Strong Plasmon-exciton Interactions Research Articles

Plasmon–Exciton Strong Coupling Effects of the Chiral Hybrid Nanostructures Based on the Plexcitonic Born–Kuhn Model

A deep understanding of the optical properties of the chiral plexcitonic systems not only adds a new dimension to the exploration of the strong plasmon–exciton interaction but also provides a new method to design chiroptical devices. We develop a plexcitonic Born–Kuhn model to explore the strong coupling dynamics of the chiral hybrid nanostructures made of corner-stacked nanorod dimers and molecules from the chiroptical perspective. As a consequence of strong interaction between plasmons and excitons, double normal mode splittings/Rabi splittings and an anti-crossing phenomenon appear in circular dichroism (CD) spectroscopy. The character of dual plexciton (symmetric and anti-symmetric plexciton) and associated chiroptical properties have been demonstrated. The interplay between mirror symmetry breaking and plasmon–plasmon/plasmon–exciton interaction leads to an optimal configuration with the maximal chiroptical response. Moreover, chirality (of incident light) selective excitation of plexcitonic modes has been achieved by structural design. Finally, we verify our theory experimentally by synthesizing chiral plasmonic nanodimers and coating these structures with J-aggregate dye molecules.

Plexcitonic Optical Chirality: Strong Exciton-Plasmon Coupling in Chiral J-Aggregate-Metal Nanoparticle Complexes.

Understanding the unique characteristics of plexcitons, hybridized states resulting from the strong coupling between plasmons and excitons, is vital for both fundamental studies and practical applications in nano-optics. However, the research of plexcitons from the perspective of chiral optics has been rarely reported. Here, we experimentally investigate the optical chirality of plexcitonic systems consisting of composite metal nanoparticles and chiral J-aggregates in the strong coupling regime. Mode splitting and anticrossing behavior are observed in both the circular dichroism (CD) and extinction spectra of the hybrid nanosystems. A large mode splitting (at zero detuning) of up to 136 meV/214 meV in CD/extinction measurements confirms that the systems attain the strong coupling regime. This phenomenon indicates that the formation of plexcitons modifies not only the extinction but also the optical chirality of the hybrid systems. We develop a quasistatic theory to elucidate the chiral optical responses of hybrid systems. Furthermore, we propose and justify a criterion of strong plasmon-exciton interaction: the mode splitting in the CD spectra (at zero detuning) is larger than half of that in the extinction spectra. Our findings give a chiral perspective on the study of strong plasmon-exciton coupling and have potential applications in the chiral optical field.

Selective far-field addressing of coupled quantum dots in a plasmonic nanocavity

Plasmon–emitter hybrid nanocavity systems exhibit strong plasmon–exciton interactions at the single-emitter level, showing great potential as testbeds and building blocks for quantum optics and informatics. However, reported experiments involve only one addressable emitting site, which limits their relevance for many fundamental questions and devices involving interactions among emitters. Here we open up this critical degree of freedom by demonstrating selective far-field excitation and detection of two coupled quantum dot emitters in a U-shaped gold nanostructure. The gold nanostructure functions as a nanocavity to enhance emitter interactions and a nanoantenna to make the emitters selectively excitable and detectable. When we selectively excite or detect either emitter, we observe photon emission predominantly from the target emitter with up to 132-fold Purcell-enhanced emission rate, indicating individual addressability and strong plasmon–exciton interactions. Our work represents a step towards a broad class of plasmonic devices that will enable faster, more compact optics, communication and computation.

Controlled growth and optical response of a semi-hollow plasmonic nanocavity and ultrathin sulfide nanosheets on Au/Ag platelets.

Herein, we report a strategy to construct a semi-hollow plasmonic nanocavity and grow ultrathin sulfide nanosheets inside. The competition and cooperation of Au deposition with Ag etching based on flat Ag nanoplates are proposed. For the establishment of the semi-hollow nanocavity, Au shells are grown on Ag nanoplates, which serve as a stable frame, followed by partial etching of the Ag nanoplates. By controlling the thickness of the initial Ag nanoplates or the injected amount of etchant, the nanocavity size is fine-tuned. Significantly, the remaining unetched Ag layers provide a flat platform for the growth of 2D ultrathin sulfides of Ag2S and CdS inside the semi-hollow plasmonic nanocavity. Strong plasmon resonance and large local field enhancement are exhibited inside the plasmonic cavity where the ultrathin semiconductor sulfides are grown, indicating strong plasmon-exciton interactions in the hybrids. Furthermore, this synthetic approach is extended to grow other metal sulfides such as Bi2S3 and PbS. The combination of a flat plasmonic cavity with ultrathin semiconductor nanosheets in this study provides a new strategy for the development of unique plasmon-based hybrids with excellent optical properties.

Exploring the coherent interaction in a hybrid system of hollow gold nanoprisms and cyanine dye J-aggregates: role of plasmon-hybridization mediated local electric-field enhancement.

In this work, we probed the possibility of observing strong plasmon-exciton interactions in hollow gold nanoprism-J-aggregate nanocomposites. Several different hollow gold nanoprisms (HGNs) with different aspect ratios were synthesized. This allowed us to systematically tune the LSPR energies through the exciton energy of the PIC-J-aggregate, which in turn allowed us to have direct determination of the coupling strength of HGN-J-aggregate composites. Hybrid nanosystems were prepared by adsorbing and assembling 1,1'-diethyl-2,2'-cyanine (pseudoisocyanine or PIC) iodide onto the surface of hollow gold nanoprisms. Plasmon-exciton interactions were studied using extinction spectroscopy. The experimental results were analysed, and complemented by the results obtained from numerical simulations. Our results reveal that the HGN-PIC-J-aggregate hybrid nanosystem shows coherent coupling between the localized surface plasmons of the HGN and excitons of the PIC-J-aggregate, as obvious from the observation of a clear transparency dip and the formation of two new hybrid plexcitonic modes in the plexcitonic spectra. Anti-crossing behaviour of the plexcitonic modes, together with large Rabi splitting and coupling constant, asserts strong coupling between the plasmon and the exciton, overwhelming the decoherence effects, in our hybrid nanosystem. Analysis of the calculated near-field distribution establishes that the plasmon-hybridization mediated large electric-field enhancement holds the key to the strong coupling.

Role of material loss and mode volume of plasmonic nanocavities for strong plasmon-exciton interactions

We investigate the role of material loss and mode volume of plasmonic nanostructures on strong plasmon-exciton coupling. We find that the field enhancement, and therefore loss, is not important for the magnitude of the Rabi splitting as such, but instead it is determined by the mode volume. Nevertheless, for reaching true strong coupling condition, that is, coupling greater than any dissipation, it is important to compromise losses. We also show that using such popular geometries as a dimer of two spheres or bow-tie nanoantennas, does not allow compressing the mode volume much in comparison to a single nanoparticle case, except for very narrow gaps, but rather it allows for efficient extraction of the mode out of the metal thus making it more accessible for excitons to interact with. Even more efficient mode extraction is achieved when high refractive index dielectric is placed in the gap. Our findings may find practical use for quantum plasmonics applications.

Coupling Enhancement and Giant Rabi-Splitting in Large Arrays of Tunable Plexcitonic Substrates

Advances in active manipulation of light at the nanoscale are rapidly emerging with the concept of plexcitonic coupling at the interface between plasmonics nanostructures and excitonic molecules. In this work, we devise a simple fabrication scheme to produce and optimize large area tunable plasmonic substrates for strong plasmon-exciton interactions. By tuning the diameter of the nanoholes using a simple plasma etching process, we demonstrate the potential of our approach to deliver tunable plasmonic substrates. Thus, large enhancements of fluorescence and Raman scattering could be measured. Moreover, hybridized states appearing in the presence of excitonic molecules (RG6) give rise to anticrossing behaviors in extinction spectroscopy, a phenomenon also known as Rabi-splitting. The results demonstrate the great potential of our large nanofabricated arrays as plexcitonic substrates for numerous applications, including sensors, light harvesters, and all-optical switches.

Sign-reversed and magnitude-enhanced nonlinear absorption of Au–CdS core–shell hetero-nanorods

We synthesis uniform Au–CdS core–shell hetero-nanorods and demonstrate the effective plasmon–exciton interaction induced optical nonlinear enhancement in metal–semiconductor hetero-nanostructures. After growing CdS semiconductor shell onto the Au nanorods, the longitudinal plasmon resonance exhibits considerable red-shift with enlarged absorption intensity. Nonlinear absorption responses transform from saturable absorption to reverse saturable absorption, and effective nonlinear absorption coefficient β is increased from −7.7 to +22.2 cm/GW. The observed behaviors indicate strong plasmon–exciton interaction and great local field enhancement.

Tailoring hot-exciton emission and lifetimes in semiconducting nanowires via whispering-gallery nanocavity plasmons

The manipulation of radiative properties of light emitters coupled with surface plasmons is important for engineering new nanoscale optoelectronic devices, including lasers, detectors and single photon emitters. However, so far the radiative rates of excited states in semiconductors and molecular systems have been enhanced only moderately, typically by a factor of 10-50, producing emission mostly from thermalized excitons. Here, we show the generation of dominant hot-exciton emission, that is, luminescence from non-thermalized excitons that are enhanced by the highly concentrated electromagnetic fields supported by the resonant whispering-gallery plasmonic nanocavities of CdS-SiO(2)-Ag core-shell nanowire devices. By tuning the plasmonic cavity size to match the whispering-gallery resonances, an almost complete transition from thermalized exciton to hot-exciton emission can be achieved, which reflects exceptionally high radiative rate enhancement of >10(3) and sub-picosecond lifetimes. Core-shell plasmonic nanowires are an ideal test bed for studying and controlling strong plasmon-exciton interaction at the nanoscale and opens new avenues for applications in ultrafast nanophotonic devices.

Anisotropic Emission from Multilayered Plasmon Resonator Nanocomposites of Isotropic Semiconductor Quantum Dots

We propose and demonstrate a nanocomposite localized surface plasmon resonator embedded into an artificial three-dimensional construction. Colloidal semiconductor quantum dots are assembled between layers of metal nanoparticles to create a highly strong plasmon-exciton interaction in the plasmonic cavity. In such a multilayered plasmonic resonator architecture of isotropic CdTe quantum dots, we observed polarized light emission of 80% in the vertical polarization with an enhancement factor of 4.4, resulting in a steady-state anisotropy value of 0.26 and reaching the highest quantum efficiency level of 30% ever reported for such CdTe quantum dot solids. Our electromagnetic simulation results are in good agreement with the experimental characterization data showing a significant emission enhancement in the vertical polarization, for which their fluorescence decay lifetimes are substantially shortened by consecutive replication of our unit cell architecture design. Such strongly plasmon-exciton coupling nanocomposites hold great promise for future exploitation and development of quantum dot plasmonic biophotonics and quantum dot plasmonic optoelectronics.