Design and Optimization of Wire-array Optical Metamaterial with Alloy Elements Fabricated through Thermal Drawing
- Author(s): Yang, Yingchao
- Advisor(s): Li, Xiaochun
- et al.
The objective of this study is to design and optimize metamaterial, especially those functioning in the near-IR and visible light range, with alloy wire array in a dielectric material to be fabricated by thermal drawing, overcoming the long time challenge in scalable fabrication of optical metamaterials. The feature size, wire diameter, of the metamaterial made of the wire-array structure, is demanded to be less than one-fifth of the incident light wavelength. Therefore, metal wire arrays with a wire diameter and a pitch size both less than 500 nm are required. This type of metamaterial, due to its three dimensional nature, is very hard to fabricate using conventional nanofabrication technologies. Thermal drawing technology is widely considered to be the most suitable method for fabricating this type of particular structures. Yet long, continuous metal wires with a diameter less than 500 nm have not been achieved by thermal drawing, due to an unavoidable instability in molten metal, a fluid with low viscosity and a high interfacial energy with dielectric cladding materials. To tackle this problem, noble metal based alloys are proposed to replace pure noble metals because these alloys can become semi-solid with a much higher viscosity within specific temperature windows, possible allowing the thermal drawing of long continuous micro/nano metal wires. It is expected that the optical properties of the alloys would be not as satisfying as pure noble metals. However, since currently there are no other promising way to realize the novel type of metamaterial from theoretical designs, the alloy thermal drawing approach would be groundbreaking to overcome the long-standing fabrication limit.
In this study, first a material selection criteria is established. The dielectric material needs to be transparent, and has low extinction coefficient and suitable refractive index. Metal has to have a large enough localized surface plasmon at the interested wavelength range. Not only optical properties should be considered. Several other aspects including phase diagram, potential reaction between the cladding material and the metal core, viscosity profile of the dielectric cladding, and oxidation under high temperature. Using this criteria, a proper combination of alloy and cladding materials could be determined for a successful thermal drawing.
Then two noble metal based alloy systems, silver-gold and silver-copper alloys are selected to be embedded inside a dielectric cladding with a dielectric constant of 2.4. Matlab calculation based on Maxwell-Garnett Theory is used to theoretically predict the optical performance of alloy wire array structures with a metal filling fraction between 0.227 and 0.325. The results show that the optical metamaterial with aligned wire arrays made of these two alloy systems could yield the desired negative refractive index, as light bends 8 degrees away from the normal axis, not at the same plane as the reflected wave but at same plane as the incident wave. COMSOL simulations are also carried out and the results validate the Matlab predictions. The phenomena by the negative refractive index are visualized in the COMSOL results.
One major source of defects and their effects during thermal drawing are studied as well. It is crucial to control temperature during the thermal drawing process. Yet in reality the furnace is never perfect to heat up the whole preform uniformly during drawing. Simulation of the process suggests that the temperature difference induced along the radial direction of the cylindrical preform is problematic since the viscosity of the alloys in the semi-solid state is highly temperature sensitive. Precautions should be taken to reduce the temperature difference as much as possible to prevent the failure of the metamaterial by defects such as missing wires.
In summary, this study successfully conducted research in design and optimization of optical metamaterial with aligned alloy wire array fabricated by thermal drawing. It overcomes the long time fabrication limit for wire-array metamaterial functioning in near-IR and visible light region. Analytical and numerical approaches have been applied to verify that using alloy of noble metals, instead of pure metals, is highly promising to make a breakthrough into the currently stagnant fabrication of wire-array optical metamaterials. Experimental verification as well as wire arrangement optimization are recommended to be explored in the future.