We have designed a light-guide for lighting applications, including automotive headlamps. The light-guide is designed based on a free-form slab. Our intention has been to optimize the design for creating legal patterns. One of our objectives has been to investigate how the changes in the shape of the freeform light-guide affects the light pattern, and how the light source geometry affects the design. For prototyping purpose, we use a particular polymer that exhibits optical performance close to acrylic (PMMA).
A one-dimensional, single-material polarizing photonic bandgap structure is designed and fabricated using e-beam PVD and oblique angle deposition technique. In order to obtain high- and low-index layers, we deposited alternate layers of titanium dioxide (TiO<sub>2</sub>) at deposition angles of 0° and 70°on top of a fused silica substrate. This approach is chosen since at deposition angle of zero degree, deposited TiO<sub>2</sub> using e-beam PVD, show a negligible birefringence while the obliquely deposited TiO<sub>2 </sub>acts as a biaxial material with significant birefringent behavior. As a result, deposition of a bilayer film at two angles is analogous to using two different materials with the advantage of simplifying fabrication and modeling this polarizing device. The bandgap of the bilayer structure is modeled in a way that only a specific wavelength with certain polarization, <i>p </i>polarization, could pass through while the <i>s </i>polarization is reflected. For modeling we used Transfer Matrix Method and numerical FDTD analysis to simulate behavior of the 1D photonic band gap structure. The simulations produce better than 98% reflection for <i>s</i> polarization and almost no reflection for <i>p</i> polarization for the center wavelength of 632.8 nm. The fabricated device shows 94% reflection for s polarization and less than 6% reflection for <i>p</i> polarization at the red HeNe laser wavelength at an incident angle of 70°. The results demonstrate that a 1D multi-layer photonic crystal, fabricated from a single material, can be designed to selectively reflect or transmit<i> p</i> or<i> s</i> polarization of an incident light beam.
We investigate negative index of refraction in plasmonic metamaterials with an emphasis on distinguishing and isolating contributions to negative refraction from spatial dispersion, as a function of metamaterial dimensions on the scale of the wavelength. We explain the design approach using genetic algorithm and provide sample applications including negative refraction.
We explain the design of one dimensional Hyperbolic Metamaterials (HMM) using a genetic algorithm (GA) and provide sample applications including the realization of negative refraction. The design method is a powerful optimization approach to find the optimal performance of such structures, which “naturally” finds HMM structures that are globally optimized for specific applications. We explain how a fitness function can be incorporated into the GA for different metamaterial properties.
We investigate a novel light conversion scheme in nanostructures for the highly demanding field of plasmonic solar cells. In our study, we incorporate vertical nanorods made of semiconductor materials, which are coupled optically to plasmonic nanoantennas for optimal absorption of sunlight. Utilizing the unique properties of localized surface plasmon resonances, we create dedicated nanoantenna elements such that the emission pattern is effectively directed toward the absorber material. In our approach, we use a computational finite element method to investigate the effects of size and shape of metallic nanoparticles to obtain an asymmetric radiation pattern that matches the geometry of our design.
We investigate the incorporation of an epsilon-near-zero (ENZ) material into a waveguide structure in order to suppress dispersion associated with the interaction of light with material in the core, guiding layer. ENZ metamaterials can provide a mechanism for air-core waveguides by introduction of a cladding medium exhibiting a refractive index less than unity. We study the application of aluminum zinc oxide (AZO), a transparent conducting oxide, as the candidate for ENZ waveguides. For this purpose, we design a metamaterial cladding layer with ENZ properties derived from nanoparticles of AZO, and investigate the resulting loss and dispersion of guided optical signals.
Recent progress in the area of hyperbolic metamaterials (HMMs) has sparked interest in transparent conducting oxides (TCOs) that behave as plasmonic media in the near-IR and at optical frequencies for imaging and sensing applications. It has been shown that by depositing alternating layers of negative-epsilon/positive-epsilon materials, a medium can be created with unusual index values such as near zero. HMMs support high-k waves corresponding to a diverging photonic density of states (PDOS), the quantity determining phenomena such as spontaneous and thermal emission. Also, modeling such structures allows evanescent fields containing sub-wavelength information to be coupled to propagating radiation. We investigate the optical, electronic, and physical properties of radio frequency plasma-assisted molecular beam epitaxial (RF-MBE) growth of alternating layers of ZnO and TCO of uniform thickness for HMM applications. Preliminary work creating HMMs with ZnO and Al-doped ZnO (AZO) has shown a negative real part of the permittivity at near-IR whose modulus is proportional to the number density of Al dopant. However, increasing the Al content of the AZO increases the transmission losses to unacceptable levels for device applications at industry standard wavelengths. A TCO with conductivity and physical structure superior to that of AZO is gallium-doped ZnO (GZO). Uniformly grown GZO has been demonstrated to possess improved crystal quality over AZO due to the higher diffusivity of Al in the ZnO. AZO and GZO HMM structures grown by RF-MBE are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Hall effect, four-point probing, deeplevel transient spectroscopy (DLTS), ellipsometry, visible and ultraviolet spectroscopy (UV-VIS) and in-situ reflection high energy electron diffraction (RHEED).
We investigated the optical characteristics and polarization insensitivity of an epsilon-near-zero metamaterial structure comprising aluminum-doped zinc oxide nanoparticles (NPs) hosted by a medium of ligands. By the use of an equivalent circuit model for the pairs of NPs, or dimers, and also of fullwave simulations, we studied the response of this self-assembled metamaterial for near-infrared applications. Considering the coupling of localized surface plasmons, we demonstrated the dominance of a certain dimer configuration and then applied this result to the whole medium as a simplifying approximation for a random structure. The consequent results showed a polarization insensitivity and also a general redshift in the plasmon resonance of the structure.
Transparent conducting oxides (TCO) are an interesting class of plasmonic materials, which are under intensive
study for their use in low-loss metamaterials and a range of applications such as sensing, imaging and transformation
optics. Here, using both full-wave simulations and an equivalent circuit model for pairs of nanoparticles
of aluminum doped zinc oxide (AZO), we study the plasmonic effects for low loss low index metamaterials for
infrared applications. The behavior of localized surface plasmon resonances (LSPR) of AZO nanoparticle dimers
embedded in a host polymer medium is investigated for different dimer orientations with respect to the indicent
electromagnetic wave. In doing this, the role of dressed polarizability to enhance and quench the plasmonic
effects is also considered. The effects of the nanoparticles relative size and the spacing between them are studied.
Understanding these resonances and their dependence on dimer orientations, provides a means to design metamaterial
structures for use in the near infrared (NIR) region with epsilon-near-zero properties leading also to
low index metamaterials. In our studies, we demonstrate how nanospheres with radii less than 100 nm that are
distributed with an average spacing less than their diameter, can result in an effective medium with refractive
index less than one. We utilize a full-wave frequency domain finite element method in conjunction with an
equivalent-circuit model for the nanoscale dimers in order to describe the spectral response of the bulk low index
properties. We also present a statistical analysis to obtain the effective refractive index for incident light having