Ultrafast photoinduced anisotropy in GeSe2thin film.

In this Letter, we demonstrate for the first time that anisotropy can be induced at ultrafast time scales in an otherwise isotropic a-GeSe2 thin film using polarized femtosecond light. This photoinduced anisotropy (PA) spans the bandgap to the sub-bandgap region and self-annihilates over picosecond time scales. The ultrafast decay rate of PA is a clear indication that the observed effect is due to photoinduced transient defects in the sub-bandgap region and associated structural rearrangement in the near-bandgap region.

Ultrafast light induced unusually broad transient absorption in the sub-bandgap region of GeSe2 thin film.

In this paper, we show for the first time that ultrafast light illumination can induce an unusually broad transient optical absorption (TA), spanning of ≈ 200 nm in the sub-bandgap region of chalcogenide GeSe2 thin films, which we interpret as being a manifestation of creation and annihilation of light induced defects. Further, TA in ultrashort time scales show a maximum at longer wavelength, however blue shifts as time evolves, which provides the first direct evidence of the multiple decay mechanisms of these defects. Detailed global analysis of the kinetic data clearly demonstrates that two and three decay constants are required to quantitatively model the experimental data at ps and ns respectively.

Crossover from photodarkening to photobleaching in a-Ge(x)Se(100-x) thin films.

In this Letter, we present the interesting results of photodarkening (PD), transition toward photostability, and a slow crossover from PD to photobleaching when composition of the chalcogenide glassy thin film changes from Ge-deficient to rich. A subsequent Raman analysis on these as-prepared and irradiated samples provide the direct evidence of photoinduced structural rearrangement, i.e., photocrystallization of Se and the removal of edge-sharing GeSe4 tetrahedra. Further, our experimental results clearly demonstrate that light-induced effects can be effectively controlled by choosing the right composition and provide valuable information on synthesizing photostable/sensitive glasses.

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 Ge(19)As(21)Se(60) thin films, when illuminated with a laser of wavelength 671 nm. 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. Both PD and PB follow stretched exponential 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 effectively independent of each other.

Role of rigidity and temperature in the kinetics of photodarkening in Ge(x)As(45-x)Se55 thin films.

We present insightful results on the kinetics of photodarkening (PD) in Ge(x)As(45-x)Se55 glasses at the ambient and liquid helium temperatures when the network rigidity is increased by varying x from 0 to 16. We observe a many fold change in PD and its kinetics with decreasing network flexibility and temperature. Moreover, temporal evolution of PD shows a dramatic change with increasing x.