Enhancement of coherent phonon amplitude in phase-change materials by near-infrared laser irradiation

We have examined the effect of pump-probe photon energy on the amplitude of coherent optical phonons in a prototypical phase change material using a femtosecond time-resolved transmission technique. The photon energy was varied between 0.8 and 1.0 eV (corresponding to the wavelengths of 1550 and 1200 nm), a range over which there is significant optical contrast between the crystalline and amorphous phases of Ge₂Sb₂Te₅ (GST225). It was found that in crystalline GST225, the coherent phonon amplitude monotonically increases as the photon energy increases, indicating that the phonon amplitude is enhanced by interband optical absorption, which is associated with the imaginary part of the dielectric function. In amorphous GST225, in contrast, the coherent phonon amplitude does not depend on the photon energy, remaining almost constant over the tuning range. A possible contribution from the polarizability associated with the resonant bonding nature of GST225 is discussed.