Session Z18: Enhanced Optical Properties Using Plasmonics, Metamaterials, and Nanoparticles

Nonresonant Broadband Funneling of Light via Ultrasubwavelength Channels

Efficient control of light-matter interaction, which is key to many photonics applications such as detectors, sensors and novel light sources, can be achieved by enhancing and funneling light efficiently through deep subwavelength channels. Thus far, this has been accomplished by exciting the structural surface plasmon resonances of perforated nanostructured metal films, a phenomenon known as extraordinary optical transmission. The resonant nature of the phenomenon makes it inherently narrowband. Here, we introduce a new paradigm structure consisting of double-grooved metallic structure that possesses all the capabilities of extraordinary optical transmission platforms, yet operates nonresonantly and across broadband (Phys. Rev. Lett. 107, 163902(2011)). As a result, our proposed platform demonstrates efficient ultrabroadband funneling of optical power confined to an area as small as $\sim $ ($\lambda $/500)$^{2}$, where optical fields are enhanced, thus exhibiting functional possibilities beyond resonant platforms. We explain the nonresonant mechanism underlying this phenomenon with a simple quasistatic picture that shows excellent agreement with our numerical simulations.   

Measurement of Near-Field Thermal Radiation through a Nanometer-Sized Gap

Radiation heat transfer in nanostructures can differ significantly from that in macrostructures due to wave effects. Theory has predicted that thermal radiation heat transfer between two surfaces separated by tens of nanometers can exceed that of Planck's blackbody radiation law by several orders of magnitude. Our AFM-inspired heat flux sensor, comprising of a sphere attached to the tip of a bimetallic cantilever, can measure the radiation exchange across nanometer-scale gaps between a sphere and a flat surface. The objective of this work is to experimentally study thermal radiative transfer at very small separation gaps. In previous experiment, our group has successfully measured near-field radiative heat transfer through gap as small as 30 nm. In this work, we extend this technique to decrease the gap down to a few nanometers and show that existing fluctuating electrodynamics theory cannot predict experimental results in the extreme limit of small separation between two surfaces. Our experiments raise interesting question on the convergence of radiation heat transfer mechanism and interfacial heat conduction mechanism. Theoretical approaches bridging these two regimes will be discussed.   

Making metals transparency for white light by surface plasmons

We demonstrate both experimentally and theoretically that metallic gratings consisting of narrow slits become transparent for extremely broad bandwidths under oblique incidence. This phenomenon can be explained by a concrete picture in which the incident wave drives free electrons on the conducting surfaces and part of the slit walls to form surface plasmons (SPs). The SPs then propagate on the slit walls but are abruptly discontinued by the bottom edges to form oscillating charges that emit the transmitted wave. This picture explicitly demonstrates the conversion between light and SPs and indicates clear guidelines for enhancing SP excitation and propagation. Making structured metals transparent may lead to a variety of applications. References: Xian-Rong Huang, Ru-Wen Peng, and Ren-Hao Fan, Phys. Rev. Lett. (2010)105, 243901; and Ren-Hao Fan, Ru-Wen Peng, Xian-Rong Huang, Jia Li, Qing Hu, and Mu Wang, manuscript prepared(2011).   

Fabrication of Broad Band Mid-Infrared Absorber based on Periodic Dielectric-Thin Metal Film Multilayer Structures

We present results from measurements on periodic multilayer structure of alternating dielectric and thin metal layers to achieve a broadband absorber of mid-infrared radiation. We examine the effect on performance of a back-reflective metallic bottom layer, surface roughness at interfaces, the metal conductance, the thickness of dielectric layers, and a patterned anti-reflective layer. We determine optimum structure parameters for absorption of a 500 K-black body spectrum, and find that the numerical results agree well with the measured absorption spectra. We also investigate the possibility of fabricating a patterned anti-reflective layer to further increase the absorption.   

Simultaneous Bulk and Surface Plasmon Resonance and Radiative Polaritons excited in RuO$_2$ films grown on glass and on TiO$_2$ (001)

Conducting oxides, such as RuO$_2$, have a much lower carrier concentration as compared to metals, leading to a lower plasma frequency of $3.3eV$ which lies in the infrared (IR) region. This unique feature of conducting oxides allows for simultaneous observation of surface and bulk polariton modes in the IR range. Here we have investigated bulk and surface plasmons as well as radiative polaritons in RuO$_2$ thin films. The RuO$_2$ thin films investigated were grown using DC magnetron sputtering on glass and on TiO$_2$ (001). We have used X-ray Diffraction and Reflection High-Energy Electron Diffraction to characterize the microstructure of these samples. Four-point probe and ellipsometry were used to investigate the electrical conductivity and the optical properties. The optical measurements were carried out using HeNe red laser ($632nm$) and IR laser ($1520nm$) radiations to illuminate RuO$_2$ thin films. We will show that bulk plasmons can be excited in RuO$_2$ thin films in the visible red region, while simultaneous bulk plasmons as well as surface plasmons excitation are observed in the IR region. We also studied the substrate influence on the radiative polaritons in the middle IR region ($20$-$2.2um$) by measuring films grown on glass and on TiO$_2$ (001).   

Simulations of enhanced absorption in composite embedded, insulated metal nanopatterns for ultrathin film photovoltaics

In recent work [1], a concept of employing embedded metallic nanopatterns (EMN) in ultrathin film solar cells was discussed. Elsewhere in this conference, Fan {\it et al.} advance this with a scheme for embedded insulated metallic nanopatterns (EIMN) that is designed to avoid deleterious carrier recombination as would result from bare metal inclusions in a PV film. However, a practical route to fabricating EIMNs of desired shapes for eventual scale production is nontrivial. Here, we introduce two notions toward that goal, nano-stamping and spin-coating, of compact arrays of metallic core/insulating shell nanoparticles (MNP). We show by simulations that optical absorption of an EIMN composed of arrays of core-shell MNPs having SiO2 coatings is essentially the same as that of an EMN composed of solid metals without insulation, with absorption concentrated in the surrounding PV medium. These concepts may provide practical routes for scalability of EIMN-based ultrathin film plasmonic solar cells.\\[4pt] [1] F. Ye, M. J. Burns, M. J. Naughton, Proc. SPIE {\bf 8111}, 811103 (2011), and this conference.   

Embedded insulated metallic nanopatterns for enhanced optical absorption and photovoltaics

Recently, we have shown embedded metallic nanopatterns (EMN) in ultrathin PV films to be candidates for high efficiency thin-film solar cells, owing to prominent metamaterial/plasmonic-enhanced light trapping, as compared to unpatterned, surface- or bottom-patterned [1]. We also showed that hot electron effects emerge in ultrathin a-Si-based solar cells [2]. The EMN in the semiconductor layer, however, can also serve as a source of recombination for photogenerated electrons and holes, leading to decreased current. Here, we propose the idea of an embedded insulated metallic nanopattern (EIMN) to efficiently avoid the recombination effect while maintaining high light absorption in an ultrathin film format in which hot electron physics can contribute. Simulations show that an EIMN with a ~10 nm layer of dielectric insulation provides essentially the same absorption as its EMN counterpart. Measurements on several EMN structures will be presented. This EIMN architecture may provide a practical route to high efficiency, hot electron solar cell technology using ultrathin films.$\\$[1]F. Ye, M.J. Burns, M.J. Naughton, Proc. SPIE {\bf8111}, 811103 (2011).$\\$[2]K. Kempa, M.J. Naughton, Z.F. Ren, A. Herczynski, T. Kirkpatrick, J. Rybczynski, Y. Gao, Appl. Phys. Lett. {\bf95}, 233121(2009)   

An effective medium formulation to estimate the plasmonic dispersion of a randomly distributed gallium nanoparticle ensemble

Quantum confinement causes the dielectric function of nanometer-sized metal particles to depart from metallic bulk dispersion in a manner correlating with the size, shape, and spacing of the nanostructures. An improved effective medium approximation is formulated to reconcile angle-dependent ultraviolet/visible spectroscopy and spectroscopic ellipsometry measurements of the collective optical dispersion of randomly distributed hemispherical gallium nanoparticles deposited on a sapphire surface using ultra high vacuum molecular beam epitaxy. Atomic force microscopy and scanning electron microscopy analyze the size distribution of the nanoparticle ensembles to estimate their volume fraction. The optical constants are then estimated using a modified Maxwell-Garnett effective medium approximation that treats the ambient vacuum as a host and the bare sapphire substrate as a semi-infinite layer. The refined dielectric function improves estimates of the collective plasmonic response of the nanoparticle ensemble.   

Plasmon responses and optical chirality of helical nanoparticle assemblies

Helical gold nanoparticle assemblies exhibit strong circular dichorism (CD) in the plasmonic band. This CD effect comes from dipolar Coulomb and electromagnetic interactions between spherical gold nanoparticles. A typical CD spectrum of chiral plasmonic assembly includes positive and negative bands. The shape of CD spectra is sensitive to geometrical parameters of the assembly. In this study, we show that the CD signal is stable against structural defects, which makes experimental realizations of strong CD effect feasible. To date, several recent experimental papers reported CD effects in helical plasmonic systems. In addition, we found that the sign of CD signal can flip as a function of the inter-particle distance and, for very long helices, the long-range electrodynamic interactions become essential. These results are important for designing nanocomposite materials with strong optical chirality in the visible wavelength range.   

Determining 3D Relative Orientation between Plasmonic Nanosparticles

Polarization resolved far-field scattering measurements were used to determine 3D relative orientation between plasmonic nanosparticles placed few nanometers apart. The metallic nanostructure was assembled using the atomic force microscope (AFM) nanomanipulation method. When a gold nanosphere (150 nm in diameter) was placed within a few nanometers to the end of a gold nanorod (20 nm in diameter and 180 nm in length), near field coupling between them introduced new features in the scattering spectra. Specifically, a Fano resonance emerged, due to the interference between a dark mode, corresponding to the quadrupole charge distribution on the rod, and the bright, dipole mode of the sphere. As linear polarizer in the path of the incident and emission light was rotated, the scattering spectra evolve systematically, enabling us to determine the 3D relative orientation between these two plasmonic nanoparticles. This orientation information cannot be accurately and dynamically obtained using other scanning probe techniques, especially when one nanoparticle is partially hidden under the other nanoparticles.   

Plasmonic Circular Dichroism Effect in Nanomaterials

By employing a numerical solution of Maxwell's equations beyond the dipole limit, we studied the circular dichroism (CD) signal of a chiral molecule in the presence of a gold nanopartical (NP) dimer. The CD signals come from two parts: The first one is Coulomb interaction within the molecule-dimer complex giving rise to the plasmon peak in the CD spectrum, while the other one is the plasmonic enhancement of the absorption process in a chiral molecule. Typically, the CD signals of chiral molecules are very weak in the visible range, but are strong in the UV range. In the presence of gold dimer, however, we found a strong CD signal emerges in the visible range of photon energies, where the plasmon effect makes the main contribution to CD signal at the plasmon frequency. Furthermore, we propose that, by using the plasmon-induced CD signals, one can design optical sensors to study chirality of biomolecules.   

Metamaterial Enhanced Terahertz Spectroscopy of Biomolecules

As the field of metamaterials experiences exponential growth, an increasing number of studies have focused on understanding near-field coupling between arrays of metamaterials and their local environment. Examples include a second array of metamaterials, phonon bands in semiconductors, and even small molecules. In this work, we demonstrate metamaterial coupling to a thin film of biomolecules. Protein films are deposited onto arrays of gold split ring resonators (SRRs) on thin silicon nitride substrates. As the LC resonance of the SRR is tuned across the low frequency terahertz (THz) modes of the biomolecule, hybridization results in mode splitting. The thin substrates (400nm) and the large electric field enhancement intrinsic to the SRRs provide ultrasensitive detection of the THz modes in the protein, allowing for the THz response to be measured from thin films which would otherwise not be observable.   

Poynting vector in magnetic materials: from thermomagnetic effects to metamaterials

In a finite sample, besides the bulk currents given by the Kubo formula, additional charge and energy are transferred by surface magnetization currents. We show that for the energy current, the corresponding surface correction to the Kubo's current is expressed in terms of the magnetization component of the Poynting vector, MxE. Magnetization currents, like persistent currents, are dissipationless and do not transfer entropy. Only in this way, one can obtain the Nernst and Ettingshausen coefficients that satisfy to the Onsager relation. In the microscopic transport theory, for maintaining gauge invariance in a magnetic field, the heat current operator should include the magnetic term. Both magnetization-related effects - the dissipationless nature and strong surface contribution to the energy transfer - have been also overlooked in the recent works on the Poynting vector in metamaterials. The paradoxes in this area are resolved, if one accurately considers a balance of electromagnetic energy transferred by bulk and surface magnetization currents.   

Polarization Conversion Using the Cavity Resonances of Plasmonic Patch Nanoantennas

The control of light polarization is essential to all optical experiments and photonic devices. Here we propose and demonstrate a novel method to realize the polarization conversion by using an array of non-chiral plasmonic patch nano-antennas. The patch nano-antennas are composed of a 2D array of elliptical Ag nano-disks and a base Ag substrate spaced by an ultra-thin dielectric layer. The Ag nano-disks and the base metal substrate would form an array of plasmonic nanocavities with ultrasmall mode volumes. We will show the excitation of the fundamental mode of the plasmonic cavity resonance could introduce a 90$^{\circ}$ phase delay to the reflected light from the patch antenna. This phase delay can be utilized to realize the polarization conversion between linearly polarized light and elliptically or circularly polarized light.   

Plasmonic nanowire transmision lines

Metallic nanowires could be used as nano-optical transmission lines. The important factors characterizing such lines are the subwavelength operation, long propagation length, and low penetration of the propagating modes into the environment outside wires. In this work we study these factors in silver nanowires operating as surface plasmon polariton (SPP) waveguides, by employing the finite difference time domain (FDTD) and the finite difference frequency domain (FDFD) simulations. In addition to the dispersion relation of the SPP mode, we investigate the inter-wire crosstalk, an important feature of the nano-optical circuits. We compare our results with the available experimental results.~