Optical activity in Ge₂Sb₂Te₅ (GST)

Ge2Sb2Te5 (GST) is an established phase-change material that undergoes fast reversible transitions between amorphous and crystalline states with a high electro-optical contrast, enabling applications in non-volatile optical and electronic memories and optically-switchable structured metamaterials. We have recently demonstrated that optical activity can be induced in pure and doped GST thin films using polarised light, opening up the possibility of controlled induction of anisotropic phase transition in these and related materials for optoelectronic and photonic applications. While the phase transition has generally been understood to proceed via a thermal mechanism, our work strongly suggests that there is an electronic component of crystallization induced by the handedness of circularly polarised nanosecond laser pulses. Significant optical activity in the inorganic thin films, measured by circular dichroism spectroscopy at a synchrotron beamline, implies the existence of chiral structures or motifs. Optically active and inactive regions in the film have been studied using electron diffraction and spectroscopic techniques in order to obtain a structural picture that can be correlated to the optical changes observed. We also propose several mechanisms for the observed effects, which may be extended to other material systems and harnessed in photonic or chiroptical applications