Fabricating Metamaterials Using the Fiber Drawing Method

Summary

Metamaterials at terahertz frequencies offer unique opportunities, but are challenging to fabricate in bulk. We adapt the fabrication procedure for microstructured polymer optical fibers to inexpensively fabricate metamaterials potentially on an industrial scale. We produce polymethylmethacrylate fibers containing ~10 μm diameter indium wires separated by ~100 μm, which exhibit a terahertz plasmonic response.

Abstract

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate ¹. They owe their electromagnetic properties to the structure of their constituents, instead of the atoms that compose them. For example, sub-wavelength metal wires can be arranged to possess an effective electric permittivity that is either positive or negative at a given frequency, in contrast to the metals themselves ². This unprecedented control over the behaviour of light can potentially lead to a number of novel devices, such as invisibility cloaks ³, negative refractive index materials ⁴, and lenses that resolve objects below the diffraction limit ⁵. However, metamaterials operating at optical, mid-infrared and terahertz frequencies are conventionally made using nano- and micro-fabrication techniques that are expensive and produce samples that are at most a few centimetres in size ^6-7. Here we present a fabrication method to produce hundreds of meters of metal wire metamaterials in fiber form, which exhibit a terahertz plasmonic response ⁸. We combine the stack-and-draw technique used to produce microstructured polymer optical fiber ⁹ with the Taylor-wire process ¹⁰, using indium wires inside polymethylmethacrylate (PMMA) tubes. PMMA is chosen because it is an easy to handle, drawable dielectric with suitable optical properties in the terahertz region; indium because it has a melting temperature of 156.6 °C which is appropriate for codrawing with PMMA. We include an indium wire of 1 mm diameter and 99.99% purity in a PMMA tube with 1 mm inner diameter (ID) and 12 mm outside diameter (OD) which is sealed at one end. The tube is evacuated and drawn down to an outer diameter of 1.2 mm. The resulting fiber is then cut into smaller pieces, and stacked into a larger PMMA tube. This stack is sealed at one end and fed into a furnace while being rapidly drawn, reducing the diameter of the structure by a factor of 10, and increasing the length by a factor of 100. Such fibers possess features on the micro- and nano- scale, are inherently flexible, mass-producible, and can be woven to exhibit electromagnetic properties that are not found in nature. They represent a promising platform for a number of novel devices from terahertz to optical frequencies, such as invisible fibers, woven negative refractive index cloths, and super-resolving lenses.

Protocol

Overview The composite indium/PMMA fiber (Figure 3) is produced by drawing a stack of PMMA fibers including a single indium wire (Figure 2), which themselves have to be prepared from available PMMA tubes and wires. The steps presented are: Produce a PMMA fiber that contains a single indium wire of diameter appropriate for manual stacking. For this, first prepare a PMMA tube that can accommodate a 1 mm indium wire (Section 1), then inc…

Representative Results

Metamaterial fibers were produced using the technique described. They was assembled from a preform of 1 mm PMMA fibers containing 100 μm diameter continuous indium wires, shown in Figure 2, which in turn had themselves been drawn from a preform of 1 mm indium wires contained inside a 10 mm polymer jacket, which was produced by sleeving appropriately sized polymer tubes, as shown in the schematic of Figure 1. A microscope image of the cross section of an example of a metamaterial fiber…

Discussion

The technique presented here allows the fabrication of kilometers of continuous three-dimensional metamaterials with microscale feature sizes, possessing a plasmonic response (and thus a tailored electric permittivity) in the THz range, effectively behaving as a high-pass filter. This can be experimentally characterized using terahertz time-domain spectroscopy ¹¹. Such fiber-shaped metamaterials can be cut and stacked into bulk materials to realize a large number of devices, or woven into other structures, for…

Materials

Name of Reagent/Material	Company	Catalogue Number	Comments
Indium 99.99% Wire, 1 mm diameter	AIM Specialty	Available on request	www.aimspecialty.com http://www.aimspecialty.com/Portals/0/Files/Indium.pdf
2-Propanol(Isopropanol)	Sigma-Aldrich	Product Number 190764	http://www.sigmaaldrich.com/chemistry/solvents/products.html?TablePage=17292086
Adhesive tape	Staples
One Wrap PTFE Tape, 5 ml x 12 mmW x 0.2 mmT	RS Components	RS Stock Number 231-964	http://uk.rs-online.com/web/p/ptfe-tapes/0231964/
50 Micron Aluminium Foil Tape	Advance Adhesive Tapes	AT506	http://www.advancetapes.com/Products/types/9/page1/81
Blu-tak	Bostik		http://www.blutack.com/index.html
Araldite Quick Set	Selleys		http://selleys.com.au/adhesives/household-adhesive/araldite/quick-set
PMMA tubes: – ID 6 mm, OD 12 mm – ID 9 mm, OD 12 mm	B & M Plastics: Plastic Fabrication	Available on request	http://www.bmplastics.com.au/about-us.htm
			Equipment Requirements
			Fibre draw tower with furnaces of maximum temperatures of at least 200 °C (Heathway Polymer Draw Tower with Preform and Fibre draw facilities). A photograph of the draw tower is shown in Figure 5. Annealing oven of maximum temperatures of at least 90 °C. Optical microscope. Hot air gun. Vacuum pump. Top preform extender (metal tube of 30 cm length and 12 mm diameter). Primary draw bottom extender (metal tube of 100 cm length and 12 mm diameter). Secondary draw bottom extender (PMMA tube of 20 cm length and 12 mm diameter).