Interacting plexcitons for designed ultrafast optical nonlinearity in a monolayer semiconductor

Microcavity exciton polaritons (also referred to as microcavity polaritons here) are formed by quantum hybridization of semiconductor excitons and microcavity photons. Benefitting from their mixed light and matter nature, microcavity polaritons have yielded many observations on collective quantum effects, such as Bose-Einstein condensation, superfluidity, and nonclassical light, and can be engineered for a wealth of technological applications, including low-threshold polariton lasers, all-optical logic circuits, and even ultrafast optical switches at the single-photon limit. Of these fascinating phenomena, the intrinsic strong nonlinearity inherited from the exciton component is the key to most of the distinctive physical features of polaritons. However, in traditional Wannier-Mott and Frenkel semiconductor materials with hydrogenic-type excitonic states, the polariton nonlinearity and exciton-photon coupling strength are two quantities with an evitable trade-off, as the polariton nonlinearity scales with the exciton Bohr radius, while the coupling strength is inversely proportional to the exciton Bohr radius. This contradiction severely limits the development of room-temperature polaritonic devices, in which sufficiently strong nonlinearity and coupling strength are simultaneously required.

 

Recently emerged monolayer (ML) transition metal dichalcogenides (TMDs) offer a unique solution to this trade-off due to their modified Coulomb potential beyond the hydrogenic picture, which essentially improves the exciton nonlinearity despite the ~ 1 nm exciton Bohr radius. Nonetheless, such an increase in nonlinearity is still not enough to meet the requirements of practical applications. One strategy for higher room-temperature nonlinearity in TMD-based polaritons without compromising the stability is to squeeze them into very tiny, deep-subwavelength areas to achieve adequately strong interactions. Plasmon-exciton polaritons (i.e., plexcitons), as a special example of hybrid polaritons constructed by plasmon-exciton coupled modes, are naturally localized in their spatial distribution as they adopt highly plasmon-like charge and field characteristics, and thus very hopeful to reach this prospect. Although robust coupling strengths in TMD-based plexcitons have been intensively reported, research on their nonlinearity remains scarce, and the fundamental issue of whether the nanoscale confinement nature will impart plexcitons with superior nonlinearity remains ambiguous.

In a new paper published in Light Science & Application, a team of scientists, led by Professor Tian Jiang from College of Advanced Interdisciplinary Studies, National University of Defense Technology, China, and Professor Lei Shi, Key Laboratory of Micro- and Nano-Photonic Structures (Ministry of Education), and State Key Laboratory of Surface Physics, Department of Physics, Fudan University, China, have systematically studied the plexciton nonlinearity in a silver (Ag) nanodisk (ND)-ML tungsten-disulfide (WS₂) hybrid sample. By applying femtosecond pump-probe measurements, they first revealed the ultrafast photophysical dynamics of the hybrid sample. Then, they provided experimental evidence of nontrivial higher-order nonlinearity in plexcitons arising from the excitation-induced dephasing effect of their exciton component. Combining this giant nonlinear interaction with the great tunability of plexcitons, they finally implemented active design of ultrafast nonlinear absorption responses in plexcitons at comparatively low excitation energy, which highlights the great potential of plexcitons in nonlinear optical applications.

The prepared sample in this work is schematically shown in Figure 1, in which the plasmonic arrays composed of disk-shaped Ag nanoparticles were directly patterned onto ML WS₂ flakes supported by a quartz substrate, facilitating the formation of coherent plexcitonic states. The subsequent pump-probe measurements reveal the subtle dynamics of plexcitons in Figure 2a, identifying three different photophysical processes of plexcitons with distinguished characteristic lifetimes, including coherent plexcitons, incoherent plasmon/exciton populations, and heat effect. Moreover, the plexcitons are found to sustain enough strong nonlinearity at room temperature due to their subwavelength-confined nature in spatial distribution, and the nonlinearity are mainly originated from the first-order saturation interaction and higher-order excitation-induced dephasing interaction between plexcitons (Figure 2b). Ultimately, active manipulation of ultrafast nonlinear responses (NLA) is successfully achieved in plexcitonic systems by virtue of their giant nonlinearity and great tunability, as displayed in Figure 3. Such substantial progress in plexcitons is expected to open new routes for high-speed all-optical switching and energy-efficient all-optical data processing under room conditions.

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