Terahertz metamaterials for linear polarization conversion and anomalous refraction.

Polarization is one of the basic properties of electromagnetic waves conveying valuable information in signal transmission and sensitive measurements. Conventional methods for advanced polarization control impose demanding requirements on material properties and attain only limited performance. We demonstrated ultrathin, broadband, and highly efficient metamaterial-based terahertz polarization converters that are capable of rotating a linear polarization state into its orthogonal one. On the basis of these results, we created metamaterial structures capable of realizing near-perfect anomalous refraction. Our work opens new opportunities for creating high-performance photonic devices and enables emergent metamaterial functionalities for applications in the technologically difficult terahertz-frequency regime.

Three-dimensional nanostructures as highly efficient generators of second harmonic light.

Plasmonic nanostructures enable the generation of large electromagnetic fields confined to small volumes, potentially providing a route for the development of nanoengineered nonlinear optical media. A metal-capped hemispherical nanoparticle, also known as a nanocup, generates second harmonic light with increasing intensity as the angle between the incident fundamental beam and the nanocup symmetry axis is increased. Nanoparticle orientation also modifies the emission direction of the second harmonic light. With conversion efficiencies similar to those of inorganic SHG crystals, these structures provide a promising approach for the design and fabrication of stable, synthetic second-order nonlinear optical materials tailored for specific wavelengths.

Plexciton dynamics: exciton-plasmon coupling in a J-aggregate-Au nanoshell complex provides a mechanism for nonlinearity.

Coherently coupled plasmons and excitons give rise to new optical excitations--plexcitons--due to the strong coupling of these two oscillator systems. Time-resolved studies of J-aggregate-Au nanoshell complexes when the nanoshell plasmon and J-aggregate exciton energies are degenerate probe the dynamical behavior of this coupled system. Transient absorption of the interacting plasmon-exciton system is observed, in dramatic contrast to the photoinduced transmission of the pristine J-aggregate. An additional, transient Fano-shaped modulation within the Fano dip is also observable. The behavior of the J-aggregate-Au nanoshell complex is described by a combined one-exciton and two-exciton state model coupled to the nanoshell plasmon.

Nanostructure-mediated launching and detection of 2D surface plasmons.

Au nanoparticles deposited on a metallic film act as nanoantenna receivers and transmitters for the coupling of free-space radiation into, and out of, 2D surface plasmons. Nanosteps, sub-10-nm gaps between metallic films of differing thickness, can also launch and detect surface plasmons. Here we use both types of structures to locally launch propagating surface plasmon waves and probe their properties. Nanoparticle-launched surface plasmons emerge as two lobes of nominally 90 degree angular width, propagating along the direction of incident polarization. Alternatively, plasmons can be launched unidirectionally, by asymmetric illumination of a nanoparticle receiver.

Metallic nanoshells with semiconductor cores: optical characteristics modified by core medium properties.

It is well-known that the geometry of a nanoshell controls the resonance frequencies of its plasmon modes; however, the properties of the core material also strongly influence its optical properties. Here we report the synthesis of Au nanoshells with semiconductor cores of cuprous oxide and examine their optical characteristics. This material system allows us to systematically examine the role of core material on nanoshell optical properties, comparing Cu(2)O core nanoshells (ε(c) ∼ 7) to lower core dielectric constant SiO(2) core nanoshells (ε(c) = 2) and higher dielectric constant mixed valency iron oxide nanoshells (ε(c) = 12). Increasing the core dielectric constant increases nanoparticle absorption efficiency, reduces plasmon line width, and modifies plasmon energies. Modifying the core medium provides an additional means of tailoring both the near- and far-field optical properties in this unique nanoparticle system.

Optically-driven collapse of a plasmonic nanogap self-monitored by optical frequency mixing.

A nanoparticle separated from a metallic surface by a few-nanometer thick polymer layer forms a nanoscale junction, or nanogap. Illuminating this structure with ultrashort optical pulses, exciting the plasmon resonance, results in a continuous, monitorable collapse of the nanogap. The four-wave mixing signal generated by this illumination of the nanogap provides a simultaneous monitoring of the collapse, increasing dramatically upon gap closure. Collapse is irreversible, occurring with simultaneous ablation of the dielectric from the junction.

Fluorescence enhancement by Au nanostructures: nanoshells and nanorods.

Metallic nanoparticles influence the quantum yield and lifetime of adjacent fluorophores in a manner dependent on the properties of the nanostructure. Here we directly compare the fluorescence enhancement of the near-infrared fluorophore IR800 by Au nanoshells (NSs) and Au nanorods (NRs), where human serum albumin (HSA) serves as a spacer layer between the nanoparticle and the fluorophore. Our measurements reveal that the quantum yield of IR800 is enhanced from approximately 7% as an isolated fluorophore to 86% in a NSs-HSA-IR800 complex and 74% in a NRs-HSA-IR800 complex. This dramatic increase in fluorescence shows tremendous potential for contrast enhancement in fluorescence-based bioimaging.

Tailoring plasmonic substrates for surface enhanced spectroscopies.

Our understanding of how the geometry of metallic nanostructures controls the properties of their surface plasmons, based on plasmon hybridization, is useful for developing high-performance substrates for surface enhanced spectroscopies. In this tutorial review, we outline the design of metallic nanostructures tailored specifically for providing electromagnetic enhancements for surface enhanced Raman scattering (SERS). The concepts developed for nanoshell-based substrates can be generalized to other nanoparticle geometries and scaled to other spectroscopies, such as surface enhanced infrared absorption spectroscopy (SEIRA).

Nanoparticle-induced enhancement and suppression of photocurrent in a silicon photodiode.

Nanoparticles are capable of both enhancing and suppressing the photocurrent in a silicon diode when deposited on the active face of the device. Photocurrent imaging of the individual nanoparticles and nanoparticle aggregates responsible for this effect reveals that Au nanospheres, nanoshells, and nanoshell dimers each exhibit unique wavelength-dependent suppression-enhancement characteristics. In contrast, silica nanospheres provide a sizable and relatively uniform photocurrent enhancement across the same spectral range (532-980 nm). Unusual light-harvesting behavior observed correlates with a highly complex energy flow (optical "vortexing") for the forward scattered light of plasmon resonant nanoparticles into the device.

Nanoparticle-mediated coupling of light into a nanowire.

We show that a nanoparticle can serve as an efficient antenna for coupling of visible light into propagating plasmons of an Ag nanowire. For long wires, the coupling is maximal for incident light polarized perpendicular to the nanowire. For sub-10-mum nanowires, the polarization corresponding to maximum emission from the ends of the nanowire was found to be strongly dependent on the nanowire geometry and position of the vicinal nanoparticle. This nanoparticle antenna-based approach offers a potential strategy for optimizing plasmon coupling into nanoscale metallic waveguides.

Electromigrated nanoscale gaps for surface-enhanced Raman spectroscopy.

Single-molecule detection with chemical specificity is a powerful and much desired tool for biology, chemistry, physics, and sensing technologies. Surface-enhanced spectroscopies enable single-molecule studies, yet reliable substrates of adequate sensitivity are in short supply. We present a simple, scaleable substrate for surface-enhanced Raman spectroscopy (SERS) incorporating nanometer-scale electromigrated gaps between extended electrodes. Molecules in the nanogap active regions exhibit hallmarks of very high Raman sensitivity, including blinking and spectral diffusion. Electrodynamic simulations show plasmonic focusing, giving electromagnetic enhancements approaching those needed for single-molecule SERS.

Profiling the near field of a plasmonic nanoparticle with Raman-based molecular rulers.

The enhanced local optical fields at the surface of illuminated metallic nanoparticles and nanostructures are of intense fundamental and technological interest. Here we report a self-consistent measurement of the spatial extent of the fringing field above a plasmonic nanoparticle surface. Bifunctional DNA-based adsorbate molecules are used as nanoscale optical rulers, providing two distinct surface enhanced Raman scattering signals that vary independently in intensity as a function of distance from the nanoparticle surface. While the measurement technique is calibrated on gold nanoshell surfaces with controlled and predictable electromagnetic nanoenvironments, this approach is broadly adaptable to a wide range of plasmonic geometries.

Determining the conformation of thiolated poly(ethylene glycol) on Au nanoshells by surface-enhanced Raman scattering spectroscopic assay.

The packing density of thiolated poly(ethylene glycol) (PEG) adsorbates on Au nanoshells is determined by exploiting the surface-enhanced Raman scattering response of individual nanoshell substrates. By incorporating the linker molecule p-mercaptoaniline (pMA), the number of 2000 MW and 5000 MW PEG molecules on each nanoparticle is determined by interpolation of the Langmuir isotherm for pMA. We conclude that both PEG adsorbates maintain a compact "brush" rather than an extended "mushroom" configuration on nanoshell surfaces.

Surface-enhanced Raman scattering from individual au nanoparticles and nanoparticle dimer substrates.

Surface-enhanced Raman scattering (SERS) intensities for individual Au nanospheres, nanoshells, and nanosphere and nanoshell dimers coated with nonresonant molecules are measured, where the precise nanoscale geometry of each monomer and dimer is identified through in situ atomic force microscopy. The observed intensities correlate with the integrated quartic local electromagnetic field calculated for each specific nanostructure geometry. In this study, we find that suitably fabricated nanoshells can provide SERS enhancements comparable to nanosphere dimers.

Cu nanoshells: effects of interband transitions on the nanoparticle plasmon resonance.

The optical properties of metals arise both from optical excitation of interband transitions and their collective electronic, or plasmon, response. Here, we examine the optical properties of Cu, whose strong interband transitions dominate its optical response in the visible region of the spectrum, in a nanoshell geometry. This nanostructure permits the geometrical tuning of the nanoparticle plasmon energy relative to the onset of interband transitions in the metal. Spectral overlap of the interband transitions of Cu with the nanoshell plasmon resonance results in a striking double-peaked plasmon resonance, a unique phenomenon previously unobserved in other noble or coinage metal nanostructures.