Transmission based measurements of femtosecond laser induced nonlinear absorption inside polymers

Interaction of intense ultrashort laser pulses with wide bandgap materials is dominated by nonlinear absorption that can be measured by monitoring the transmission of incident pulse or a weak probe pulse. Transmission of incident pulses focused inside glass exhibit a gradual decline after nonlinear absorption sets in at a certain threshold laser fluence. In contrast, we demonstrate a step-function like self-limiting behaviour in transmission of incident pulses inside polymers, dropping abruptly to ∼ 20% (∼ 35%) in PDMS (PMMA) and remaining nearly constant beyond the threshold fluence. The self-limiting threshold, which decreases with the number of incident pulses, is also associated with the onset of blackening in polymers. By measuring the transmission of a probe pulse we show that the degree of blackening increases with the number of pump laser pulses reaching a saturation where only ∼ 40% of the probe is transmitted. Spectroscopic characterization of laser irradiated polymers indicates a localized reduction in the band gap when compared with pristine polymers. We exploit laser-induced blackening to fabricate embedded micro-optical white light filters by varying the laser fluence and the number of modified layers.