Daniel B. Fullager

Infrared dielectric response of nanoscribe IP-dip and IP-L monomers after polymerization from 250 cm^−1 to 6000 cm^−1

Direct laser writing via two photon polymerization has enabled previously unavailable degrees of freedom in the additive fabrication of micro-to-meso scale structures. The structures produced by these techniques are ideally suited to create optical devices which operate from the THz regime to the near infrared spectrum into the visible spectral range. Here we report on the infrared dielectric response of two monomers IP-dip and IP-L after polymerization which are frequently employed in commercial two photon lithography tools from nanoscribe over the spectral range of 250 cm−1 to 6000 cm−1. A parameterized dielectric function model is presented and discussed.

Epitaxial thin films for hyperbolic metamaterials

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).

Nanoscale electrodynamics of evanescent fields

An analysis is presented of k-space coupling of energy from an object into one or more proximal resonant scatterers. The choice of basis function provides insight into coupling mechanisms and efficiency which leads to the design of effective resonant scatterers that can direct energy and/or information associated with high-k evanescent fields away from the object. We discuss the trade-offs between the k-space and ω-space coupling as a function of the Q of the resonant scatterer. At the nanoscale, this has applications for super-Planckian heat removal as well as superresolution imaging.

Imaging Structures Near Resonance

Imaging from strongly scattering objects requires nonlinear inverse algorithms and multiple scattering can yield improved resolution. However, near resonance, weak scattering inverse methods can still prove useful. We investigate this phenomenon using strongly resonant meta-atoms.

Genetic algorithm for true negative index in plasmonic metamaterials

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.

Design theory of thin film hyperbolic metamaterial colimators

Hyperbolic metamaterial (HMM) research has led to the fabrication of devices which have unbounded k-space ellipsoids. Alternating layers of films with alternating signs of relative permittivity or permeability in a given direction enable multi-layer surfaces that are, in theory, either perfectly reflective or transmissive at an angle dependent upon the free space wave vector and ratios of the permittivity or permeability in the normal and transverse directions. By having knowledge of the electromagnetic properties of the constituent materials of a multi-layer HMM over a given bandwidth, the functionality of these structures can be altered by changing the fill fraction of the constituents. One potential device design that results is that of a flat electromagnetic wave collimator. The degree to which a multi-layer HMM collimates comes from the contrast in the magnitudes of the relative permeability or permittivity in the normal and transverse directions. With a large material parameter contrast at a given frequency, the number of transverse wave vectors that allow for successful EM wave propagation at the HMM/atmosphere interface approaches zero. This leads to propagation of a narrow angular cone of waves relative to the surface normal of the HMM. Herein we show that analytical calculations are in relatively good agreement with finite element method electromagnetic simulations performed in COMSOL’s RF module and compare dispersion relations of known materials to the resulting collimation generated in a corresponding HMM. We thereby use existing material data and predictive theories show how to tailor the frequency response of HMMs.

Fundamentals of Engineering Plasmonic Optical Materials

We consider two approaches for creating low-loss plasmonic optical materials, specifically semiconductor doping and nanopatterning. Simple Drude/Drude-Lorentz models for permittivity provide interesting insights into the relative merits of doping versus material structuring.

ENZ waveguide of Al-doped zinc oxide for telecommunication applications

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.

Design of hyperbolic metamaterials by genetic algorithm

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.

Tuning of low-index bandwidth in metamaterials

We investigate the bandwidth of optical metamaterials having low ( n<1) refractive index. From studies of displacement currents in nanostructures, a method of tuning the bandwidth and obtaining reduced loss is described.