Nanoscale light localization in plasmonic structures enables large changes in the optical properties of materials within small volumes to manifest themselves via very strong changes in light transmission and emission. I will illustrate this concept with several examples, including i) a dramatic modulation of the complex dielectric function of conducting oxide thin films by carrier modulation in nanometer-thickness layers in plasmonic metal-insulator-metal waveguide structures, and ii) large increases in the spontaneous emission rate for for "Wantecas" or waveguide/antenna/cavity structures containing III-V semiconductor and quantum dot heterostructures.

In this talk, I give an overview of my group’s recent work in exploring the role of nonlinearity in several plasmonic phenomena which involve extreme metamaterials, e.g., epsilon-near-zero (ENZ) structures. We will discuss our results in combining scattering-cancellation-based plasmonic cloaking with the second-harmonic generation, the role of optical nanoantennas in enhancing the nonlinearity in optical materials, and the effect of nonlinear optical materials when combined with ENZ structures to provide nonlinear lumped optical elements in metamaterial-inspired optical nanocircuitries – metactronics (N. Engheta, Science, 317, pp. 1698-1702 (2007)). The enhancement of second-harmonic generation using proper design of nanoantennas may lead to composites with strong second harmonic effects. The nonlinear elements, that are formed by the combination of ENZ structures with nonlinear optical materials, may act as optical switches at the nanoscale, leading to the possibility of optical manipulation and switching using metactronic circuits. In this talk, we will present our recent results in these areas, will discuss the physical meaning behind our theoretical findings, and will forecast future directions in these areas.

Surface plasmon polaritons propagating through a circular V-groove in a Au surface give rise to whispering gallery resonances. We have investigated the nature, ordering and confinement of these plasmonic resonances both in theory and experiment. The ring resonator supports a rich set of modes with increasing radial and azimuthal order that can be controlled by the diameter, depth and opening angle of the circular groove. Here, we show that the plasmonic whispering gallery modes can confine light to a very small volume, resulting in a broadband Purcell factor as large as 2000. The plasmonic ring resonances can be used to study enhanced light-matter interaction.Ring resonators were made in a single-crystalline Au surface by focused ion beam milling. Ring diameters of 200-1000 nm were studied, with groove depths of 100-500 nm. We excite the resonator modes using a 30 keV electron beam in a scanning electron microscope equipped with a new mirror design, that enables –for the first time– the angle-resolved collection of cathodoluminescence radiation from the sample. We study the angle-resolved emission patterns from ring resonators with intentional shape distortion, in order to break the mode symmetry, and study coupled resonators, in which two ring cavities are placed within each other’s near field.Theoretical studies of the dispersion in linear V-grooves were made using boundary-element-method (BEM) calculations of the local density of optical states (LDOS). We find that the dispersion is higher for grooves with a narrow opening angle and is highest for the lowest order mode (mode index up to 3). In a circular V-groove, these groove modes are resonant when the circumference of the ring equals an integer number of plasmon wavelengths. We determine the resonances of these circular ring resonators using axially symmetric BEM. We find that the resonant wavelengths agree well with the dispersion of plasmons in a linear groove, even in grooves with a circumference of only a single plasmon wavelength. This shows that groove plasmons are strongly confined to the groove and that their dispersion is not influenced significantly by a sharp bend.From the spectral width of the resonances we derive the quality factor Q=15-50. These values are slightly lower than one would expect based on the dispersion of groove plasmons, which is attributed to radiation losses of the whispering gallery resonant modes. Using BEM we calculate the electric field in the groove and find that for rings with a shallow groove (100 nm) with a small groove width (10 nm) the mode volume of the lowest-order ring resonances can be as small as 0.00073 λ^3. As a result, the Purcell factor of the ring resonances can be well over 2000. The high Purcell factor at low Q enables strong and broadband interaction between emitters and modes of the resonator. Preliminary measurements on the enhanced spontaneous emission of ATTO 680 dye in the V-groove cavities will be presented.

Transformation optics has provided a new design methodology allowing an unprecedented manipulation of light propagation, enabling exciting new applications. However, transformation optics can also help to improve the performance and functionality of conventional optical elements. Since typical optical elements possess only a single functionality, and work only in a single direction, they cannot be utilized for light manipulation in the other two spatial directions. We demonstrate that transformation optics can provide a new route for designing and integrating multiple and dissimilar optical elements into one footprint. Such a level of interchangeable dual-functionality cannot be obtained with conventional optical designs. Our approach is based on a quasi-conformal mapping of the optical space in order to realize two different elements within the same footprint for the device. We implement the spatially transformed permittivity profile into a silicon-on-insulator waveguide system by simply pattern the waveguide slab with sub-wavelength air holes. In this fashion different optical elements with two independent functionalities are realized. The performance of the optical elements is measured at 1500 nm and compared to numerical simulations. Using transformation optics as a design methodology provides more flexibility in architecture together with a way to build compact highly integrated optical devices. Furthermore, the technique is fully compatible with standard semiconductor fabrication technologies used for electronics and silicon photonics.