## 1 Introduction
For the appearance of a visible flash at a wavelength of \(\lambda\) = 1.064 \(\upmu\)m for the nanosecond range, the radiation (threshold) intensity should be \(I\)-10\({}^{11}\) W/cm\({}^{2}\), whereas for the picosecond range the threshold intensity \(I\)-10\({}^{12}\) W/cm\({}^{2}\), and for pulses with a duration of 10-100 fs the threshold intensity \(I\)-10\({}^{13}\) W/cm\({}^{2}\); see the review [13]. In this work, we study a special mechanism of optical breakdown at a wavelength of \(\lambda\) = 1.064 \(\upmu\)m, where breakdown centers are clusters of nanobubbles filled with dissolved gas (atmospheric air). As shown in our previous works [14; 15], nanobubbles and clusters of nanobubbles always exist in water, which is saturated with dissolved gas and contains an ionic additive. It seems obvious that such clusters also exist in the intraocular fluid, since this fluid is very close in its properties (basically, the content of dissolved ions) to physiological saline solution, which contains an ionic component. Therefore, the excitation of optical breakdown on clusters of nanobubbles, in our opinion, looks promising for laser applications in ophthalmology. Indeed, the best treatment for pupillary-block glaucoma is to create a tiny hole in the iris (known as an iridotomy). The iridotomy allows the flow of fluid to the front of the eye to be restored, bypassing the location of the blockade. A laser beam is used to burn a small hole in the iris, which releases pressure in the eye. The purpose of this procedure is precisely to reduce the pressure inside the eye; that is, to reduce the likelihood of glaucoma. We hypothesize that this operation can be carried out due to breakdown inside nanobubble clusters, and the threshold intensity for such a specific breakdown turns out to be significantly lower than the threshold intensities indicated in review [13]. As will be shown in Appendix A, optical breakdown on clusters of nanobubbles that serve as breakdown centers occurs in two stages. The first stage is the coalescence (merging together) of the clusters, which is accompanied by the formation of micron-sized gas bubbles. If the laser pulse has not ended at the moment of formation of such a bubble, and the energy of the laser pulse is still large enough, then laser-induced plasma is generated inside the bubble, and a light flash of breakdown occurs due to the bremsstrahlung effect, which is the basic end point of optical breakdown in liquids. In the case of breakdown on the nanobubble clusters, the cluster coalescence is generated the earlier and the higher the laser pulse intensity (see Appendix A), and at sufficiently high intensities an optical breakdown flash always takes place, which leads to undesirable side effects associated with the mechanical damage of the eye tissues. The purpose of this work is to find the experimental conditions under which laser-stimulated coalescence occurs, but the energy of the laser pulse is still low, and visible breakdown flash is not excited. Now, it is necessary to explain how gas nanobubbles and their clusters arise in a liquid far from boiling point. Recently, a large number of publications, devoted to the study of the nanobubbles in the bulk of a liquid, have appeared; see, for example, [16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. The effects, associated with the clustering of gas nanobubbles, have also been recently studied; see [26; 27]; the study of the properties of nanobubbles and their clusters seems to be a hot topic. When studying such tiny bubbles in a liquid, the question of their stability arose; see, for example, review [28]. One of the mechanisms that can provide the stability of nanobubbles is the compensation of surface tension forces by the electrostatic repulsion of anions, adsorbed on the nanobubble surface; see [29]. Therefore, such nanobubbles were termed substons (bubble, stabilized by ions).