Controlled laser radiates through opaque materials
Opaque materials like white paint, an eggshell or human tissue diffuse incident light. Diffuse light is omnidirectional. Generally speaking, it is not possible to have a beam of light transmitted through these materials. However, researchers at the FOM Foundation and the MESA+ Research Institute, University of Twente, have been succeeding in using diffusion exactly for focussing on light. By using interference they had opaque objects - e.g. an eggshell - transmitting a beam of light or even focussing rays of light. This technology is a step forward to examining opaque tissue or separately focussing on deep cells. Their findings will soon be published in the authoritative journal Optics Letters.
Light is a plane wave. Take a cross section at right angles to a beam of light and notice that all spots on the cross section share the same phase. Such a segment is called a wavefront. If the bundle of light is dropped on an opaque material such as an eggshell, this material will scatter the light. The wavefront becomes perturbed and very soon, the perturbation is so serious that the light has become diffuse: it is omnidirectional and cannot be depicted. However, we do notice a diffuse slight spot if, for instance, we have light dropped on an eggshell or on paper.
When the same experiment is carried out with laser light, an unpredictable interference pattern arises due to the way the laser waves are interacting: areas arise that have either a high intensity or a low intensity. Laser light gives a sharp interference pattern, also called a speckled pattern, because all waves in the bundle share the same wavelength. This makes laser light so special: even after scattering a couple of hundred times, the phase of the light can still be defined nicely.
'Anti-scattering'
The scientists at the University of Twente have used the phase data in order to smooth the wavefronts that emerges from the material and thus, to remove the perturbation. Actually, they have inversed the scattering process with that. Firstly, they transmitted the laser bundle through a modulator. By using the modulator they were able to scatter the bundle meticulously. Then, the scattered bundle was dropped on an opaque material that scattered the bundle of light in the opposite direction. After scattering hundreds of times a focussed laser bundle arises through interference. They carried out the experiment by using materials like white paint, a milk tooth, petals and an eggshell. Each material requires a particular focus of the modulator. So, the wavefront and the piece of material will match like key and lock.
Scientists considered diffusion always to be an irreversible process. The researchers at the University of Twente have now proven the opposite for light. This implies that opaque material does not have to be an insurmountable barrier any longer, say, for microscopy. They are now going to develop this technology any further, in order to be able to look inside opaque tissue or to focus on separate cells.
References: 'Focusing coherent light through opaque strongly scattering media', by Ivo Vellekoop and Allard Mosk, http://ol.osa.org/upcoming_pdf.cfm?id=80762.
For more information, please contact Ivo Vellekoop, University of Twente, (053) 489 53 90.
Information is also to be found on the website: http://cops.tnw.utwente.nl/.
An animated illustration of the experiment is to be found on: http://cops.tnw.utwente.nl/research/animation_focus/speckle.html.