Ashwin Gopinath

Deterministic aperiodic arrays of metal nanoparticles for surface-enhanced Raman scattering (SERS)

Deterministic Aperiodic (DA) arrays of gold (Au) nanoparticles are proposed as a novel approach for the engineering of reproducible surface enhanced Raman scattering (SERS) substrates. A set of DA and periodic arrays of cylindrical and triangular Au nanoparticles with diameters ranging between 50-110 nm and inter-particle separations between 25-100 nm were fabricated by e-beam lithography on quartz substrates. Using a molecular monolayer of pMA (p-mercaptoaniline) as a Raman reporter, we show that higher values of SERS enhancement factors can be achieved in DA structures compared to their periodic counterparts, and discuss the specific scaling rules of DA arrays with different morphologies. Electromagnetic field calculations based on the semi-analytical generalized Mie theory (GMT) fully support our findings and demonstrate the importance of morphology-dependent diffractive coupling (long-range interactions) for the engineering of the SERS response of DA arrays. Finally, we discuss optimization strategies based on the control of particles sizes and shapes, and we demonstrate that spatially-averaged SERS enhancement factors of the order of approximately 10(7) can be reproducibly obtained using DA arrays of Au nano-triangles. The ability to rigorously design lithographically fabricated DA arrays of metal nanoparticles enables the optimization and control of highly localized plasmonic fields for a variety of chip-scale devices, such as more reproducible SERS substrates, label-free bio-sensors and non-linear elements for nano-plasmonics.

Photonic-Plasmonic Scattering Resonances in Deterministic Aperiodic Structures

In this paper, we combine experimental dark-field scattering spectroscopy and accurate electrodynamics calculations to investigate the scattering properties of two-dimensional plasmonic lattices based on the concept of aperiodic order. In particular, by discussing visible light scattering from periodic, Fibonacci, Thue-Morse and Rudin-Shapiro lattices fabricated by electron-beam lithography on transparent quartz substrates, we demonstrate that deterministic aperiodic Au nanoparticle arrays give rise to broad plasmonic resonances spanning the entire visible spectrum. In addition, we show that far-field diffractive coupling is responsible for the formation of characteristic photonic-plasmonic scattering modes in aperiodic arrays of metal nanoparticles. Accurate scattering simulations based on the generalized Mie theory approach support our experimental results. The possibility of engineering complex metal nanoparticle arrays with distinctive plasmonic resonances extending across the entire visible spectrum can have a significant impact on the design and fabrication of novel nanodevices based on broadband plasmonic enhancement.

Rapid Nanoimprinting of Silk Fibroin Films for Biophotonic Applications

With soft microand nanopatterned materials becoming increasingly useful for various optical, mechanical, electronic, microfluidic, and optofluidic devices, the extension of this paradigm to a pure protein-based material substrate would provide entirely new options for such devices. Silk fibroin is an appealing biopolymer for forming such devices because of its optical properties, mechanical properties, all aqueous processing, relatively easy chemical and biological functionalization, and biocompatibility. Biologically functionalized silk fibroin films can be patterned on the micro and nanoscale using a soft lithography casting technique while maintaining the biological activity of the embedded proteins. The combination of these properties could enable a new class of active optofluidic devices that merge high-quality photonic structures whose very material constituent responds, through the embedded proteins, to analytes infused through integrated microfluidics. However, the silk fibroin casting process takes 12–36 h, hindering the ability to rapidly produce multiple devices and the resulting silk structures contain artifacts due to drying and liftoff. In this communication, we will show that silk has the properties of an ideal nanoimprint resist enabling rapid device fabrication, which in combination with its optical properties and biocompatibility make it a new technology platform that seamlessly combines nanophotonics, biopolymeric and biocompatible materials. Optofluidics, though a relatively new field, is already undergoing evolution, finding applications to an ever-increasing range of problems, including varieties of biological sensing and detection. Initially optofluidics was developed as a fusion of microfluidics and photonics to enable compact, novel optical modulation technologies. The union of optical and fluidic confining structures, however, led optofluidic devices to be applied to sensing problems especially looking toward highly parallel, sensitive and low analyte volume applications. A further development of the optofluidic paradigm, introduced here through the use of silk, is to ‘‘activate’’ the constituent material of the device to make it chemically sensitive to species flowed past it. Typically, optofluidic devices are fabricated from materials usually found in photonics or microfluidics such as silica, silicon, polydimethylsiloxane or polymethacrylmethacrylate and other polymers. These materials, while possessing suitable and well-characterized optical and material properties are not inherently chemically sensitive or specific. It is possible to functionalize the surfaces of these materials with chemical reagents, however, a much broader range of sensitivities and specificities can be achieved if proteins or enzymes are used as the sensitizing agents. The use of proteins presents an issue in itself. Binding proteins (or chemicals receptive to them) to inorganic or synthetic polymer surfaces is complex. Ideally, a material such as silk fibroin that posseses excellent optical and mechanical qualities can be formed into a variety of optofluidic geometries and maintains the activity of embedded proteins is needed for realizing active optofluidic devices. A proof of concept presented here is to build a self-sensing nanoscale imprinted optofluidic device based on imprinted silk doped with lysed red blood cells. The device can be thought of as ‘‘self-analyzing’’ in that the single optofluidic component provides both chemical and spectral analysis due to the activation of the constituent imprinted silk. Nanoimprinting is a high-throughput lithography technique in which a mold is pressed onto a thermoplastic material heated above its glass-transition temperature. The softened material conforms to the mold due to applied pressure. Sub-100 nm structures by nanoimprint lithography were first demonstrated in polymethylmethacrylate (PMMA) and now structures as small as 10 nm are routinely achieved in PMMA. An ideal nanoimprint resist combines rapid imprinting times with low temperature and pressure as well as low surface energy to aid in mold removal. As such, the mold is often coated with a low surface energy surfactant. Nanoimprinting of biopolymers presents additional challenges because of a restricted parameter space that limits the ranges of temperature and pressures usable. However, in this communication, we demonstrate that silk fibroin films exhibit many characteristics of an ideal nanoimprint resist, which in combination with its optical properties and biocompatibility make it a new technology platform that seamlessly combines

Electromagnetic coupling and plasmon localization in deterministic aperiodic arrays

In this paper we explore the potential of one-dimensional and two-dimensional deterministic aperiodic plasmonic arrays for the design of electromagnetic coupling and plasmon-enhanced, sub-wavelength optical fields on chip-scale devices. In particular, we investigate the spectral, far-field and near-field optical properties of metal nanoparticle arrays generated according to simple deterministic sequences characterized by fractal Fourier spectra. Additionally, we will consider the case of flat Fourier-transform sequences, which reproduce the behavior of purely random systems to an arbitrary degree of accuracy. Based on the coupled dipole approach (CDA) and finite difference time domain (FDTD) simulations, we study the radiative (long-range) and quasi-static (short-range) electromagnetic coupling in deterministic aperiodic plasmon arrays of metal nanoparticles. In addition, we investigate the local field enhancement and the enhancement scaling in periodic and aperiodic arrays with increasing degree of complexity. We believe that the accurate control of electromagnetic coupling and sub-wavelength field enhancement in deterministic aperiodic environments will enable novel nanodevice applications in areas such as field-enhanced nanosensors, engineered SERS substrates and optical nano-antenna arrays.

Spectral analysis of induced color change on periodically nanopatterned silk films

We demonstrate controllable structural color based on periodic nanopatterned 2D lattices in pure protein films of silk fibroin. We show here periodic lattices in silk fibroin films with feature sizes of hundreds of nanometers that exhibit different colors as a function of varying lattice spacing. Further, when varying the index of refraction contrast between the nanopatterned lattice and its surrounding environment by applying liquids on top of the lattices, colorimetric shifts are observed. The effect is characterized experimentally and theoretically and a simple example of glucose concentration sensing is presented. This is the first example of a functional sensor based on silk fibroin optics.

Quasi-periodic distribution of plasmon modes in two-dimensional Fibonacci arrays of metal nanoparticles

In this paper we investigate for the first time the near-field optical behavior of two-dimensional Fibonacci plasmonic lattices fabricated by electron-beam lithography on transparent quartz substrates. In particular, by performing near-field optical microscopy measurements and three dimensional Finite Difference Time Domain simulations we demonstrate that near-field coupling of nanoparticle dimers in Fibonacci arrays results in a quasi-periodic lattice of localized nanoparticle plasmons. The possibility to accurately predict the spatial distribution of enhanced localized plasmon modes in quasi-periodic Fibonacci arrays can have a significant impact for the design and fabrication of novel nano-plasmonics devices.

Optical gap formation and localization properties of optical modes in deterministic aperiodic photonic structures

We theoretically investigate the spectral and localization properties of two-dimensional (2D) deterministic aperiodic (DA) arrays of photonic nanopillars characterized by singular continuous (Thue-Morse sequence) and absolutely continuous (Rudin-Shapiro sequence) Fourier spectra. A rigorous and efficient numerical technique based on the 2D Generalized Multiparticle Mie Theory is used to study the formation of optical gaps and the confinement properties of eigenmodes supported by DA photonic lattices. In particular, we demonstrate the coexistence of optical modes with various degrees of localization (localized, extended and critical) and show that in-plane and out-of-plane optical energy confinement of extended critical modes can be optimally balanced. These results make aperiodic photonic structures very attractive for the engineering of novel passive and active photonic devices, such as low-threshold microlasers, sensitive detectors and bio-chemical sensors.

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.

Enhancement of the 1.54 μm Er3+ emission from quasiperiodic plasmonic arrays

Periodic and Fibonacci Au nanoparticle arrays of varying interparticle separations were fabricated on light emitting Er:SiNx films using electron beam lithography. A 3.6 times enhancement of the photoluminescence (PL) intensity accompanied by a reduction in the Er3+ emission lifetime at 1.54 μm has been observed in Fibonacci quasiperiodic arrays and explained with radiating plasmon theory. Our results are further supported by transmission measurements through the Fibonacci and periodic nanoparticle arrays with interparticle separation in the 25–500 nm range. This work demonstrates the potential of quasiperiodic nanoparticle arrays for the engineering of light emitting devices based on the silicon technology.

Near-field excitation and near-field detection of propagating surface plasmon polaritons on Au waveguide structures

The propagation of surface plasmon polaritons guided along Au metal waveguides fabricated by electron-beam lithography is experimentally investigated using simultaneous near-field excitation and detection of plasmon-polariton modes localized at the air/Au interface. The directly measured propagation characteristics of surface plasmon-polaritons agree well with simulation results obtained using full-vector calculations and the analytic dispersion of asymmetric plasmonic waveguides for thin Au films. Our results demonstrate that near-field excitation/detection schemes are well suited for direct imaging and characterization of propagating surface plasmon-fields bound to thin-film metal layers, and can be used for fast and reliable characterization of plasmonic waveguide elements and nanodevices.