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Nanometric gaps in noble metals can harness surface plasmons, collective excitations of the conduction electrons, for extreme subwavelength localization of electromagnetic energy. Positioning molecules within such metallic nanogaps dramatically enhances light-matter interactions, increasing absorption, emission, and, most notably, surface-enhanced Raman scattering (SERS). However, the lack of reproducible high-throughput fabrication techniques with nanometric control over the gap size has limited practical applications. Here we show sub-10-nm metallic nanogap arrays with precise control of the gap's size, position, shape, and orientation. The vertically oriented plasmonic nanogaps are formed between two metal structures by a sacrificial layer of ultrathin alumina grown using atomic layer deposition. We show increasing local SERS enhancements of up to 10(9) as the nanogap size decreases to 5 nm. Because these sub-10-nm gaps can be fabricated at high densities using conventional optical lithography over an entire wafer, these results will have significant implications for spectroscopy and nanophotonics.

We demonstrate three-dimensional plasmonic nanofocusing of light with patterned metallic pyramids obtained via template stripping. Gratings on the faces of these pyramids convert linearly polarized light into plasmons that propagate toward and converge at a approximately 10 nm apex. Experiments and computer simulations confirm that optical energy is focused into a nanoscale volume (5 x 10(-5) wavelength(3)). Because these structures are easily and reproducibly fabricated, our results could benefit many applications, including imaging, sensing, lithography, and nonlinear spectroscopy.

Fabricating plasmonic nanostructures with robust optical and chemical properties remains a challenging task, especially with silver, which has superior optical properties but poor environmental stability. In this work, conformal atomic layer deposition (ALD) of thin alumina overlayers is used to precisely tune the optical transmission properties of periodic nanohole arrays made in gold and silver films. Experiments and computer simulations confirm that ALD overlayers with optimized thicknesses tune and enhance the transmitted intensity due to refractive index matching effects and by modifying the dielectric properties of each nanohole. Furthermore, encapsulating silver nanohole arrays with thin alumina overlayers protects the patterned surfaces against unwanted oxidation and contamination. The ability to precisely tune the optical properties while simultaneously providing robust chemical stability can benefit a broad range of applications, including biosensing and fluorescence imaging.

Integration of solid-state biosensors and lipid bilayer membranes is important for membrane protein research and drug discovery. In these sensors, it is critical that the solid-state sensing material does not have adverse effects on the conformation or functionality of membrane-bound molecules. In this work, pore-spanning lipid membranes are formed over an array of periodic nanopores in free-standing gold films for surface plasmon resonance (SPR) kinetic binding assays. The ability to perform kinetic assays with a transmembrane protein is demonstrated with α-hemolysin (α-HL). The incorporation of α-HL into the membrane followed by specific antibody binding (anti-α-HL) red-shifts the plasmon resonance of the gold nanopore array, which is optically monitored in real time. Subsequent fluorescence imaging reveals that the antibodies primarily bind in nanopore regions, indicating that α-HL incorporation preferentially occurs into areas of pore-spanning lipid membranes.

We present a simple and massively parallel nanofabrication technique to produce self-assembled periodic nanohole arrays over a millimeter-sized area of metallic film, with a tunable hole shape, diameter, and periodicity. Using this method, 30 x 30 microm(2) defect-free areas of 300 nm diameter or smaller holes were obtained in silver; this area threshold is critical because it is larger than the visible wavelength propagation length of surface plasmon waves ( approximately 27 microm) in the silver film. Measured optical transmission spectra show highly homogeneous characteristics across the millimeter-size patterned area, and they are in good agreement with FDTD simulations. The simulations also reveal intense electric fields concentrated near the air/silver interface, which was used for surface-enhanced Raman spectroscopy (SERS). Enhancement factors (EFs) measured with different hole shape and excitation wavelengths on the self-assembled nanohole arrays were 10(4)-10(6). With an additional Ag electroless plating step, the EF was further increased up to 3 x 10(6). The periodic nanohole arrays produced using this tunable self-assembly method show great promise as inexpensive SERS substrates as well as surface plasmon resonance biosensing platforms.

Technologies based on surface plasmon resonance (SPR) have allowed rapid, label-free characterization of protein-protein and protein-small molecule interactions. SPR has become the gold standard in industrial and academic settings, in which the interaction between a pair of soluble binding partners is characterized in detail or a library of molecules is screened for binding against a single soluble protein. In spite of these successes, SPR is only beginning to be adapted to the needs of membrane-bound proteins which are difficult to study in situ but represent promising targets for drug and biomarker development. Existing technologies, such as BIAcoreTM, have been adapted for membrane protein analysis by building supported lipid layers or capturing lipid vesicles on existing chips. Newer technologies, still in development, will allow membrane proteins to be presented in native or near-native formats. These include SPR nanopore arrays, in which lipid bilayers containing membrane proteins stably span small pores that are addressable from both sides of the bilayer. Here, we discuss current SPR instrumentation and the potential for SPR nanopore arrays to enable quantitative, high-throughput screening of G protein coupled receptor ligands and applications in basic cellular biology.

We propose a method of optical data storage that exploits the small dimensions of metallic nano-particles and/or nano-structures to achieve high storage densities. The resonant behavior of these particles (both individually and in small clusters) in the presence of ultraviolet, visible, and near-infrared light may be used to retrieve pre-recorded information by far-field spectroscopic optical detection. In plasmonic data storage, a very short (approximately few femtoseconds) laser pulse is focused to a diffraction-limited spot over a small region of an optical disk containing metallic nano-structures. The digital data stored in each bit-cell, comprising multiple bits of information, modifies the spectrum of the incident light pulse. This spectrum is subsequently detected, upon reflection/transmission, with the aid of an optical spectrum analyzer. We present theoretical as well as preliminary experimental results that confirm the potential of plasmonic nano-structures for high-density optical data storage applications.

Surface plasmons are electromagnetic waves that can exist at metal interfaces because of coupling between light and free electrons. Restricted to travel along the interface, these waves can be channeled, concentrated, or otherwise manipulated by surface patterning. However, because surface roughness and other inhomogeneities have so far limited surface-plasmon propagation in real plasmonic devices, simple high-throughput methods are needed to fabricate high-quality patterned metals. We combined template stripping with precisely patterned silicon substrates to obtain ultrasmooth pure metal films with grooves, bumps, pyramids, ridges, and holes. Measured surface-plasmon-propagation lengths on the resulting surfaces approach theoretical values for perfectly flat films. With the use of our method, we demonstrated structures that exhibit Raman scattering enhancements above 10(7) for sensing applications and multilayer films for optical metamaterials.

We present nanohole arrays in a gold film integrated with a six-channel microfluidic chip for parallel measurements of molecular binding kinetics. Surface plasmon resonance effects in the nanohole arrays enable real-time, label-free measurements of molecular binding events in each channel, while adjacent negative reference channels can record measurement artifacts such as bulk solution index changes, temperature variations, or changing light absorption in the liquid. With the use of this platform, streptavidin-biotin specific binding kinetics are measured at various concentrations with negative controls. A high-density microarray of 252 biosensing pixels is also demonstrated with a packing density of 10(6) sensing elements/cm(2), which can potentially be coupled with a massively parallel array of microfluidic channels for protein microarray applications.

We present nanohole arrays in thin gold films as sub-micron resolution surface plasmon resonance (SPR) imaging pixels in a microarray format. With SPR imaging, the resolution is not limited by diffraction, but by the propagation of surface plasmon waves to adjacent sensing areas, or nanohole arrays, causing unwanted interference. For ultimate scalability, several issues need to be addressed, including: (1) as several nanohole arrays are brought close to each other, surface plasmon interference introduces large sources of error; and (2) as the size of the nanohole array is reduced, i.e. fewer holes, detection sensitivity suffers. To address these scalability issues, we surround each biosensing pixel (a 3-by-3 nanohole array) with plasmonic Bragg mirrors, blocking interference between adjacent SPR sensing pixels for high-density packing, while maintaining the sensitivity of a 50 x larger footprint pixel (a 16-by-16 nanohole array). We measure real-time, label-free streptavidin-biotin binding kinetics with a microarray of 600 sub-micron biosensing pixels at a packing density of more than 10(7) per cm(2).

We integrate periodic nanohole arrays in a thin gold film with lipid membranes in a microfluidic channel. Surface plasmon-enhanced light transmission through the periodic nanohole arrays enables real-time label-free sensing of molecular binding on the lipid membrane surface. This membrane biosensor can potentially act as a natural platform for studying binding kinetics of proteins with their binding partners anchored in the lipid membrane. We also present the concept of using nanopore arrays for kinetic assays of transmembrane proteins in a free-standing lipid membrane.

Surface plasmon resonance (SPR) imaging is a powerful technique for high-throughput, real-time, label-free characterization of molecular interactions in a microarray format. In this paper, we demonstrate SPR imaging with nanohole arrays illuminated by a laser source. Periodic nanoholes couple incident photons into SPs, obviating the need for the prism used in conventional SPR instruments, while a laser source provides the intensity, stability and spectral coherence to improve the detection sensitivity. The formation of a self-assembled monolayer of alkanethiolates on gold changed the laser transmission by more than 35%, and binding kinetics were measured in parallel from a 5 x 3 microarray of nanohole sensors. These results demonstrate the potential of nanohole sensors for high-throughput SPR imaging on microarrays.

Protein microarray technology, in which a large number of capture ligands are spatially arrayed at a high density, presents an attractive method for high-throughput proteomic analysis. Toward this end, we demonstrate the first cell-based protein detection in a microsystem, wherein Escherichia coli cells are genetically engineered to express the desired capture proteins on the membrane surface and are spatially arrayed as sensing elements in a microfluidic device. An E. coli clone expressing peptide ligands with high affinity and high specificity for target molecules was isolated a priori. Then these cells were electrokinetically immobilized on gold electrodes using dielectrophoresis, thus allowing each sensor element to be electrically addressable. Flow cytometry and subsequent fluorescence analysis verified the highly specific capture and detection of target molecules by the bacteria. Finally, through the coexpression of peptide-based capture ligands on the cell surface and fluorescent protein in the cytoplasm, we demonstrate an effective means of directly linking the fluorescence intensity to the density of capture ligands.

Free-space-based channelized dynamic spectral equalizers are theoretically investigated by solving the temporal-frequency-dependent power-coupling integral for commonly used active device technologies: liquid-crystal modulators, tilting micromirror arrays, and deformable gratings. Channel-filter characteristics, such as bandwidth and interchannel transition, are found to depend on the different attenuation mechanisms provided by the active devices. Such information is required for choosing the proper device parameters in designing channel equalizers and similar free-space spatially dispersed subsystems.