On the Nature of Processes Excited by Nanosecond Laser Pulses near the Water Surface
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he Nature of Processes Excited by Nanosecond Laser Pulses near the Water Surface A. F. Bunkina, M. A. Davydova,*, S. M. Pershina, and S. N. Fedorova a
Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow, 119991 Russia * e-mail: [email protected] Received April 10, 2019; revised August 22, 2019; accepted October 5, 2019
Abstract—For the first time to our knowledge, a number of Stokes and anti-Stokes lines with a shift in magnitude corresponding to stimulated Mandelstam—Brillouin scattering (SBS) lines in water are recorded in the SBS spectrum excited by nanosecond laser pulse focusing “under the free water surface.” In our opinion, this spectrum is caused by the nonlinear interaction of waves in the external cavity formed by a distributed mirror and the water surface. Keywords: stimulated scattering, lasing threshold, four-photon interaction DOI: 10.3103/S1068335620100048
Stimulated Mandelstam—Brillouin scattering (SBS) accompanied by a number of competing nonlinear processes is to date studied in detail [1]. However, interest in various aspects of this phenomenon is retained, in particular because it should be considered in various fields of application of pulsed lasers [2, 3]. The objective of this work is to study the SBS nature near the liquid surface. The block diagram of measurements is shown in Fig. 1. Radiation of pulsed YAG : Nd3+ laser (ТЕМ00, the emission wavelength λ = 0.532 nm, the maximum pulse energy Ep ~ 3 mJ; the emission pulse length is τ ~ 10 ns, the pulse repetition rate is 1 Hz) was directed vertically downward on the free surface of liquid under study (double distilled water), being in a glass cylindrical cell opened from above with an inner diameter of 25 mm. The cell bottom was turned by 4°—5° with respect to the setup optical axis. Laser radiation was focused on the liquid surface using a lens with f = 30 mm (the beam parameters were estimated as follows: the waist diameter is d ~ 3.2 μm and the waist length is l ~ 40 μm). Fine tuning of the laser beam incident on the free liquid surface was performed similarly to tuning of the optical cavity mirror. To study the surface liquid layer over depth, a cell could be moved along the vertical axis using a micrometer screw with a step of 5 μm. The spectrum of “backward” radiation was measured using a Fabry—Perot interferometer with dispersion region Δν = 0.625 cm–1 or Δν = 2.5 cm–1. The spectra were photographed using a CMOS camera without acquisition (“single-shot” measurements) and numerically processed in the Labview software environment. The incident radiation energy was monitored using an IMO-2N device. Measurements were performed at room temperature. The following results were obtained. (i) The caustic (laser beam waist) is completely in water, about 10 mm below the surface; the strong backward SBS signal takes place (only the first Stokes component, the shift is Δνs ~ 0.245 cm–1 which corresponds to the SBS shift in water (Fig. 2); the SBS threshold in this setup geometry Eth ~ 0.38 mJ; with increasing pumping, the fir
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