Abstract

We performed a series of experiments on suspensions of carbon particles in liquids (ink) and carbon particles deposited on glass to determine the mechanisms for the observed optical-limiting behavior. Both materials show reduced transmittance for increasing fluence (energy per unit area). We found that nonlinear scattering dominates the transmissive losses and that the limiting is fluence dependent, so that limiters based on black ink are effective for nanosecond pulses but not for picosecond pulses. Additionally, the nonlinear scattering and the limiting behavior cease after repeated irradiation. For the liquid, flowing eliminates this effect. All the data obtained are consistent with a model of direct heating of the microscopic-sized carbon particles by linear absorption with subsequent optical breakdown initiated by thermally ionized carriers. A simple calculation gives temperatures higher than the sublimation temperature at the onset of limiting. Emission spectra measurements show singly ionized carbon emission lines with a hot blackbody background emission consistent with temperatures of ≃4000 K. A rapid expansion of the microscopic plasmas generated by the breakdown will effectively scatter further input light. Indeed, in time-resolved experiments the trailing portion of the pulse is most heavily scattered. The time-resolved transmittance of a weak cw probe beam also follows the temporal dependence of the singly ionized carbon emission (≃102 ns). We directly monitored the expansion of the scattering centers by angularly resolving the scattered light for different input fluences and fitting to Mie scattering theory. Since the carbon is black and the microplasmas are initiated by linear absorption, the limiting is extremely broadband. Within the context of this model we discuss the limitations and optimization of ink-based optical limiters.

© 1992 Optical Society of America

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