Jesse A. Rodriguez

An approach is presented for characterizing heterogeneous magnetized plasma discharge tubes through the scattering of electromagnetic plane waves. Here, we formulate the analytical problem of electromagnetic scattering from a gyrotropic plasma column. The scattering accounts for the heterogeneous composition of the cylindrical discharge plasma and facilitates determining its propensity for gyrotropic scattering, particularly when electron collisional damping may be prevalent. The analytical results are validated using computational simulations. Scattered fields from the magnetized plasma are measured experimentally, and, by comparing the analytical and experimental results, the unknown parameters of the discharge, i.e., characteristic plasma and electron collisional damping frequencies, are determined. The technique is relatively straight-forward to use and removes the need for commercial computational electromagnetic simulations when experimental data on scattering characteristics of such cylindrical discharge plasmas are available.

We present a theoretical analysis and experimental demonstration of a microwave circulator that uses a magnetized plasma as the gyrotropic element. Unlike traditional circulators that exploit the anisotropic permeability of ferrite, here, we exploit the anisotropic dielectric constant of a magnetized plasma to achieve non-reciprocity. The advantage of a plasma-loaded circulator is that it allows for tunability, and modulation that is limited in speed by the ionization/recombination time of the plasma. The theoretical analysis treats the problem of electromagnetic scattering from a heterogeneous gyrotropic plasma rod to confirm scattering behavior and to guide in defining the design parameter space covered by more detailed computational simulations. Proof-of-concept experiments are carried out using a custom-fabricated low-frequency gas discharge tube as the plasma element. Here, we confirm the plasma-frequency dependent tunability predicted by the simulations, demonstrating this tunability in the experiments by varying the applied discharge voltage. These initial experiments suggest that isolation in excess of 25 dB is achievable, in reasonable agreement with the theory and simulations for studies in the S and C ranges of the microwave spectrum.

In an expansion of a previous study [1], we apply inverse design methods to produce two-dimensional plasma metamaterial devices with realistic plasma elements which incorporate quartz envelopes, collisionality (loss), non-uniform density profiles, and resistance to experimental error/perturbation. Backpropagated finite difference frequency domain simulations are used to design waveguides and demultiplexers operating under the transverse magnetic polarization. Optimal devices with realistic elements are compared to previous devices with idealized elements, and several parameter initialization schemes for the optimization algorithm are explored. Demultiplexing and waveguiding are demonstrated for microwave-regime devices composed of plasma elements with reasonable space-averaged plasma frequencies ~10 GHz and a collision frequency ~1 GHz, allowing for future in-situ training and experimental realization of these designs.

In an expansion of a previous study (Rodríguez et al 2021 Phys. Rev. Appl. 16 014023), we apply inverse design methods to produce two-dimensional plasma metamaterial devices with realistic plasma elements which incorporate quartz envelopes, collisionality (loss), non-uniform density profiles, and resistance to experimental error/perturbation. Finite difference frequency domain simulations are used along with forward-mode differentiation to design waveguides and demultiplexers operating under the transverse magnetic polarization. Optimal devices with realistic elements are compared to previous devices with idealized elements, and several parameter initialization schemes for the optimization algorithm are explored, yielding a robust procedure for producing such devices. Demultiplexing and waveguiding are demonstrated for microwave-regime devices composed of plasma elements with reasonable space-averaged plasma frequencies ∼ 10 GHz and a collision frequency ∼ 1 GHz, allowing for future in-situ training and experimental realization of these designs.

We experimentally demonstrate the controlled scattering of incident transverse-electric electromagnetic waves from a gyrotropic magnetized plasma cylindrical discharge. Scattered electromagnetic waves can bend left and right by changing the external magnetic field of a plasma rod. Measured scattered wavefronts are in good agreement with electromagnetic simulations. A gyrotropic response is observed for incident wave frequencies ranging from 3.5 to 5.6 GHz for conditions corresponding to a ratio of cyclotron frequency to plasma frequency, ωce/ωp≈ 0.16. The observation of a gyrotropic response from cylindrical plasma discharges paves the way for their use as building blocks for future devices such as magnetized plasma photonic crystals, topological insulators, plasma metamaterials, non-reciprocal waveguide structures, and other devices, which require a tunable gyrotropic response from centimeter to meter-scale materials with application-specific geometry.

We apply inverse design methods to produce two-dimensional plasma metamaterial (PMM) devices. Backpropagated finite difference frequency domain (FDFD) simulations are used to design waveguides and demultiplexers operating under both transverse electric (TE) and transverse magnetic (TM) modes. Demultiplexing and waveguiding are demonstrated for devices composed of plasma elements with reasonable plasma densities ~7 GHz, allowing for future in-situ training and experimental realization of these designs. We also explore the possible applicability of PMMs to nonlinear boolean operations for use in optical computing. Functionally complete logical connectives (OR and AND) are achieved in the TM mode.

The introduction of 3D printing has enabled fabrication of photonic crystal devices with complex crystal structures that would be challenging to construct using other fabrication methods. In this paper, we construct and characterize a photonic crystal consisting of two 4 × 8 × 8 cubic lattices composed of spherical silicon nitride elements straddling a layer of 8 plasma discharge tubes, creating a 3D hybrid plasma photonic crystal device. Integrating under-dense gaseous plasma elements provides a unique coupling dynamic between the dielectric spheres and the cylindrical plasma discharges, creating a monolithic hybrid photonic crystal with solid state and reconfigurable elements. The device has resonant modes that have attenuation peaks that are either switchable, tunable in amplitude, or tunable in frequency with variations in plasma density. The response of these bands seen with varying plasma density is confirmed through simulations when effects due to the heating of the photonic crystal from the gaseous plasma elements are accounted for in the experiments. We discuss how this reconfigurable device may be used and expanded upon for applications in photonic artificial neural networks and optical computing systems.

We report on dual-polarization Dirac cones in a simple square lattice two-dimensional (2D) photonic crystal (PC) based on transmission at accidental degeneracies centered at the k = 0 symmetry ( Γ ) point. Finite difference time domain simulations are used to identify the material and geometric parameters for Dirac-like dispersion. A configuration that produces a Dirac-like point for both transverse electric and transverse magnetic polarizations at the same frequency is presented. The PC dispersion shows the expected threefold degenerate linear branch crossings at the Dirac-like point. Full-field electromagnetic wave simulations exhibit some common behaviors of devices based on Dirac-like dispersion, such as cloaking and waveguiding. The configuration works for a considerable range of the parameter space, and thus is experimentally realizable with a wide range of materials.

A three-dimensional woodpile structure tunable plasma photonic crystal is designed, simulated, and experimentally characterized over the S – X band of the electromagnetic (EM) spectrum. The measurements confirm that the EM response is rich in dynamics. The photonic crystal’s reconfigurability achieved through individual discharge control of the properties of the woodpile plasma columns offers an unprecedented opportunity to better resolve the interactions between both the Bragg and localized surface plasmon modes which dominate the spectrum at lower frequencies. Both the experiments and simulations reveal evidence for the coupling of Bragg and surface plasmon modes through a Fano resonance.