RESEARCH

Vibrational Strong coupling

Vibrational absorbers that interact with resonantly-matched optical cavity modes can generate new hybrid light-matter states called polaritonic states. The vibrational strong coupling (VSC) has a potential to modify material properties, especially chemical reactivity of molecules. First, we demonstrated the dependence of VSC on spatial distributions of vibrational absorbers within a Fabry-Pérot microcavity. Additionally, we showed the possibility of coupling between two remotely-located vibrational absorbers via a single optical cavity field. Recently, we showed experimental evidence that chemical reactivity of a simple addition reaction can be modulated by VSC.

References
[1] W. Ahn, J. F. Triana, F. Recabal, F. Herrera, B. S. Simpkins, “Modification of Ground-State Chemical Reactivity via Light-Matter Coherence in Infrared Cavities.”, Science, 380, 1165 – 1168 (2023) 
[2] W. Ahn, I. Vurgaftman, A. D. Dunkelberger, J. C. Owrutsky, B. S. Simpkins, “Vibrational Strong Coupling Controlled by Spatial Distribution of Molecules within the Optical Cavity”, ACS Photonics 5, 158 – 166 (2018)
[3] A. D. Dunkelberger, R. B. Davison, W. Ahn, B. S. Simpkins, J. C. Owrutsky, “Ultrafast Transmission Modulation and Recovery via Vibrational Strong Coupling”, Journal of Physical Chemistry A. 122, 965 – 971 (2018)
[4] W. Ahn, B. S. Simpkins, “Raman Scattering Under Strong Vibration-Cavity Coupling”, Journal of Physical Chemistry C, 125, 830 – 835 (2021)

excitonic strong coupling

Photons confined in an optical cavity can strongly couple to quantum emitters such as quantum dots, j-aggregates, and organic dye molecules, creating a new hybridized energy state called polaritons. The formation of these half-light half-matter quasi-particles has excellent potential as they can provide a photonic means to modify materials’ physical and chemical properties. The so-called excitonic strong coupling (ESC) is evidenced by Rabi splitting (Ω) in the optical spectra of the cavity. The magnitude of Ω correlates with emission properties of the material, but the modulation of the coupling strength is usually challenging to obtain due to the fixed mode volume of the microcavity. We demonstrated electrochemically tunable excitonic strong coupling in a system where organic dye molecules are strongly coupled to surface plasmon polaritons (SPPs) propagating on the surface of a gold film. By using a reversible redox cycle of the dye molecules, we were able to show that Ω values can be tuned from a strong to ultrastrong coupling regime in the electrochemically-actuated coupled exciton-polariton system. We are currently developing the coupled light-matter systems in the strong coupling regime and investigating the effect of ESC on material properties.    

References
[1] W. Ahn, B. S. Simpkins, “Spectroelectrochemical Measurement and Modulation of Exciton-Polaritons”, APL Photonics, 5, 076107 (2020)

plasmon-induced hot carrier generation for photocatalysis

Noble metals that generate hot carriers by plasmon decay promote efficient charge separation with visible light irradiation, opening up new prospects in the field of photocatalysis and photovoltaics. While localized surface plasmon resonance (LSPR)-induced hot carrier generation has been evidenced using diverse metal nanostructures, inhomogeneous metal-semiconductor mixtures hinder efficient control over photocarrier generation and carrier-mediated photochemistry. We generated surface plasmon polaritons (SPPs) at the interface between a noble metal film and aqueous solution by which simultaneous optical and electrochemical interrogation is possible for plasmon-mediated chemistry. We demonstrated electrochemical photovoltage and photocurrent responses as SPP-induced hot carriers drove methanol oxidation and the anodic half-reaction of water splitting. We are currently designing and developing novel plasmonic and nanophotonic systems that can further improve the efficiency of hot carrier-driven processes, especially for photocatalysis that eliminates or mitigates global warming contributors.

References
[1] W. Ahn, D. C. Ratchford, P. E. Pehrsson, B. S. Simpkins, “Surface Plasmon Polariton-Induced Hot Carrier Generation for Photocatalysis”, Nanoscale, 9, 3010 – 3022 (2017)
[2] W. Ahn, I. Vurgaftman, J. J. Pietron, P. E. Pehrsson, B. S. Simpkins, “Energy-Tunable Photocatalysis by Hot Carriers Generated by Surface Plasmon Polaritons”, Journal of Materials Chemistry A. 7, 7015 – 7024 (2019)