Abstract
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 similar to 20% (similar to 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 similar to 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. (C) 2016 Elsevier B.V. All rights reserved.