MIT researchers have created high-performance mirrors in the shape of hair-like flexible fibres that could be woven into cloth or incorporated in paper.
Applications may include fabrics with embedded ‘bar codes’ that identify the wearer, potentially useful in the battle suits of future soldiers; or a lightweight cloth that reflects radiation, protecting from blasts of heat. These mirrors could also be used as filters for telecommunications applications.
The work reportedly builds on the omnidirectional dielectric reflector (dubbed the ‘perfect mirror’) created in 1998. The ‘perfect mirror’ is said to combine the best characteristics of the familiar metallic mirror with those of the dielectric mirror. The latter is a second type of mirror composed of alternating layers of non-metallic materials that allow much greater control over the mirror’s reflectivity, but can only reflect light from a limited set of angles and are polarisation sensitive.
MIT’s perfect mirror can reflect light from all angles and polarisation’s, just like metallic mirrors, but unlike its metal counterpart can also be ‘tuned’ to reflect certain wavelength ranges while transmitting others. Consequently, an array of mirror fibres or even a single fibre can be ‘tuned’ to reflect light at different wavelengths to create a kind of optical bar code that could be woven into fabric or incorporated into paper.
‘We’ve opened a new avenue of applications for these high-performance optical devices,’ said Shandon D. Hart, a graduate student in the Department of Materials Science and Engineering (MSE).
Polymer fibres have been quite successful commercially with applications stemming from their superior mechanical properties and low cost. Yet although they’ve been optimised for everything from strength to moisture resistance, little has been done to control their optical properties, said Yoel Fink, leader of the current research team. To address that challenge, the Fink team created a polymer fibre that is sheathed with 21 layers of alternating index of refraction, forming a cylindrical ‘perfect mirror.’
The team first created a macroscopic cylinder, or preform, some 20 to 30 centimetres long by 25 millimetres in diameter. It contained the same 21 layers of dielectric materials surrounding a polymer core as the final fibres, but unlike the microscopic features of the ultimate fibres, each layer of the preform could be seen with the naked eye.
The preform was subsequently fed into a tube furnace that is part of an optical-fibre draw tower recently constructed in the Fink lab. From there, it was drawn into hundreds of meters of thread-like fibre with 21 microscopic layers.
Each layer is a few hundred nanometers thick, thus spanning 9 orders of dimensional magnitude in a single processing step. ‘The amazing thing is that the resulting fibre retained the same structure as the macroscopic preform cylinder over extended distances,’ said Fink.
Key to the success of the drawing process is the identification of a pair of materials which have substantially different indices of refraction yet similar thermo-mechanical properties which enable them to be thermally processed at the same temperature.
Fink notes that drawing fibres is a process commonly used to create the glass threads of fibre-optics. But while the typical glass thread has only a few and fairly large internal features, the mirror threads have over 21 and the thickness control is well below one micron. ‘So we’re taking the process a step further and getting very high control over the microstructure of the fibre with a very small number of defects overall.’