The Photoinduced Response of Antimony from Femtoseconds to Minutes

Antimony is an interesting candidate for photonic memory applications due to a number of desirable properties. These include a large optical contrast over a wide wavelength range between crystalline and amorphous solid states. In addition, switching between the states is possible on nanosecond timescales by applying short heating pulses. The amorphous (glass) state is then attained by melting and rapid quenching through a supercooled liquid regime, whereas recrystallisation requires only a more moderate temperature increase. While initial and final states in such a switching cycle are easily characterized, little is known about the optical properties on the path to forming a glass. Here we resolve the entire switching cycle of antimony with femtosecond resolution in stroboscopic optical pump-probe measurements and combine the experimental results with ab-initio molecular dynamics simulations. The glass formation process of antimony is revealed to be a complex multi-step process in which the intermediate transient states exhibit distinct optical properties with even larger contrasts than those observed between crystal and glass. Our findings indicate that the dissolution and formation of a structural distortion motif common to PCMs plays a significant role in the property contrast between distorted crystals and undistorted liquids. Moreover, it is the origin of the large optical response observed in both the ultrafast regime and during the transition from the metallic supercooled liquid to the semiconducting glass. The quantitative understanding that is provided forms the basis for its exploitation in high bandwidth photonic applications.