Reconfigurable optical assembly of nanostructures

Like many scientific breakthroughs, the recent discovery by Dr Haider Butt and two colleagues – that it’s possible to produce reconfigurable optical devices such as erasable and rewritable holograms in a few nanoseconds without any tools or manufacturing processes – happened ‘purely by chance’.

The revelation came when the three researchers found that when you shine high-powered laser light on to randomly arranged nanoparticles (NPs), they move. This means their assemblies can be dynamically manipulated to create more sophisticated nanostructures than ever before.

A major area of interest in science today is the development of programmable materials that change their physical properties. Mechanisms to configure nanostructures in 3D space are essential in nanotechnology, photonics and materials science, with common methods being light related, often using lasers.

Until now, the application of optical forces has been mostly restricted to optical tweezers, but Haider and his colleagues aim to expand this through holographic assemblies of NPs that will allow a new generation of programmable composites for tunable metamaterials, data storage devices, sensors and displays.

Haider, Lecturer in Micro Engineering and Nanotechnology at Birmingham, has been working collaboratively for several years with friends and former fellow Cambridge University researchers Yunuen Montelongo, an holography expert now at Imperial College London, and Biotechnology Engineer Ali K Yetisen, now at Harvard Medical School, on the development of bio sensors, but recently realised their work could have wider applications.

This groundbreaking, multidisciplinary research resulted in a paper, titled ‘Reconfigurable optical assembly of nanostructures’, which was published in the journal Nature Communications earlier this year and won the College’s Paper of the Month award in June (the second time Haider has won the accolade).

‘What we did, and what this paper is about, is we used light as an optical force to arrange NPs,’ explains Pakistani-born Haider, who won the prestigious Philip Leverhulme Prize for his work in 2013 and this year was awarded a Leverhulme Fellowship.

‘Within a flexible material, hydrogel, we are able to produce NPs – which are like balls made out of metal, but a million times smaller than the width of a hair – and randomly arrange them in the gel. An important principle of physics is that light interacts with nanostructures: when they are random, light is confused – it can’t make sense of it, so it just diffuses. So there’s no proper effect that we can see.

‘We used high-powered layers of light, which we call an interference pattern, produced by combining multiple beams of pulsed lasers (which release all the energy within a few nanoseconds). And purely by chance we were able to show that when this light illuminated the random NPs, they moved: the light was strong enough to change the orientation of the NPs and make them move within the material and into the dark regions.’

Eventually, instead of randomly arranged NPs, they were set out in periodic layers. This uniform arrangement is meaningful to light, which interacts with the layers and scatters accordingly.

‘So it tells us what it has just seen, as with X-rays, and that’s music to us because we can really play with it,’ says Haider. ‘Previously, scientists were able to focus a laser and use it to pick up a single NP, but our technique enables the relocation of thousands of NPs simultaneously. This means that without sophisticated clean room-based manufacturing processes we can make much larger optical devices.’

One of the most exciting applications is the writing – and, crucially, the erasing and rewriting, which hasn’t been done before – of holograms.

‘We can change the orientation of NPs as we choose, so can write them in 3D. By shining another laser, we can rewrite them to a different image. So this is the key thing: we are using optical forces to reorient NPs in 3D. By doing this, we can write an infinite amount of arrangements. We can write images. We can have a display system, which will work on lasers. So we can erase an image and write another one. It’s significant because it’s much faster: within a few nanoseconds.

‘This can have applications in the security industry – for example, producing security holograms on documents that can be changed if necessary – data storage, sensors and displays, as well as in aerospace, the military and a number of other industries.’