Sylvanus Y. Lee

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.

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.

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.

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.

Angularly independent structural color of nanostructured metal surfaces

We design and demonstrate angularly insensitive structural color from nanostructured metal surfaces. Plasmon-enhanced light scattering of aperiodic pinwheel gold nanoparticles on gold thin film is observed in green by dark-field scattering and variable-angle reflection spectroscopies.

Aperiodic Order in Nanoplasmonics

In this chapter, we review our work on the engineering of aperiodic order for nanoplasmonics device applications. In particular, we discuss the optical response of arrays of metallic nanoparticles with Fourier spectral features that interpolate in a tunable fashion between periodic crystals and disordered random media, referred to as Deterministic Aperiodic Nano Structures (DANS). These plasmonic structures, conceived by designing spatial frequencies in aperiodic Fourier space, give rise to characteristic scattering resonances and localized mode patterns enhancing the intensity of optical near fields over planar surfaces and broad frequency spectra. Moreover, the distinctive interplay between photonic diffraction and near field plasmonic localization in DANS provides novel opportunities to manipulate light-matter interactions on the nanoscale for device applications to optical biosensing, plasmon-enhanced light sources, solar cells, nonlinear frequency generation, and singular optics.

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

We design and demonstrate angle-insensitive structural color by engineering plasmonenhanced light scattering from homogenized Pinwheel arrays of Au nanoparticles on Au substrate for plasmonic applications, like displays, security tagging and solar cells.

Microfluidic Integration of Aperiodic Gold Nanoparticle Arrays for Local Refractive Index Sensing

We present microfluidic integration of a novel optical sensing technique based on distinctive structural color modifications in aperiodic nano-structures. The liquid-induced index sensitivity of the proposed devices is quantified by autocorrelation analysis in the visible spectral range.

Biosensing with Colorimetric Signatures of Deterministic Aperiodic Metal Nanoparticle Arrays

A novel optical sensing technique based on distinctive colorimetric signatures and spectral shifts of deterministic aperiodic arrays is demonstrated by protein monolayer sensing in the visible spectral range using inexpensive dark-field spectroscopy and autocorrelation analysis.

Engineering photonic-plasmonic aperiodic surfaces for optical biosensing

The ability to reproducibly and accurately control light matter interaction on the nanoscale is at the core of the field of optical biosensing enabled by the engineering of nanophotonic and nanoplasmonic structures. Efficient schemes for electromagnetic field localization and enhancement over precisely defined sub-wavelength spatial regions is essential to truly benefit from these emerging technologies. In particular, the engineering of deterministic media without translational invariance offers an almost unexplored potential for the manipulation of optical states with vastly tunable transport and localization properties over broadband frequency spectra. In this paper, we discuss deterministic aperiodic plasmonic and photonic nanostructures for optical biosensing applications based on fingerprinting Surface Enhanced Raman Scattering (SERS) in metal nanoparticle arrays and engineered light scattering from nanostructured dielectric surfaces with low refractive index (quartz).