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Is equilibrium overrated? Processing of optical materials at finite time scales

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In materials processing of crystalline materials, the first “go-to” bit of information is the equilibrium phase diagram –an indication of the way things might be, given sufficient time. One can approach that structure by melting, slow cooling and controlling nucleation, as is done in the semiconductor industry. Molecular Beam Epitaxy (MBE) uses high temperatures and the mobility of arriving atoms to make single-crystal thin films of these materials.

This talk will examine cases where fabrication of materials with optical properties of interest occurs far from equilibrium. Group IV semiconductor-core optical fibres have recently emerged as a platform for non-linear optics and mid-to far IR transmission. In processing of these materials, the atypically large thermal gradient established during laser heating has permitted the fabrication of single crystals with geometrical aspect ratios approaching a million. This works only at high solidification rates for elemental systems. These rates need to be slower for alloys due to the miscibility gap that hampers the growth of bulk material, but the preferential segregation of one component allows interesting structuring of the semiconductor core. In depositing semiconductor films for electrical devices, crystal perfection is essential, but if a large energy-gap material is being used as a host for atomic species that only require local order, the constraints are relaxed. ZnS:Cr, of interest as a mid-IR laser host is just such a material, and the results of slow vs rapid deposition will be presented.

These two cases illustrate the value of investigating a wide range of processing conditions, taking into account the requirements for a particular application in addition to the established convention for what is deemed “perfect”.

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