Session Y34: Quanum Effects in Plasmonic Metamaterials

Plasmonic refraction in random nanowire networks

Metallic random networks have been proposed as replacements for indium tin oxide (ITO) in transparent conductor applications. They offer improved electro-optical performance, mechanical flexibility, and low manufacturing costs. It has been recently demonstrated, that plasmonic refraction is responsible for increased transmission beyond the geometric limit in metallic nanowire arrays (up to 18{\%} in random and up to 130{\%} in periodic arrays)[1]. In this work, we study in detail the increased light transmission due to plasmons in various metallic networks, and discuss means to maximize this effects (for a given network resistance), as well as we propose an extended mean-field theory of light transmission through random metallic networks, which agrees very well with experiments and simulations.   

Understanding of extraordinary optical transmission in metallic nano-hole array via plasmon dispersion

Extraordinary optical transmission (EOT) is a phenomenon that shows high optical transmission through sub-wavelength hole array on metallic thin film. Understanding of this EOT behavior and designing the structure for tunable optical transmission are crucial for applications to EOT-based devices such as biosensors, color filters, and transparent electrodes. In this study, the transmission properties of EOT in Ag nano-hole array with variable periods are investigated through a comparison between optical transmission and surface plasmon polariton (SPP) dispersion, which is believed to be directly correlated with EOT. To examine the optical transmission properties, Ag nano-hole arrays with variable periods are fabricated using electron-beam lithography. The measured optical transmission spectra are in good agreement with calculated spectra. We found that these EOT behaviors can be explained by analyzing the parameters of the SPP dispersion such as the group velocity and the momentum matching between SPPs and photons. Also, we show that EOT-based Ag nano-hole arrays with high optical transmission in broad wavelength region can be utilized for transparent electrodes. Our results indicate that EOT can be engineered in a wide range by tuning the SPP dispersion.   

Plasmonically enhanced photoluminescence dependence on thickness variations and other nanostructure geometries

Plasmonic nanostructures produce local electric fields with values higher than that of the incoming light because when the light encounters the plasmonic nanostructure, it creates charge density oscillations, called plasmons. These plasmons create a local electric field which varies in strength by location. Near the edges and gaps of the nanostructure, the electric field is amplified, and a stronger electric field than that of the incoming light is created. These enhanced electric fields make possible signal amplification of otherwise very weak signals. Amplification of weak signals allows for single molecule spectroscopy such as surface-enhanced Raman spectroscopy. In this paper, various structure geometries are combined with cadmium-selenide quantum dots in order to study the effect of the plasmonic enhancement on photoluminescence. The geometries of the structures have been varied by size, allowing for the tunability of the structures for desired wavelengths. Photoluminescence, although different from Raman spectroscopy, still allows for the examination of the increase of the local enhancement of the electric field. This work plans to experimentally verify numerical simulations of local enhancement for these various geometries.   

Photothermal Plasmonic Effects and Localization of Excess Temperature Using Metal Nanostructures

Heat dissipation is an essential aspect of photo-excited nano-systems. Nanoparticles (NPs) and nano-engineered materials have recently been used for their heat generation properties in applications such as bio-sensing, photo-chemistry, and energy harvesting. In most nano-heating applications, efficient localization of temperature remains a challenge. We theoretically study heat dissipation for spherical NPs. We introduce figures of merit for the localization of temperature and power efficiency of photo-heating. Our investigation focuses on two complex systems. The first one is composed of two large spherical NPs, working as a nano-optical antenna, along with one small NP in the middle, functioning as an electromagnetic and thermal hot spot. We show that based upon the geometry and sizes of the first structure, the temperature is ''focused'' in the hot spot but the power efficiency of the trimer is much smaller compared with a single NP system. To solve this issue, we use a small nanorod instead of a small NP for our second structure. We observe a plasmonic Fano effect in the second complex, which results in strong enhancement in the local excess of temperature, temperature localization, and power efficiency.   

Plasmon drag effect in profile-modulated surfaces with periodic and random modulation.

Photoinduced currents associated with plasmon excitation and propagation (plasmon drag effect) are theoretically and experimentally studied in flat, rough and profile modulated metal films. We demonstrate great enhancement of the effect in rough and nanostructured surfaces and possibility to control the amplitude and polarity of photoinduced electric signals with surface geometry and illumination conditions. We discuss the modified electromagnetic momentum loss approach which can correctly describe the photoinduced voltages associated with propagating surface plasmon polaritons. Direct proportionality of energy and momentum transfer in interactions of plasmons and free electrons in metal is predicted and proven to be valid for surfaces with relatively low height modulation amplitudes. We also suggest an equivalent circuit model, which can provide qualitative description of the plasmon-induced electrical effects in modulated surfaces and surfaces with random roughness.   

Suppression of quenching in metal nanoparticle based spasers

Radiation of a dipole in a close proximity to the metal surface is quenched due to nonradiative energy transfer to optically inactive excitations in metal. We show that in spaser action, involving ensemble of pumped two-level systems interacting with a resonant plasmon mode in a metal nanoparticle, quenching is suppressed. We develop a model that incorporates coupling of gain molecules to nonresonant nanoparticle modes, which are responsible for quenching of plasmon-enhanced fluorescence, and show the effect of quenching on spaser threshold is reduced for larger gain densities. We derive explicit condition relating gain molecule ensemble size and their average proximity to the metal surface which governs the importance of quenching effects in metal nanoparticle based spasers.   

Quantum and nonlocal phenomena in plasmonic nanoparticles

The field of plasmonics is widely explored with a classical mindset, while recent experimental efforts now reveal plasmon phenomena beyond expectations rooted in classical electrodynamics [1]. In particular, intrinsic length scales of the electron gas are anticipated to manifest in a nonlocal plasmonic response [2] and other quantum corrections to the light-matter interactions [3]. I will discuss theory and experimental efforts to understand nonlocal dynamics (size-dependent frequency shifts and damping) in metallic nanoparticles with true nanoscale dimensions [4], providing also a link between the observed spectral shifts and the fraction of electromagnetic energy attributed to quantum degrees of freedom [5]. \begin{itemize} \item[[1]] S.I. Bozhevolnyi \& N.A. Mortensen, "Plasmonics for emerging quantum technologies", doi:10.1515/nanoph-2016-0179 \item[[2]] S. Raza, S.I. Bozhevolnyi, M. Wubs \& N.A. Mortensen, "Nonlocal optical response in metallic nanostructues", J. Phys. Cond. Matter. {\bf 27}, 183204 (2015) \item[[3]] T. Christensen, W. Yan, A.-P. Jauho, M. Solja{\v c}i{\'c} \& N.A. Mortensen, "Quantum corrections in nanoplasmonics: shape, scale, and material", arXiv:1608.05421 \item[[4]] S. Raza \emph{et al.}, "Multipole plasmons and their disappearance in few-nanometer silver nanoparticles", Nature Communications {\bf 6}, 8788 (2015) \item[[5]] W. Yan \& N.A. Mortensen, "Nonclassical effects in plasmonics: An energy perspective to quantify nonclassical effects", Phys. Rev. B {\bf 93}, 115439 (2016) \end{itemize}   

Coalescence Processes of Ag Nanoparticles. A case of Surface Plasmon Resonances.

A novel process, the formation and growth of metallic Ag nanowires (NWs) and nanoparticles (NPs) on Ag$_{\mathrm{2}}$WO$_{\mathrm{4}}$ upon electron beam irradiation has been discovered and was extensively investigated by different experimental techniques, transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS) characterization, among others. Ag metallic NWs grow on Ag$_{\mathrm{2}}$WO$_{\mathrm{4}}$, and further observation showed the formation of Ag NPs formed from the Ag filaments during exposition to the electron beam. The present work concentrates in the study and the understanding of physical processes that occur with these new Ag NPs an their interaction. The experiments found the formation of Ag NPs of sizes from 2 to 15 nm. These NPs produced in vacuum show very interesting behavior displaying coalescence effects with two nearby NPs forming other large particles. Associating theory and computer simulations we model this evolution process that is driven by plasma resonance effects due to the electron beam.   

Aluminum Nanowire Arrays via Soft Nanoimprint Lithography

We have previously reported a method to fabricate freestanding, vertically-oriented, and lithographically-ordered Al nanowire arrays via directed assembly, and demonstrated their utility as a plasmonic waveguide\footnote{N. T. Nesbitt, J. M. Merlo, A. H. Rose, Y. M. Calm, K. Kempa, M. J. Burns, \& M. J. Naughton, \textit{Nano Lett.} \textbf{15}, 7294-7299 (2015)}. However, the process, a variation on the preparation of anodized aluminum oxide (AAO), involved imprinting Al with a hard stamp, which wore down the stamp and had a low yield of Al NWs. Here we show a new nanoimprint lithography (NIL) technique that uses a soft stamp to pattern a mask on the Al; it provides a greater yield of Al NWs and is less destructive to the stamp, providing a path to applications that require NW arrays over macroscopic areas.   

Plasmon-Exciton Coupling Using DNA Templates

In the strong coupling regime, coherent energy exchange between plasmons and excitons is a phenomenon which displays distinct hybrid states. We employ the DNA origami technique to precisely position metallic nanoparticles in a defined spatial arrangement and fixed interparticle spacing. We adjust the plasmon resonance of the structure to accurately match the energy absorption of a molecular exciton (J-aggregate) by varying the nanoparticle diameter between 30~nm and 60~nm while keeping their separation distance constant ($\sim$ 5~nm). Using this pre-programmable self-assembly approach, we obtained strong plasmon-exciton coupling and studied farfield scattering at the single-structure level, displaying normal mode splitting up to 170~meV. The ability to custom-tune the plasmon frequency and to provide strong field confinement makes DNA-origami the ideal template to bottom-up assembly plasmon-exciton systems operating at room temperature and optical frequencies.   

Computational Design of Tunable UV-Vis-IR Filters Based on Silver Nanoparticle Arrays

We propose design strategies to develop selective optical filters in the UV-Vis-IR spectrum using the surface plasmon response of silver nanoparticle arrays. Our finite-difference time-domain simulations allow us to rapidly evaluate many nanostructures comprising simple geometries while varying their shape, height, width, and spacing. Our results allow us to identify trends in the filtering spectra as well as the relative amount of absorption and reflection. Optical filtering with nanoparticles is applicable to any transparent substrate and can be easily adapted to existing manufacturing processes while keeping the total cost of materials low.   

Electrochemically Induced Insulator-Metal-Insulator Transformations of Vanadium Dioxide Nanocrystal Films

Vanadium dioxide (VO2) undergoes significant optical, electronic, and structural changes as it transforms between the low-temperature monoclinic and high-temperature rutile phases. The low-temperature state is insulating and transparent, while the high-temperature state is metallic and IR blocking. Alternative stimuli have been utilized to trigger insulator-to-metal transformations in VO2, including electrochemical gating. Here, VO2 nanocrystal films have been prepared by solution deposition of V2O3 nanocrystals followed by oxidative annealing. Nanocrystalline VO2 films are electrochemically reduced, inducing changes in their electronic and optical properties. We observe a reversible transition between infrared transparent insulating phases and a darkened metallic phase by in situ visible–near-infrared spectroelectrochemistry and correlate these observations with structural and electronic changes monitored by X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, and conductivity measurements. Reduction causes an initial transformation to a metallic, IR-colored distorted monoclinic phase. However, an unexpected reversible transition from conductive, reduced monoclinic VO2 to an infrared-transparent insulating phase is observed upon further reduction.   

Enhanced Broadband Photoresponse in Plasmonic Nanoparticles decorated ZnO Nanowire Film fabricated by Laser Ablation method

ZnO is a high band gap semiconductor which is widely used as an UV photo-detector. However, one of the draw backs of ZnO based photo-detectors is its lack of response in the visible, in particular above a wavelength ($\lambda )$ of 450 nm which limits its use as broadband photodetector. Here, we report that the photoresponse of ZnO nanowire (NW) based photodetector can be significantly enhanced in wide spectral range (350 to 650nm) using ligand free attachment of plasmonic Au-nanoparticles (NP) on its surface by laser ablation process. This simple fabrication method increases responsitivity ($R)$ (2 to 4 order) of Au-ZnO device in a window of 500\textless $\lambda $\textless 625 nm in which bare ZnO-NW photodetector has very low $R$. The enhanced broad band photoresponse is strongly linked to the enhancement of the absorption in the spectral range of 300 nm to 700 nm due to Au-NP attachment. We found that both $R$ and photocurrent decay time in Au-ZnO device can be tuned controllably by increasing Au NP concentration by just varying the no. of laser shots used for the ablation process. This simple, single step, laser ablation based plasmonic nanoparticle attachment process can be further utilized to make other semiconductor nanoparticle based devices.