Femtosecond lattice and spin dynamics in solid-state materials

Abstract

Irradiation of femtosecond laser pulses on solid-state materials induces non-equilibrium electronic states, whose lattice and structural properties are far-from equilibrium, e. g., exciting electrons from bonding into anti-bonding states would lead to the breaking of longer (weaker) bonds in phase-change materials. Under such the non-equilibrium conditions, one expects that atomic rearrangements are possible to occur, depending on the electronic excitation density. We have investigated lattice dynamics in phase-change materials, such as Ge₂Sb₂Te₅ (GST225), under non-equilibrium conditions. In addition to GST225, interfacial phase-change memory (iPCM) is more interesting material than GST225, since it would be easier to realize ultrafast lattice switching, based on dominant atomic rearrangement of Ge atoms near the interfaces of superlattice (SL) structure. Recently, it has been actively reported that under a certain structural condition, the iPCM structure may become a topological insulator (TI).
Here, we present a study of non-equilibrium atomic rearrangements in phase-change materials, based on breaking of resonant bonds in the crystalline phase, using amplified femtosecond pulses with less than 40 fs duration and wavelength in the infrared region. In addition, the results for femtosecond spin dynamics in TIs, such as Sb₂Te₃, based on both inverse Faraday effect (IFE) and optical Kerr effect (OKE), are reported, in which transient magnetization by femtosecond laser pulses is induced. The polarization dependence on the OKE signal is governed by the thickness of the sample due to a 3D-2D TI crossover or to existence of a Dirac semimetal phase.