Mazhar E. Nasir

Eliminating material constraints for nonlinearity with plasmonic metamaterials.

Nonlinear optical materials comprise the foundation of modern photonics, offering functionalities ranging from ultrafast lasers to optical switching, harmonic and soliton generation. Optical nonlinearities are typically strong near the electronic resonances of a material and thus provide limited tuneability for practical use. Here we show that in plasmonic nanorod metamaterials, the Kerr-type nonlinearity is not limited by the nonlinear properties of the constituents. Compared with golds nonlinearity, the measured nonlinear absorption and refraction demonstrate more than two orders of magnitude enhancement over a broad spectral range that can be engineered via geometrical parameters. Depending on the metamaterials effective plasma frequency, either a focusing or defocusing nonlinearity is observed. The ability to obtain strong and fast optical nonlinearities in a given spectral range makes these metamaterials a flexible platform for the development of low-intensity nonlinear applications.

Bulk plasmon-polaritons in hyperbolic nanorod metamaterial waveguides

Hyperbolic metamaterials comprised of an array of plasmonic nanorods provide a unique platform for designing optical sensors and integrating nonlinear and active nanophotonic functionalities. In this work, the waveguiding properties and mode structure of planar anisotropic metamaterial waveguides are characterized experimentally and theoretically. While ordinary modes are the typical guided modes of the highly anisotropic waveguides, extraordinary modes, below the effective plasma frequency, exist in a hyperbolic metamaterial slab in the form of bulk plasmon-polaritons, in analogy to planar-cavity exciton-polaritons in semiconductors. They may have very low or negative group velocity with high effective refractive indices (up to 10) and have an unusual cut-off from the high-frequency side, providing deep-subwavelength (λ0/6–λ0/8 waveguide thickness) single-mode guiding. These properties, dictated by the hyperbolic anisotropy of the metamaterial, may be tuned by altering the geometrical parameters of the nanorod composite.

Hydrogen Detected by the Naked Eye: Optical Hydrogen Gas Sensors Based on Core/Shell Plasmonic Nanorod Metamaterials

Gold-core/palladium-shell metamaterials for hydrogen detection are presented. The more than 30% change in both the reflection and transmission from the metamaterial layer that is observed when the layer is exposed to 2% hydrogen mixture is clearly noticeable to the naked eye as a change in the brightness of light transmitted by the metamaterial. This sensor should make a contribution to the safety of processes involving hydrogen.

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.

Tuning the effective plasma frequency of nanorod metamaterials from visible to telecom wavelengths

Hyperbolic plasmonic metamaterials are important for designing sensing, nonlinear, and emission functionalities, which are, to a large extent, determined by the epsilon-near-zero behaviour observed close to an effective plasma frequency of the metamaterial. Here, we describe a method for tuning the effective plasma frequency of a gold nanorod-based metamaterial throughout the visible and near-infrared spectral ranges. These metamaterials, fabricated by two-step anodization in selenic acid and chemical post-processing, consist of nanorods with diameters of around 10 nm and interrod distances of around 100 nm and have a low effective plasma frequency down to a wavelength range below 1200 nm. Such metamaterials open up new possibilities for a variety of applications in the fields of bio- and chemical sensing, nonlinearity enhancement, and fluorescence control in the infrared.

Reactive tunnel junctions in electrically driven plasmonic nanorod metamaterials

Non-equilibrium hot carriers formed near the interfaces of semiconductors or metals play a crucial role in chemical catalysis and optoelectronic processes. In addition to optical illumination, an efficient way to generate hot carriers is by excitation with tunnelling electrons. Here, we show that the generation of hot electrons makes the nanoscale tunnel junctions highly reactive and facilitates strongly confined chemical reactions that can, in turn, modulate the tunnelling processes. We designed a device containing an array of electrically driven plasmonic nanorods with up to 1011 tunnel junctions per square centimetre, which demonstrates hot-electron activation of oxidation and reduction reactions in the junctions, induced by the presence of O2 and H2 molecules, respectively. The kinetics of the reactions can be monitored in situ following the radiative decay of tunnelling-induced surface plasmons. This electrically driven plasmonic nanorod metamaterial platform can be useful for the development of nanoscale chemical and optoelectronic devices based on electron tunnelling.Chemical reactions in nanoscale gaps can be controlled by hot electrons generated by voltage-induced tunnelling.

Into the deep UV: self-assembled hyperbolic metamaterials for the ultraviolet range (Conference Presentation)

Optical metamaterials have been shown to offer a number of useful properties, including enhancement and control of spontaneous emission, chemical and biosensing and nonlinear optical control and manipulation. To date, however, the vast majority of research has been conducted into the properties of visible (or longer) wavelength systems. The materials typically chosen for this work, such as the coinage metals (Ag, Au, Cu) for visible wavelength applications become unsuitable for use in the ultraviolet due to inter- and intraband driven absorption. In order to develop metamaterials in the ultraviolet different materials need to be considered. Ultraviolet metamaterials are proposed to have additional benefits and functionalities in addition to those present in previously considered systems. For instance, the autofluorescence of biological materials typically lies in the UV range (DNA fluoresces at around 260 nm) and this can be coupled to the optical response of UV metamaterials to allow label-free fluorescence (allowing the studying of biological materials and cells in a near-native state, without the use of potentially bio-perturbing dyes) as well as detection at lower concentrations. UV-range substrates are also of interest for SERS applications due to the fact that the enhancement scales as ν^4, dramatically increasing the efficiency of the process. Here we demonstrate the development and characterisation of a large area, self assembled metamaterial for use in the ultraviolet wavelength range. Anodised aluminium oxide (AAO) provides a template for the growth of nanorods of deep-UV suitable metals, Aluminium and Gallium. These metamaterials consist of nanorods with geometric parameters smaller than the free-space wavelength of UV light (diameter around 25 nm, inter-rod separation around 60 nm) grown vertically from a UV-suitable substrate. The precise geometric parameters are controlled by the anodisation conditions, allowing tunability of the spectral response of the metamaterial. Aluminium is well documented to be the best choice of material for UV plasmonic and metamaterial use, due to its large, negative real permittivity and low imaginary permittivity in the UV range, and gallium presents interesting behaviour due to its relatively low melting point (30°C), with the liquid and solid state showing significant differences in their optical properties. Both systems have been optically characterised across the UV and visible wavelength ranges and compared with numerical modelling in order to analyse and describe their behaviour.

Controlling field enhancement with plasmonic nanocone metamaterials

The plasmonic resonances of metamaterials based on Au nanorod arrays give rise to high field enhancement combined with strong field localisation which can be controlled by nanoscale geometrical effects within the nanorod array. These have led to extraordinary refractive index sensitivity, enhanced nonlinear optical effects and enhanced spontaneous emission rate near and inside the metamaterial [1-4]. Additional flexibility in engineering the spectrum, magnitude and spatial distribution of the field enhancement can be achieved with more complex nanostructures replacing the cylindrical nanorods in the metamaterial with nanocones.

Overcoming material limitations of nonlinear dynamics using metamaterial resonances

Nonlinear optical processes in highly conductive materials, produce fast and strong changes in their refractive index. Using nano structuring, further enhancement of this Kerr-type nonlinearity, determined by free-electron dynamics in strong electromagnetic fields, can be achieved through increased light-matter interactions [1]. However, although the initial change in optical properties occurs over femtosecond time scales, the relaxation of the excited electrons is governed by electron cooling, which may take place generally over several picoseconds.

Hot-carrier generation in plasmonic SiO 2 -Au core-shell nanoparticles

After the innovative method reported by Fujishima and Honda for hydrogen generation by the UV-light-induced electrochemical water splitting on TiO2 electrodes [1], many other approaches were developed using hot-carrier generation by light in various semiconductors and metallic nanostructures [2-4]. For example, as an alternative to electrochemical cell systems, semiconductor particles suspended in water became a new field of study [2], where photocatalysis takes place on the surface of the particles. Hydrogen evolution from plasmonic metal-semiconductor heterostructures has also been observed with Au nanorods partially coated with TiO2 which provide separated Au/TiO2 regions [3]. In this case, hot electrons are generated in the plasmonic Au nanorods upon light illumination and subsequently some of them filtered out through the Au/TiO2 Schottky barrier into TiO2 and become available for photoreduction.