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Plasmonics

Where does the strong nonlinearity of plexcitons come from?

Tang Y, Zhang Y, Liu Q, et al. · Light, science & applications · 2022

Open access · cc by · source: Europe PMC

Mixed plasmon-exciton states in a silver-nanodisk/WS2 system respond nonlinearly at about ten times lower pulse energy than bare WS2, and the effect comes from the exciton part.

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Study at a glance

Design
Other — Room-temperature extinction and femtosecond pump-probe reflection on Ag nanodisk arrays patterned on monolayer WS2, with selective exciton or plasmon pumping and coupled-oscillator fits
N
Series of fabricated samples varying disk diameter (80 to 140 nm) and Al2O3 spacer thickness; no single sample count
Population
Ag nanodisk arrays on monolayer WS2 flakes on quartz
Outcome
Plexciton resonance energies, Rabi splitting dynamics, fluence-dependent line shape and nonlinear absorption

Structured fields used in claim comparison tables when every cited study has a complete layer.

Key findings

The plasmon and exciton coupled with a strength of about 92.6 meV, placing the system in an intermediate coupling regime, and the Rabi splitting recovered in about 0.6 ps after excitation. Pumping only the exciton reproduced the full plexciton response, while pumping only the plasmon did almost nothing, so the nonlinearity is excitonic. Fits needed saturation plus excitation-induced dephasing (linewidth broadening) to match the data, and the same nonlinear signal required almost 10 times less pulse energy than in bare WS2. Changing spacer thickness or disk diameter switched the response between reverse saturable and saturable absorption.

Methodology

The researchers patterned arrays of silver nanodisks onto single-layer WS2, varying disk diameter so the plasmon resonance swept through the exciton to form hybrid plexciton states. They used femtosecond pump-probe reflection spectroscopy at room temperature, pumping either the plexciton, only the exciton, or only the plasmon, and fitted spectra with a coupled oscillator model. They then changed spacer thickness and disk size to shape the nonlinear absorption.

Limitations

The roughly 200 fs time resolution could not resolve the coherent plexciton dynamics directly, so nonlinearity was sampled at a single delay of 100 fs. The authors could not estimate the plexciton density, so no interaction constant was quantified. The system never reached true strong coupling, where the mechanism might differ, and a small deviation at low fluence was left unexplained.

How this study connects

Role on claims

Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.

  • SupportsPlasmonic hot electrons and strong couplingconcept

    In plexcitons the nonlinearity came from the exciton side.

    In silver nanodisks coupled to WS2 (coupling about 92.6 meV, intermediate regime), selectively pumping the exciton reproduced the full nonlinear response while pumping the plasmon did almost nothing, and the nonlinearity needed about 10 times less pulse energy than bare WS2.

    Evidence for the claim as stated.

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