Laser Pulses In Liquids Research Articles

Scattering of Ginzburg–Frank and Cherenkov Types Under Self-Focusing of Nanosecond Laser Pulses in Liquids

We study the dynamics of nonlinear optical processes such as self-focusing, self-phase modulation, and stimulated Raman scattering in Kerr-liquids under the nanosecond laser pulse excitation. The results prove the existence of the transition Ginzburg–Frank-type effect, which promotes the appearance of new spectral components of the laser radiation at the medium boundary. The generation of extended anti-Stokes frequency-angular bands of stimulated Raman scattering is explained. When the velocity of a self-focusing focal spot matches the phase velocity of the non-linear polarization at the anti-Stokes Raman frequency and the phase velocity of the scattered axial radiation, the most intense frequency-angular bands appear. They are described by the equations typical of the Cherenkov radiation.

Stimulated Thermal Scattering Induced by Two-Photon Absorption and Experimental Observation of the Genuine Stimulated Brillouin Scattering in the Near-Ultraviolet Region

Part of the experimental measurements of the frequency shift and the phase-conjugation (PC) fidelity gained from previous studies of stimulated scattering (SS) of nanosecond ( ) near-ultraviolet (UV) ( ) laser pulses in liquids (hexane, heptane, and others) are found to disagree with the theory of SS, which takes into account only the linear (single-photon) light absorption. To resolve the inconsistency, SS of XeCl excimer laser radiation ( ) with the duration of in liquid hexane is investigated experimentally. A theoretical analysis of the results obtained revealed three new nonlinear optical phenomena induced by the heating due to two-photon absorption: stimulated thermal scattering (two-photon STS-2), phase mismatch for stimulated Brillouin scattering (SBS), and phase self-modulation. Experimental SS spectrum contains two new lines - a previously unknown two-photon STS-2 line and a genuine SBS line newly discovered in the near-UV region

Cavitation dynamics and directional microbubble ejection induced by intense femtosecond laser pulses in liquids

We study cavitation dynamics when focusing ring-shaped femtosecond laser beams in water. This focusing geometry reduces detrimental nonlinear beam distortions and enhances energy deposition within the medium, localized at the focal spot. We observe remarkable postcollapse dynamics of elongated cavitation bubbles with high-speed ejection of microbubbles out of the laser focal region. Bubbles are ejected along the laser axis in both directions (away and towards the laser). The initial shape of the cavitation bubble is also seen to either enhance or completely suppress jet formation during collapse. In the absence of jetting, microbubble ejection occurs orthogonal to the laser propagation axis.

Stimulated thermal scattering induced by two-photon absorption and experimental observation of genuine stimulated brillouin scattering in the near-ultraviolet region

Some of the experimental measurements of the frequency shift and phase-conjugation fidelity gained from previous studies of stimulated scattering (SS) of nanosecond (5 ÷ 10 ns) near-ultraviolet (uv) (λ = 193 ÷ 351 nm) laser pulses in liquids (hexane, heptane, and others) are found to disagree with the theory of SS, which takes into account only the linear (single-photon) light absorption. To resolve the inconsistency, SS of XeCl excimer laser radiation (λ = 308 nm) with the duration of 8 ns in liquid hexane is investigated experimentally. A theoretical analysis of the results obtained revealed three nonlinear optical phenomena induced by the heating due to two-photon absorption: stimulated thermal scattering (two-photon STS-2), phase mismatch for stimulated Brillouin scattering (SBS), and phase self-modulation. The experimental SS spectrum contains two additional lines - a two-photon STS-2 line and a genuine SBS line in the near-uv region.

Generation of nanostructures on metals by laser ablation in liquids: new results

Surface nanostructuring of titanium, nickel, molybdenum, and tungsten by ablation with pico- and femtosecond laser pulses in liquids is studied experimentally for the first time. The morphology and properties of obtained nanostructures are investigated using a field emission scanning electron microscope and Raman spectroscopy. The size of nanostructures depends on the laser pulse duration and energy density and on the target material. As a rule, the size distribution of structures is bimodal. Potential applications of such nanostructured substrates are discussed.

Stimulated thermal scattering induced by two-photon absorption and experimental observation of genuine stimulated Brillouin scattering in the near-ultraviolet region

Some of the experimental measurements of the frequency shift and phase conjugation fidelity gained from previous studies of stimulated scattering (SS) of nanosecond $(5\ensuremath{-}10\text{ }\text{ns})$ near-ultraviolet (uv) $(\ensuremath{\lambda}=193\ensuremath{-}351\text{ }\text{nm})$ laser pulses in liquids (hexane, heptane, and others) are found to disagree with the theory of SS, which takes into account only linear (single-photon) light absorption. To resolve the inconsistency, SS of XeCl excimer laser radiation $(\ensuremath{\lambda}=308\text{ }\text{nm})$ with the duration of 8 ns in liquid hexane is investigated experimentally. A theoretical analysis of the results obtained revealed three nonlinear optical phenomena induced by the heating due to two-photon absorption: stimulated thermal scattering (two-photon STS-2), phase mismatch for stimulated Brillouin scattering (SBS), and phase self-modulation. The experimental SS spectrum contains two additional lines---a two-photon STS-2 line and a genuine SBS line in the near-uv region.

Self-organized bundle of lasing filaments in dense media

The filamentation of powerful, ultrashort laser pulses in liquids is investigated. It is shown that the multiple filaments produced in ethanol can be modified into controllable, regular patterns by doping the medium with dyes at high concentrations. Experimental results are confirmed by three-dimensional numerical simulations. Pump-dump pulse experiments furthermore reveal that the stimulated emission from excited dye molecules in the bundle can be locked in phase and leads to bright interference patterns.

Nonlinear propagation of fs laser pulses in liquids and evolution of supercontinuum generation

Nonlinear propagation of fs laser pulses in liquids and the dynamic processes of filamentation such as self-focusing, intensity clamping, and evolution of white light production have been analyzed by using one- and two-photon fluorescence. The energy losses of laser pulses caused by multiphoton absorption and conical emission have been measured respectively by z-scan technique. Numerical simulations of fs laser propagation in water have been made to explain the evolution of white light production as well as the small-scale filaments in liquids we have observed by a nonlinear fluorescence technique.

Stimulated Raman scattering of femtosecond Bessel pulses

It is demonstrated experimentally that a Bessel pump pulse provides group-velocity-matched Raman conversion of 100 fs, 1 GW laser pulses in liquids. By virtue of the peculiar geometry of Bessel beams, self-phase-modulation, and thereby continuum generation from the pump, is fully suppressed, while the beam quality of the emitted Stokes radiation is close to a perfect Gaussian beam. Conversion efficiencies are sufficient for the Stokes light to generate a continuum at elevated pump levels.

Generation of picosecond light continua by parametric four-photon interactions in liquids and solids

Light continua extending from the i.r. to the u.v. are generated with picosecond laser pulses in liquids and solids. It is demonstrated that the light spectra in water are produced by stimulated parametric fourphoton interactions, which are resonantly enhanced by single and difference frequency resonances of the non-linear susceptibility X (3). A theory of the various parametric four-photon interaction processes is developed and the resonant structure of X (3) is discussed. The experimental results are compared with theoretical calculations. The generation of light continua by other nonlinear optical processes is briefly discussed.

Observation of backward - stimulated Raman scattering generated by picosecond laser pulses in liquids

Stimulated Raman scattering in the backward direction is observed in organic liquids under 5300-\AA{} picosecond excitation. The spectrum and intensity of the backward-directed light is measured and compared with the emission in the forward direction. The Stokes spectrum of the light emitted in backward direction consists of stimulated Raman light, while in the forward direction both stimulated Raman and self-phase-modulated light is detected. No stimulated Brillouin or Rayleigh light is detected in the backward direction. The experimental results are in qualitative agreement with transient Raman theory.