Ultrafast optical response of the amorphous and crystalline states of the phase change materialGe2Sb2Te5

Abstract

We examine the ultrafast optical response of the crystalline and amorphous phases of the phase change material Ge2Sb2Te5 below the phase transformation threshold. Simultaneous measurement of the transmissivity and reflectivity of thin film samples yields the time-dependent evolution of the dielectric function for both phases. We then identify how lattice motion and electronic excitation manifest in the dielectric response. The dielectric response of both phases is large but markedly different. At 800 nm, the changes in amorphous GST are well described by the Drude response of the generated photo-carriers, whereas the crystalline phase is better described by the depopulation of resonant bonds. We find that the generated coherent phonons have a greater influence in the amorphous phase than the crystalline phase. Furthermore, coherent phonons do not influence resonant bonding. For fluences up to 50% of the transformation threshold, the structure does not exhibit bond softening in either phase, enabling large changes of the optical properties without structural modification.

Giant Photoinduced Chirality in Thin Film Ge2Sb2Te5

“…In support of our hypothesis that light affects phase transitions in other ways than thermally, recent experiments have attributed photoinduced phase transitions to electronic excitation at subnanosecond timescales . It has also been reported that photoinduced crystallization of amorphous selenium films is polarization dependent .…”

Photo-induced Tellurium segregation in $\mathrm{MoTe_2}$

“…The emergent mode at 3.6 THz is considered to be the A 1 mode of tellurium (Te) [30][31][32][33][34] due to segregation of atomic Te from the MoTe 2 samples as a result of surface melting induced by heat-accumulation effects of high repetitive pulsed laser irradiation. The Te segregation observed to date in coherent phonon spectra appearing at 3.6 THz has been occasionally observed in other telluride compounds, such as Ge 2 Sb 2 Te 5 [35], Sb 2 Te 3 [36,37], CdTe [38], ZnTe [39] as well as static-measurement studies on MoTe 2 [18]. On the other hand, additional phonon modes related to the different structural phases were not observed after the laser irradiation: the 2H-to-1T' and 1T'-to-2H phase transitions were not observed.…”

Sub‐Picosecond Non‐Equilibrium States in the Amorphous Phase of GeTe Phase‐Change Material Thin Films

“…Miller et al. [ 21 ] obtained the similar tendencies and timescales for optical excitation of Ge 2 Sb 2 Te 5 , this alloy being also a prototypical PCM. The variation of the pump fluence has no effect on the recorded transients of ε r .…”