Experimental and theoretical investigation of waveguided plasmonic surface lattice resonances.

Plasmonic nanostructures are good candidates for refractive index sensing applications through the surface plasmon resonance due to their strong dependence on the surrounding dielectric media. However, typically low quality-factor limits their application in sensing devices. To improve the quality-factor, we have experimentally and theoretically investigated two-dimensional gold nanoparticle gratings situated on top of a waveguide. The coupling between the localized surface plasmon and waveguide modes results in Fano-type resonances, with high quality-factors, very similar to plasmonic surface lattice resonances. By combining plasmonic surface lattice resonance and waveguide theory, we present a theoretical framework describing the structures. By immersing the fabricated samples in three different media we find a sensitivity of ∼50 nm/RIU and figure of merit of 8.9, and demonstrate good agreement with the theory presented. Further analysis show that the sensitivity is very dependent on the waveguide parameters, grating constant and the dielectric environment, and by tuning these parameters we obtain a theoretical sensitivity of 887 nm/RIU.

Rotationally tunable varifocal 3D metalens.

Varifocal optics have a variety of applications in imaging systems. Metasurfaces offer control of the phase, transmission, and polarization of light using subwavelength engineered structures. However, conventional metasurface designs lack dynamic wavefront shaping which limits their application. In this work, we design and fabricate 3D doublet metalenses with a tunable focal length. The phase control of light is obtained through the mutual rotation of the singlet structures. Inspired by Moiré lenses, the proposed structure consists of two all-dielectric metasurfaces. The singlets have reverse-phase profiles resulting in the cancellation of the phase shift in the nominal position. In this design, we show that the mutual rotation of the elements produces different wavefronts with quadratic radial dependence. Thus, an input plane wave is converted to spherical wavefronts whose focal length depends on the rotation. We use a combination of a nanopillar and a phase plate as the unit cell structure working at a wavelength of 1500 nm. Our design holds promise for a range of applications such as zoom lenses, microscopy, and augmented reality.

Hybrid metasurfaces for simultaneous focusing and filtering.

This work presents the design and fabrication of polymeric, structural optical filters that simultaneously focus light. These filters represent a novel, to the best of our knowledge, design at the boundary between diffractive optics and metasurfaces that may provide significant advantages for both digital and hyperspectral imaging. Filters for visible and near-infrared wavelengths were designed using finite-difference time-domain (FDTD) simulations. Prototype filters were fabricated using two-photon lithography, a form of nanoscale 3D printing, and have geometries suitable to replication by molding. The experimentally measured spectral transmission and focused spot size of each filter show excellent agreement with simulation.

Effect of silica-core gold-shell nanoparticles on the kinetics of biohydrogen production and pollutant hydrogenation via organic acid photofermentation over enhanced near-infrared illumination.

A biological photoinduced fermentation process provides an alternative to traditional hydrogen productions. In this study, biohydrogen production was investigated at near IR region coupled to a near-field enhancement by silica-core gold-shell nanoparticles (NPs) over a range of acetate concentrations (5-40 mM) and light intensities (11-160 W/m2). The kinetic data were modeled using modified Monod equations containing light intensity effects. The yields of H2 and CO2 produced per acetate were determined as 2.31 mol-H2/mol-Ac and 0.83 mol-CO2/mol-Ac and increased to 4.38 mmol-H2/mmol-Ma and 2.62 mmol-CO2/mmol-Ma when malate was used. Maximum increases in H2 and CO2 productions by 115% and 113% were observed by adding NPs without affecting the bacterial growth rates (6.1-8.2 mg-DCM/L/hour) while the highest hydrogen production rate was determined as 0.81 mmol/L/hour. Model simulations showed that the energy conversion efficiency increased with NPs concentration but decreased with the intensity. Complete hydrogenation application was demonstrated with toxic 2-chlorobiphenyl using Pd catalysts.

Altering the radiation chemistry of electron-beam lithography with a reactive gas: a study of Teflon AF patterning under water vapor.

Variable-pressure electron-beam lithography (VP-EBL) employs an ambient gas at subatmospheric pressure to reduce charging of insulating films and substrates during electron exposure. In this work, VP-EBL proves to be an efficient method for patterning a widely used, but challenging to process, fluoropolymer, Teflon AF. However, rather than solely mitigating charging, the ambient gas is found to alter the radiation chemistry of the exposure process. Specifically, irradiating Teflon AF under water vapor increases the dissolution rate of the exposed regions in non-fluorinated solvents and enables complete patterning in a positive tone process. When compared to conventional e-beam resists, the contrast (≈4), clearing dose (<700 µC cm-2), and resolution (≈40 nm half-pitch) of Teflon AF are adequate. However, these figures of merit are quite remarkable when the process is considered as a means for directly patterning a functional material with extremely low surface energy, dielectric constant, and refractive index. Intriguingly, VP-EBL of Teflon AF under water vapor also exhibits non-reciprocity, through dose-rate dependence, and exhibits anomalous proximity effects. Thus, the influence of the ambient gas on radiation chemistry must be considered for VP-EBL, and some of the resulting effects may offer significant benefits for patterning both functional and lithographic materials.

Rhodopseudomonas palustris-based conversion of organic acids to hydrogen using plasmonic nanoparticles and near-infrared light.

The simultaneous elimination of organic waste and the production of clean fuels will have an immense impact on both the society and the industrial manufacturing sector. The enhanced understanding of the interface between nanoparticles and photo-responsive bacteria will further advance the knowledge of their interactions with biological systems. Although literature shows the production of gases by photobacteria, herein, we demonstrated the integration of photonics, biology, and nanostructured plasmonic materials for hydrogen production with a lower greenhouse CO2 gas content at quantified light energy intensity and wavelength. Phototrophic purple non-sulfur bacteria were able to generate hydrogen as a byproduct of nitrogen fixation using the energy absorbed from visible and near-IR (NIR) light. This type of biological hydrogen production has suffered from low efficiency of converting light energy into hydrogen in part due to light sources that do not exploit the organisms' capacity for NIR absorption. We used NIR light sources and optically resonant gold-silica core-shell nanoparticles to increase the light utilization of the bacteria to convert waste organic acids such as acetic and maleic acids to hydrogen. The batch growth studies for the small cultures (40 mL) of Rhodopseudomonas palustris demonstrated >2.5-fold increase in hydrogen production when grown under an NIR source (167 ± 18 µmol H2) compared to that for a broad-band light source (60 ± 6 µmol H2) at equal light intensity (130 W m-2). The addition of the mPEG-coated optically resonant gold-silica core-shell nanoparticles in the solution further improved the hydrogen production from 167 ± 18 to 398 ± 108 µmol H2 at 130 W m-2. The average hydrogen production rate with the nanoparticles was 127 ± 35 µmol L-1 h-1 at 130 W m-2.