Thermally driven optical switching in Bi nanostructures

Thermally induced transmission and reflectivity changes are investigated in thin films formed by Bi nanostructures embedded in amorphous Al 2O3. The Bi nanostructures are formed by coalesced nanoparticles forming a quasi-network close to the percolation threshold. Upon heating above the Bi melting temperature (>574 K), the transmission of the film increases abruptly, up to 18% in respect to the initial value, which is related to Bi melting. Upon cooling, the high transmission state remains up to temperatures as low as 436 K, thus evidencing a wide melting-solidification hysteresis cycle. The existence of transient morphological changes within the embedded nanostructure related to the contraction of Bi upon melting seems to have a significant contribution to the large transmission contrast between the molten and solid states of the Bi nanostructures. This large contrast together with the large width of the cycle makes this nanostructured film promising for a thermally driven optical switch.