The human eye, sensors and photoreceptors can be damaged if they are submitted to laser beams with high intensities and low divergence. In the military field, an optical sensor can be exposed beyond its capacities and be damaged by pulsed laser radiations. In order to have a protection against such threats, passive optical limiting is used1. It is based on nonlinear effects which allow to reduce the intensity of aggressive laser radiations. Nowadays, the development of nonlinear filters for optical limiting is a challenge in order to have protection against threats caused by a pulsed laser beam. As a matter of fact, tactical lasers are a threat toward sensing devices between hundreds of meters and some kilometers.2,3
This work has for purpose the synthesis of methacrylate-based hybrid polymer systems in order to study their nonlinear properties for the optical limiting in the near infrared part of the electromagnetic spectrum. Several polymers have been selected with specific properties, such as a high transmittance (>80%) over a broad spectrum and a significant third order nonlinear susceptibility χ3. Poly(methyl methacrylate) (PMMA) is considered as the standard host polymer material because of its high transparency up to 1.6μm but also due to its good accessibility. However, to avoid some issues (e.g. exothermy of the polymerization and Tromsdorff-Norish effect during the radical polymerization) regarding the use of methyl methacrylate (MMA) and to improve the nonlinear properties like the nonlinear and damage thresholds, new polymers or hybrid systems have been synthesized. In the present work, a thermoset matrix is studied corresponding to a mix between a reactive oligomer and a monomer both with methacrylate functions. The use of an oligomer and a monomer with a high boiling temperature as precursors allows to prevent the monomer evaporation, resulting in the formation of bubbles in the final material, related to the reaction exotherm.
The attenuation or "limitation" of an incoming laser threat to a safe level of radiations using an external element is called optical limitation. The concept of passive laser protection requires the use of materials with nonlinear optical properties that are self-activated when the incident laser threat is above a certain level of intensity. Conjugated polymers are tailored for optical limitation due to their versatility, their non-negligible nonlinear absorption properties and their uniform transmittance characteristics over a broad spectral range, the latter property avoiding color distortions to the observer. Usually, to obtain an optical limiting filter, nanomaterials are dispersed in a polymer host. In this work, we consider polyvinylcarbazole (PVK) as a new polymer host for optical limiting filters. PVK is a high Tg polymer exhibiting a high linear transmittance over a broad spectral range, up to 1.6 μm. The adiabatic bulk polymerization method has been employed to thermally polymerize the monomer in the presence of an initiator. Traces of remaining monomer have qualitatively been monitored using FTIR spectroscopy. Optical limiting experiments have been conducted at the wavelength of 1.06 μm using a Nd:YAG laser source in the nanosecond pulsed regime at a low pulse repetition rate. PVK-based filters present a low nonlinear threshold (i.e. activation of the nonlinear attenuation), a factor of 10 lower than the usual PMMA-based filters. Their global nonlinear attenuation is as high as OD = 2.0.
The widespread usage of harmful pulsed laser sources emitting brief but intense radiations implies to search for appropriate and convenient forms of protection. Nonlinear optical nanomaterials can serve this purpose when properly embedded in a solid medium, resulting in nanohybrid passive optical limiting filters. This work focuses on the optical limiting behavior of polymer-dye nonlinear nanohybrids, for which we combined azophloxine, a red azo-dye, with two polymer hosts, namely PMMA and polylactide (PLA). Transmittance measurements in the nonlinear regime were performed at the wavelength of 1064 nm with nanosecond pulses at a low pulse repetition rate. The nonlinear optical properties resulting from energy dependent transmittance assessments reveal that the dye concentration is of major relevance regarding the PMMA nanohybrids, and to a lesser extend for the PLA based systems. The PLA synthesis concept described in this study offers an easy way to directly attach the dye covalently to the polymer chain. The originality and novelty of this synthesis technique is to be pointed out since it has never been mentioned elsewhere to date. For the various types of nanohybrids investigated, significant differences in the optical limiting response were observed. A molecular model claiming for the dye aggregation in the PMMA nanohybrids is discussed. Two different absorption regimes responsible for the optical limiting action have been identified to be reverse saturable – excited state absorption on the one hand and multi-photons absorption on the other hand.
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