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1Institute of Photonics and Optical Sciences (IPOS), School of Physics, University of Sydney
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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.
Tuniz, A., Lwin, R., Argyros, A., Fleming, S. C., Kuhlmey, B. T. Fabricating Metamaterials Using the Fiber Drawing Method. J. Vis. Exp. (68), e4299, doi:10.3791/4299 (2012).
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.
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:
Sections 4 and 5 detail the drawing processes used in sections 2 and 3.
1. Fabricating the PMMA Jacketing Tube
The PMMA jacketing tube used to structure the 1 mm indium wire is made by stretching and sleeving standard PMMA tubes in the primary draw process (Section 4) to make a final PMMA jacketing tube of ID 1 mm and OD 12 mm.
2. Fabricating the Indium Filled Fiber
The 1 mm indium wire is sleeved and stretched in the PMMA jacketing tube made in Section 1 using the secondary draw process (Section 5) to produce indium filled fiber with a final OD 1.2 mm.
3. Fabricating the Indium Stacked Fiber
The indium stacked fiber is fabricated by first stacking the indium filled fibers produced in Secton 2 in a larger PMMA preform jacketing tube, which is then stretched and sleeved to the desired fiber dimensions using the secondary draw process (Section 5).
4. Primary Draw Process
The primary draw process is used to stretch preforms to outer diameters greater than 1 mm. The following procedure is used in Section 1: Fabricating the PMMA Jacketing Tube.
5. Secondary Draw Process
The secondary draw process is used to stretch preforms to ODs smaller than 1 mm. The following procedure is used in Section 2: Fabricating the indium filled fiber and 3: Fabricating the indium stacked fiber.
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 with plasmonic response in the THz range is shown in Figure 3.
The plasmonic response manifests itself such that at low frequencies the material behaves like a metal (low transmission) and at high frequencies like a dielectric (high transmission), with the plasma frequency defining the boundary between the two behaviours. In this specific case, the plasma frequency is expected at 1.2 THz, however our technique permits this to be readily changed by varying the draw speed, which in turn changes the radius and separation of the wires, as presented in Ref.8. The resulting high-pass filtering behavior of the metamaterial fiber, for incident THz waves with their electric fields directed along the wires, can be measured via terahertz time domain spectroscopy 11.
Figure 4.i.a shows experimental measurements of this fibre type drawn to three different dimensions. This agrees very well with theory, shown in Figure 4.i.b. In both cases the plasma frequency dependence on diameter is apparent. Analysis of the particular fiber shown in Figure 3 gives the plasmonic response shown in Figure 4.ii where the plasma frequency is at 0.6 THz.
Figure 1. Multiple sleeved jacket cross section schematic with single indium wire. 1 is the indium wire, 2 is the 1st jacketing PMMA tube, 3 is the 2nd, and 4 is the 3rd.
Figure 2. Top view and side view of 1 mm PMMA fibre with a single 100 μm indium wire.
Figure 3. (Composite) optical microscope cross sectional image of the 5 μm indium wires separated by 50 μm in a PMMA fiber. (40x objective lens).
Figure 4. (i) Schematic of the experimental setup for measuring metamaterial ﬁber transmittance (ii) (a) Experimental and (b) simulated (finite element method) transmittance for arrays of metamaterial fibers of different diameters (electric field parallel to the wires), as presented in Ref. 8, showing very good agreement. A scanning electron microscope image of the 590 μm fiber is shown in the inset of (a). An image of the simulated geometry is shown in the inset of (b). The smallest fiber had ~8 μm diameter wires separated by ~100 μm. The shaded region illustrates where the medium cannot be seen as homogeneous. The plasmonic transition region shifts to lower frequencies as we increase the fiber diameter (obtained simply by changing the draw speed), resulting in a shifting of the high-pass filtering behavior. After Ref. 8. (iii) Simulated transmittance for an array of the metamaterial fiber shown in Figure 3, using the same methods and optical parameters presented in Ref. 8. Note that in this case the fiber would exhibit a plasma frequency around 0.6 THz. Click here to view larger figure.
Figure 5. Top section of the fibre draw tower on the secondary side. Note in particular the chuck feed (top) and the furnace (middle), connected to the control unit (right).
Figure 6. (Left to right) Bottom extender, preform, and top extender.
Figure 7. Attaching top extender - with PTFE (left) and reflective tape (right).
Figure 8. Attaching bottom extender - reflective tape.
Figure 9. Hot air gun crimp.
Figure 10. Inserting tube in jacket (left) and with PTFE seal (right).
Figure 11. Inserting indium wire into PMMA tube.
Figure 12. Inserting indium wire stacked bundle into PMMA tube.
Figure 13. Top to bottom: attaching vacuum tube to preform, clamping preform in the 3 jaw chuck feed unit, and feeding into the furnace.
Figure 14. Pre-heat profile.
Figure 15. Primary tension profile.
Figure 16. + Drop-down preform section.
|Preform OD (mm)||Feed Rate (mm/min)||Draw Rate (mm/min)||Furnace Temperature (°C)|
Table 1. Primary draw conditions.
|Preform OD (mm)||Feed Rate (mm/min)||Furnace Temperature (°C)||Draw Tension (g)|
Table 2. Secondary draw conditions.
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 example negative refractive index materials, when combined with metamaterial fibers possessing a negative magnetic permeability in this range 12. Note that magnetically responsive fibers may also be fabricated in bulk by a variation on the technique presented here 13.
No conflicts of interest declared.
This research was supported under Australian Research Council's Discovery Projects funding scheme (project number DP120103942). B. T. K. and A. A. are the recipients of an Australian Research Council Future Fellowship (FT0991895) and Australian Research Fellowship (DP1093789) respectively.
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