Vashista C. de Silva

Metamagnetics with rainbow colors.

A family of coupled nanostrips with varying dimensions is demonstrated exhibiting optical magnetic responses across the whole visible spectrum, from red to blue. We refer to such a phenomenon as rainbow magnetism. The experimental and analytical studies of such structures provide us with a universal building block and a general recipe for producing controllable optical magnetism for various practical implementations.

Scattering suppression in epsilon-near-zero plasmonic fractal shells

Significant extinction from the visible to mid-infrared makes fractal shells very attractive as aerosolized obscurants. In contrast to the planar fractal films, where the absorption and reflection equally contribute to the extinction, the shells’ extinction is caused mainly by the absorption. The Mie scattering resonance at 560 nm of a silica core with 780 nm diameter is suppressed by 75% and only partially substituted by the absorption in the shell so that the total transmission is noticeably increased. The silica vibrational stretching band at 9 μm in absorption also disappears. Effective medium theory supports our experiments and indicates that light goes mostly through the epsilon-near-zero shell with approximately wavelength independent absorption rate.

Photomodification of Semicontinuous Silver Films with ps Pulses -- New Spectrum-Structure Optimization Technique

Semicontinuous silver films were photomodified with picosecond laser operating at 10.6µm. Slow spectral and structural changes were obtained. This technique allows the creation of filters for mid-IR wavelengths and optimization of films for sensing applications.

Cooperative bi-exponential decay of dye emission coupled via plasmons

Bi-exponential decay of dye fluorescence near the surface of plasmonic metamaterials and core-shell nanoparticles is shown to be an intrinsic property of the coupled system. Indeed, the Dicke, cooperative states involve two groups of transitions: super-radiant, from the most excited to the ground states and sub-radiant, which cannot reach the ground state. The relaxation in the sub-radiant system occurs mainly due to the interaction with the plasmon modes. Our theory shows that the relaxation leads to the population of the sub-radiant states by dephasing the super-radiant Dicke states giving rise to the bi-exponential decay in agreement with the experiments. We use a set of metamaterial samples consisting of gratings of paired silver nanostrips coated with Rh800 dye molecules, having resonances in the same spectral range. The bi-exponential decay is demonstrated for Au\SiO2\ATTO655 core-shell nanoparticles as well, which persists even when averaging over a broad range of the coupling parameter.

Optimization and photomodification of extremely broadband optical response of plasmonic core-shell obscurants.

Plasmonic resonances of the metallic shells depend on their nanostructure and geometry of the core, which can be optimized for the broadband extinction normalized by mass. The fractal nanostructures can provide a broadband extinction. It allows as well for a laser photoburning of holes in the extinction spectra and consequently windows of transparency in a controlled manner. The studied core-shell microparticles synthesized using colloidal chemistry consist of gold fractal nanostructures grown on precipitated calcium carbonate (PCC) microparticles or silica (SiO2) microspheres. The optimization includes different core sizes and shapes, and shell nanostructures. It shows that the rich surface of the PCC flakes is the best core for the fractal shells providing the highest mass normalized extinction over the extremely broad spectral range. The mass normalized extinction cross section up to 3m2/g has been demonstrated in the broad spectral range from the visible to mid-infrared. Essentially, the broadband response is a characteristic feature of each core-shell microparticle in contrast to a combination of several structures resonant at different wavelengths, for example nanorods with different aspect ratios. The photomodification at an IR wavelength makes the window of transparency at the longer wavelength side.

Metamagnetics for Visible Wavelengths (491 - 754 nm)

We designed, fabricated and experimentally validated a representative number of periodic arrays of magnetically resonant silver nanostrips. Our studies confirmed that the coupled-strip design can provide controllable magnetic responses in the entire visible range.

Fabrication of Metamagnetics for Visible Wavelengths

An improvement in the surface roughness of coupled silver nanostrips produces stronger magnetic response. We designed and experimentally demonstrated that improved coupled strips can provide controllable metamagnetism from 491 nm to 754 nm.