Publications

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This report details the modeling results for the response of a finite-length dissipative conductor interacting with a conducting ground to a hypothetical nuclear device with the same output energy spectrum as the Fat Man device. We use a frequency-domain method based on transmission line theory and implemented it in a code we call ATLOG - Analytic Transmission Line Over Ground. Select results are compared to ones computed using the circuit simulator Xyce. Intentionally Left Blank

This report details the modeling results for the response of a finite-length dissipative conductor interacting with a conducting ground to a hypothetical nuclear device with the same output energy spectrum as the Fat Man device. We use a time-domain method based on transmission line theory that allows accounting for time-varying air conductivities. We implemented such method in a code we call ATLOG - Analytic Transmission Line Over Ground. Results are compared the frequency-domain version of ATLOG previously developed and to the circuit simulator Xyce in some instances. Intentionally Left Blank

This report details the modeling results for the response of a finite-length dissipative conductor interacting with a conducting ground to the Bell Labs electromagnetic pulse excitation. We use both a frequency-domain and a time-domain method based on transmission line theory through a code we call ATLOG - Analytic Transmission Line Over Ground. Results are compared to the circuit simulator Xyce for selected cases. Intentionally Left Blank

This report details the comparison of ATLOG modeling results for the response of a finite-length dissipative aerial conductor interacting with a conducting ground to a measurement taken November 2016 at the High-Energy Radiation Megavolt Electron Source (HERMES) facility. We use the ATLOG time-domain method based on transmission line theory. Good agreement is observed between simulations and experiments. Intentionally Left Blank

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We demonstrate the active tuning of all-dielectric metasurfaces exhibiting high-quality factor (high-Q) resonances. The active control is provided by embedding the asymmetric silicon meta-atoms with liquid crystals, which allows the relative index of refraction to be controlled through heating. It is found that high quality factor resonances (Q = 270 ± 30) can be tuned over more than three resonance widths. Our results demonstrate the feasibility of using all-dielectric metasurfaces to construct tunable narrow-band filters.

Dielectric metasurfaces that exploit the different Mie resonances of nanoscale dielectric resonators are a powerful platform for manipulating electromagnetic fields and can provide novel optical behavior. In this work, we experimentally demonstrate independent tuning of the magnetic dipole resonances relative to the electric dipole resonances of split dielectric resonators (SDRs). By increasing the split dimension, we observe a blue shift of the magnetic dipole resonance toward the electric dipole resonance. Therefore, SDRs provide the ability to directly control the interaction between the two dipole resonances within the same resonator. For example, we achieve the first Kerker condition by spectrally overlapping the electric and magnetic dipole resonances and observe significantly suppressed backward scattering. Moreover, we show that a single SDR can be used as an optical nanoantenna that provides strong unidirectional emission from an electric dipole source.

Here, epsilon-near-zero materials provide a new path for tailoring light-matter interactions at the nanoscale. In this paper, we analyze a compact electroabsorption modulator based on epsilon-near-zero confinement in transparent conducting oxide films. The non-resonant modulator operates through field-effect carrier density tuning. We compare the performance of modulators composed of two different conducting oxides, namely indium oxide (In2O3) and cadmium oxide (CdO), and show that better modulation performance is achieved when using high-mobility (i.e. low-loss) epsilon-near-zero materials such as CdO. In particular, we show that non-resonant electroabsorption modulators with sub-micron lengths and greater than 5 dB extinction ratios may be achieved through the proper selection of high-mobility transparent conducting oxides, opening a path for device miniaturization and increased modulation depth.

Ultrafast control of the polarization state of light may enable a plethora of applications in optics, chemistry and biology. However, conventional polarizing elements, such as polarizers and waveplates, are either static or possess only gigahertz switching speeds. Here, with the aid of high-mobility indium-doped cadmium oxide (CdO) as the gateway plasmonic material, we realize a high-quality factor Berreman-type perfect absorber at a wavelength of 2.08 μm. On sub-bandgap optical pumping, the perfect absorption resonance strongly redshifts because of the transient increase of the ensemble-averaged effective electron mass of CdO, which leads to an absolute change in the p-polarized reflectance from 1.0 to 86.3%. By combining the exceedingly high modulation depth with the polarization selectivity of the perfect absorber, we experimentally demonstrate a reflective polarizer with a polarization extinction ratio of 91 that can be switched on and off within 800 fs.

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The goal of this paper is to investigate full-wave simulations of realistic implementations of multifunctional nanoantenna enabled detectors (NEDs). We realize a 2×2 pixelated array structure that supports two wavelengths of operation. After designing each resonating structure independently using full-wave simulations with periodic boundary conditions mimicking the whole infinite array, we construct a supercell made of a 2×2 pixelated array with periodic boundary conditions mimicking the full NED. In the NED, each pixel comprises 10-20 nanoantennas. Our simulations account for the cross-talk between contiguous pixels. We observe that, even though there are finite extent effects, the pixels work as designed, each responding at the respective wavelength of operation. We want to stress that realistic simulations of multifunctional NEDs need to be performed to verify the design functionality by taking into account finite extent and cross-talk effects.

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This report details the effect of antenna loading on the interior near - field response of a resonating cylindrical cavity characterized by a leaky aperture. We find a large field variation within the cavity when a long antenna is introduced within the interior and the antenna load is varied from 0 to 50 Ohms. We also find the effect of absorption losses to be negligible. In order to accurately characterize the coupling into the cavity, a non - perturbing sensor (such as a monopole) is recommended. With this approach, the interior field distribution and peak levels characterizing the cavity will be fairly well preserved. In addition to studying the impact of antenna loading on the interior near - field response, the resonant frequencies for the cylindrical structure perturbed by a subwavelength aperture are found to be well estimated by analytical computations.

Three-dimensional (3D) metafilms composed of periodic arrays of silicon unit cells containing single and multiple micrometer-scale vertical split ring resonators (SRRs) per unit cell were fabricated. In contrast to planar and stacked planar structures, these 3D metafilms have a thickness t ∼ λd/4, allowing for classical thin film effects in the long wavelength limit. The infrared specular far-field scattering response was measured for metafilms containing one and two resonators per unit cell and compared to numerical simulations. Excellent agreement in the frequency region below the onset of diffractive scattering was obtained. For dense arrays of unit cells containing single SRRs, normally incident linearly polarized plane waves which do not excite a resonant response result in thin film interference fringes in the reflected spectra and are virtually indistinguishable from the scattering response of an undecorated array of unit cells. For the resonant linear polarization, the specular reflection for arrays is highly dependent on the SRR orientation on the vertical face for gap-up, gap-down, and gap-right orientations. For dense arrays of unit cells containing two SRRs per unit cell positioned on adjacent faces, the specular reflection spectra are slightly modified due to near-field coupling between the orthogonally oriented SRRs but otherwise exhibit reflection spectra largely representative of the corresponding single-SRR unit cell structures. The ability to pack the unit cell with multiple inclusions which can be independently excited by choice of incident polarization suggests the construction of dual-channel films where the scattering response is selected by altering the incident polarization.

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This paper details a model for the response of a finite- or an infinite-length wire interacting with a conducting ground to an electromagnetic pulse excitation. We develop a frequency–domain method based on transmission line theory that we name ATLOG–Analytic Transmission Line Over Ground. This method is developed as an alternative to full-wave methods, as it delivers a fast and reliable solution. It allows for the treatment of finite or infinite lossy, coated wires, and lossy grounds. The cases of wire above ground, as well as resting on the ground and buried beneath the ground are treated. The reported method is general and the time response of the induced current is obtained using an inverse Fourier transform of the current in the frequency domain. The focus is on the characteristics and propagation of the transmission line mode. Comparisons with full-wave simulations strengthen the validity of the proposed method.

We demonstrate the ultrafast formation of terahertz (THz) metasurfaces through all-optical creation of spatially modulated carrier density profiles in a deep-subwavelength GaAs film. The switch-on of the transient plasmon mode, governed by the GaAs effective electron mass and electron− phonon interactions, is revealed by structured-optical pump THz probe spectroscopy, on a time scale of 500 fs. By modulating the carrier density using different pump fluences, we observe a wide tuning of the electric dipole resonance of the transient GaAs metasurface from 0.5 THz to 1.7 THz. Furthermore, we numerically demonstrate that the metasurface presented here can be generalized to more complex architectures for realizing functionalities such as perfect absorption, leading to a 30 dB modulation depth. The platform also provides a pathway to achieve ultrafast manipulation of infrared beams in the linear and, potentially, nonlinear regime.

This paper investigates three-dimensional cut wire pair (CWP) behavior in vertically oriented meta-atoms. We first analyze CWP metamaterial inclusions using full-wave electromagnetic simulations. The scattering behavior of the vertical CWP differs substantially from that of the planar version of the same structure. In particular, we show that the vertical CWP supports a magnetic resonance that is solely excited by the incident magnetic field. This is in stark contrast to the bianisotropic resonant excitation of in-plane CWPs. We further show that this CWP behavior can occur in other vertical metamaterial resonators, such as back-to-back linear dipoles and back-to-back split ring resonators (SRRs), due to the strong coupling between the closely spaced metallic elements in the back-to-back configuration. In the case of SRRs, the vertical CWP mode (unexplored in previous literature) can be excited with a magnetic field that is parallel to both SRR loops, and exists in addition to the familiar fundamental resonances of the individual SRRs. In order to fully describe the scattering behavior from such dense arrays of three-dimensional structures, coupling effects between the close-packed inclusions must be included. Here, the new flexibility afforded by using vertical resonators allows us to controllably create purely electric inclusions, purely magnetic inclusions, as well as bianisotropic inclusions, and vastly increases the degrees of freedom for the design of metafilms.

We experimentally demonstrate high-contrast, ultrafast switching of infrared light at 2.1 μm via intraband pumping of a high quality factor perfect absorber made from a highly doped cadmium oxide thin film.

We study the role of carrier mobility in transparent conducting oxides integrated into epsilon-near-zero modulators. High-mobility materials including CdO enable sub-micron length electroabsorption modulators through >4dB/μm extinction ratios.

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We present a new approach to dielectric metasurface design that relies on a single resonator per unit cell and produces robust, high quality factor Fano resonances. Our approach utilizes symmetry breaking of highly symmetric resonator geometries, such as cubes, to induce couplings between the otherwise orthogonal resonator modes. In particular, we design perturbations that couple "bright" dipole modes to "dark" dipole modes whose radiative decay is suppressed by local field effects in the array. Our approach is widely scalable from the near-infrared to radio frequencies. We first unravel the Fano resonance behavior through numerical simulations of a germanium resonator-based metasurface that achieves a quality factor of ∼1300 at ∼10.8 μm. Then, we present two experimental demonstrations operating in the near-infrared (∼1 μm): a silicon-based implementation that achieves a quality factor of ∼350; and a gallium arsenide-based structure that achieves a quality factor of ∼600, the highest near-infrared quality factor experimentally demonstrated to date with this kind of metasurface. Importantly, large electromagnetic field enhancements appear within the resonators at the Fano resonant frequencies. We envision that combining high quality factor, high field enhancement resonances with nonlinear and active/gain materials such as gallium arsenide will lead to new classes of active optical devices.

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The development of novel thermal sources that control the emission spectrum and the angular emission pattern is of fundamental importance. In this paper, we investigate the thermal emission properties of semiconductor hyperbolic metamaterials (SHMs). Our structure does not require the use of any periodic corrugation to provide monochromatic and directional emission properties. We show that these properties arise because of epsilon-near-zero conditions in SHMs. The thermal emission is dominated by the epsilon-near-zero effect in the doped quantum wells composing the SHM. Furthermore, different properties are observed for s and p polarizations, following the characteristics of the strong anisotropy of hyperbolic metamaterials.

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Cable shielding to protect against coupling of electromagnetic radiation into a component or circuit, particularly over large frequency bands, is at times a challenging task. It is general understanding that increasing the number of shields of a cable will improve the shielding performance. However, there are situations in which a cable with multiple shields may perform similar to or in some cases worse than a cable with a single shield, and this analysis has seldom been discussed in the literature. We intend to shed more light onto this topic in this paper.

Metamaterial dielectric resonators represent a promising path toward low-loss metamaterials at optical frequencies. In this paper we utilize perturbations of high symmetry resonator geometries, such as cubes, either to overlap the electric and magnetic dipole resonances, thereby enabling directional scattering and Huygens' metasurfaces, or to induce couplings between the otherwise orthogonal resonator modes to achieve high-quality factor Fano resonances. Our results are fully scalable across any frequency bands where high-permittivity dielectric materials are available, including microwave, THz, and infrared frequencies.

Narrow-bandgap semiconductors such as alloys of InAsAlSb and their heterostructures are considered promising candidates for next generation infrared photodetectors and devices. The prospect of actively tuning the spectral responsivity of these detectors at the pixel level is very appealing. In principle, this could be achieved with a tunable metasurface fabricated monolithically on the detector pixel. Here, we present first steps towards that goal using a complementary metasurface strongly coupled to an epsilon-near-zero (ENZ) mode operating in the long-wave region of the infrared spectrum. We fabricate such a coupled system using the same epitaxial layers used for infrared pixels in a focal plane array and demonstrate the existence of ENZ modes in high mobility layers of InAsSb. We confirm that the coupling strength between the ENZ mode and the metasurface depends on the ENZ layer thickness and demonstrate a transmission modulation on the order of 25%. We further show numerically the expected tunable spectral behavior of such coupled system under reverse and forward bias, which could be used in future electrically tunable detectors.

Narrow-bandgap semiconductors such as alloys of InAsAlSb and their heterostructures are considered promising candidates for next generation infrared photodetectors and devices. The prospect of actively tuning the spectral responsivity of these detectors at the pixel level is very appealing. In principle, this could be achieved with a tunable metasurface fabricated monolithically on the detector pixel. Here, we present first steps towards that goal using a complementary metasurface strongly coupled to an epsilon-near-zero (ENZ) mode operating in the long-wave region of the infrared spectrum. We fabricate such a coupled system using the same epitaxial layers used for infrared pixels in a focal plane array and demonstrate the existence of ENZ modes in high mobility layers of InAsSb. We confirm that the coupling strength between the ENZ mode and the metasurface depends on the ENZ layer thickness and demonstrate a transmission modulation on the order of 25%. We further show numerically the expected tunable spectral behavior of such coupled system under reverse and forward bias, which could be used in future electrically tunable detectors.

This report examines bounds on the penetrant power through ports of entry into a conductive cavity. We first replace the cavity by a load and consider the maximum power transfer properties of an antenna or an aperture. We consider how limitations on the load quality factor place limits on received power. For general frequency ranges we model the backing region by means of a uniformly distributed matched load along a slot aperture and adjust its value for maximum power transfer. This result is derived in closed form using a transmission line model for the aperture. This result illustrates the reduction in received power for low frequencies with finitely conducting wall materials. At high frequencies it approaches the receiving cross section of a linear array having the slot length dimension. Next we examine a slot aperture in a conducting rectangular enclosure and determine how the cavity wall losses and resulting quality factor limit the penetrant power. Detailed simulations and experimental measurements are compared with each other and with the bounding results to assess the accuracy of the bounds. These comparisons also indicate limitations on the accuracy of the models due to perturbing influences in construction, such as bolted joints.

We investigate optical polariton modes supported by subwavelength-thick degenerately doped semiconductor nanolayers (e.g. indium tin oxide) on glass in the epsilon-near-zero (ENZ) regime. The dispersions of the radiative (R, on the left of the light line) and non-radiative (NR, on the right of the light line) ENZ polariton modes are experimentally measured and theoretically analyzed through the transfer matrix method and the complex-frequency/real-wavenumber analysis, which are in remarkable agreement. We observe directional near-perfect absorption using the Kretschmann geometry for incidence conditions close to the NR-ENZ polariton mode dispersion. Along with field enhancement, this provides us with an unexplored pathway to enhance nonlinear optical processes and to open up directions for ultrafast, tunable thermal emission.

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By using a multipole-conformal mapping expansion for the wire charges we examine the accuracy of approximations for the transfer elastance (elastance is the inverse of capacitance) of a one dimensional array of wires (wire grid). A simple uniform fit is constructed by introduction of the decay factor from bipolar coordinates into existing formulas for this elastance.

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