Modeling optical constants from the absorption of organic thin films using a modified Lorentz oscillator model

Abstract

Optical constants of organic thin films can be evaluated using the Lorentz oscillator model (LOM) which fails to fit inhomogeneously broadened absorption of highly concentrated molecular films. In modified LOM (MLOM), the inhomogeneous broadening is implemented through a frequency-dependent adjustable broadening function. In this work, we evaluate the optical constants of rhodamine 6G doped poly-vinyl alcohol thin films with varying doping concentration (including also extensively high concentrations) using MLOM, which outperforms LOM by showing a better agreement with the experimental results. Our proposed method provides a way to accurately determine optical constants of isotropic organic thin films only from their absorption spectra without spectroscopic ellipsometry.

Controlling Molecular Photoisomerization in Photonic Cavities through Polariton Funneling

“…87 Instead, this overall rate enhancement effect can be understood as arising from a cavity-induced renormalization of the photonic density of states, as was recently inferred for different reactions. 56,88 This effect does not require strong light-matter coupling. Nevertheless, our results indicate that enhanced absorption at the LP branch followed by conversion of LPs into dark states does enable more efficient use of photons resonant with the LP branch compared to that outside the cavity.…”

“…Multiple studies have shown that relaxation dynamics in molecular polaritons are dominated by dark states. ,, Indeed, excitation into any state (UP, LP, or dark states) tends to localize both spatially and energetically into the dark state manifold because of the latter’s large density of states. ,,, This aspect is particularly relevant in molecular polaritons with large disorder, since the latter ensures energetic overlap between polariton branches and molecular states over a large range of detunings. , The trend we observe in γ (Figure d) can be rationalized within this context and suggests that photochemistry in our systems is principally driven by (relatively) localized dark states: as the detuning between the excitation and bare exciton resonance increases, we expect a smaller LP → dark state transfer rate. ,, The other decay pathway for LPs is to the ground state, typically on sub-100 fs time scales, dictated primarily by cavity losses. Given this kinetic competition between polariton localization into dark states and polariton decay to the ground state, large detuning (small LP → dark state rate) results in a lower dark state yield and thus lower photochemical yields (Figure d).…”

“…Thus, appropriate tuning of the cavity resonance steers excited-state reaction pathways in chemical mixtures by funneling the absorbed energy toward a target reactant state, similar to light-harvesting schemes, though the overall photon efficiency limits of this process in cavities remains to be investigated. These results suggest that strong light-matter coupling in chemical mixtures allows steering chemical dynamics in a potentially more profound way than by acting as an optical filter, which was recently suggested to be the case for chemical systems with single-component reactants. ,, This approach will be particularly valuable to adopt in mixtures of reactants that have drastically different photochemical pathways and product states. Electronic strong-coupling in this context would provide a compelling platform to achieve precise reaction specificity, product selectivity, including suppression of undesired products, and reconfigurable photoswitching.…”

“…89,90 The trend we observe in γ (Figure 3d) can be rationalized within this context and suggests that photochemistry in our systems is principally driven by (relatively) localized dark states: as the detuning between the excitation and bare exciton resonance increases, we expect a smaller LP → dark state transfer rate. 41,62,88 The other decay pathway for LPs is to the ground state, typically on sub-100 fs time scales, dictated primarily by cavity losses. Given this kinetic competition between polariton localization into dark states and polariton decay to the ground state, large detuning (small LP → dark state rate) results in a lower dark state yield and thus lower photochemical yields (Figure 3d).…”

“…Therefore, cavity coupling, particularly under conditions where the LP is significantly detuned from dark states through ultrastrong coupling or large detuning, can yield photoisomerization suppression even without fundamentally modifying molecular potential surfaces. 41,88 We emphasize, however, that the absolute photoisomerization rate enhancement (per irradiation photon rather than absorbed photon, RE in Figure 3a) can be large in microcavities since it allows low-energy light below the molecular resonance to be efficiently absorbed by the system and transferred into reactive dark states.…”