## 3 Results  
### Experiment with Low-Intensity Nanosecond Laser Pulses  
The idea of the experiment with low-intensity nanosecond laser pulses, the schematic setup of which is exhibited in Figure 3, is as follows. According to our model (see Appendix A), the optical breakdown in individual bubstons is caused by the collisions of electrons, driven by the electric field of the light wave, with water molecules of the bubston liquid walls. If a bubston cluster appears in the focal area of a laser beam, an electron avalanche develops inside each bubston. Obviously, the collisions of electrons with the liquid walls of the bubston cluster are elastic; that is, the electrons efficiently accumulate energy and heat up, see Ref. [43] for more details. In this case, a rapid expansion of individual bubstons within the cluster occurs, which ends with the coalescence (merging together) of the cluster and the formation of a macroscopic vapor-gas bubble of micron size (the bubble size is about the size of the bubston cluster itself). We called this process stimulated optical coalescence. As shown in Appendix A, if the laser pulse has not ended by the time of the formation of the macroscopic bubble, the electrons continue to oscillate due to the electric field of the light wave, but in this case the electrons will collide with heavy particles (fragments of the liquid walls of the bubston cluster), and during such collisions the electrons will transfer their momentum to heavy particles and experience deceleration, which will be accompanied by the emission of bremsstrahlung quanta, see Appendix A. The further evolution of a macroscopic bubble includes the growth of a vapor-gas bubble up to millimeter size, pulsations and collapse, which is accompanied by the generation of a shock wave. It is obvious that such an optical breakdown mode is not applicable in ophthalmic applications, since undesirable side effects inevitably arise in this case. At the same time, at low energies of laser pulses, it is possible to choose such a regime when the entire pulse energy will be spent solely to the stimulation of the optical coalescence of a bubston cluster. According to the estimates presented in Appendix A, the intensity of laser radiation at the wavelength \(\lambda=1064\) nm, which is sufficient to complete the coalescence process, is about \(10^{7}\) W/cm\({}^{2}\). For that intensity, the coalescence time for a bubston cluster with a size of 1 micron (such a cluster includes several hundred individual bubstons) is about \(t_{coal}=10^{-10}\) s; i.e., for nanosecond pulses, coalescence finishes at the front of the laser pulse. Obviously, for higher intensities, stimulated optical coalescence can occur much earlier, see Appendix A. The micron-sized bubble formed as a result of coalescence is unstable and collapses according to diffusion kinetics. Indeed, it was theoretically shown in [44] that the lifetime of unsteady bubbles is proportional to the square of their radius; in accordance with the estimates given in this work, the lifetime of a micron non-stabilized micron bubble at room temperature (that is, far from the boiling point) is of the order of 10 msec. In the experiment, the schematic setup of which is illustrated in Figure 3, we attempted first to demonstrate the fact that the coalescence stage does exist in the optical breakdown, and second to estimate the coalescence threshold. To observe the coalescence, it was very important to make sure that the directions of beams at wavelengths of 1064 and 532 nm strictly coincide with one another along the entire path of both beams. To satisfy this condition, the position of the DKDP crystal was aligned in such a way that a characteristic dark area should appear in the center of the green spot, projected on a luminescence screen (see Figure 6). This dark area appears in the case of a complete coincidence of both beams for the following reason.