arXiv subjects

Kenneth W. Allen

Publications and source records attributed to Kenneth W. Allen.

13 recordsLinked to original sources

arXiv2024

Equivalent Circuit Modeling and Design of a Reconfigurable Loaded Dogbone Metasurface Element

The accelerating trend of active metasurfaces -- such as those incorporating non-Foster matching, programmable control, or space-time modulation -- adds complexity to the computational electromagnetic (CEM) simulation landscape. In this work, we present an equivalent circuit model (ECM) for a particular periodic array element -- the dogbone element -- that incorporates an arbitrary sub-cell electronic circuit. The ECM is capable of predicting the overall surface impedance with any changes in the circuit's impedance. Three full-wave CEM simulations fully determine the EC, and the scattering parameters computed from the ECM for an embedded circuit, however complex, should be substantially similar to those produced by the full-wave code. With metasurfaces for which it is possible to obtain an ECM by this method, the reflection or transmission behavior of the surface under varying circuit conditions can be computed quickly without need of any further full-wave solutions. After highlighting the importance of fully characterizing parasitics in embedded components, we apply the method to the design of a varactor-based tunable metasurface, which upon fabrication and focused beam measurement demonstrates excellent model-measure agreement.

physics.app-ph↗

arXiv2024

Multimodal Effective Permittivity Model for Metasurfaces Embedded in Layered Media

The dielectric layers surrounding a metasurface have a large impact on its frequency and angular response. The notion of effective permittivity captures this dependence by suggesting that a layered dielectric environment will perturb metasurface resonances in the same manner as an infinite environment of that effective permittivity. A model for effective permittivity is presented in this work. In contrast to previous empirically derived models, this multi-term model is a robust approximation based on the modal expansion of fields at the metasurface. A simple dipole metasurface array is simulated to demonstrate the accuracy of the model, which is shown to be improved over other models. Finally, we show that it is possible for dielectric layers to produce an effective permeability different than unity, which must be considered in conjunction with higher-order equivalent circuit modeling to effectively predict metasurface impedance variation.

physics.app-ph↗

arXiv2023

Focusing and Diffraction of Light by Periodic Si Micropyramidal Arrays

This research was devoted to modeling of the optical properties of Si micropyramids aimed at designing optimal structures for applications as light concentrators in mid-wave infrared (MWIR) focal place arrays (FPAs). It is shown that completely different optical properties of such structures can be realized using two types of boundary conditions (BCs): i) periodical and ii) perfectly matched layer. The first type (periodical BC) allowed us to describe the Talbot effect under plane wave coherent illumination conditions. This effect was experimentally demonstrated in the proposed structures. The second type (perfectly matched layer BC) allows describing the optical properties of individual micropyramids concentrating or focusing light on the photodetector. The optimal geometries of micropyramids required for maximizing the intensity of photonic nanojets emerging from their truncated tips are determined.

physics.optics↗

arXiv2023

Photodetector Focal Plane Arrays Integrated with Silicon Micropyramidal Structures in MWIR

Light-concentrating truncated Si micropyramidal arrays with 54.7 degree sidewall angles were successfully integrated with PtSi Schottky barrier photodetectors. Four different devices consisting of 10 x 10 photodetectors with 60 um pitch combined in parallel were tested, where significant enhancement capability was demonstrated by the Si micropyramids. The device consisting of one hundred 22 um square detectors monolithically integrated with the light-concentrating micropyramidal array displayed signal enhancement of up to 4 times compared to the same size 22 um square photodetector device without the light concentrators.

physics.optics↗

arXiv2019

Reconfigurable Electromagnetic Structures: X-Band and Beyond

Developing reconfigurable millimeter-wave (mmWave) antennas and devices is an outstanding challenge, with switch technologies being a primary impediment. Recently, it has been shown that vanadium dioxide (VO2), a thermochromic material whose resistance changes with temperature, could provide a path forward in developing reconfigurable mmWave devices. As an initial step towards this vision, we investigate the integration of VO2 switches in reconfigurable components at 15 GHz. In particular, a frequency reconfigurable antenna and a reconfigurable phase shifter are shown. The low loss and minimal parasitics of VO2 technology have the potential to enable devices at 15 GHz and beyond.

physics.app-ph↗

arXiv2017

Aa Unbalanced Feed Design for Wideband Phased Arrays

In this work, a planar phased array antenna was engineered with ultra-wideband (UWB) performance that covers portions of the L-, S-, and C-bands. The unit cell design contains a driven layer that is directly connected to a single coaxial feed and a parasitic layer located above the driven layer separated by free space. This design does not require a balun or any vias between the antenna ground plane and the driven layer, resulting in a simple antenna stack-up consisting of only planar layers, aside from the feed structure. As a consequence, the complexity, and potentially sensitivity to mechanical tolerances, is reduced. The simulated results of this unbalanced phased array are discussed and experimentally validated.

physics.app-ph↗

arXiv2017

Whispering gallery mode hybridization in photonic molecules

This work takes inspiration from chemistry where the spectral characteristics of the molecules are determined by hybridization of electronic states evolving from the individual atomic orbitals. Based on analogy between quantum mechanics and the classical electrodynamics, we sorted dielectric microspheres with almost identical positions of their whispering gallery mode (WGM) resonances. Using these microspheres as classical photonic atoms, we assembled them in a wide range of structures including linear chains and planar photonic molecules. We studied WGM hybridization effects in such structures using side coupling by tapered microfibers as well as finite difference time domain modeling. We demonstrated that the patterns of WGM spectral splitting are representative of the symmetry, number of constituting atoms and topology of the photonic molecules which in principle can be viewed as "spectral signatures" of various molecules. We also show new ways of controlling WGM coupling constants in such molecules. Excellent agreement was found between measured transmission spectra and spectral signatures of photonic molecules predicted by simulation.

physics.optics↗

arXiv2016

Label-Free Nanoscopy with Contact Microlenses: Super-Resolution Mechanisms and Limitations

Despite all the success with developing super-resolution imaging techniques, the Abbe limit poses a severe fundamental restriction on the resolution of far-field imaging systems based on diffraction of light. Imaging with contact microlenses, such as microspheres or microfibers, can increase the resolution by a factor of two beyond the Abbe limit. The theoretical mechanisms of these methods are debated in the literature. In this work, we focus on the recently expressed idea that optical coupling between closely spaced nanoscale objects can lead to the formation of the modes that drastically impact the imaging properties. These coupling effects emerge in nanoplasmonic or nanocavity clusters, photonic molecules, or various arrays under resonant excitation conditions. The coherent nature of imaging processes is key to understanding their physical mechanisms. We used a cluster of point dipoles, as a simple model system, to study and compare the consequences of coherent and incoherent imaging. Using finite difference time domain modeling, we show that the coherent images are full of artefacts. The out-of-phase oscillations produce zero-intensity points that can be observed with practically unlimited resolution (determined by the noise). We showed that depending on the phase distribution, the nanoplasmonic cluster can appear with the arbitrary shape, and such images were obtained experimentally.

physics.optics↗

arXiv2016

iQ Cavity for Material Permittivity Characterization

We present an X-band waveguide (WR90) and UHF waveguide (WR1500) measurement method that permits the extraction of the complex permittivity for low dielectric loss tangent material specimen. The extraction method relies on computational electromagnetic (CEM) simulations; coupled with a genetic algorithm; to fit the experimental measurement and the simulated transmitted scattering parameter (S21) of the TE10 mode through the waveguide with the material specimen partially filling the cross-section. This technique provides the material measurement community with the ability to accurately extract material properties of low-loss material specimen.

physics.ins-det↗

arXiv2016

An X-Band Waveguide Measurement Technique for the Accurate Characterization of Materials with Low Dielectric Loss Permittivity

In this work, we present a new X-band waveguide (WR90) measurement method that permits the broadband characterization of the complex permittivity for low dielectric loss tangent material specimens with improved accuracy. An electrically-long polypropylene specimen that partially fills the cross-section is inserted into the waveguide and the transmitted scattering parameter (S21) is measured. The extraction method relies on computational electromagnetic simulations, coupled with a genetic algorithm, to match the experimental S21 measurement. The sensitivity of the technique to sample length was explored by simulating specimen lengths from 2.54 to 15.24 cm, in 2.54 cm increments. Analysis of our simulated data predicts the technique will have the sensitivity to measure loss tangent values on the order of 10e-3 for materials such as polymers with relatively low real permittivity values. The ability to accurately characterize low-loss dielectric material specimens of polypropylene is demonstrated experimentally. The method was validated by excellent agreement with a free-space focused-beam system measurement of a polypropylene sheet. This technique provides the material measurement community with the ability to accurately extract material properties of low-loss material specimen over the entire X-band range. This technique could easily be extended to other frequency bands.

physics.ins-det↗

arXiv2016

Permittivity and permeability determination for high index specimens using partially filled shorted rectangular waveguides

A method for determining the permittivity and permeability for specimens with high refractive index and variable shape is investigated. The method extracts the permeability and permittivity tensor elements from reflection measurements made with a partially-filled shorted rectangular waveguide on an electrically small specimen. Measurements are performed for two isotropic, heavily loaded coaxial magnetic composites. Supporting measurements from a stripline cavity and coaxial airline are used to validate the method. The results demonstrate the methods ability to handle frequency dispersive and high index materials.

physics.ins-det↗

arXiv2015

Movable thin films with embedded high-index microspheres for super-resolution microscopy

Microsphere-assisted imaging emerged as a surprisingly simple way of achieving optical super-resolution imaging. In this work, we use movable PDMS thin films with embedded high-index barium titanate glass microspheres a sample scanning capability was developed, thus removing the main limitation of this technology based on its small field-of-view.

physics.optics↗

arXiv2014

Super-Resolution Imaging by Arrays of High-Index Spheres Embedded in Transparent Matrices

We fabricated thin-films made from polydimethylsiloxane (PDMS) with embedded high-index (n~1.9-2.2) microspheres for super-resolution imaging applications. To control the position of microspheres, such films can be translated along the surface of the nanoplasmonic structure to be imaged. Microsphere-assisted imaging, through these matrices, provided lateral resolution of ~λ/7 in nanoplasmonic dimer arrays with an illuminating wavelength λ=405 nm. Such thin films can be used as contact optical components to boost the resolution capability of conventional microscopes.

physics.optics↗