Advancing Basic Science for Humanity
2014 Nanoscience Prize Explanatory Notes
The ability to image objects with visible light is at the heart of our perception of the world, and is essential in fields ranging from cancer diagnostics to astronomy. Up until only a few years ago, the resolution of optical imaging was thought to be limited by the wavelength of light, making it impossible to image nano-objects.
In making their award, the Kavli Prize Committee in Nanoscience has selected three scientists who have demonstrated how to break this resolution limit and how to use light to interact with matter with nanoscale precision, allowing the development of new and more efficient imaging techniques and photonic devices.
Thomas W. Ebbesen was primarily working on carbon nanostructures when he made an extraordinary observation in a different field. He noticed that light could be transmitted through a metal plate with a pattern of holes that were much smaller than the wavelength. The phenomenon was unexpected because it was assumed that, when a beam of light hits a hole smaller than its wavelength, it is affected by very poor transmission and strong diffraction.
Focused ion beam micrograph image of the bull’s eye structure on a 300 nm-thick silver film used to demonstrated transmission through a subwavelength hole. The hole has a diameter of 250 nm, the grooves periodicity is 500 nm and their depth is 60 nm. (Science 297, 820-822 (2002))
More systematic experiments showed that this extraordinary optical transmission, as it was named in the seminal paper that reported the results in 1998, was only occurring for specific wavelengths, and there was a well-defined relationship between these wavelengths and the periodicity of the holes. The phenomenon was related to the interaction of the light beam with surface plasmons. These are collective electron states in a metal that can harvest the incident light at a wavelength determined by the geometry of the metallic surface, that is, the periodicity and shape of the holes. The effect of the plasmons was so strong that they could channel part of the light hitting the metal surface, and send it through the holes, resulting in a transmission higher than 100%.
Ebbesen kept working on this topic and shortly after he and his colleagues showed that light could also be transmitted through a single hole in a metal plate measuring only a couple of hundred nanometres in diameter. This worked as long as the hole was surrounded by a periodic structure of grooves in the metal that allowed a light beam to be coupled with the plasmons. What is more, not only can light be transmitted through such a small hole; it can also be beamed in a collimated ray. Ebbesen and co-workers studied a so-called bull’s eye structure, consisting in a periodic pattern of concentric grooves in a silver plate around a 200 nm hole. If the same structure is fabricated also on the output side of the hole, the light exits as in an almost parallel beam.
The extraordinary light transmission through sub-wavelength holes has several applications in diverse fields of photonics. Because of the dependence of the transmitted wavelength on the geometry of the metal plate, it is possible to design very selective optical filters or polarizers. The creation of collimated and very small beams can be used in photonic devices like lasers. Furthermore, it was shown that the fluorescence of a single molecule collected through one such hole is substantially enhanced. Finally, it is, in principle, possible to use the phenomenon to obtain very small features in lithography, the technique of forming patterns onto materials that allows subsequent formation of devices and circuits
In the late 1980s, while working on his postgraduate project in Heidelberg, Stefan W. Hell became interested in ways to improve the resolution of confocal microscopy. In particular, he convinced himself that it would be possible to break the barrier of 200 nm resolution in optical microscopy imposed by Ernst Abbe’s law in 1873, assumed as a fundamental limit by the whole scientific community.
According to Abbe’s law, the optical resolution of a microscope is limited by the smallest spot at which a microscope objective can focus a laser beam. For example, in fluorescence microscopy, molecular dyes known as fluorophores are injected into a tissue and illuminated to produce an image. Fluorophores within the laser spot are all illuminated at the same time, resulting in a resolution which is at best 200 nm.
STED microscopy image of protein complexes (right), revealing a much higher resolution than conventional confocal microscopy (left). The scale bar is 500 nm. (Adapted from Biophysical Journal 105, L01–L03 (2013))
Hell realized that the best way to overcome the limitations set by Abbe’s law was to affect the electronic states of the fluorophores. In standard fluorescence microscopy, a laser beam is used to excite the fluorophores from their ground states to a state with higher energy. After a certain time, the fluorophores decay back to their ground state by the emission of light. In 1994, while working in Turku, Finland, Hell published a theoretical paper explaining the concept of what is now known as stimulated emission depletion (STED) microscopy. Beside the laser beam used in fluorescence microscopy, a second, donut-shaped beam was superimposed to the first one to force all fluorophores located outside the central hole of the donut back to the ground state without emitting light. By varying the intensity of the laser and the shape of the donut, it would be possible to improve the resolution, in principle to a few nanometres.
The concept was intriguing but it had to wait a few years to be widely accepted. In 2000, Hell and co-workers published another paper, this time experimental, demonstrating the use of STED microscopy to obtain images with a resolution of just over 100 nm. In the following years, Hell’s research helped improving the resolution down to a few tens of nanometres and to develop other forms of microscopy, based on the concepts of STED, but necessitating much lower laser power. One such example is reversible saturable optical fluorescence transition (RESOLFT) microscopy.
Hell’s work has always been driven by the principle that the power of an optical microscope is ultimately determined by its resolution, and that the ability to distinguish details only a few nanometres apart would lead to essential information on how proteins, lipids and even neurons interact with each other. A beautiful example of the success of his approach is the imaging, in 2012, of the brain of a living mouse with a 70 nm resolution.
John B. Pendry of Imperial College London designed artificial materials with properties not found in nature, and showed that such materials can be used to fabricate optical lenses with a resolution smaller than the wavelength of light, thus allowing imaging of objects with nanoscale dimensions.
The materials inspired by Pendry’s vision since the 1990’s are called ‘metamaterials’. They are composed by nanoscale metallic or dielectric elements, with a geometry that determines the optical properties. What is more, this geometry can be engineered to obtain the desired response to electric and magnetic fields. Pendry’s work also predicts metamaterials with a negative refractive index, a feature not found in nature, and resulting in dramatically different ways in which light “bends” as it traverses two different materials. Negative index materials were first experimentally demonstrated by David Smith in 2000.
In 1968, Victor Veselago predicted that a slab of material with negative refractive index can focus light. Building on this work, Pendry theoretically demonstrated in 2000 that such a slab not only can focus light, but also that it does so with sub-wavelength resolution, which cannot be achieved with standard lenses. Pendry named this slab of metamaterial a ‘perfect lens’, because it can focus light of a given wavelength with unlimited resolution, if the index of refraction is exactly -1. This is due to the action of surface plasmons on both sides of the slab, as they amplify the components of the electromagnetic radiation - the so-called ‘near field’ - that carries sub-wavelength information of the object. This is information that is normally lost in conventional optical lenses.
Electric field patterns of microwave radiation with a frequency of 8.5 GHz as it propagates through a metamaterial cloak (the region between the two black circles) wrapped around a copper cylinder (inner black circle). The wave front propagates from left to right. Outside the cloak, the wave front continues relatively unperturbed, making the cylinder partially invisible. (Adapted from Science 314, 977-980 (2006))
Lenses with sub-wavelength resolution built from silver metamaterials have been experimentally demonstrated in 2005 by two independent groups in Berkeley and Canterbury. Such devices can be applied in a vast range of technologies, from imaging of small biological structures such as DNA molecules, to manufacturing of electronic devices with smaller dimensions or optical data storage with increased information density.
The applications of metamaterials go beyond perfect lenses; they can also be used selectively to absorb different wavelengths of radiation, or to realise the ‘invisibility cloak’, proposed by Pendry in the early 2000s. When shaped around an object, the metamaterial cloak can hide it from electromagnetic radiation, making it invisible to an observer. Cloacking is analogous to creating a mirage; light is bent in a way that our brain reconstructs. In 2006, Pendry, David Smith and co-workers demonstrated cloaking at microwave frequencies. Although cloaking has only been demonstrated for small objects and may not be the most promising application of metamaterials, the concept is interesting as it challenges our intuition and understanding of the behaviour of light.
Professor Arne Brataas, of the Norwegian University of Science and Technology, and chairman of the Kavli Nanoscience Prize Committee, said: “Thomas W. Ebbesen, Stefan W. Hell, and John B. Pendry have independently advanced our ability to ‘see’ nano-scale objects using visible light. They have greatly advanced our understanding of nano-optics and the applications of their insights promise to have an enduring benefit to a wide range of fields from physics and chemistry to the biological and biomedical sciences”.
—Elisa De Ranieri and Fabio Pulizzi