Single vs double anti-crossing in the strong coupling between surface plasmons and molecular excitons

Abstract

Strong coupling between surface plasmons and molecular excitons may lead to the formation of new hybrid states—polaritons—that are part light and part matter in character. A key signature of this strong coupling is an anti-crossing of the exciton and surface plasmon modes on a dispersion diagram. In a recent report on strong coupling between the plasmon modes of a small silver nano-rod and a molecular dye, it was shown that when the oscillator strength of the exciton is large enough, an additional anti-crossing feature may arise in the spectral region where the real part of the permittivity of the excitonic material is zero. However, the physics behind this double anti-crossing feature is still unclear. Here, we make use of extensive transfer matrix simulations to explore this phenomenon. We show that for low oscillator strengths of the excitonic resonance, there is a single anti-crossing arising from strong coupling between the surface plasmon and the excitonic resonance, which is associated with the formation of upper and lower plasmon–exciton polaritons. As the oscillator strength is increased, we find that a new mode emerges between these upper and lower polariton states and show that this new mode is an excitonic surface mode. Our study also features an exploration of the role played by the orientation of the excitonic dipole moment and the relationship between the modes we observe and the transverse and longitudinal resonances associated with the excitonic response. We also investigate why this type of double splitting is rarely observed in experiments.

Origin of an Anticrossing between a Leaky Photonic Mode and an Epsilon-Near-Zero Point of Silver

“…In most studies of strong coupling with ENZ materials, ENZ modes are involved. ,, ENZ modes are surface modes that occur in deep-subwavelength films; such surface modes can only couple to p -polarized light with momentum greater than that of free-space light. , Within the radiative region, thin ENZ films can also support Berreman modes. − Berreman modes provide strong out-of-plane electric-field enhancement due to the continuity of the normal component of D⃗ = ε E⃗ ; therefore, the excitation of Berreman modes also requires TM-polarized light. Longitudinal resonances can exist around the ENZ point of a material with low loss . The longitudinal resonance arises due to the presence of surface charge, which also requires electric field normal to the surface to excite it.…”

“…Longitudinal resonances can exist around the ENZ point of a material with low loss. 32 The longitudinal resonance arises due to the presence of surface charge, which also requires electric field normal to the surface to excite it. Furthermore, Ag possesses a second ENZ point at 4.6 eV, at which no anticrossing or absorption is observed.…”

Intensity figures of merit of a metal-insulator-metal thin-film stack in the spatial and frequency domains

“…These planar nanoscopic thin-film stacks are versatile since one can study the associated observables of the modes when one: (i) varies two independent variables, the in-plane wave vector k ρ /k 0 and the frequency ω, (ii) modifies the nature of the material constituents and the thin-film stack thicknesses. The resonant phenomena present in these MIM thin film stacks, due to the interaction between SPP and PWG modes giving rise to avoided crossings, have been discussed in several review papers by Barnes et al [3][4][5], Achlan [6] and Raseev and Achlan [7]. Miroshnichenko et al [8] and Luk'yanchuk et al [9] have discussed these thin film stacks and nanostructures identifying Fano resonance shapes.…”

Tuning surface plasmon-exciton coupling via thickness controlling of excitonic layer

“…The resonant angle must be smaller than 90 • , thus the thickness of CsPbBr 3 layer must be smaller than 29 nm and the corresponding Rabi energy is about 90 meV. Theoretical studies of Rabi energy in Krestchmann configuration have been reported previously [20,22,27]. Pang has revealed that the Rabi energy will increase with the increment of the thickness of the Ag layer [22].…”

“…In their work, the increment of Ag layer thickness will not cause the shift of E SP (θ), thus the resonant condition will be always fulfilled. Tan has discussed the influence of the oscillator strength on the Rabi splitting [27]. They concluded that with large oscillator strength, a new mode emerges and induces a double anticrossing structure.…”