Thermodynamics Doesn't Care about you

Jose Ocaña-Pujol works on optical metamaterials, materials whose properties come from their geometry rather than their chemistry. His group makes them in about an hour instead of days, and he studies whether they hold up at high temperatures. He also learned, after a swim in a cold Lake Zurich, that thermodynamics always wins.

About the guest

Jose Luis Ocaña-Pujol was a PhD student at ETH Zurich in Switzerland when we recorded this episode in 2022. He studied nanoscience at the Autonomous University of Barcelona, then took a master’s in Science for Energy and Sustainability in Amsterdam, which included research at AMOLF and a part-time job in energy analysis. His 2022 paper with colleagues at ETH Zurich, Strain-Driven Thermal and Optical Instability in Silver/Amorphous-Silicon Hyperbolic Metamaterials, was published in Advanced Optical Materials.

Jose L Ocana-Pujol curated the Real Scientists Nano Twitter account (@RealSci_Nano) from October 30 to November 7 2022. Read all of Jose’s tweets from that week.

Key highlights in this episode

• 03:06 Properties from geometry. Metamaterials get unusual properties, such as negative refraction or shrinking sideways when squeezed, from patterns smaller than the wave they interact with.
• 08:02 Fast, scalable, and heat-proof. His group makes metamaterials by magnetron sputtering in about an hour, for uses like thermophotovoltaics, which turn the light from very hot objects into electricity.
• 11:37 From Barcelona to Zurich. One of the first nanoscience degrees in Spain, a master’s designed to get him out of research, and a lab in Amsterdam that pulled him back in.
• 17:12 Slicing the cake. In silver-silicon multilayers, the bottom layers broke down first while the optical properties stayed the same. 3D imaging and simulations showed the cause was strain, and the stacking order controls where it starts.
• 24:04 Three unusual wishes. More specialisation instead of expecting scientists to do everything, greater inclusivity for researchers facing visa barriers, and journals that publish smaller results and failed experiments.

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Why it matters

The first metamaterials were made top-down, which was difficult and time-consuming. Jose’s group designs metamaterials that can be made in minutes or hours rather than days, and that hold up at the high temperatures needed for applications such as hyperthermia and thermophotovoltaics. His paper showed two things. Fast deposition gives rough interfaces, and that disorder actually improved how much light the material absorbs. And when the layers did break down, the cause was stress from the different thermal expansion of the materials, which multilayer theory usually overlooks. Changing the stacking order changed where the breakdown started.

Terms explained

In the guest’s own words, with a link to the moment they explain it.

• Metamaterial: a material whose properties come from its geometry, the shapes and patterns it is built from, rather than its chemistry. This can give properties not normally found in nature. (Jose at 03:06)
• Feature size: for a metamaterial to work with visible light (about 500 to 800 nanometres), its features need to be at least about five times smaller than the wavelength. (Jose at 08:36)
• Focused ion beam (FIB) cross-section: sending ions to cut through a sample, like cutting a cake, so you can look at the layer structure inside. (Jose at 20:01)

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