Thesis (Ph.D.)--University of Nebraska-Lincoln, 2008. / Title from title screen (site viewed Mar. 10, 2009). PDF text: xv, 182 p. : ill. (some col.) ; 6 Mb. UMI publication number: AAT 3331444. Includes bibliographical references. Also available in microfilm and microfiche formats.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

It has been shown that the energy exchange between two objects can be greatly enhanced when the separation between the objects is on the order of the wavelength of thermal emission. The earliest theoretical and computational work focused on simple planar and spherical geometries, or they resorted to approximations that separated the object to outside of the thermal wavelength \(\lambda_T = hc/(k_BT)\). Since those original works, the study of near-field energy exchange has expanded to object shapes that can be described by a separable coordinate system using a spectral expansion of the dyadic Green function of the system. The boundary element method has also been used to study arbitrary shapes in thermal equilibrium. Application of these new expansion methods to general shapes out of thermal equilibrium will facilitate in the optimization of nanoscale structures. A three step process is used to investigate the effects of object shape on the total and directionality of the energy exchange between objects. First, a general expression for the energy flux between the objects will be formulated. Second, a computational method to evaluate the expression will be implemented. Finally, the effects of varying the surface geometry will be explored. The computational results demonstrate that the total energy exchange between two bodies is influenced by the surface shape of the objects even when the surface areas are held constant. While the primary increase over the classical blackbody energy exchange \(\sigma T^4 A\) is primarily governed by separation of the surfaces, we show that the view factors from classical far-field radiative transfer can be used to predict the change in the total energy exchange from a reference configuration at the same separation when the surface area of the two objects is comparable. Additionally, we demonstrate that the spatial distribution of the energy exchange can be localized into small spatial region with a peak value increased over \SI{30}{\percent} by using two objects with dramatically different projected areas.

All substances above zero kelvin temperature emit fluctuating electromagnetic waves due to the random motions of charge carriers. Controlling the spectral and directional radiative properties of surfaces has wide applications in energy harvesting and thermal management. Artificial metamaterials have attracted much attention in the last decade due to their unprecedented optical and thermal properties beyond those existing in nature. This dissertation aims at tailoring radiative properties at infrared regime and enhancing the near-field radiative heat transfer by employing metamaterials. A comprehensive study is performed to investigate the extraordinary transmission, negative refraction, and tunable perfect absorption of infrared light. A polarizer is designed with an extremely high extinction ratio based on the extraordinary transmission through perforated metallic films. The extraordinary transmission of metallic gratings can be enhanced and tuned if a single layer of graphene is covered on top. Metallic metamaterials are not the unique candidate supporting exotic optical properties. Thin films of doped silicon nanowires can support negative refraction of infrared light due to the presence of hyperbolic dispersion. Long doped-silicon nanowires are found to exhibit broadband tunable perfect absorption. Besides the unique far-field properties, near-field radiative heat transfer can be mediated by metamaterials. Bringing objects with different temperatures close can enhance the radiative heat flux by orders of magnitude beyond the limit set by the Stefan-Boltzmann law. Metamaterials provide ways to make the energy transport more efficient. Very high radiative heat fluxes are shown based on carbon nanotubes, nanowires, and nanoholes using effective medium theory (EMT). The quantitative application condition of EMT is presented for metallodielectric metamaterials. Exact formulations including the scattering theory and Green’s function method are employed to investigate one- and two-dimensional gratings as well as metasurfaces when the period is not sufficiently small. New routes for enhancing near-field radiative energy transport are opened based on proposed hybridization of graphene plasmons with hyperbolic modes, hybridization of graphene plasmons with surface phonon modes, or hyperbolic graphene plasmons with open surface plasmon dispersion relation. Noncontact solid-state refrigeration is theoretically demonstrated to be feasible based on near-field thermal radiation. In addition, the investigation of near-field momentum exchange (Casimir force) between metamaterials is also conducted. Simultaneous enhancement of the near-field energy transport and suppress of the momentum exchange is theoretically achieved. A design based on repulsive Casimir force is proposed to achieve tunable stable levitation. The dissertation helps to understand the fundamental radiative energy transport and momentum exchange of metamaterials, and has significant impacts on practical applications such as design of nanoscale thermal and optical devices, local thermal management, thermal imaging beyond the diffraction limit, and thermophotovoltaic energy harvesting.

In the year 2000 John Pendry described a new kind of lens that could focus both the propagating and evanescent components of light. This ‘super’ lens, which took the form of a thin slab of silver with a negative effective index of refraction under certain conditions, had the ability to reproduce images much smaller than the wavelength of light, seemingly in violation of the diffraction limit that governed the performance of conventional optics. Despite significant controversy regarding the purported operation of such superlenses, the first experimental samples were fabricated in 2005, with features as small as 63 nm successfully imaged with 365 nm light. These results put to rest disbelief in the feasibility of superlenses and ushered in an era of intense interest in near-field phenomena and negative index materials (NIMs). Despite sustained effort, progress on the practical implementation of superlenses was slow, with a further five years passing before improved experimental results were published. In the meantime, a proliferation of analytical and modelling studies appeared on the behaviour and properties of superlenses, as well as numerous suggestions for improved physical designs, very few of which had accompanying experimental evidence. The primary aim of this thesis arose from these many proposals, namely, to reconcile predictions made about the behaviour of superlenses with observed experimental results. The measurement of the theoretical and practical behaviour of superlenses is addressed in this thesis by the development of a set of characterisation metrics that can be used to describe the imaging performance of a number of near-field imaging systems. These metrics are initially calculated via transfer matrix modelling (TMM), which is a one-dimensional analytical technique traditionally used to find the transmission and reflection coefficients of planar structures. Two families of metrics are derived; one that describes imaging systems in terms of their abilities in generic situations and the other that gives the suitability of an imaging system for application to a given class of object. Transfer functions, bandwidth and peak wavenumber measurements form this first group of characterisation functions, while contrast, pseudo-contrast and correlation coefficients are used to assess the quality of imaging systems when exposed to well-defined input profiles. Both sets of metrics show that the performance of superlenses is highly application-specific, with the fidelity or otherwise of a generated image dependent more on the construction of the superlens than on the maximum spatial frequencies present in the object. The results from the characterisation metrics are also used to guide the design of hypothetical superlens structures; these suggest that sub-diffraction limited resolution may still be available with almost a full wavelength separation between object and image. The quantitative accuracy of the TMM method is assessed by comparison to full-field vector simulations performed via finite element modelling (FEM), these reveal systematic inadequacies in the application of the TMM technique to superlensing applications. These inadequacies stem from near-field mask-lens interactions that are present in superlens experiments but are not accounted for in TMM calculations. A new technique, based on a modified transfer matrix model (M-TMM), is proposed that accounts for the effects between masks and superlenses by approximating masks as solid slabs of known thickness. Results generated via M-TMM are shown to be in better agreement with FEM models than similar TMM data, even when the duty cycle of the actual mask becomes significant and the approximation in M-TMM is at its most coarse. Finally, experiments are designed and executed that directly measure the transfer functions of superlenses and other near-field imaging techniques. The problem of intimate contact between optics components, which normally hinders any such attempts to perform lithography in the near-field, is mitigated by including a flexible layer of poly (dimethylsiloxane) (PDMS) between various components in the mask:lens:resist stack. Furthermore, high spatial frequency data corresponding to low nanometre-scale features are retrieved from masks with periodic, micron-scale patterns, greatly easing the requirements on mask construction for these experiments. The end results show good agreement with FEM and M-TMM data and satisfy the aim of this thesis, which was to bridge the divide between the performance expected and experienced from silver superlenses.

No description available.

In recent times, near-field optical microscopy has received increasing attention for its ability to obtain high-resolution images beyond the diffraction limit. Near-field optical microscopy is achieved via the positioning and manipulation of a probe on a scale less than the wavelength of the incident light. Despite many variations in the mechanical design of near-field optical microscopes almost all rely on direct mechanical access of a cantilever or a derivative form to probe the sample. This constricts the study to surface examinations in simple sample environments. Distance regulation between the sample surface and the delicate probe requires its own feedback mechanism. Determination of feedback is achieved through monitoring the shift of resonance of one arm of a 'tuning fork', which is caused by the interaction of the probes tip with the Van der Waals force. Van der Waals force emanates from atom-atom interaction at the top of the sample surface. Environmental contamination of the sample surface with additional molecules such as water makes accurate measurement of these forces particularly challenging. The near-field study of living biological material is extremely difficult as an aqueous environment is required for its extended survival. Probe-sample interactions within an aqueous environment that result in strong detectable signal is a challenging problem that receives considerable attention and is a focus of this thesis. In order to increase the detectible signal a localised field enhancement in the probing region is required. The excitation of an optically resonant probe by morphology dependent resonance (MDR) provides a strong localised field enhancement. Efficient MDR excitation requires important coupling conditions be met, of which the localisation of the incident excitation is a critical factor. Evanescent coupling by frustrated total internal reflection to a MDR microcavity provides an ideal method for localised excitation. However it has severe drawbacks if the probe is to be manipulated in a scanning process. Tightly focusing the incident illumination by a high numerical aperture objective lens provides the degree of freedom to enable both MDR excitation and remote manipulation. Two-photon nonlinear excitation is shown to couple efficiently to MDR modes due to the high spatial localisation of the incident excitation in three-dimensions. The dependence of incident excitation localisation by high numerical aperture objective on MDR efficiency is thoroughly examined in this thesis. The excitation of MDR can be enhanced by up to 10 times with the localisation of the incident illumination from the centre of the microcavity to its perimeter. Illuminating through a high numerical aperture objective enables the remote noninvasive manipulation of a microcavity probe by laser trapping. The transfer of photon momentum from the reflection and refraction of the trapping beam is sufficient enough to exert piconewtons of force on a trapped particle. This allows the particle to be held and scanned in a predictable fashion in all three-dimensions. Optical trapping removes the need for invasive mechanical access to the sample surface and provides a means of remote distance regulation between the trapped probe and the sample. The femtosecond pulsed beam utilised in this thesis allows the simultaneous induction of two-photon excitation and laser trapping. It is found in this thesis that a MDR microcavity can be excited and translated in an efficient manner. The application of this technique to laser trapped near-field microscopy and single molecule detection is of particular interest. Monitoring the response of the MDR signal as it is scanned over a sample object enables a near-field image to be built up. As the enhanced evanescent field from the propagation of MDR modes around a microcavity interacts with different parts of the sample, a measurable difference in energy leakage from the cavity modes occurs. The definitive spectral properties of MDR enables a multidimensional approach to imaging and sensing, a focus of this thesis. Examining the spectral modality of the MDR signal can lead to a contrast enhancement in laser trapped imaging. Observing a single MDR mode during the scanning process can increase the image contrast by up to 1:23 times compared to that of the integrated MDR fluorescence spectrum. The work presented in this thesis leads to the possibility of two-photon fluorescence excitation of MDR in combination with laser trapping becoming a valuable tool in near- field imaging, sensing and single molecule detection in vivo. It has been demonstrated that particle scanned, two-photon fluorescence excitation of MDR, by laser trapping 'tweezers' can provide a contrast enhancement and multiple imaging modalities. The spectral imaging modality has particular benefits for image contrast enhancements.

This thesis will focus on spherical near-field antenna measurements and the methods developed or modified for the work of this thesis to estimate the uncertainty in a particular far-field radiation pattern. We will discuss the need for error assessment in spherical near-field antenna measurements. A procedure will be proposed that, in an automated fashion, can be used to determine the overall uncertainty in the measured far-field radiation pattern of a particular antenna. This overall uncertainty will be the result of a combination of several known sources of error common to SNF measurements. This procedure will consist of several standard SNF measurements, some newly developed tests, and several stages of post-processing of the measured data. The automated procedure will be tested on four antennas of various operating frequencies and directivities to verify its functionality. Finally, total uncertainty data will be presented to the reader in several formats.

This thesis will focus on spherical near-field antenna measurements and the methods developed or modified for the work of this thesis to estimate the uncertainty in a particular far-field radiation pattern. We will discuss the need for error assessment in spherical near-field antenna measurements. A procedure will be proposed that, in an automated fashion, can be used to determine the overall uncertainty in the measured far-field radiation pattern of a particular antenna. This overall uncertainty will be the result of a combination of several known sources of error common to SNF measurements. This procedure will consist of several standard SNF measurements, some newly developed tests, and several stages of post-processing of the measured data. The automated procedure will be tested on four antennas of various operating frequencies and directivities to verify its functionality. Finally, total uncertainty data will be presented to the reader in several formats.

Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references. Available also from UMI Company.