Brian Wells

Looking into meta-atoms of plasmonic nanowire metamaterial.

Nanowire-based plasmonic metamaterials exhibit many intriguing properties related to the hyperbolic dispersion, negative refraction, epsilon-near-zero behavior, strong Purcell effect, and nonlinearities. We have experimentally and numerically studied the electromagnetic modes of individual nanowires (meta-atoms) forming the metamaterial. High-resolution, scattering-type near-field optical microscopy has been used to visualize the intensity and phase of the modes. Numerical and analytical modeling of the mode structure is in agreement with the experimental observations and indicates the presence of the nonlocal response associated with cylindrical surface plasmons of nanowires.

Hyperbolic and plasmonic properties of Silicon/Ag aligned nanowire arrays

The hyperbolic and plasmonic properties of silicon nanowire/Ag arrays have been investigated. The aligned nanowire arrays were formed and coated by atomic layer deposition of Ag, which itself is a metamaterial due to its unique mosaic film structure. The theoretical and numerical studies suggest that the fabricated arrays have hyperbolic dispersion in the visible and IR ranges of the spectrum. The theoretical predictions have been indirectly confirmed by polarized reflection spectra, showing reduction of the reflection in p polarization in comparison to that in s polarization. Studies of dye emission on top of Si/Ag nanowire arrays show strong emission quenching and shortening of dye emission kinetics. This behavior is also consistent with the predictions for hyperbolic media. The measured SERS signals were enhanced by almost an order of magnitude for closely packed and aligned nanowires, compared to random nanowire composites. These results agree with electric field simulations of these array structures.

Spontaneous emission in non-local materials

Light–matter interactions can be strongly modified by the surrounding environment. Here, we report on the first experimental observation of molecular spontaneous emission inside a highly non-local metamaterial based on a plasmonic nanorod assembly. We show that the emission process is dominated not only by the topology of its local effective medium dispersion, but also by the non-local response of the composite, so that metamaterials with different geometric parameters but the same local effective medium properties exhibit different Purcell factors. A record-high enhancement of a decay rate is observed, in agreement with the developed quantitative description of the Purcell effect in a non-local medium. An engineered material non-locality introduces an additional degree of freedom into quantum electrodynamics, enabling new applications in quantum information processing, photochemistry, imaging and sensing with macroscopic composites.

Metamaterials-based Salisbury screens with reduced angular sensitivity

We demonstrate that the incorporation of nonlocal nanowire metamaterials into Salisbury screens allows for a substantial reduction of the dependence of incident angle on the absorption maximum. Realizations of angle-independent Salisbury screens for the near-IR, mid-IR, and GHz frequencies are proposed and their performances are analyzed analytically and numerically. It is shown that nonlocal effective medium theory adequately describes the angular dependence of nanowire-based Salisbury screens.

Spontaneous Emission in Nonlocal Metamaterials with Spatial Dispersion

Recent successes in fabrication, characterization, numerical computations, and theory have brought to life a new class of composite materials with engineered optical properties, metamaterials. Uniaxial anisotropic artificially created structures based on plasmonic nanowire arrays have emerged as a versatile platform for negative refraction, subwavelength optics, biosensing, acoustic sensing, and nonlinearity engineering. It has been demonstrated, both experimentally and theoretically, that the optical response of plasmonic nanowire arrays is strongly affected by nonlocal electromagnetism, a phenomenon where permittivity of metamaterial strongly depends not only on the frequency, but also on wavevector of the plane wave interacting with this structure. Nonlocal dielectric response leads to excitation of additional electromagnetic wave that does not exist in conventional, local, metamaterials. The dispersion of this wave can be engineered by adjusting composition and geometry of metamaterial. In this chapter we present comprehensive review of nonlocal electromagnetic properties in plasmonic nanowire metamaterials. We begin by introducing the material platform, explain the theoretical approach for nonlocal homogenization, and finally discuss the implication of material nonlocality for emission of light in nonlocal environment.

Nonlinear optics in nonlocal nanowire metamaterials(Conference Presentation)

Plasmonic nanowire metamaterials, arrays of aligned plasmonic nanowires grown inside an insulating substrate, have recently emerged as a flexible platform for engineering refraction, diffraction, and density of photonic states, as well as for applications in bio- and acoustic sensing. Majority of unique optical phenomena associated with nanowire metamaterials have been linked to the collective excitation of cylindrical surface plasmons propagating on individual nanowires. From the effective medium standpoint, this collective excitation can be described as an additional electromagnetic wave, emanating from nonlocal effective permittivity of metamaterial. The electromagnetic fields associated with such mode can are strongly inhomogeneous on the scale of the unit cell. In this work we analyze the effect of the strong field variation inside nanowire metamaterial on second harmonic generation (SHG). We show that second harmonic generation is strongly enhanced in the frequency region where metamaterial is nonlocal. Overall, the composite is predicted to outperform its homogeneous metal counterparts by several orders of magnitude. Quantitative description of SHG in nanowire medium is developed. The results suggest that bulk second harmonic polarizability emerges as result of collective surface-enhanced SHG by individual components of the composite.

Light emission in nonlocal plasmonic nanowire metamaterials

We analyze, analytically and computationally, light emission in nonlocal plasmonic nanowire metamaterials and analyze the contribution of longitudinal wave to the density of optical states in the system.

Light emission in nonlocal plasmonic metamaterials (Presentation Recording)

Plasmonic metamaterial composites are often considered to be promising building blocks for a number of applications that include subwavelength light manipulation, imaging, and quantum optics engineering. These applications often rely on effective medium response of metamaterial composites and require metamaterial to operate in exotic (hyperbolic, or epsilon-near-zero) regimes. However, the behaviour of metamaterials is often different from the predictions of effective medium. In this work we aim to understand the implications of composite nature of metamaterials on their optical properties. Plasmonic nanowire metamaterials are a convenient metamaterial platform that is capable of realization of ENZ, hyperbolic, and elliptic responses depending on light frequency and metamaterial geometry. In this work we show that the response of metamaterial in elliptical regime may be strongly affected by the additional electromagnetic wave that represents collective excitation of cylindrical surface plasmons in nanowire arrays. We present an analytical description of optical properties of additional wave and analyse the effect of this mode on quantum emitters inside nanorod metamaterials.

Additional Waves in Nonlocal Nanowire Metamaterials

We analyze, analytically and numerically, optical properties of plasmonic nanowire composites that form a promising platform for nanophotonics, present description of nonlocal electromagnetism in this media, and discuss implications of nonlocal response for different applications.

Angle-independent Salisbury screens based on nonlocal nanowire metamaterials

We demonstrate that nonlocal nanowire metamaterials can help to alleviate one of the main limitations of Salisbury screens, their dependence on the incident angle.