[13a-2G-1~10] 3.6 Ultrashort-pulse and high-intensity lasers

[13a-2G-1] Ultrafast time-resolved electron diffraction revealing the nonthermal dynamics of near-UV photoexcitation-induced amorphization in Ge₂Sb₂Te₅

〇Masaki Hada^1,2, Wataru Oba³, Kuwahara Masashi⁴, Ikufumi Katayama³, Toshiharu Saiki⁵, Jun Takeda³, Kazutaka Nakamura^1,6 (1.Tokyo Ints. Tech. MSL, 2.JST-PRESTO, 3.Yokohama Nat. Univ., 4.AIST, 5.Keio Univ., 6.JST-CREST)

Keywords:Ultrafast phenomena,Phase transition,Electron diffraction

Because of their robust switching capability, Ge₂Sb₂Te₅ have been used for optical storages devices. These phase transitions are achieved by laser irradiation via thermal processes. Recent studies have suggested the potential of nonthermal phase transitions in Ge₂Sb₂Te₅ triggered by ultrashort optical pulses; however, a detailed understanding of the amorphization governed by nonthermal processes is still lacking. Here we performed ultrafast time-resolved electron diffraction and single-shot optical pump-probe measurements followed by femtosecond near-ultraviolet pulse irradiation to study the structural dynamics of polycrystalline Ge₂Sb₂Te₅. The experimental results present a nonthermal crystal-to-amorphous phase transition of Ge₂Sb₂Te₅ initiated by the displacements of Ge atoms. Above the fluence threshold, we also found that the permanent amorphization caused by multi-displacement effects is accompanied by a partial hexagonal crystallization.