Laser-Induced Defects in a Bulk GaAs Used as an Output Coupler in an Nd？:？YAG Laser Cavity

By using bulk GaAs as an output coupler as well as a saturable absorber, with changing of concentration of defects inside the laser cavity, Q-switching of flashlamp-pumped Nd？:？YAG laser has been studied. It is shown that the formation of the laser pulses, in a wide range of duration, which were reported in a lot of previous papers is caused by the intracavity changes of the defect concentration in GaAs. The factor of increase in defect concentration is estimated by modeling of the pulse formation process. 1. Introduction Recently, GaAs crystals are frequently used as nonlinear-optical elements, saturable absorbers for passive Q-switching and mode locking, and an output mirror [1–10]. In comparison with active Q-switching, passive technologies can considerably simplify a design, improve the efficiency and reliability, and reduce the cost of the laser sources. In [6], a semiconductor GaAs crystal with a thickness of 0.5？mm is used as a saturable absorber, and the pulses as short as 10？ps are received while in other papers the obtaining of durations 3？ns [7], 15.5？ns [8], 78？ns [9], and from 90 up to 800？ns [10] is reported. Though in these papers, it is shown that GaAs can be effectively used as a saturable absorber, the mechanism of formation of pulses up to now is not found out in detail. So, for example, even if the distinctions in conditions of these experiments will be taken into account, it is difficult to explain such large differences in the results, as the initial parameters of the used crystals did not differ significantly. Note that in all these experiments were used either a focusing lens or a spherical mirror for increasing the intensity of radiation on the GaAs surface planes to reach optimum efficiency of a saturable absorber. It is also necessary to note that in all these experiments the concentration of defects was supposed to be constant. At the same time, from the experiments on research of laser-matter interactions, it is known that intensive laser radiation can result in formation of defects in semiconductor crystals [11–15]. So, in [15], at a long irradiation of a GaAs crystal with the sequence of pulses of the Nd？:？YAG laser with an intensity lower than the threshold for an optical breakdown, the increase in concentration of defects more than one order was revealed. Since it is possible to assume that the use of GaAs in combination with a lens inside the resonator, during the operation of the laser, the increase in concentration of defects of a GaAs crystal is possible as well. This process would result in a change of the optical