Laser-induced defect reactions governing the damage performance of KDP and DKDP.

摘要

Laser-induced damage in optical components at relatively low power levels remains a key limitation on the power handling capabilities of optical systems. The formation of damage sites is due to the presence of defect structures (damage precursors) that interact with the laser light. This problem represents a major challenge in the use of optical materials for various applications extending from miniaturized optoelectronic devices to large-aperture laser systems for inertial confinement fusion. KH2PO4 and its analog KDxH2-xPO4, also known as KDP and DKDP, are representative examples of nonlinear optical materials where exposure to nanosecond laser pulses leads to micron-sized damage sites within the bulk at fluences of more than an order of magnitude below the intrinsic breakdown threshold.; The identification of the exact structure of the precursors has been unapproachable due to their size and sparse distribution. The laser-defect interaction mechanisms governing the initiation of the damage sites are also unknown. In KDP/DKDP and other optical materials, a key behavior of the damage precursors is that the overall damage performance is improved with pre-exposure to sub-damage laser fluences, a phenomenon often referred to as laser conditioning. The lack of understanding of the mechanisms involved during damage initiation and conditioning makes it difficult to develop laser conditioning 2 and other protocols to optimally improve the damage performance. It also hinders identification of the structure of the damage precursors needed to ultimately eradicate the damage problem. It is therefore imperative to develop new methods to reveal the optical properties of the damage precursors.; In this dissertation, the interaction of the damage precursors in KDP/DKDP with laser pulses of variable fluence and frequency is investigated using several experimental methods. A damage testing approach is implemented that takes advantage of the change in the damage characteristics of the material following exposure to laser pulses by measuring the ensuing density of damage sites. The results suggest that there are two types of precursors, each leading to damage initiation over a different frequency range. For both precursor populations, the pathways leading to damage initiation are different than those leading to conditioning. Furthermore, among one precursor population, there are multiple pathways leading to conditioning that are dependent on the pre-exposure frequency and fluence. From these results, a method is developed for predicting the damage performance during harmonic conversion where multiple wavelengths are present. Moreover, laser pre-irradiation parameters are prescribed for maximizing the conditioning effect.