Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides (1605.03359v1)

Abstract: Atomically thin transition metal dichalcogenides (TMDs) are direct-gap semiconductors with strong light-matter and Coulomb interaction. The latter accounts for tightly bound excitons, which dominate the optical properties of these technologically promising materials. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. In a recent study, an experimental evidence for the existence of such dark states has been demonstrated, as well as their strong impact on the quantum efficiency of light emission in TMDs. Here, we reveal the microscopic origin of the excitonic coherence lifetime in two representative TMD materials (WS$_2$ and MoSe$_2$) within a joint study combining microscopic theory with optical experiments. We show that the excitonic coherence lifetime is determined by phonon-induced intra- and intervalley scattering into dark excitonic states. Remarkably, and in accordance with the theoretical prediction, we find an efficient exciton relaxation in WS$_2$ through phonon emission at all temperatures.

• The paper demonstrates that excitonic linewidths arise from both radiative recombination and phonon-mediated nonradiative scattering.
• It reveals distinct exciton dynamics in WS2 and MoSe2, with intervalley and intravalley processes driving their coherence lifetimes.
• Temperature-dependent measurements show a super-linear increase in linewidth, informing the design of future TMD-based optoelectronic devices.

Excitonic Linewidth and Coherence Lifetime in Monolayer Transition Metal Dichalcogenides

The paper "Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides" investigates the microscopic mechanisms underlying the coherence properties of excitons in monolayer transition metal dichalcogenides (TMDs). These materials, such as WS$_2$ and MoSe$_2$, are recognized for their direct bandgap and strong light-matter interactions, which make them promising candidates for optoelectronic applications.

Methodological Insights

The authors employ a combined theoretical and experimental approach to explore the temperature-dependent excitonic coherence properties of TMDs. The theoretical framework is based on the solution of the Bloch and Wannier equations to model the exciton dynamics within these two-dimensional systems. The optical matrix elements and phonon-induced scattering lifetimes are calculated, emphasizing the role of both intra- and intervalley scattering processes. The experimental aspect involves temperature-dependent optical spectroscopic measurements to quantify the linewidths of excitonic resonances.

Key Results

1. Radiative and Non-Radiative Contributions: The excitonic linewidth is shown to have both radiative and phonon-mediated non-radiative components. At lower temperatures, radiative recombination primarily determines the coherence lifetime, whereas at higher temperatures, phonon interactions become more significant.
2. Material-Specific Dynamics: A notable differentiation in exciton dynamics is observed between WS$_2$ and MoSe$_2$.
   • In WS$_2$, efficient intervalley scattering to dark states is facilitated by phonon emission, resulting in a pronounced non-radiative linewidth component even at low temperatures. The dark $K-\Lambda$ exciton states lie energetically below the bright states, making scattering processes more effective.
   • In contrast, MoSe$_2$ exhibits a dominance of intravalley scattering with acoustic phonons. The $K-\Lambda$ dark exciton states are energetically above the bright states in MoSe$_2$, reducing the effectiveness of phonon emission processes at low temperatures.
3. Temperature Dependence: For both materials, the linewidth increases super-linearly with temperature due to both acoustic and optical phonon interactions, demonstrating distinct material-specific scattering mechanisms.

Implications and Future Directions

This paper provides crucial insights into the temperature-dependent coherence properties of excitons in TMDs, with direct implications for the design of next-generation optoelectronic devices. A comprehensive understanding of both radiative and non-radiative processes and their temperature dependence allows for enhanced control over exciton dynamics, paving the way for improved photonic materials.

Future work could explore extending the theoretical model to account for additional two-dimensional materials with varying compositions and structures. Additionally, further experimental studies under varied environmental conditions, such as different substrates or external fields, could provide deeper insights into excitonic interactions and their tunability in TMDs.

The integration of these findings into practical device engineering could significantly impact the development and optimization of TMD-based technologies, including light-emitting diodes, photovoltaics, and excitonic circuits, ensuring that the unique properties of these materials are fully leveraged.