Session K32: Plasmonics and Beyond I: Resonant Coupling

Coupling of Acoustic Vibrations to Plasmon Resonances in Metal Nanoparticles

Measurements of acoustic vibrations in nanoparticles provide a unique opportunity to study mechanical phenomena at nanometer length scales and picosecond time scales. Phonon vibrations of plasmonic nanoparticles are of particular interest, due to their large extinction efficiencies, and high sensitivity to surrounding medium. There are two mechanisms that transduce the mechanical oscillations into plasmon resonance shift: (1) changes in polarizability; and (2) changes in electron density. These mechanisms have been used to explain qualitatively the origin of the transient-absorption signals, however, a quantitative connection has not yet been made except for simple geometries. Here, we present a method to quantitatively determine the coupling between vibrational modes and plasmon modes in noble-metal nanoparticles including spheres, shells, rods and cubes. We separately determine the parts of the optical response that are due to shape changes and to changes in electron density, and we relate the optical signals to the symmetries of the vibrational and plasmon modes. These results clarify reported experimental results, and should help guide the optimization of future experiments.   

Quantum effects in active linear and non-linear plasmonics.

The unique properties of localized surface plasmons have turned plasmonic nanoparticles into a suitable platform for novel and more efficient optoelectronic processes. Therefore, the development of practical approaches to actively control~the plasmon excitations is a major fundamental and practical challenge. Using Time Dependent Density Functional Theory we explore the possibility of all electrical control of the optical properties of different plasmonic systems such as isolated nanoparticles as well as nanoparticle dimers, and core-shell nanoparticles with sub nm gaps. We demonstrate that for plasmonic systems with narrow gaps, the quantum regime owing to the electron tunneling offers the possibility of fast and reversible control of the plasmon resonances, by application of an external dc bias. Along with all-electrical control of the linear response, we also show that the external polarizing DC field can be used to actively control high-harmonic generation from plasmonic nanoparticles.   

Plasmon-Induced Resonant Energy Transfer: a coherent dipole-dipole coupling mechanism

Metal-insulator-semiconductor core-shell nanoparticles have been used to demonstrate a dipole-dipole coupling mechanism that is entirely dependent on the dephasing time of the localized plasmonic resonance [1]. Consequently, the short-time scale of the plasmons leads to broad energy uncertainty that allows for excitation of charge carriers in the semiconductor via stimulation of photons with energies below the energy band gap. In addition, this coherent energy transfer process overcomes interfacial losses often associated with direct charge transfer. This work explores the efficiency of the energy transfer process, the dipole-dipole coupling strength with dipole separation, shell thickness and plasmonic resonance overlap. We demonstrate limits where the coherent nature of the coupling is switched off and charge transfer processes can dominate. Experiments are performed using transient absorption spectroscopy. Results are compared to calculations using a quantum master equation. These nanostructures show strong potential for improving solar light-harvesting for power and fuel generation. \newline [1] J. Li \textit{et al}, Nature Photonics \textbf{9}, 601 (2015).   

Nanoparticle Lasing Spasers.

Plasmon nanolasers, or spasers (surface plasmon amplification by stimulated emission of radiation) are devices based on plasmonic cavities and gain media that can compensate loss and achieve amplification of nano-localized electromagnetic fields. Several nanocavity architectures have been reported for spasers, such as a metal film-dielectric spacer-semiconductor nanowire configuration or arrays of plasmonic cavities, where the unit cells are nanoparticles or nanoholes. We will discuss two platforms based on nanoparticle arrays that support lattice plasmons for far-field directional emission that can achieve tunable lasing at room temperature. Also, we will describe competing and unique loss mechanisms in nanoparticle cavity arrays as well as the design principles for an optimized unidirectional lasing device by examining different plasmonic materials, unit cell shapes, and gain materials.   

A Self-Consistent Scheme for Optical Response of large Hybrid Networks of Semiconductor Quantum Dots and Plasmonic Metal Nanoparticles

We discuss a self-consistent scheme for treating the optical response of large, hybrid networks of semiconducting quantum dots (SQDs) and plasmonic metallic nanoparticles (MNPs). Our method is efficient and scalable and becomes exact in the limiting case of weakly interacting SQDs. The self-consistent equations obtained for the steady state are analogous to the Heisenberg equations of motion for the density matrix of a SQD placed in an effective electric field computed within the discrete dipole approximation (DDA). Illustrative applications of the theory to square and honeycomb SQD, MNP and hybrid SDQ/MNP lattices as well as SQD-MNP dimers are presented. Our results demonstrate that hybrid SQD-MNP lattices can provide flexible platforms for light manipulation with tunable resonant characteristics.   

Colloidal aluminum nanoparticles with tunable localized surface plasmon resonances for energy applications

Localized surface plasmon resonances (LSPRs) of noble metal nanoparticles are of interest for energy applications due to their visible and near infrared wavelength sensitivity. However, application of these materials in optoelectronic devices is limited by their rarity and high cost. Earth-abundant, inexpensive and non-toxic aluminum is a promising alternative material with a plasmon resonance that can also be tuned via size-, shape- and surface-oxide-control. Here, we employ solution-processed methods to synthesize stable colloidal aluminum nanoparticles. We systematically investigate parameters in the synthesis that control size, shape and oxidation of the aluminum nanoparticles and tune their LSPRs over the ultraviolet and visible spectral regions. We optically characterize the nanoparticle solutions and evaluate their potential for future integration into photovoltaic, photocatalytic and photosensing systems.   

Exciton-plasmon interactions in carbon nanotube arrays.

The response properties of semiconducting carbon nanotubes (CNs) allow for the excitation of both plasmons and excitons at optical frequencies, which can interact with each other to give rise to a variety of phenomena and applications [1-3]. If carbon nanotubes are aligned in a periodic array, then energy bands can be formed due to the array periodicity. Using a quantum electrodynamics approach, the energy dispersion relation for the coupled exciton and plasmon excitations in the CN array is theoretically analyzed. The predicted result is the formation of photonic bands, which may give rise to tunable optoelectronic devices and other applications. [1] I.V.Bondarev, L.M.Woods, and K.Tatur, PRB 80, 085407 (2009); [2] I.V.Bondarev, PRB 85, 035448 (2012); [3] I.V.Bondarev and A.V.Meliksetyan, PRB 89, 045414 (2014).   

Quantum Beats from Entangled Localized Surface Plasmons

Recent experiments report observations of quantum interference between plasmon resonances, inviting descriptions of plasmon-photon interaction using methods from quantum optics. Here we demonstrate, using a Heisenberg-Langevin approach, that the radiation emitted from the localized surface plasmon resonances of a mixed-metal heterodimer may exhibit observable, beat frequency interferences at a far-field detector, known as quantum beats. This prediction represents a correspondence between V-type atoms of quantum optics and the familiar heterodimer system of plasmonics. We explore this analogy in depth and find that although both systems support quantum beats, the heterodimer emits photons in bunches due to the bosonic nature of the plasmon. This highlights a significant difference between the properties of atomic and plasmonic systems.   

Toward Quantum Plasmonics with Plasmon Drag Effect. Theory and Experiment

Giant plasmon drag effect observed in plasmonic metal films and nanostructures brings new fundamental insights into ways in which light-matter interaction occurs. We demonstrate analytically, numerically and experimentally that rectified drag forces acting upon electrons in plasmonic metals are intimately related to the absorption of plasmonic excitations. The plasmon energy quanta absorbed by the metal plasma are associated with momentum quanta, which are also transferred to electrons upon energy absorption. We show that this picture directly applies to plasmon drag effect in a variety of systems, and, to our knowledge for the first time, is capable to explain and predict the magnitude of the effect not only qualitatively, but with close quantitative agreement. The plasmon drag effect opens new avenues for plasmonic-based electronics providing opportunities for incorporation of plasmonic circuits into electronic devices, and for optical sensing offering a new operational principle and an opportunity to substitute the bulky optical set-ups with diffraction limited sensing by electronics. Our work not only adds more clarity into the mechanism behind the plasmon drag effect but also contributes to the emerging field of quantum plasmonics.   

Cavity-coupled molecular vibrational spectra and dynamics

Coherent coupling between an optical transition and confined optical mode, when sufficiently strong, gives rise to new modes separated by the vacuum Rabi splitting. Such systems have been investigated for electronic-state transitions, for quantum wells and dots, however, only very recently have vibrational transitions been explored. Both static and dynamic results are described for vibrational bands strongly coupled to optical cavities. First, we experimentally and numerically describe coupling between a Fabry-Perot cavity and carbonyl stretch (\textasciitilde 1730 cm$^{\mathrm{{\-}1}})$ in poly-methylmethacrylate as a function of several parameters of the system including absorber strength and concentration as well as cavity length. Similar studies are carried out for anions both in solution and exchanged into cationic polymers. Ultrafast pump-probe studies are performed on W(CO)$_{\mathrm{6}}$ in solution which reveals changes to the transient spectra and modified relaxation rates. We believe these modified relaxation rates are a consequence of the energy separation between the vibration-cavity polariton modes and excited state transitions. Cavity-modified vibrational states and energy transfer may provide a new avenue for systematic control of molecular processes and chemistry.   

Non-adiabatic Dynamics of Molecules in Optical Cavities

Molecular systems coupled to optical cavities are promising candidates for a novel kind of photo chemistry. Strong coupling to the vacuum field of the cavity can modify the potential energy surfaces opening up new reaction pathways. We present a derivation of the non-adiabatic couplings for single molecules in the strong coupling regime. The possibilities for photo chemistry are demonstrated for a set of model systems representing typical situations found in molecules.