Label-free plasmonic biosensors rely either on surface plasmon polaritons or on localized surface plasmons on continuous or nanostructured noble-metal surfaces to detect molecular-binding events. Despite undisputed advantages, including spectral tunability, strong enhancement of the local electric field and much better adaptability to modern nanobiotechnology architectures, localized plasmons demonstrate orders of magnitude lower sensitivity compared with their guided counterparts. Here, we demonstrate an improvement in biosensing technology using a plasmonic metamaterial that is capable of supporting a guided mode in a porous nanorod layer. Benefiting from a substantial overlap between the probing field and the active biological substance incorporated between the nanorods and a strong plasmon-mediated energy confinement inside the layer, this metamaterial provides an enhanced sensitivity to refractive-index variations of the medium between the rods (more than 30,000 nm per refractive-index unit). We demonstrate the feasibility of our approach using a standard streptavidin-biotin affinity model and record considerable improvement in the detection limit of small analytes compared with conventional label-free plasmonic devices.

Wave interference is a fundamental manifestation of the superposition principle with numerous applications. While in conventional optics interference occurs between waves undergoing different phase advances during propagation, we show that the vectorial structure of the near-field of an emitter is essential for controlling its radiation as it interferes with itself on interaction with a mediating object. We demonstrate that the near field interference of a circularly polarized dipole results in the unidirectional excitation of guided electromagnetic modes in the near-field, with no preferred far-field radiation direction. By mimicking the dipole with a single illuminated slit in a gold film, we measured unidirectional surface-plasmon excitation in a spatially symmetric structure. The surface wave direction is switchable with the polarization.Interference is the cornerstone of various phenomena in nature enabling numerous applications. In optics, it is intensively used in microscopy, stellar measurements, spectroscopy, and communication technologies, among many others, and is the basis behind the concepts of reflection, refraction and light bending (1, 2). Typically, interference occurs due to the relative phase lag of different propagating waves. On the other hand, nanophotonics -the branch of optics studying the interaction of light with subwavelength nanoscale structures-deals inherently with phenomena that occur via near-field interactions before appreciable phase lags can be accumulated (3). A radiationless form of interference in the near field (4) is behind new exciting applications such as the focusing of evanescent components to achieve subwavelength resolution in imaging (5-8). Near field interference achieved through the full coherent control of the phase and amplitude of excitation light allows asymmetric spatial field localization (9, 10) and selection of propagation paths at intersections of waveguides (11).We demonstrate near field interference by considering a single source of radiation coupled to a mode with a vectorial structure of electromagnetic field. Using an additional degree of freedom provided by the vectorial character of the field, control over the near-field interference can be achieved. We show that an elliptically polarized dipole can produce destructive or constructive interference of different evanescent components in its near field, and as a result, excite electromagnetic modes in neighbouring material structures, such as dielectric and plasmonic waveguides and diffraction gratings, with a controlled directionality of propagation.

All-optical signal processing enables modulation and transmission speeds not achievable using electronics alone. However, its practical applications are limited by the inherently weak nonlinear effects that govern photon-photon interactions in conventional materials, particularly at high switching rates. Here, we show that the recently discovered nonlocal optical behaviour of plasmonic nanorod metamaterials enables an enhanced, ultrafast, nonlinear optical response. We observe a large (80%) change of transmission through a subwavelength thick slab of metamaterial subjected to a low control light fluence of 7 mJ cm(-2), with switching frequencies in the terahertz range. We show that both the response time and the nonlinearity can be engineered by appropriate design of the metamaterial nanostructure. The use of nonlocality to enhance the nonlinear optical response of metamaterials, demonstrated here in plasmonic nanorod composites, could lead to ultrafast, low-power all-optical information processing in subwavelength-scale devices.

Controlling coherent electromagnetic interactions in molecular systems is a problem of both fundamental interest and important applicative potential in the development of photonic and opto-electronic devices. The strength of these interactions determines both the absorption and emission properties of molecules coupled to nanostructures, effectively governing the optical properties of such a composite metamaterial. Here we report on the observation of strong coupling between a plasmon supported by an assembly of oriented gold nanorods (ANR) and a molecular exciton. We show that the coupling is easily engineered and is deterministic as both spatial and spectral overlap between the plasmonic structure and molecular aggregates are controlled. We think that these results in conjunction with the flexible geometry of the ANR are of potential significance to the development of plasmonic molecular devices.