Beyond the diffraction limit

By Olivier Rekers
Due to the diffraction limit, it is hard to look at things that are smaller than the wavelength of the imaging light. But with the help of a 'hyperlens,' it is possible to produce magnified images of objects that are smaller than the wavelength of the imaging light. Hyperlenses and their close cousins, superlenses, have received a lot of attention recently because they have the potential to provide detailed information in living biological systems, unlike other high resolution imaging systems such as scanning tunneling microscopy.

How do hyperlenses avoid the diffraction limit? To understand that we need to understand what happens to the light when it strikes an object smaller than its wavelength. When light hits any object, information about the object is encoded in the light by changing the amplitude, phase, and direction that the light travels. When the object is smaller than the wavelength of light, the part of the light that carries the information doesn't propagate like normal light, instead it vanishes just a short distance from the object—after traveling one wavelength it is already half gone. These so-called evanescent waves are the key to breaking through the diffraction limit. One property of evanescent waves is that by controlling the refractive index it is possible to create a situation where they do not vanish, but rather propagate like normal light and can then be used to image the very small object. Two papers published in Science contain experimental details of hyperlenses that do just this.

A 'hyperlens' is made out of a cylindrical layered object that has dielectric constants of different signs across the layer (radial axis) and along the layers (tangential axis). It is both possible to use a half-cylindrical or a cylindrical lens. The papers summarized here report on one of each.

The two groups differ in their methods for achieving the necessary strong anisotropy in their 'hyperlens' medium. The group of Liu used a curved, periodic stack of silver and aluminum oxide. This stack is deposited on a concave quarts substrate. The object to be imageed is placed in contact with the lens—in this case a chromium layer inscribed with a pattern. With use of a conventional lens is it possible to make a projection of a sub-wavelength structure.

The group of Smolyaninov combine the idea of a hyperlens with the earlier concept of a superlens. A superlens is made of a single layer of a meta-material with a negative refractive index. In this case, the negative refractive index is created by depositing concentric rings of poly (methyl methacrylate) on a golden film surface. The evanescent wave impinging on the gold surface excites a surface plasmon polariton wave, which experiences the structure as a negative refractive index. Snell's law then ensures that the superlens magnifies a sub-wavelength sample in a ring—the magnification increases as you travel outwards from the center point. Near the edge of the superlens, the magnification is sufficient that it is possible to see use a microscope objective to image the sample.

Both lenses have significant advantages. Conventional microscopy is limited due to the diffraction limit. This limit makes it impossible to see things smaller than 200 nm. Thus, viruses, proteins, DNA molecules, and many other samples that are impossible to clearly visualize with a regular microscope may soon be accessible to visible light microscopy. Used in combination with labeling or spectroscopic techniques that enable the observer to identify different structures this could become a very useful tool in identifying molecular pathways.

Molecular Voyeurism: coming to a microscope near you

The development of atomic force microscope and scanning tunneling microscopes (STM) provide resolution that no other imaging technique can touch. These microscope techniques have progressed to the point that if an atom is dropped onto a specially prepared surface, called atomically flat, then the microscope will provide you with a picture of the atom—the proverbial red blob on a black background. The same is true of small molecules, where each atom is imaged and the molecular structure can be clearly seen. However, getting a picture out takes time, so a complete picture tends to be a time average of the molecule. What that really means is that researchers only get clear pictures of molecules that sit still.

In the Brevia section of Science, researchers are reporting imaging of the step-wise motion of a single molecule. To achieve this, the scientists fixed carbon nanotubes of various widths to a surface. They then deposited hydrocarbon molecules in the carbon nanotubes. This confines the molecules to motion in one dimension, which is important to keep the molecule within the field of view of the STM. They show that some hydrocarbons stick and no motion is observed, however, others moved slowly along the carbon nanotube (the movies are very blurry).

So is this a big advance in microscopy? No, but it is a nice demonstration of single molecule imaging. To put this in context, researchers in the nanotechnology area want to make small molecules that move in a controlled way along a surface. To do this, it will help to be able to observe how such motion works in one dimension—giving chemists more knowledge with which to choose functional groups from their catalog.