The terahertz (THz) region of the electromagnetic spectrum holds promise for spectroscopic imaging of illicit and hazardous materials, and chemical fingerprinting using moment of inertia vibrational transitions. Passive and active devices operating at THz frequencies are currently a challenge, and a promising emerging technology for such devices is optical metamaterials. In particular, a chem/bio sensing scheme based on the sensitivity of metamaterials to their dielectric environment has been proposed but may be limited due to the large concentration of electric flux in the substrate. In addition, there is an interest in fabricating 3D metamaterials, which is a challenge at these and shorter wavelengths due to fabrication constraints. In order to address both of these problems, we have developed a process to fabricate THz metamaterials on free-standing, 1 micron thick silicon nitride membranes. We will present THz transmission spectra and the corresponding simulation results for these metamaterials, comparing their performance with previously fabricated metamaterials on various thick substrates. Finally, we will present a scheme for implementing a 3D THz metamaterial based on stacking and possibly liftoff of these silicon nitride membranes.

Surface plasmon waveguides show promise as sub-wavelength signal elements in integrated optoelectronic devices. Modulation of these signals requires designs that are compatible with existing semiconductor fabrication technologies, such as MOS structures. We show that waveguides of this type are not only practical, but offer distinct advantages. In particular, an asymmetric geometry is shown to support very long-range modes. In preliminary experimental results we report the observation of these modes in an visible wavelength analog structure. Our models predict that these characteristics can be maintained while shifting the operating conditions to the fiber communications band in waveguides constructed in silicon.

We show the approaches in achieving high resolution Raman and white light imaging. In Raman imaging, a dielectric microsphere is trapped by the incoming laser, which was focused onto the sample by the microsphere. The microsphere was also used to collect the scattered Raman signals. We show the capability of this method in imaging various types of samples, such as Si devices and gold nanopattern. This method is comparatively easier to perform, better repeatability, and stronger signal than the normal near-field Raman techniques. Besides the Raman imaging, we also show a far-field confocal white light reflection imaging system that can be used for the fast imaging and characterization of nanostructures. This system uses a xenon (Xe) lamp as the incident light source and tunable aperture to enhance the spatial resolution. It has a spatial resolution of around 370 nm at a wavelength of 590 nm. With our system, we can clearly resolve images of 300 nm nanoparticles arranged in 2D honeycomb arrays with a period of 500 nm. Localized surface plasmons (LSPs) of isolated single and dimer gold nanospheres were also studied and the resonance energy difference between their LSPs was extracted.

Gold inverse spherical nanoscale voids have been fabricated in linear arrays for directional plasmon measurements in the visible spectral range. We show that by KOH anisotropic etching in Si, we are able to make V-grooves in which latex spheres of the order of 500 nm self-assemble with largely defect-free cubic symmetry. Both single layer and multilayer assembly in a face-centered close-packed (FCC) lattice can be achieved by varying the width of the trenches. This template is subsequently used for electrodeposition of gold to create the inverse spherical nanovoids.

Interaction of light with plasmonic nano-antennas is investigated. First, an extensive parametric study is performed on the material and geometrical effects on dipole and bow-tie nano-antennas. The transmission efficiency is studied for various parameters including length, thickness, width, and composition of the antenna as well as the wavelength of incident light. The modeling and simulation of these structures is done using 3-D finite element method based full-wave solutions of Maxwell's equations. Next, a modeling-based automated design optimization framework is developed to optimize nano-antennas. The electromagnetic model is integrated with optimization solvers such as gradient-based optimization tools and genetic algorithms.

We propose a new approach towards all-optical active plasmonics based on ordered Au nanodisk arrays embedded in azobenzene-doped liquid crystals (LCs). Upon photoirradiation, the doped LCs went through phase transition induced by trans-cis photoisomerization of the azobenzene molecules. The phase transition led to the change in the refractive index of the LCs experienced by incident light, and enabled reversible tuning of the localized surface plasmon resonance (LSPR) of the embedded Au nanodisks. The tuning utilized the sensitivity of the LSPR of the Au nanodisks to the change in the surroundings' refractive index. Experimental observations on both peak shift and intensity change of the LSPR matched those from discrete dipole approximation (DDA) calculations.

A simple method to develop TiO2, Ag or Au-doped TiO2 thin films on cotton textiles for advanced applications, is reported. The homogeneous TiO2 thin films have been deposited on cotton textiles by using sol-gel method at low temperature (100° C), whereas Ag and Au nanoparticles were then deposited on the pre-existent TiO2 films by photoreduction. The Ag/TiO2 covered cotton fibres show multichromic behaviour (grey colour under visible light and brown colour upon ultraviolet light exposure) as well as photoactivity. The Au-TiO2 film coated the cotton textile produces a purple colour with excellent self cleaning properties. The original and treated fibres have been characterized by several techniques (SEM, HRTEM, FTIR, Raman, UV–vis spectroscopy and XRD).

Photoactive yellow protein (PYP) is a small cytosolic photoreceptor that actuates the negative phototactic response in its host organism Halorhodospira halophila. It has an optical absorption maximum at 446 nm (blue light). We report an initial study of the photocycle of PYP at the single molecule level using “high enhancement factor” surface-enhanced Raman scattering (SERS)-active nanostructures with 514 nm laser excitation. The SERS-active “nanometal-on-semiconductor” structures are prepared employing a redox technique on thin germanium films, coated on glass slides. Single molecule spectra are observed in terms of sudden appearance of discernable Raman peaks with spectral fluctuations. The single molecule spectra capture protonation, photo-isomerization, and H-bond breaking - the steps that are instrumental in the photocycle of PYP. This is indicative of single PYP molecules diffusing to high-enhancement-factor SERS sites, and undergoing photo-cycle under 514 nm excitation.

Surface plasmon propagation on gold over 0.1 – 1.6 micrometer distances for plasmon energies in the range 1.6 – 3.5 eV was characterized. Surface plasmons were excited by an electron beam near a grating milled in the gold. The spectra of out-coupled radiation reveal increasingly strong propagation losses as surface plasmon energy increases above 2.8 eV, but little effect in the range 1.6 – 2.8 eV. These results are in partial agreement with theoretical expectations.