JUSTIN ZOPPE. Researcher of the Polyfunctional Polymeric Materials research group (POLY2) and a Serra Húnter lecturer at the Department of Materials Science and Engineering of the Universitat Politècnica de Catalunya – BarcelonaTech (UPC).
The ability to manipulate light and to control how it interacts with matter is fundamental in science, technology and engineering. In Herbert George Wells’ 1897 novel, “The Invisible Man,” the protagonist explored an extraordinary possibility: “…a method by which it would be possible, without changing any other property of matter, (except, in some instances, colours) – to lower the refractive index of a substance, solid or liquid, to that of air – so far as all practical purposes are concerned.” In this case, the protagonist proposes changing the way matter bends light to render it invisible. In the 21st century, concepts such as invisibility, that used to be considered science fiction, are slowly becoming closer to reality, in part due to the development of materials with unnatural abilities, also known as metamaterials.
Metamaterials are engineered to exhibit properties not found in nature. They are typically composed of repeating patterns, or arrays, of much smaller subunits. Indeed, it is the patterned structure of the metamaterial, and not the material itself, which gives rise to its unnatural properties. Among the different types of metamaterials, electromagnetic metamaterials interact with light in ways that bring about extraordinary characteristics, such as negative refraction and artificial magnetism. In order for a metamaterial to interact with ultraviolet or visible light, the scale of the repeating patterns needs to be on the order of hundreds of nanometers or less, which is extremely difficult and costly to fabricate using currently available technologies. This is especially the case for so-called chiral metamaterials, which are made up of non-symmetrical subunits, like helices. This represents a significant challenge to be addressed in materials engineering for future optical devices.
The CELICOIDS project, which is funded by the European Research Council, responds to this growing need for simpler metamaterial fabrication technologies and proposes the development of new types of chiral nanostructures to control light-matter interactions. The project focuses on self-assembly of nanoparticles, which is a bottom-up fabrication technique that relies simply on physical interactions between particles in order to form the desired pattern, without the need of complex and expensive equipment.
A great example of a chiral metamaterial is a nanohelix array. One can imagine it as a flat surface with a precisely arranged pattern of springs, as in a box-spring mattress, on the nanoscale. This kind of periodic arrangement of chiral nanostructures strongly interacts with circularly polarized light, a type of light wave that propagates on a helical path. Circularly polarized light can be either right- or left-handed, that is rotating clockwise or counter-clockwise, and is routinely used in optical devices. For example, circular dichroism, the differential absorption of right- and left-handed circularly polarized light, is a powerful analytical tool in chemistry and biochemistry in order to characterize the 3-dimensional chemical structure of molecules. The main problem is that the optical responses of chiral molecules are very weak and chiral metamaterials are one of the most promising solutions to enhance the detection of molecular chirality. This is where CELICOIDS again comes into play.
The research group Polyfunctional Polymeric Materials (POLY2) of the Universitat Politècnica de Catalunya – BarcelonaTech (UPC) is leading the CELICOIDS project, whose objective is to investigate the bottom-up self-assembly of modified nanorods to fabricate a new class of metamaterial, metallic nanohelicoids. The nanorods are obtained from cellulose, a natural polysaccharide extracted from paper, cotton or other plant fibres. When a suspension of the modified cellulose nanorods is poured onto a surface, they self-assemble to form helical structures as they dry. Once these structures are impregnated with metals such as gold, they will guide the formation of metallic nanohelicoids. After crystallizing the metal and removing the modified cellulose template, the ultimate goal is to achieve arrays of new metallic helicoidal structures, which are similar to Archimedes screws on the nanoscale. Such a structure, when combined in a solution of chiral molecules, will likely amplify the overall signal of detection in circular dichroism spectroscopy through their unnatural electromagnetic properties. These properties will open new prospects for optical instruments routinely used in chemistry, biochemistry and pharmacology. For instance, they could facilitate the detection of chiral biomarkers present at very low concentrations for disease diagnosis, prognosis, adverse drug-effect monitoring and personalized medicine. Novel functionalities are likewise envisioned, applicable to future devices for invisibility cloaking and super-resolution imaging in medicine, Earth observation and space exploration.
Image: a graphic image showing an example of the chirality property.