Imaging Beyond the Diffraction Limit

Hannah Barnard is a third year PhD student in the XM² Metamaterials CDT within Physics at the University of Exeter. Her supervisors are Professor Geoff Nash (Director of Natural Sciences), Dr Isaac Luxmoore (Senior lecturer in Engineering) and Dr Jonathan Philips (Future Leaders Fellow in LSI). Her PhD is an interdisciplinary study into using metamaterials for enhanced spectroscopy of biological molecules.

My journey into research began while studying for an Undergraduate degree in Natural Sciences here at Exeter. Quickly, my horizons broadened and I learnt just how interdisciplinary modern science and research has to be. I have always been passionate about breaking down the barriers in science- not only between the traditional sciences, but also to science itself; making it accessible for all. When I first began my PhD, I knew that working across the disciplines at a fundamental research level was going to be tough. But I also knew it would be worthwhile; it would provide me with an invaluable opportunity for meaningful contribution and to disseminate scientific knowledge more widely.

The problem:

Presently, I sit in the Metamaterials CDT within Physics at the University, working on a project to enhance our ability to image and detect very small entities. Originally, I decided to work in this field because of the far reaching implications of success. Currently, all imaging techniques are limited by the ‘diffraction limit of light’. In simple terms, this means that light of a particular wavelength can only interact with objects roughly the same size as that wavelength or larger. It turns out, that many of the objects scientists might want to measure are much smaller than the wavelength of light, and so it falls to researchers such as myself to try and overcome this fundamental limit. One example of such entities is a biological molecule like a protein. These are complex, dynamic molecules, involved in sustaining life, progressing disease, and even overcoming infection by means of our immune system. In order to fully understand their function within our bodies, it would be very useful to design a system that can interact with, and therefore tell us some information about, these tiny molecules.

The solution:

That’s where the physics comes in. I work within the field of Metamaterials, which encompasses any man-made material made up of some small periodic structure of so called ‘meta-atoms’. These ‘meta-atoms’ are analogous to regular atoms within a material, except they are designed and made by scientists out of pre-existing materials. In doing so, new, novel materials are created that have unique and sometimes unusual properties. In general, the new material will interact with light in a way that is not otherwise possible with natural materials. In my research, I focus on designing materials that confine light to very tiny spatial regions. Such confinement facilitates interaction with molecules that would otherwise be too small.

In addition to the metamaterial itself, the type of light being used is very important. For traditional microscopy, one uses light within the optical range – in other words, light our eyes can detect. However, for my particular research, I don’t want to produce a traditional image of the sample, like one you might see down a microscope. Instead I want to perform spectroscopy, a technique used to probe the constituents of an unknown sample. Instead of producing an image, my work produces a list of ingredients (or atoms) that make up that sample. In order to do this, I use infrared light instead of visible, which has a wavelength of roughly 2000-6000nm – around 100 times the size of a protein molecule, and far too large to interact with tiny molecules on its own. However, by designing a metamaterial that interacts with the infrared light, I can achieve confinement down to a region of several hundred nanometers (nm) in size. This is far closer to the dimensions of the molecules themselves. Thus, such a metamaterial can be used to perform enhanced spectroscopy, informing us about the composition of an unknown sample of molecules.

Outlook:

At the moment, this technology is still in its infancy in a number of ways. The design process for a metamaterial is lengthy and requires computer software capable of intensive simulations in order to test different designs and optimise the confinement of the light. Fabrication is also time consuming and problematic; the metamaterials themselves are made up of ‘meta-atoms’ roughly 500nm in size, which means very expensive equipment (such as electron beam lithographers) are required to fabricate the designs. In addition, the yield of in-house fabrication tends to be around 30%. Measuring of the resulting confinement of a finished metamaterial is also challenging due to the small size of the end material and the relatively weak signal produced.

Despite all of this, the field shows enormous potential. Several groups have been successful in using metamaterials to probe very small samples of various molecules, which would otherwise be invisible to infrared light. Personally, I have succeeded in producing metamaterials that confine light to tiny regions and manipulate it such that it can interact with chiral molecules, a special type of molecule where two mirror-image versions of the same molecule exist.

What is really crucial here, is the ability to apply metamaterial enhanced spectroscopy to very small sample sizes and potentially even individual molecules in the future. This would allow molecule by molecule analysis of samples, shining new light on the mechanisms and functional configurations of proteins, drug constructs, and complex protein conjugates within the body, totally revolutionising the field.