Twisted semiconductors could help project moving holograms
A new method for mass-assembling semiconductors into fusilli pasta shapes could one day lead to moving holograms projected right from your smartphone.
Since their invention in the 1960s, static holograms have found applications in everything from data storage to credit card authentication. But holographic moving images are still stuck in the realm of science fiction.
Now Nicholas Kotov at the University of Michigan and his colleagues hope to change that using spiral semiconductors.
To make a hologram, information about an object is recorded into a light-sensitive material, such as photographic film or plates. When it is lit in just the right way – often with lasers – the recorded pattern is recreated in three-dimensional space.
Regular holograms are frozen light waves. Getting them to move requires a material that can twist light in specific ways – say, get them to change phase or polarisation very quickly – so they act, in essence, like a flip book.
Semiconductors are good materials for this sort of thing because they are easy to work with and some can emit light, but they typically take the shape of sheets or wires. However, Kotov realised that if they could be fabricated in spiral shapes at the nanoscale, they could act as a waveguide: light passing through would naturally follow the twists in the material.
Kotov got this idea when he noticed a similarity between certain synthetic composite substances called metamaterials that have a spiral structure and twisted nanostructures found in nature, most notably in proteins. He thought it should be possible to make a twisted semiconducting material by coating semiconductor particles with amino acids, a key component in proteins that determines how they twist.
These spiral semiconductors could then be easily incorporated into electronic devices like smartphones or displays, thereby enabling control over light properties such as polarisation, phase and colour.
Cadmium telluride nanoparticles were chosen as the semiconductor because they can emit light.
The team mixed the amino acid-coated nanoparticle solution in a vial with methanol, and the resulting chemical reaction caused the nanoparticles to self-assemble into the desired spiralling fusilli shape, 98 per cent of which twisted in the same direction.
“We were quite assured by this experiment that our crazy idea that maybe we can harness the toolbox of biology to meet the needs of the semiconductor industry is not so crazy,” says Kotov.
But there is more work to be done before you could project a tiny holographic Princess Leia from your smartphone. “If one wants to make a hologram out of these materials, it is essential to assemble the interference pattern out of the material in some way,” says Nasser Peyghambarian, an optical scientist at the University of Arizona in Tucson. “Even if they did that, it still produces a still hologram, not a moving hologram. There already exist many good materials for static holograms.”
Kotov emphasised that this is just a first step. “It is something that we envisioned, but it’s not yet a reality,” he says.