## 3 Results
### Experiment with Low-Intensity Nanosecond Laser Pulses
Radiation at the second harmonic is stronger deflected by the focusing lens; therefore, its focus is localized in front of the focus of the beam at the first harmonic along the propagation of both beams. Here, we should take into account that the radiation at wavelength \(\lambda=1064\) nm is slightly absorbed in water (the absorbance is 0.14 cm\({}^{-1}\)), while absorption in water at wavelength \(\lambda=532\) nm can be neglected. Since \(dn/dT<0\), where \(n\) is the refractive index of water (see Ref. [45]), we are dealing with a weak negative thermal lens, arising along the propagation of both beams. This thermal lens efficiently deflects the second harmonic rays away from the focus area. It is clear that thedark area in the center of the green spot can appear only in cases where the directions of rays of the first and second harmonics exactly coincide. The fact that darkening is associated namely with the weak thermal lens was verified by placing a spectral filter, opaque for IR radiation but transparent for the visible region, in front of the cell with liquid; in this case, darkening disappeared. Under the irradiation of water with laser pulses with a certain intensity at the wavelength of 1064 nm, instantaneous diffraction patterns appeared on the luminescence screen. We used a mask cutting the central radiation maxima for the first and second harmonics (Figure 7) to prevent direct light incidence on the CCD array of the camera, which was used for snap-shooting the patterns on the screen. The size of the object, resulting in the diffraction, was estimated by the period of diffraction rings; this size was about 1-5 \(\upmu\)m. Thus, it was found that the laser shooting of water results in the formation of objects of such sizes. These objects instantly disappear if water is under normal conditions. If water was maintained at a temperature slightly less than the boiling point, we observed the formation of vapor bubbles under the same experimental conditions; i.e., liquid boiled before reaching the boiling point. Thus, we claim that the objects at which the second harmonic beam is diffracted are vapor-gas bubbles. In this experiment, it is important that the coalescence regime arose at intensities of the order of 10\({}^{6}\)-10\({}^{7}\) W\(\cdot\)cm\({}^{-2}\), which is close to the estimate in Appendix A. A macroscopic vapor-gas bubble (which is a negative lens with respect to laser radiation) is formed at the laser pulse front. To initiate an optical breakdown inside such a bubble, laser radiation should be introduced into such a bubble, which obviously requires a higher laser pulse energy compared to the energy applied to generate this bubble. It is clear that, at the energy of threshold of stimulated optical coalescence, the laser beam cannot penetrate into the bubble, and no optical breakdown spark (plasma flash) occurs. Figure 6: Light spot of the second harmonic. The dark area in the spot center is caused by the thermal lens. In Figure 8, we exhibit an oscillogram of the second harmonic pulse at sensor (7) in Figure 3. The duration of this pulse is 8.5 ns, which approximately coincides with the duration of the pump pulse \(\tau=10\) ns, which was to be expected. In this experiment, we did not have the opportunity to measure the lifetime of the gas bubble after the completion of the stimulated optical coalescence. It is clear, however, that the spherical bubbles formed during coalescence disappear almost instantly after completing the laser pulse, since there is no stabilization mechanism for these bubbles, see Ref. [44] for more details. Summing up, we can claim that at weak laser intensities the optical breakdown, which is characterized by a bright flash, does not occur because of low energy in the pulse, whereas micron-sized spheres are formed due to the stimulated optical coalescence of the bub  
Figure 8: Temporal profile of the intensity of the second-harmonic pulse, which diffracted on a gas-vapor bubble, arisen as a result of forced optical coalescence.