In-person MSE Colloquium: Steve Yalisove, Ultrafast laser enhanced mass transport: 10 to 20 orders of magnitude increase at 10 picoseconds at a time

Abstract

Mass transport in solids at room temperature are typically 15 to 20 orders of magnitude slower than diffusion in liquids. We have normally assumed that we needed to reach a homologous temperature of about 0.7 or 0.8 to anneal a material. At that temperature, mass transport is usually 4-8 orders of magnitude lower than the liquid. Our recent work to uncover the physical mechanisms of high spatial frequency laser induced periodic surface structures (HSLIPSS or just HSFL) has deepened our understanding of the role an ultrafast laser plays in generating dissociated Frenkel pairs in semiconductors. The short laser pulse imposes a large electro-magnetic field seen by atoms in a solid that is sufficient to excite valence electrons into the conduction band, creating point defects, for about 2% of the atoms in the excitation region. Because the atoms are already vibrating with the Maxwell-Boltzman velocity distribution at room temperature, those atoms who lose those valence electrons can freely drift until the electrons relax back to the lowered energy state (about 1-10ps). This traps atoms in interstitial locations and leaves vacancies in their wake. Our model worked very well with surface plasmon-polariton models to match our results in GaAs to the 1-2% level. Furthermore, subsequent laser irradiations are able to greatly enhance the mass transport of these defects since we significantly soften the bonds of a far greater fraction of atoms than we actually break. Indeed, we see 200 nm corrugation of the surface affect only 1000 irradiations in one second. We are at fluences well below the melt threshold and no material leave, it just is redistributed. We will present our results that show that the mass transport needed to redistribute this amount of material is the mass transport you would typically see in a molten material. However, our material never got hotter than 800 degrees K. This is, in the most conservative case, 12 orders of magnitude higher than normal mass transport at 800K. We will also present work that demonstrates the enhanced mass transport in Si, and the oxidation of Si.

Bio

Steve Yalisove obtained a PhD in Materials Science and Engineering at the University of Pennsylvania in 1986. After a post doc at Bell Laboratories, he joined the Michigan faculty in 1989. In 1996 he was a Fulbright scholar at the FOM institute in the Netherlands. He is currently the Associate Director of the Materials Laboratory at the Center for Ultrafast Optical Sciences at the University of Michigan. Yalisove’s current research focuses on understanding the relationships between atomic structure and materials properties at surfaces and interfaces in a wide variety of material systems. He has made important contributions to the fields of surface science, thin film growth, evolution of thin film morphology, and most recently, the interaction of high intensity femtosecond laser pulses and materials. Ultrafast laser/material interaction is being studied in his group to understand the fundamental mechanisms which drive ablation and collateral damage. His work focuses on the modification and material removal processes in metals, semiconductors and ceramics as well as organic materials including graphene and carbon nanotubes. Yalisove uses a variety of characterization techniques including pump-probe ultrafast microscopy, femtosecond Laser Induced Breakdown Spectroscopy (fsLIBS), dual pulse LIBS, optical and scanning electron microscopy, transmission electron microscopy, and a variety of in-situ probes. Recently his group discovered a novel approach to nano and micro fluidic channel manufacturing using ultrafast lasers. Additionally, his group has been investigating the role that fs lasers can play in modification of interfaces for joining materials. A recent discovery of non-thermal point defect injection mechanisms (well below the melt threshold) offer a novel approach for doping materials and fabrication of unique sensors. His group is also studying how short intense pulses of light can alter the electronic structure of a material where long lived events can be generated. His group is also using still higher fluence to induce shock waves and push materials into extreme states for creating materials that would not exist otherwise. He is also very interested in revolutionizing engineering education.

Zoom option

https://osu.zoom.us/s/99442121610

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