NOVEL FABRICS SEE THE LIGHT
31 October 2004 - Massachusetts Institute of Technology

In work on smart fabrics and new computer interfaces that could lead to applications including multifunctional textile fabrics and all-optical computer interfaces, MIT researchers report the creation of flexible fibres and fabrics that can not only sense light, but also analyse its colours.

'These novel fibre structures offer a unique possibility for constructing an optoelectronic functional fabric because the fibres are both flexible and mechanically tough, and can thus be woven,' say the researchers. 'Interesting device applications follow not only from the ability to engineer the single-fibre properties, but also from the specifics of fiber arrangements into larger assemblies.'

The team's leader, Yoel Fink, notes that 'the technique we developed allows us to bring together two disparate technologies: those involved in creating optical fibres and those for electronic components.' The work 'challenges the traditional barrier between semiconductor devices and fibre-optic processing,' said Fink, the Thomas B King Assistant Professor of Materials Science and Engineering.

The result? The team can create devices which marry the ease of fabrication, length and flexibility associated with optical fibres with the many integrated functions associated with semiconductor devices. 'Being able, for the first time, to precisely control the behaviour of electrons, photons and their interactions within a fibre framework leads naturally to the exciting possibility of eventually creating intrinsically smart fabrics,' said co-worker John D Joannopoulos, the Francis Wright Davis Professor of Physics.

Already the team has created two different prototype fibres with the new technology. The first is a fibre that simultaneously conducts two types of information carriers: electrons and photons. The photons are guided in a hollow core lined by a highly confining reflective surface dubbed 'the perfect mirror' when Fink invented it in 1998 as an MIT graduate student. The electrons are conducted through metal microwires that surround the fibre core. The photons and electrons do not interact as they are confined to different spatial locations within the fibre.

A second fibre utilises an interaction between photons and electrons. This fibre photodetector was designed to be sensitive to external illumination at specific wavelengths of light. It is made of a cylindrical semiconductor core contacted by four metal microwires that are surrounded by an optical cavity structure. The electrical conductance of this fibre was found to increase dramatically upon illumination with light at the wavelength it was designed to detect.

Some of the most exciting and novel potential applications stem from assembling the fibres into woven structures. As the authors point out, 'It is the assembly of such fibres into 2-D grids or webs that enables the identification of the location of an illumination point on a surface,' and does so with a very small number of fibres.

Embedding these grids in computer screens or onto projection boards could therefore provide a new type of interface, said Fink. 'Instead of having a mechanical mouse, you could just use a light beam, like a laser pointer, to communicate with the computer because the screen would know where it was being hit.'

'Just as in the movie 'Honey, I Shrunk the Kids,' wouldn't it be wonderful if you could fabricate something on the macroscale, then shrink it to a microscopic size?' said Fink. 'That's what we did. But the magic shrink-down apparatus we used is not a 'shrinking beam' from science fiction ... it's a furnace.'

The team first created a macroscopic cylinder, or preform, some 20 centimetres long by 35 millimetres in diameter containing a low-melting-temperature conductor, an amorphous semiconductor, and a high-glass transition thermoplastic insulator. The preform shares the final geometry of the fibre, but lacks functionality due to the absence of intimate contact between its constituents and proper element dimensions.

The preform is subsequently fed into a tube furnace where it is heated and drawn into a fibre that does exhibit both electrical and optical functionalities. 'These follow from the excellent contact, appropriate element dimensions, and the fact that the resulting fibre retains the same structure as the macroscopic preform cylinder throughout the drawing process,' said Bayindir, who designed and synthesised the semiconducting glasses, assembled the preforms and drew the fibres presented in the paper.

The researchers conclude that their new ability to interface materials with widely disparate electrical and optical properties in a fibre, achieve submicrometre features, and realise arbitrary geometries over extended fibre lengths presents 'the opportunity to deliver novel semiconductor device functionalities at fibre-optic length scales and cost.'

This work is funded by the Defense Advanced Research Projects Agency, the Army Research Office, the Office of Naval Research, the Air Force Office of Scientific Research, the Department of Energy, MIT's Institute for Soldier Nanotechnology, and the Materials Research Science and Engineering Center (MRSEC) program of the National Science Foundation.

http://mit-pbg.mit.edu

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