Abstract
We proposed broadband single-shot femtosecond spectroscopy in a time-frequency space using a
reflective echelon mirror that makes a spatially encoded time-delay for a white-light continuum probe
pulse. It produces a temporal step of 66 fs and an overall time-delay of 33 ps. With this technique, we
observed an ultrafast crystalline-to-amorphous phase-change in chalcogenide alloys: Ge2Sb2Te5 (GST)
and GeTe thin films. We even measured the absorbance change that accompanied the ultrafast
amorphization for laser fluences above the critical value, where a permanent recording mark was formed.
The observed rise time to reach amorphization was 130~200 fs both in GST and GeTe thin films,
suggesting that ultrafast amorphization can be attributed to the rearrangement of Ge atoms from an
octahedral to a tetrahedral structure.
