Scattering Lens Yields Unprecedented Sharp Images

Elbert G. van Putten

Author: E.G. van Putten

Affiliation: Complex Photonic Systems, MESA+ Institute for Nanotechnology, University of Twente, The Netherlands

It is generally believed that disorder always degrades the sharpness of optical images. Now scientists of the MESA+ Institute at the University of Twente, University of Florence and the FOM Institute AMOLF have shown that a scattering and disordered layer in conjunction with a high refractive index material can be used as an imaging device with a sub-100 nm resolution thereby beating the most expensive microscope objectives. The robustness of this scattering lens against distortion and aberrations, together with the ease of manufacturing and its very high resolution are highly favorable features to improve the performance of a wide range of cutting-edge microscopy techniques.

Even the most expensive microscope objectives offer only a limited resolution. This restriction is due to the wave nature of light that force any focus to be larger than half the wavelength of light (the diffraction limit). This theoretical limit is usually impossible to reach due to practical problems like aberrations that cause focal distortion. Paradoxically a completely disordered layer naturally creates very small and intense light spots when illuminated by a laser. The price to pay is that these spots, which are known as speckle, are arranged in a dense and random pattern making them useless for imaging purposes.

The new scattering lens developed by the scientists, uses light scattering to couple light efficiently into a high refractive index material. By a fine control over the light that illuminates the disordered layer they can concentrate the speckle spots in the same place, effectively creating a single very small focus. Taking advantage of what is known as the "memory effect" the scientists were able to scan this nano-sized focus in the object plane of the lens. They then placed small gold nano particles in the object plane and used the scattering lens to resolve the particles with a sub-100 nm resolution.

Figure 1. Comparison of light focusing with a conventional lens and a scattering lens. (a) A plane light wave sent through a normal lens forms a focus. The focal size is determined by the range of angles in the converging beam as and by the refractive index of the medium that the light is propagating in. The microscope image shows a collection of gold spheres as imaged with a commercial high quality oil immersion microscope objective. Inset on left is a photo of an ordinary lens. (b) The scientists send a shaped wave through a scattering layer on top of a high refractive index material. The wave front is carefully shaped so that, after traveling through the layer, it forms a perfectly spherical, converging wave front. The large range of angles contributing to the converging beam, combined with the high refractive index, give rise to a nanometer-sized focal spot. The microscope image shows the same collection of gold spheres as in (a) imaged with the scattering lens. Inset on left is a photo of the lens with the scattering layer on top.

The combination of a high-index scattering material with the complete control over the illumination provides the first lens to create such a small and scannable focus, which makes it a favorable tool to improve the performance of all the imaging methods that require accurate focusing.

Reference
[1] E.G. van Putten, D. Akbulut, J. Bertolotti, W.L. Vos, A. Lagendijk, A.P. Mosk, “Scattering lens resolves sub-100 nm structures with visible light”, Phys. Rev. Lett. 106, 193905: 1-4 (2011). Abstract.