What determines the lifetime of a mirror when exposed to a very bright light source?
From our experience with visible light, we would guess ‘not a lot’ as the proper answer. Light is simply reflected with 99.9% efficiency. Things are notably different in the Extreme Ultraviolet range: the best mirror there reflects only 70% of the light, which implies 30% is absorbed in the mirror. That gives rise to heating and possibly degradation. A complication is the layered structure of this EUV mirror: it consists of about 50 bilayers of a metal and a spacer layer, for instance molybdenum and silicon, each some ten atoms thick! Now, metals are known to be good conductors in thick films, but what happens in films with atomic thickness, and what happens in the non-conductive spacer material?
On 4 November 2015 Dr. Slava Medvedev has defended his fundamental studies on the physics of EUV mirrors and the associated thin film materials science. He was awarded a Cum Laude for his research. Slava produced creative designs of spectrally selective mirrors with unprecedented optical properties. These have found their way in many user-cases, notably in Extreme UV photolithography, but also in spectroscopy and x-ray analysis. He was an author/co-author of six patents and nine refereed journal papers.
Slava did pioneering research on the thermal conductivity of EUV mirrors. As a part of that he found that multilayered mirrors behave very different from simple mirrors for the visible light. Their thermal conduction is over ten times lower in a direction perpendicular to the mirror surface then in the plane of the bilayers! Such an anomality determines the overall mirror’s behavior in high brightness exposure of the mirror: essential know-how for high-volume photolithography or synchrotron and free-electron laser beamlines! These require detailed understanding of fast and slow materials damage processes.
In the laudatio, professor Fred Bijkerk from the XUV Optics chair in MESA+ praised Slava Medvedev as an exceptional talent in addressing materials and optics challenges that are uncommon but very relevant for optical applications. On a very independent, but communicative way Slava was commented to be able to find original solutions in relatively new areas of optics research like anisotropic thermal-conductivity.
