Microfluidics integration of aperiodic plasmonic arrays for spatial-spectral optical detection.

We demonstrate successful integration of aperiodic arrays of metal nanoparticles with microfluidics technology for optical sensing using the spectral-colorimetric responses of nanostructured arrays to refractive index variations. Different aperiodic arrays of gold (Au) nanoparticles with varying interparticle separations and Fourier spectral properties are fabricated using Electron Beam Lithography (EBL) and integrated with polydimethylsiloxane (PDMS) microfluidics structures by soft-lithographic micro-imprint techniques. The spectral shifts of scattering spectra and the distinctive modifications of structural color patterns induced by refractive index variations were simultaneously measured inside microfluidic flow cells by dark-field spectroscopy and image correlation analysis in the visible spectral range. The integration of engineered aperiodic arrays of Au nanoparticles with microfluidics devices provides a novel sensing platform with multiplexed spatial-spectral responses for opto-fluidics applications and lab-on-a-chip optical biosensing.

Genetically engineered plasmonic nanoarrays.

In the present Letter, we demonstrate how the design of metallic nanoparticle arrays with large electric field enhancement can be performed using the basic paradigm of engineering, namely the optimization of a well-defined objective function. Such optimization is carried out by coupling a genetic algorithm with the analytical multiparticle Mie theory. General design criteria for best enhancement of electric fields are obtained, unveiling the fundamental interplay between the near-field plasmonic and radiative photonic coupling. Our optimization approach is experimentally validated by surface-enhanced Raman scattering measurements, which demonstrate how genetically optimized arrays, fabricated using electron beam lithography, lead to order of ten improvement of Raman enhancement over nanoparticle dimer antennas, and order of one hundred improvement over optimal nanoparticle gratings. A rigorous design of nanoparticle arrays with optimal field enhancement is essential to the engineering of numerous nanoscale optical devices such as plasmon-enhanced biosensors, photodetectors, light sources and more efficient nonlinear optical elements for on chip integration.

Plasmon-enhanced structural coloration of metal films with isotropic Pinwheel nanoparticle arrays.

We experimentally demonstrate angle-insensitive (i.e., isotropic) coloration of nanostructured metal surfaces by engineered light scattering from homogenized Pinwheel aperiodic arrays of gold nanoparticles deposited on gold substrates. In sharp contrast to the colorimetric responses of periodically nanopatterned surfaces, which strongly depend on the observation angle, Pinwheel nanoparticle arrays give rise to intense and isotropic structural coloration enhanced by plasmonic resonance. Pinwheel nanoparticle arrays with isotropic Fourier space were fabricated on a gold thin film and investigated using dark-field scattering and angle-resolved reflectivity measurements. Isotropic green coloration of metal films was demonstrated on Pinwheel patterns, with greatly reduced angular sensitivity and enhanced spatial uniformity of coloration compared to both periodic and random arrays. These findings, which are supported by coupled-dipole numerical simulations of differential scattering cross sections and radiation diagrams, could advance plasmonic applications to display, optical tagging and colorimetric sensing technologies.

Formation of colorimetric fingerprints on nano-patterned deterministic aperiodic surfaces.

Periodic gratings and photonic bandgap structures have been studied for decades in optical technologies. The translational invariance of periodic gratings gives rise to well-known angular and frequency filtering of the incident radiation resulting in well-defined scattered colors in response to broadband illumination. Here, we demonstrate the formation of highly complex structural color patterns, or colorimetric fingerprints, in two-dimensional (2D) deterministic aperiodic gratings using dark field scattering microscopy. The origin of colorimetric fingerprints is explained by rigorous full-wave numerical simulations based on the generalized Mie theory. We show that unlike periodic gratings, aperiodic nanopatterned surfaces feature a broadband frequency response with wide angular intensity distributions governed by the distinctive Fourier properties of the aperiodic structures. Finally, we will discuss a range of potential applications of colorimetric fingerprints for optical sensing and spectroscopy.

Spatial and spectral detection of protein monolayers with deterministic aperiodic arrays of metal nanoparticles.

Light scattering phenomena in periodic systems have been investigated for decades in optics and photonics. Their classical description relies on Bragg scattering, which gives rise to constructive interference at specific wavelengths along well defined propagation directions, depending on illumination conditions, structural periodicity, and the refractive index of the surrounding medium. In this paper, by engineering multifrequency colorimetric responses in deterministic aperiodic arrays of nanoparticles, we demonstrate significantly enhanced sensitivity to the presence of a single protein monolayer. These structures, which can be readily fabricated by conventional Electron Beam Lithography, sustain highly complex structural resonances that enable a unique optical sensing approach beyond the traditional Bragg scattering with periodic structures. By combining conventional dark-field scattering micro-spectroscopy and simple image correlation analysis, we experimentally demonstrate that deterministic aperiodic surfaces with engineered structural color are capable of detecting, in the visible spectral range, protein layers with thickness of a few tens of Angstroms.