Composite THz materials using aligned metallic and semiconductor microwires, experiments and interpretation

TL;DR

This paper presents a fabrication method and THz characterization of composite films with aligned metallic and semiconductor microwires, analyzing their optical properties and challenges in manufacturing at sub-micrometer scales.

Contribution

It introduces a stack-and-draw fabrication technique for composite THz metamaterials with detailed optical characterization and modeling of their properties.

Findings

Metallic microwire composites exhibit strong polarizing effects.

Semiconductor microwire composites are polarization independent with high refractive index.

Phase separation and nano-grid formation occur in metallic wires during fabrication.

Abstract

We report fabrication method and THz characterization of composite films containing either aligned metallic (tin alloy) microwires or chalcogenide As2Se3 microwires. The microwire arrays are made by stack-and-draw fiber fabrication technique using multi-step co-drawing of low-melting-temperature metals or semiconductor glasses together with polymers. Fibers are then stacked together and pressed into composite films. Transmission through metamaterial films is studied in the whole THz range (0.1-20 THz) using a combination of FTIR and TDS. Metal containing metamaterials are found to have strong polarizing properties, while semiconductor containing materials are polarization independent and could have a designable high refractive index. Using the transfer matrix theory, we show how to retrieve the complex polarization dependent refractive index of the composite films. We then detail the selfconsistent algorithm for retrieving the optical properties of the metal alloy used in the fabrication of the metamaterial layers by using an effective medium approximation. Finally, we study challenges in fabrication of metamaterials with sub-micrometer metallic wires by repeated stack-and-draw process by comparing samples made using 2, 3 and 4 consecutive drawings. When using metallic alloys we observe phase separation effects and nano-grids formation on small metallic wires.