Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2019, 123, 16965−16972
Strong Dipole Interaction between Chlorophyll‑a Molecules and Surface Plasmon Polaritons Astha Singh,† Geeta Sharma,† Rajib Ghosh,‡ Bhanu Pratap Singh,† and Parinda Vasa*,† †
Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India
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ABSTRACT: The ability of surface plasmon polaritons (SPPs) to localize light on the nanoscale dimensions results in significant field enhancement enabling efficient intensification of light−matter interactions. Here, we report observation of strong radiative dipole coupling between molecular excitons in chlorophyll-a (Chl-a) and propagating SPPs excited on gold thin films as well as localized SPPs of gold nanoparticles at room temperature. The optical response of the Chl-a− metal hybrid nanostructures is investigated in spectral as well as temporal domains by using various spectroscopy techniques, which reveal polariton formation with a very large coupling strength of ∼100 meV (24.2 THz). This strong dipole interaction considerably enhances the Chl-a absorption over a much wider spectrum and gives rise to SPP-exciton polariton emission. The changes in the optical response are also accompanied by ultrafast relaxation dynamics as well as suppression of luminescence yield. Our results suggest that plasmonic nanostructures can significantly alter optical response of Chl-a molecules via vacuum field interactions.
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INTRODUCTION Evolutionary pressures over centuries have led to well-adapted and efficient biological processes and structures. They have begun to inspire new approaches to fabricate artificial systems that possess the capability to mimic all or part of a biological system.1,2 Recently, artificial light-harvesting structures and photovoltaic devices have been proposed and developed, which are based on the principles of photosynthesis. A major difference compared to natural systems has been that most of the existing artificial photovoltaic devices comprise hybrid material structures that serve both as the light-harvesting component as well as for transporting charge carriers. The challenge has been to precisely control the dynamics of the system so as to achieve the same efficiency and photostability of the naturally occurring processes.2−4 Apart from possible integration into an efficient photovoltaic device, such artificial assembly might also work as a single-photon detector with ultrahigh sensitivity.3 Complex protein-pigment chlorophyll molecules present within the light-harvesting antenna of plants play an important role in light absorption and excitation energy transfer in the reaction center. Thus, chlorophyll-a (Chl-a) molecules are very promising for studies on artificial photosynthesis as they not only give an interesting biomolecular system for studying their optical response as well as the energy transfer mechanism but also offer highly potential applications in photovoltaics, artificial light-harvesting systems, and photosensors.1−5 The recent observation of room-temperature electronic quantum coherence, that is, the oscillatory motion of delocalized electronic wave functions and associated theoretical development induced a paradigm shift in © 2019 American Chemical Society
understanding the microscopic physical mechanisms involved in photosynthesis.2−7 A fascinating, but so far unexplored, perspective would be to exploit such coherent effects in artificial photosynthetic/photovoltaic systems to improve their performance. An intriguing result of quantum theory of light− matter interaction is that an emitter placed within a resonator exhibits exceptional optical properties, which are very different from those of the individual constituents. These unusual effects are a consequence of the strong light−matter interaction between the emitter and resonator, resulting in the formation of new coupled modes.8−11 Metal nanostructures owing to the large density of free electrons are known to support surface plasmon polaritons (SPPs), which can localize light on the nanoscale and function as efficient optical resonators.8−14 Therefore, even though fundamentally different, Chl-a molecular excitations and SPPs in hybrid Chl-a−metal nanostructures can exhibit strong radiative interaction.15−18 Similar to microcavities, these hybrid nanostructures are also expected to exhibit intriguing optical phenomena. For instance, the emission of a photon, which competes with solar energy conversion can be significantly suppressed, or the absorption band of Chl-a molecules can be enhanced and tuned over a wide range of wavelengths. As the coupling exists within the entire SPP mode volume, much larger than a Chl-a molecule, it can coherently couple several molecules present within the mode volume, delocalizing the electronic wave function over a Received: May 7, 2019 Revised: June 16, 2019 Published: June 17, 2019 16965
DOI: 10.1021/acs.jpcc.9b04318 J. Phys. Chem. C 2019, 123, 16965−16972
Article
The Journal of Physical Chemistry C
schematic of the sample in which a Chl-a coated gold film on a glass substrate is attached to the hemispherical prism using index-matching oil (Sigma-Aldrich). The collimated output from a temperature-stabilized tungsten−halogen lamp (Thorlabs) is incident on the metal−Chl-a or metal−SiO2−Chl-a interface, and the reflected light spectrum is recorded by a fiber-coupled CCD spectrometer Avantes. The aqueous gold colloids used here to investigate interaction with localized SPPs contained either gold nanoellipsoids or nanorods (with CTAB as the surfactant, Nanopartz Inc.) in distilled water, with
