  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 wavelength of 1550 and 1200 nm), a
range over which there is significant optical contrast between the crystalline
and amorphous phases of Ge$_{2}$Sb$_{2}$Te$_{5}$ (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 resonant bonding nature of GST225 is discussed.
