  We experimentally demonstrate the coexistence of two opposite photo-effects,
viz. fast photodarkening (PD) and slow photobleaching (PB) in Ge19As21Se60 thin
films, when illuminated with a laser of wavelength 671nm, PD appears to begin
instantaneously upon light illumination and saturates in tens of seconds. By
comparison, PB is a slower process that starts only after PD has saturated.
Although we could observe the coexistence of PD/PB even at moderate, one order
of magnitude lower intensity of 0.2 W/cm2, the kinetics of transformation is
significantly slowed down. However, both PD and PB follow stretched exponetial
dependence on time. Modeling of overall change as a linear sum of two
contributions suggests that the changes in As and Ge parts of glass network
respond to light indepndent of each other.
