Session X34: Plasmonic Metamaterials

Engineering high-k with metamaterial plasmonic structures

Metamaterial plasmonic composites offer remarkable flexibility in controlling effective dielectric properties of matter. These composites rely on trapped plasmonic resonances in metallic micro- or nanostructures embedded in dielectric or semiconducting matrices. Such composites can have very large and/or low effective dielectric functions at various frequencies, depending on the composite design. Recently, an aluminum nanoparticle composite engineered to have vanishing dielectric function at the electron-phonon interaction band achieved 3-fold increase of the superconducting Tc [1]. In the electronics industry, there is a need for materials with large (and largely real, to minimize losses) low frequency dielectric function, called high-k materials. We demonstrate that metamaterial plasmonic composites with enhanced self-inductance can be used to make high-k composites without heavy metal loading, confirming an earlier theoretical study [2]. [1] V.N. Smolyaninova, et al., ``Using metamaterial nanoengineering to triple the superconducting critical temperature of bulk aluminum'', Scientific Reports 5, 15777 (2015). [2] K. Kempa, ``Dielectric function of media based on conductive particles''. Phys. Rev. B \textbf{74}, 033411 (2006).   

Plasmonic mode coupling of silver nanoparticles through thin dielectric films

Tunable optical performance at the nanoscale can be engineered by coupling elementary plasmonic modes in new ways, for application to enhanced absorption in polymer photovoltaic devices. Plasmonic mode coupling in systems of solution-processed films with diverse and tunable morphologies represents an economic alternative to lithographically-defined systems, yet interactions between localized plasmons in solution-processable material systems have not been as well-explored as their lithographic counterparts. We seek to characterize plasmonic mode coupling in an architecture consisting of a thin dielectric spacer layer sandwiched between two layers of sparse disordered silver nanoparticles. Coupling is observed in UV-vis spectroscopy as a spectral splitting in the hybridized mode structure, as the plasmons supported on each silver layer couple through the dielectric. We explore the tunability of this coupling by modifying the spacer layer thickness, and compare behavior in systems containing non-absorbing and absorbing spacer layers.   

Modeling the generation of hot plasmonic electrons in metal nanocrystals with hot spots. A quantum model.

To efficiently harvest energy from electromagnetic radiation is a goal actively pursued in different fields within material science and chemistry. Combining plasmon-supporting nanostructures with energy harvesting systems provides access to a variety of microscopic phenomena that can increase collection rates or extend the spectrum usable by those systems. Such a pairing can be used to increase the efficiency of photochemical processes and photovoltaic systems, and to make more sensitive photodetectors. The work presented here [1] focuses on the excitation of high energy (hot) carriers within a plasmonic nanoparticle under continuous irradiation. Energetic carriers can then become available to neighboring materials, enhancing their intrinsic light-harvesting efficiency. We have used a hybrid theoretical framework adapted from the previous work [2-4], which combines classical electrodynamic calculation of the plasmonic response with a quantum description of the carriers. We use this model to describe the non-equilibrium carrier population distribution within a system of two interacting nanoparticles with an electromagnetic hot spot, attending to the effects of the field enhancement occurring because of this interaction. [1] Besteiro, L.V.; Govorov, A.O. \textit{J. Phys. Chem. C} \textbf{120}, 19329 (2016). [2] Govorov, A.O.; Zhang, H.; Gun'ko, Y.K. \textit{J. Phys. Chem. C} \textbf{117}, 16616 (2013). [3] Zhang, H.; Govorov, A.O. \textit{J. Phys. Chem. C }\textbf{118}, 7606 (2014). [4] Govorov, A.O.; Zhang, H. \textit{J. Phys. Chem. C} \textbf{119}, 6181 (2015).   

Light-matter interaction in hybrid plasmonic-photonic resonators

Plasmonic antennas and high-Q cavities are two polar opposite strategies to increase light matter interaction strength, aiming either at broad bandwidth operation at the price of Ohmic loss, or at loss-free operation, at the price of narrow bandwidth. Hybridizing antennas and cavities at first sight is a poor idea, yet in this talk I argue that in some combinations one can obtain plasmonic mode volumes at cavity Q’s, and at reduced Ohmic loss. I discuss an experiment in which we show that cavity’s with a Q of 10$^8$ can improve in Q upon introduction of metal particles. Also, I will present calculations that show that hybrids will show Fano lineshapes in local density of states (or Purcell factor), with large spontaneous emission rate enhancements over Q’s of order 1000. Finally I discuss our efforts to fabricate and optically characterize hybrid structures, and touch upon the physics of multiple antennas coupling through a single cavity.   

Strong exciton-plasmon coupling in double walled semiconducting carbon nanotubes.

We demonstrate theoretically the strong near-field exciton-plasmon coupling in a double walled semiconducting carbon nanotube (CN) system, in which the exciton residing on one tubule interacts with the near field of an interband plasmon resonance of the other concentric tubule. Because of the peculiar quasi-one-dimensional character of small-diameter CNs, both excitons and interband plasmons can coexist in the same energy range of about 1 eV in these structures[1]. Since the peak positions of the exciton and plasmon resonances are determined by the respective CN chirality indexes[2], the double walled CN combinations can be selected appropriately to have the overlapping exciton and plasmon resonances. We describe the details of the exciton-plasmon interactions in such systems, derive analytic solutions for the coupled exciton-plasmon quasiparticle dispersion, and calculate the fractions of excitons and plasmons to form hybridized quasiparticle states. We also discuss the possibility for the exciton Bose-Einstein condensation in these structures, using the analysis reported earlier for single wall CNs[3]. -- [1]I.V.Bondarev, K.Tatur, L.M.Woods, PRB 80, 085407 (2009); [2]I.V.Bondarev, PRB 85, 035448 (2012); [3]I.V.Bondarev, A.V.Meliksetyan, PRB 89, 045414 (2014).   

Characterizing Plasmonic Excitations of Quasi-2D Chains

A quantum description of the optical response of nanostructures and other atomic-scale systems is desirable for modeling systems that use plasmons for quantum information transfer, or coherent transport and interference of quantum states, as well as systems small enough for electron tunneling or quantum confinement to affect the electronic states of the system. Such a quantum description is complicated by the fact that collective and single-particle excitations can have similar energies and thus will mix. We seek to better understand the excitations of nanosystems to identify which characteristics of the excitations are most relevant to modeling their behavior. In this work we use a quasi 2-dimensional linear atomic chain as a model system, and exact diagonalization of the many-body Hamiltonian to obtain its excitations. We compare this to previous work in 1-d chains which used a combination of criteria involving a many-body state's transfer dipole moment, balance, transfer charge, dynamical response, and induced-charge distribution to identify which excitations are plasmonic in character.   

The Existence of Topological Edge States in Honeycomb Plasmonic Lattices

In this paper, we investigate the band properties of 2D honeycomb plasmonic lattices consisting of metallic nanoparticles. By means of the coupled dipole method and quasi-static approximation, we theoretically analyze the band structures stemming from near-field interaction of localized surface plasmon polaritons for both the infinite lattice and ribbons. Naturally, the interaction of point dipoles decouples into independent out-of-plane and in-plane polarizations. For the out-of-plane modes, both the bulk spectrum and the range of the momentum $k_{\parallel } $ where edge states exist in ribbons are similar to the electronic bands in graphene. Nevertheless, the in-plane polarized modes show significant differences, which do not only possess additional non-flat edge states in ribbons, but also have different distributions of the flat edge states in reciprocal space. For in-plane polarized modes, we derived the bulk-edge correspondence, namely, the relation between the number of flat edge states at a fixed $k_{\parallel } $, Zak phases of the bulk bands and the winding number associated with the bulk hamiltonian, and verified it through four typical ribbon boundaries, i.e. zigzag, bearded zigzag, armchair, and bearded armchair. Our approach gives a new topological understanding of edge states in such plasmonic systems, and may also apply to other 2D “vector wave” systems.   

Nano plasmonic Probe for Spatial Resolution Sensing.

Due to the lack of noninvasive in-vivo data collection techniques, most of the subcellular studies have been performed in-vitro. In contrast, experiments performed in vivo, such as at the single or cell population level, which studies biological phenomena in real time, better describes actual cell function. We aim to provide a proof of principle of a novel nano plasmonic probe that provides a spectral orientation and rotation sensitivite, label-free, noninvasive and high sensitive sensor. The proposed nanostructure exploits a nanogap in a partial-ring configuration that retains the advantages of small-gap antennas but increases the availability and detection volume compared to conventional plasmon-based designs. We will show first studies demonstrating the enhanced spectral fingerprints of chlorophyll b in the nanogap.   

Local density of states for nanoplasmonics

We obtain the local density of states (LDOS) for any nanoplasmonic system in the frequency range dominated by a localized surface plasmon. By including the Ohmic losses in a consistent way, we show that the plasmon LDOS is proportional to the local field intensity normalized by the absorbed power. We obtain explicit formulas for the energy transfer (ET) between quantum emitters and plasmons as well as between donors and acceptors situated near a plasmonic structure. In the latter case, we find that the plasmon-assisted ET rate is proportional to the LDOS product at the donor and acceptor positions, obtain, in a general form, the plasmon ET enhancement factor, and establish the transition onset between Forster-dominated and plasmon-dominated ET regimes.   

Quantum nonlinear light emission in nanoplasmonic waveguides

Generation of single and entangled photons in compact structures amenable to chip-based integration are important for future quantum communication and signal processing systems. Quantum nonlinear optic processes, including spontaneous parametric downconversion and spontaneous four wave mixing, are promising candidates for quantum light generation. However, the weak nonlinear interactions and high dispersion of homogeneous materials constrain the ability to simultaneously achieve the required high photonic mode density and phase matching required for enhancing nonlinear parametric processes. In this work we examine theoretically nonlinear light emission in subwavelength nanoscale plasmonic waveguides. We develop a theoretical model that describes spontaneous nonlinear downconversion with realistic models for losses and dispersion. We further show that by engineering the light dispersion in nanoplasmonic waveguides, we can simultaneously achieve phase matching, efficient mode mixing, and spontaneous photon emission. We study several waveguide motifs based on slot waveguides and hybrid plasmonic waveguides, and show that in tightly confined waveguides, photon emission rates comparable with bulk crystals are attainable with greatly reduced material interaction lengths.   

Exciton emission from bare and hybrid plasmonic GaN nanorods

We study the exciton emission of hybrid gold nanoparticle/Alq3 (aluminiumquinoline)/wurtzite GaN nanorods. GaN nanorods of \textasciitilde 1.5 $\mu $m length and 250 nm diameter were grown by plasma assisted MBE. Hybrid GaN nanorods were synthesized by organic molecular beam deposition. Temperature and power dependent time integrated (TI) and time resolved (TR) photoluminescence (PL) measurements were performed on bare and hybrid structures. Bare nanorods show donor (D$^{0}$,X) and acceptor bound (A$^{0}$,X) exciton emission at 3.473 eV and at 3.463 eV, respectively. TR-PL trace modeling reveal lifetimes of 240 ps and 1.4 ns for the (D$^{0}$,X) and (A$^{0}$,X) transition. 10 nm gold coated GaN nanorods show a significant PL quenching and (D$^{0}$,X) lifetime shortening which is tentatively attributed to impact ionization of (D$^{0}$,X) due to hot electron injection from the gold nanoparticles. This is supported by electron energy loss spectroscopy that shows a redshift of a midgap state transition indicating a reduction of a preexisting band-bending at the nanorod surface due to positive charging of the gold nanoparticles. Inserting a nominally 5 nm thick Alq$_{3}$ spacer between the nanorod and the gold reduces the PL quenching and lifetime shortening. Plasmonic nanorods with a 30 nm thick Alq$_{3}$ spacer reveal lifetimes which are nearly identical to uncoated GaN nanorods.   

High Excitation Density Effects in Plasmonic GaAs-AlGaAs-GaAs Core-Shell Nanowires.

We investigate the near-band emission of highly exited hybrid plasmonic GaAs-AlGaAs-GaAs core-shell nanowire (NW) heterostructures using time integrated (TI) photoluminescence (PL) measurements. The plasmonic structures are composed of 130 nm diameter zincblende NWs, either as bare NWs lying on an Au coated glass substrate or as Au coated NWs lying on a bare glass substrate. Intensity-dependent PL measurements on bare and plasmonic NW samples at high excitation densities reveal electron-hole-plasma (EHP) recombination. The EHP band shows a super-linear increase with increasing excitation intensity suggesting amplified spontaneous emission (ASE) at a threshold power density of around 60 microJ/cm2. Plasmonic NW samples excited above the threshold fluence reveal a weakly resolved sub-structure within the broad EHP band. The emerging sub-bands have a bandwidth which is by a factor of around 3 smaller than the width of the EHP background and are tentatively attributed to plasmonic lasing modes. This interpretation is supported by the fact that photonic lasing from 130 nm diameter thin uncoated GaAs NWs is theoretically not possible and that no sub-structure in the EHP band has been observed on bare nanowires.