Transient photodarkening induced by a nanosecond pulsed laser in amorphous and liquid As2Se3: fractal characteristics in relaxation dynamics

Both amorphous and liquid As 2 Se 3 exhibit fast decrease of optical transmittance under a nanosecond pulsed laser excitation. This light-induced effect is called fast photodarkening (PD). After the cessation of excitation (one shot), the PD recovers within 0.1 s, but remains in a wide time domain from 10 –7 to 10 –2 s. Dynamic responses of the PD-recovery are studied through the autocorrelation function (ACF) in time domain and the power spectrum (PS) in frequency domain. The first zero-crossing time in the ACF, τ 0 , a measure of decay time in the relaxation process, is scaled with the time interval of the original measured data, t m , with the relationship, τ 0 ∝ t m γ (γ ~ 1.0) in both the amorphous and liquid states. The PS is described by 1/ f α (α = 0.8–0.9). Both the ACF and PS results indicate that the structural relaxation of the As–Se network occurs with similar decay processes in “any” time region, which can be inferred to have fractal nature in time domain.