Coexistence of fast photodarkening and slow photobleaching in   Ge19As21Se60 thin films
  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. 
Pritam Khan, A. R. Barik, E. M. Vinod, K. S. Sangunni, H. Jain and K.   V. Adarsh
cond-mat.mtrl-sci physics.optics