## 5 Conclusions  
The main result of this work is as follows. It has been established that in the case of optical breakdown on bubston clusters as heterogeneous breakdown centers, the breakdown develops in two stages. At the first stage, the coalescence (merging) of the cluster occurs, followed by the formation of a macroscopic gas bubble. In this case, the size of such a bubble cannot exceed the size of the cluster itself; i.e., this size is about 1 micron. If the laser pulse has not yet finished to the moment of the coalescence completion, the secondary electrons, resulting from the ionization of the nanobubble liquid walls, keep oscillating subject to the optical wave, which leads to collisions of these electrons with heavy particles. These particles are \"fragments\" of the liquid walls of the cluster, and the collisions of electrons with these particles are obviously not elastic; that is, the electrons in these collisions lose their energy, which is accompanied by bremsstrahlung. In this case, the resulting bubble continues to expand and can reach sizes of the order of 1-2 mm, which is accompanied by a powerful shock wave, see [9; 13].  
If the intensity of laser pulses does not exceed the threshold value for the cluster coalescence, which belongs to the range of \(10^{6}\)-\(10^{7}\) W/cm\({}^{2}\), then the entire pulse energy is completely spent on the coalescence process; i.e., no flash of optical breakdown is observed, and the size of the resulting bubble does not exceed 1 micron. The lifetime of micron bubbles is limited by the rate of the diffusion of gas molecules from this bubble into the bulk of liquid. Thus, in the range of laser radiation intensities of \(10^{6}\)-\(10^{7}\) W/cm\({}^{2}\) at wavelength \(\lambda=1064\) nm, the effect of a laser pulse is reduced only to the generation of a micron-sized gas bubble. It is clear that the bubble formed as a result of coalescence does not have time to heat up; that is, the temperature of such a bubble should be close to room temperature, and the shock wave, obviously, appears neither at the end of cluster coalescence (arising of the bubble), nor at the diffusion collapse of the bubble, since the diffusion is a very slow process. For room temperature, in accordance with the results of [44], we have an estimate of \(\tau\approx 10^{-2}\) s for the lifetime of such a micron-sized bubble.  
It should be expected that when performing ophthalmic operations using laser radiation at intensities of \(10^{6}\)-\(10^{7}\) W/cm\({}^{2}\), mechanical side effects caused by the excitation of a shock wave during the generation and collapse of millimeter-sized bubbles will not manifest themselves; that is, the excitation of the stimulated optical coalescence of nanobubble clusters can be used in ophthalmologic surgery, for example, in laser iridotomy.  
Conceptualization, N.F.B.; investigation, V.A.B., A.A.S.; writing--original draft preparation, N.F.B.; supervision, N.F.B. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
This study did not require ethical approval.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.