Using laser beams to write patterns into a plastic at a scale smaller than the eye can see, a group of British researchers has developed a new, cheap way of making porous materials called 'photonic crystals', which are impenetrable to light of certain wavelengths.
The new technique is a type of freeform fabrication: a hands-off sculpture with which solid materials can be fashioned into complicated structures. One way of doing this is to build up the object layer by layer, as a series of slices. This can be done in several ways, the most elegant of which uses laser beams to harden a light-sensitive resin within a resin-filled vat. If the resin is 'cured' only where the two beams cross, the beams can 'draw' a solid plastic form into the liquid, one layer at a time. Systems like this are used in medicine, for example, to model a patient's head without the slow and uncomfortable process of taking a cast.
Making a life-sized head is one thing. But what Andrew Turberfield and colleagues at the University of Oxford, UK, set out to do was something altogether more delicate. They have used laser-based freeforming to create a material peppered with a periodic array of holes each less than a thousandth of a millimetre across -- a kind of three-dimensional mesh so fine that a bacterium would have trouble wriggling through it. They describe how in
Tiny grids like this can act as fences for keeping light out -- or in. If the gaps are properly positioned, the riddled material becomes a so-called photonic crystal, from which light is excluded. This exclusivity operates only for a particular band of wavelengths, which depends on the spacing and the arrangement of the holes. Roughly, the wavelength of the excluded light is equal to several times the width of the holes. So to keep out visible light, with wavelengths of a few hundred millionths of a millimetre, the holes must be about a thousandth of a millimetre across.
But who wants to keep out light? The reason photonic crystals might be technologically valuable is that they can confine light. A passage through a photonic crystal should act like an optical fibre, keeping light inside. But it would be more secure than optical fibres, which are typically slightly leaky. Light can't escape a photonic crystal, because it simply can't travel through the thicket of holes. So photonic crystals might, for instance, guide light around in 'photonic integrated circuits' -- microchips that run on light rather than electricity. They might also provide new kinds of miniaturized laser.
There are various ways of making photonic crystals that exclude visible light. Electron beams can be used to erode holes in a solid block of material. Alternatively, tiny spherical beads can be made to settle into regular arrays, like stacked apples. Filling in the gaps with some solid material and then dissolving the beads away, produces a hole-filled grid.
But the method described by Turberfield's group is arguably even more elegant, and potentially more versatile. They solidify a light-hardening resin into a three-dimensional grid by using the interference pattern set up between several intersecting laser beams.
When two waves -- two light beams, or two series of ripples on a pond -- cross one another, the trains of peaks and troughs can set up a stationary series of bright and dark 'interference fringes', where the disturbances either reinforce each other or cancel out. The interference pattern formed from several laser beams converging in three dimensions is more complicated. If the beams are suitably arranged, the bright regions join into a three-dimensional grid. The symmetry properties of the grid -- whether, for example, the holes are arranged at the corners of cubes, tetrahedra or other shapes -- depends on the arrangement of the beams.
The researchers used this technique to solidify a resin into various kinds of grid. They then turned these into photonic crystals by filling the pores with titanium dioxide and heating the composite to burn away the resin, leaving a solid where the holes had been and vice versa. Unfortunately, heating titanium dioxide makes it shrink and crack; but there are other, similar 'casting' procedures that don't. And, unlike some other methods for making photonic crystals, this one is cheap, quick and suitable for large-scale production.