  The near-field interaction due to a strong electromagnetic field induced by
resonant localized plasmons can result in a strong coupling of excitonic states
or formation of hybrid exciton-plasmon modes in quantum confined structures.
This coupling can be strengthed by designing a system with its vibronic states
resonant to the energy of the driving field induced by the localized plasmon
excitation. Silver nanoislands nucleated on molybdenum disulfide (MoS2) is an
ideal platform for such interaction. The influence of localized plasmons (LSP)
on the formation and dissociation of excitons due to resonant and off-resonant
optical excitation of carriers to excitonic states is studied using ultrafast
optical spectroscopy. The local field due to the Ag nanoparticles (Ag-NP)
enhances the magnitude of the Raman modes in MoS2 in the presence of resonant
excitation. An ultrashort pulsed optical excitation at 2.3 eV resonantly
excites the LSP modes and the optical near-field resonantly drive the phonon
modes, which leads to a coherent coupling of the A and B excitons in MoS2 with
the plasmon modes. The resonant excitation of the LSP modes modulate the
optical absorption of the probe field. The resonant excitation of C exciton due
to an excitation source at 3.0 eV, which is off-resonant to the LSP mode
increases the electrostatic screening in the presence of excess carriers from
Ag NPs. It results in a faster dissociation of optically generated C excitons
into free carriers that eventually increase the A and B exciton population. A
3-level density matrix theory in the presence of dressed vibronic states
induced by a localized near-field optical driving source is applied to describe
the coherent interaction process in the hybrid nano-plasmonic system.
