Joshua A. Burrow

Polarization-dependent electromagnetic responses of ultrathin and highly flexible asymmetric terahertz metasurfaces

We report the polarization-dependent electromagnetic response from a series of novel terahertz (THz) metasurfaces where asymmetry is introduced through the displacement of two adjacent metallic arms separated by a distance δ. For all polarization states, the symmetric metasurface exhibits a low quality (Q) factor fundamental dipole mode. By breaking the symmetry, we experimentally observe a secondary dipole-like mode with a Q factor nearly 9× higher than the fundamental resonance. As δ increases, the fundamental dipole mode f1 redshifts and the secondary mode f2 blueshifts creating a highly transmissive spectral window. Polarization-dependent measurements reveal a full suppression of f2 for all asymmetries at θ ≥ 60°. Furthermore, at δ ≥ 60 μm, we observe a polarization selective electromagnetic induced transparency (EIT) for the fundamental mode. This work paves the way for applications in filtering, sensing and slow-light devices common to other high Q factor THz metasurfaces with EIT-like response.

Mode hybridization in lattice induced transparency for polarization-insensitive THz metasurfaces

We demonstrate multiple lattice-induced transparencies by decoupling the first-order lattice mode achieving a measured Q factor of 40 and support measurements with numerical calculations revealing bright dipole resonances.

Electrical and optical properties of nickel-doped Ge2Sb2Te5 films produced by magnetron co-sputtering

A magnetron co-sputtering system was used for producing nickel-doped Ge2Sb2Te5 (GST-Ni) thin films. The nickel content in the thin film was adjusted by the ratio of the plasma discharge power applied to the GST and nickel targets, as well as a physical shuttering technique to further control the nickel deposition rate. The doping concentration of the film was confirmed using Energy Dispersion Spectroscopy (EDS) technique. Results from a four-point probe measurement indicate that the nickel doping can reduce the resistivity of GST in the amorphous state by nearly three orders of magnitude. The dopant’s influence on crystallization behavior was studied by analyzing X-Ray Diffraction (XRD) patterns of the pure GST and GST-Ni at different annealing temperatures. To examine the structural changes due to the nickel dopant, the thin films were investigated with the aid of Raman scattering. Additionally, we extracted the optical constants for both the amorphous and crystalline states of undoped-GST and GST-Ni films by ellipsometry. The results indicate that at low doping concentrations nickel does not appreciably affect the optical constants, but dramatically improves the electrical conductivity. Therefore, nickel-doping of GST a viable method for designing optical devices for lower operating voltages at higher switching speeds.

Improving the performance of Ge2Sb2Te5 materials via nickel doping: Towards RF-compatible phase-change devices

High-speed electrical switching of Ge2Sb2Te5 (GST) remains a challenging task due to the large impedance mismatch between the low-conductivity amorphous state and the high-conductivity crystalline state. In this letter, we demonstrate an effective doping scheme using nickel to reduce the resistivity contrast between the amorphous and crystalline states by nearly three orders of magnitude. Most importantly, our results show that doping produces the desired electrical performance without adversely affecting the films optical properties. The nickel doping level is approximately 2% and the lattice structure remains nearly unchanged when compared with undoped-GST. The refractive indices in amorphous and crystalline states were obtained using ellipsometry which echoes the results of X-ray diffraction. The materials thermal transport properties are measured using time-domain thermoreflectance, showing no change upon doping. The advantages of this doping system will open up opportunities for designing electrically reconfigurable high speed optical elements in the near-infrared spectrum.High-speed electrical switching of Ge2Sb2Te5 (GST) remains a challenging task due to the large impedance mismatch between the low-conductivity amorphous state and the high-conductivity crystalline state. In this letter, we demonstrate an effective doping scheme using nickel to reduce the resistivity contrast between the amorphous and crystalline states by nearly three orders of magnitude. Most importantly, our results show that doping produces the desired electrical performance without adversely affecting the films optical properties. The nickel doping level is approximately 2% and the lattice structure remains nearly unchanged when compared with undoped-GST. The refractive indices in amorphous and crystalline states were obtained using ellipsometry which echoes the results of X-ray diffraction. The materials thermal transport properties are measured using time-domain thermoreflectance, showing no change upon doping. The advantages of this doping system will open up opportunities for designing electrica...