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Influence of Quadrupolar Molecular Transitions within Plasmonic Cavities.

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Huang, Junyang 
Ojambati, Oluwafemi S  ORCID logo
Cuartero-Gonzalez, Alvaro  ORCID logo
Elliott, Eoin 


Optical nanocavities have revolutionized the manipulation of radiative properties of molecular and semiconductor emitters. Here, we investigate the amplified photoluminescence arising from exciting a dark transition of β-carotene molecules embedded within plasmonic nanocavities. Integrating a molecular monolayer into nanoparticle-on-mirror nanostructures unveils enhancements surpassing 4 orders of magnitude in the initially light-forbidden excitation. Such pronounced enhancements transcend conventional dipolar mechanisms, underscoring the presence of alternative enhancement pathways. Notably, Fourier-plane scattering spectroscopy shows that the photoluminescence excitation resonance aligns with a higher-order plasmonic cavity mode, which supports strong field gradients. Combining quantum chemistry calculations with electromagnetic simulations reveals an important interplay between the Franck-Condon quadrupole and Herzberg-Teller dipole contributions in governing the absorption characteristics of this dark transition. In contrast to free space, the quadrupole moment plays a significant role in photoluminescence enhancement within nanoparticle-on-mirror cavities. These findings provide an approach to access optically inactive transitions, promising advancements in spectroscopy and sensing applications.



field gradient, molecular photoluminescence, nanoparticle, plasmonics, quadrupolar transition

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ACS Nano

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American Chemical Society (ACS)
Engineering and Physical Sciences Research Council (EP/S022953/1)
EPSRC (EP/Y008162/1)
Horizon Europe UKRI Underwrite ERC (EP/Y036379/1)
Engineering and Physical Sciences Research Council (EP/L027151/1)
European Commission Horizon 2020 (H2020) ERC (883703)