Metamaterial tailors thermal emission

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay. In the last ten years, an abstract mathematical concept has been applied to materials: topology classifies geometric objects based on whether they can be continuously deformed to look like each other. Now, one research group has used this concept to develop a metamaterial which can tailor thermal emission. Rather than using a periodic system, which most topological materials employ, the team borrowed a concept from laser design and created an optical cavity using a dielectric medium sandwiched between two layers that act as mirrors: a metal substrate and a top layer of platinum. 

COSKUN KOCABAS: Cavity means you have two mirrors and you trap light between two mirrors. But our cavity is slightly different. We have a bottom mirror which is a perfectly reflecting surface but the top mirror is partially reflecting but at the same time it is a thermal emitter, so basically thermal emission is coming from the top mirror. At one resonance condition we observe this topological phase transition, so this cavity goes through a topological phase transition when we change the thickness of the top layer. So the thickness of the top layer defines the topology.

LAURA LEAY: That was Professor Coskun Kocabas from The University of Manchester in the UK. The thickness of the top layer controls the emissivity; at a critical thickness of 2.3 nm perfect emission is achieved. Under these conditions, the physics at play is similar to the physics behind how a laser converts incoherent light to coherent light. One key aspect of the research is that the interface between two surfaces is required.

COSKUN KOCABAS: There is this topological argument: if you have two topologically different objects and if you bring them together, if you form a boundary, what will happen at this boundary? Because you go from one topology to another topology, you have to go through a boundary which should emit. We look at the surface with an infra-red camera – and two cold surfaces because they have low emissivity – at the boundary we see a very hot line. So that line, that boundary is an indicator of the topological phase transition.

LAURA LEAY: The metamaterial, which is essentially a multilayer coating, can be used for thermal regulation in environments where conventional cooling cannot be used. This applies to applications in space satellites where you can’t simply blow air over an electrical component but need to absorb or reflect incoming radiation, military uses where you want to control a heat signature without causing overheating, or to infrared optics. This work was published in a recent issue of Science. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.