states originating from intervalley electron-electron scatterings. This breakthrough not only enhances our understanding of trion behaviour, but also opens new avenues for manipulating and harnessing these quasi-particles in advanced optoelectronic devices. D Materials, Excitons, and Plasmonic Coupling This research investigates the fundamental properties of two-dimensional materials, specifically monolayer tungsten disulphide and tungsten diselenide, focusing on excitons and trions, bound pairs of electrons and holes. Scientists explore how these particles behave and interact, with a key focus on manipulating valley polarization, a quantum property with potential applications in emerging fields like valleytronics. The study also incorporates plasmonics, utilizing metallic nanoparticles to enhance light-matter interactions and modify the behavior of excitons and trions. Researchers delve into the intricacies of trion behavior, including the investigation of dark trions and their activation through plasmonic coupling. They also examine valley-polarized trions, exploring the manipulation of their quantum properties, and investigate the spin states of trions, analyzing singlet and triplet configurations. The research extends to understanding the behavior of negative grey trions, formed under specific conditions of high electron density, and explores the interactions between neutral excitons and charged trions. Measurements of spin-orbit coupling in the materials provide insights into their electronic structure. By utilizing plasmonic nanoparticles, scientists enhance photoluminescence and modify exciton/trion behavior, enabling the study of long-lived excitons and trions. Investigations into intervalley interactions reveal how excitons and trions interact between different valleys in the material. This sophisticated research, combining advanced techniques and collaborative expertise, aims to deepen our understanding of 2D materials and pave the way for potential applications in optoelectronics, valleytronics, and quantum information processing.

Brightening Dark Trions with Gold Plasmons Scientists have developed a novel method to brighten dark trions in monolayer tungsten disulphide by leveraging localized surface plasmons. The process begins with mechanically exfoliating the material and transferring it onto a disordered gold film deposited on silicon dioxide. This gold film generates the localized surface plasmons crucial for interacting with the dark trions. Photoluminescence measurements are performed using a back-scattering geometry with a micro-Raman spectrometer. To investigate circular polarization, the team incorporates linear polarizers, quarter-wave plates, and half-wave plates into the optical setup. Low-temperature spectra are recorded using a temperature-controlled stage, allowing for precise control of the sample’s thermal environment. Researchers observe substantial electron doping of the tungsten disulphide monolayer when placed on the rough gold substrate, effectively converting excitons into trions at lower temperatures. This allows them to focus specifically on trion dynamics and reveal a distinct photoluminescence doublet with a peak separation of approximately 45 meV. Spatially resolved measurements confirm the uniformity of this spectral shape, ruling out local strain as the origin of the new feature. Laser power-dependent measurements demonstrate a linear relationship between integrated photoluminescence intensity and laser power, indicating that the emissions are not due to defect-bound recombination.

The team proposes that the rough gold film generates localized surface plasmons with a prominent out-of-plane electric field, which couples to the out-of-plane dipole moment of the dark trions, leading to their brightening. This plasmonic environment enhances the local density of states, increasing the intensity of the photoluminescence signal for both dark and bright trions, and provides a scalable strategy for accessing spin-forbidden states in two-dimensional materials.

Dark Trions Brightened via Gold Film Doping Scientists have achieved a breakthrough in understanding and manipulating dark trions in two-dimensional tungsten disulphide. Their work demonstrates a method for “brightening” these typically inactive states, unlocking their potential for future technologies. Experiments reveal that by placing the monolayer tungsten disulphide on a disordered gold film, researchers were able to induce substantial electron doping, effectively converting most excitons into trions, allowing them to focus exclusively on the dynamics of these trions and observe a significant enhancement of their photoluminescence.

The team observed a distinct doublet in the photoluminescence spectrum, with peaks separated by approximately 45 meV. Analysis suggests these peaks represent semi-dark and bright trion states, originating from intervalley electron-electron scattering. Further investigation ruled out localized strain as the cause of this spectral feature, confirming the unique behavior of the trions. Laser power-dependent measurements revealed a linear relationship between integrated photoluminescence intensity and laser power, with scaling factors of approximately 1. 2 and 1. 0 for the higher and lower energy peaks, respectively, excluding defect-bound recombination processes. Crucially, the disordered gold film generates localized surface plasmons with a prominent out-of-plane electric field. This field couples to the out-of-plane dipole moment of the dark trions, effectively activating them and leading to brightening. Measurements confirm a marked enhancement of the photoluminescence signal for both dark and bright trions, with the plasmonic fields coupling to their respective dipole moments. Temperature-dependent measurements, performed from 83 K to 293 K, show the emergence of the semi-dark trion peak near 115 K. Modeling the thermal behavior using the Boltzmann distribution, with an extracted energy separation of approximately 45 meV, supports the assignment of the peak energies to the semi-dark and bright trion states. This work demonstrates a pathway for controlling and utilizing dark trions, potentially enabling advancements in optoelectronic devices and other emerging technologies.

Brightening Dark Trions with Gold Plasmonics This study demonstrates the brightening of typically spin-forbidden dark trions in monolayer tungsten disulphide, achieving amplified photoluminescence emission over a temperature range of 83 K to 115 K. Researchers achieved this brightening not through traditional magnetic field application, but by utilizing localized surface plasmon modes generated within a disordered gold substrate. Analysis of the resulting photoluminescence spectrum revealed a distinct doublet, interpreted as originating from semi-dark and bright trion states, with the energy difference attributed to intervalley electron-electron scattering. 👉 More information 🗞 Brightening of dark trions in monolayer WS via localization of surface plasmons 🧠 ArXiv: https://arxiv.org/abs/2512.10856 Tags: