Laser-induced Instabilities Research Articles

Laser and Astrophysical Plasmas and Analogy between Similar Instabilities

Multipulse laser–matter interactions initiate nonlinear and nonequilibrium plasma fluid flow dynamics and their instability creating microscale vortex filaments, loop-soliton chains, and helically paired structures, similar to those at the astrophysical mega scale. We show that the equation with the Hasimoto structure describes both, the creation of loop solitons by torsion of vortex filaments and the creation of solitons by helical winding of magnetic field lines in the Crab Nebula. Our experiments demonstrate that the breakup of the loop solitons creates vortex rings with (i) quasistatic toroidal Kelvin waves and (ii) parametric oscillatory modes—i.e., with the hierarchical instability order. For the first time, we show that the same hierarchical instability at the micro- and the megascale establishes the conceptual frame for their unique classification based on the hierarchical order of Bessel functions. Present findings reveal that conditions created in the laser-target regions of a high filament density lead to their collective behavior and formation of helically paired and filament-braided “complexes”. We also show, for the first time, that morphological and topological characteristics of the filament-bundle “complexes” with the loop solitons indicate the analogy between similar laser-induced plasma instabilities and those of the Crab and Double-Helix Nebulas—thus enabling conceptualization of fundamental characteristics. These results reveal that the same rotating metric accommodates the complexity of the instabilities of helical filaments, vortex rings, and filament jets in the plasmatic micro- and megascale astrophysical objects.

Periodic Relief Fabrication and Reversible Phase Transitions in Amorphous Ge2Sb2Te5 Thin Films upon Multi-Pulse Femtosecond Irradiation

Ge2Sb2Te5 based devices attract the attention of researchers due to wide opportunities in designing phase change memory. Herein, we studied a possibility to fabricate periodic micro- and nanorelief at surfaces of Ge2Sb2Te5 thin films on silicon oxide/silicon substrates under multi-pulse femtosecond laser irradiation with the wavelength of 1250 nm. One-dimensional lattices with periods of 1250 ± 90 and 130 ± 30 nm were obtained depending on the number of acted laser pulses. Emergence of these structures can be explained by plasmon-polariton generation and laser-induced hydrodynamic instabilities, respectively. Additionally, formation of the lattices whose spatial period is close to the impacted laser wavelength can be modelled by considering the free carrier contribution under intensive photoexcitation. Raman spectroscopy revealed both crystallization and re-amorphization of the irradiated films. The obtained results show a possibility to fabricate rewritable all-dielectric data-storage devices based on Ge2Sb2Te5 with the periodic relief.

Hydrodynamic instabilities of thin Au/Pd alloy film induced by tightly focused femtosecond laser pulses

We report on detailed experimental study of various nanoscale surface hydrodynamic instabilities on thin Au/Pd alloy films induced by tightly focused single femtosecond pulses. Each type of laser-induced hydrodynamic instabilities results in the formation of corresponding resolidified surface relief nanostructure: nanojet, nanocrown or hybrid structure (a nanojet surrounded by a nanocrown), where the hybrid structure is reported for the first time. Thickness of metal films, as well as the laser pulse energy, were found to be the key parameters determining the type of the resulting surface structures. Single nanojets were revealed to appear only on films with sub-100-nm thickness, while irradiation of thicker films (120–240nm) leads to the formation of nanocrowns at near-threshold energies or hybrid structures at higher energies. The underlying formation mechanisms giving rise to all of these laser-induced nanostructures are also discussed.

Laser-induced instabilities in liquid crystal cells with a photosensitive substrate

Liquid crystal layers sandwiched between a reference plate and a photosensitive substrate were investigated. We focused on the reverse geometry, where the cell was illuminated by a laser beam from the reference side. In planar cells both static and dynamic instabilities occurred, depending on the angle between the laser polarization and the director orientation on the reference plate. In cells where the molecules were aligned along the normal of the reference plate, a dynamic pattern was observed at all angles of polarization. A simple model based on a photoinduced surface torque accounts for the findings. Light scattering studies revealed some basic properties of the instabilities.

Plasma hole boring by multiple short-pulse lasers

Channeling of a train of laser pulses into dense plasma is studied using particle-in-cell simulation. When the pulse duration and the interval between the successive laser pulses are appropriately chosen, the laser pulse train can bore into the plasma deeper than a single long-pulse laser of similar peak intensity and total energy, and laser-induced plasma instabilities are greatly suppressed by the intermittent laser-energy cut-offs. The increased penetration distance can be attributed to the repeated application of ponderomotive force, the greatly reduced plasma instabilities, as well as the continuous between-pulse plasma channel expansion due to ion inertia.

Formation and dynamics of relativistic electromagnetic solitons in plasmas containing high-and low-energy electron components

We present theoretical and numerical studies of the nonlinear interactions between intense electromagnetic waves in plasmas containing high-and low-energy electron components. Such plasmas are frequently observed in laser-plasma experiments, where the hot electron component is created by the acceleration of electrons by strong electrostatic waves that are created by the laser-induced Raman forward and backward instabilities. The two-component electron plasma is described by the Vlasov equation for the hot electrons and the hydrodynamic equations for the cold electrons, which are coupled nonlinearly to the electromagnetic wave equation and the Poisson equation for the potential. The present nonlinear system is shown to admit electromagnetic solitary waves correlated with a positive potential and trapped electron islands from the hot electron population.

Laser-induced instabilities of a continuous thermal trace on a surface

Results are presented that corroborate the important role played by laser-induced vapor-phase oxidation in the onset of an instability of the irradiation trace and formation of self-organized structures.

Laser-induced structural instabilities of porous silicon nanocrystallites

Raman and photoluminescence (PL) spectra of porous silicon (PS) were examined as functions of laser irradiation time and laser intensities. The temporal evolution shows three stages, which depend on the laser intensity as follows: (a) For relatively low intensities, the Raman and PL spectra hardly change with irradiation time during our experiments. (b) For intermediate laser levels of 0.84–1.19 W/cm 2, the Raman and PL peaks have similar quasiperiodic oscillatory behavior for some time and after that their frequencies increase and finally stay unchanged. (c) For higher irradiation levels, the frequencies and intensities of Raman and PL peaks increase monotonically and saturate finally. We attribute these results to both the laser-induced oxidation and the structural instabilities of nanocrystallites in PS. The experimental results indicate that the intense PL intensity of PS may be closely related to this structural change of microcrystallites.

Raman study of the laser-induced chemical instabilities of BaCuO 2, Ba 2Cu 3O 5.9, and Ba 2CuO 3

The chemical transformations of three barium cuprates, BaCuO 2, Ba 2Cu 3O 5.9, and Ba 2CuO 3, which are the precursors and decomposition products of the Ba-based family of superconducting cuprates, were studied through their Raman bands under various laser irradiation conditions in air. BaCuO 2 was stable up to 3 mW/μm 2. Ba 2Cu 3O 5.9 transformed into BaCuO 2 under laser irradiation of 2 mW/μm 2 or higher. Ba 2CuO 3 transformed to Ba 2Cu 3O 5.9, first under laser irradiation of ∼1 mW/μm 2, then to BaCuO 2 when the irradiation was increased to above 2 mW/μm 2. At elevated temperatures in air, BaCuO 2 seems to be the most stable of these barium cuprates and Ba 2CuO 3 the least stable.

Stochastic processes in a nonlinear Kerr-like ordered liquid

A special case of the problem determined by laser-induced instabilities (at cw input) in wave interactions with inhomogeneous anisotropic media with high threshold nonlinearity under condition of energy competition and exchange between waves with orthogonal polarizations is examined.

Laser-induced structural instabilities in amorphous materials

Amorphous materials irradiated by cw lasers undergo structural changes, which depend quasiperiodically on the irradiation time. The changes in the transmitted light intensity are related to the changes of the Raman line shape. The structural changes between metastable states are favored by the photogenerated free carriers.

Interaction of powerful laser radiation with the surfaces of semiconductors and metals: nonlinear optical effects and nonlinear optical diagnostics

This article reviews the present status of research on strongly nonequilibrium physical effects produced near and at the surfaces of metals, semiconductors, and dielectrics by high-power pulsed laser radiation, and their diagnostics by nonlinear-optical methods. The basic stages of the absorption and relaxation of the energy of laser radiation in the electronic and phonon subsystems of semiconductors and heating of the lattice are studied. The questions associated with fast laser-induced phase transformations in surface layers are discussed: melting-solidification and amorphous-solid-to-crystal and vice versa phase transitions, occurring with nano- and picosecond laser pulse durations (laser annealing). Different methods of laser diagnostics of the surfaces and nonequilibrium ultrafast processes on the surfaces of metals and semiconductors are reviewed. Special attention is devoted to nonlinear-optical methods of surface diagnostics (generation of second and third harmonics and difference frequencies in reflection, and Raman scattering of light). The high information content and sensitivity of these methods compared with other methods for studying surfaces is pointed out. The problem of the appearance of spatially periodic surface structures induced by laser radiation is examined. The theories of different laser-induced instabilities of the surface relief, giving rise to the formation of these structures (generation of surface acoustic and capillary waves and evaporation waves), are reviewed. Theoretical and experimental results are compared.

A gas target for the study of laser-induced parametric instabilities in plasmas

A hydrogen gas target is described which has been irradiated with a high intensity CO 2 laser beam to study parametric instabilities. Such a target, with n ⩽ n c, provides considerable flexibility and optical access for detailed diagnostics. Preliminary measurements are reported on stimulated Brillouin (and Compton) scattering, 2-plasmon decay and second harmonic generation, in addition to interferometric and enhanced Thomson scattering measurements, which illustrate the versatility of this target for detailed investigation of parametric instabilities.

Laser-induced instabilities in homogeneous plasmas

A linearized theory is presented for the dominant instabilities in a laser-driven homogeneous plasma. The distinction between convective and absolute instabilities, and their effect on thresholds in finite plasmas, is especially emphasized. Thus, it is found that the effective thresholds for stimulated Brillouin and Raman scattering are higher than those generally quoted. The analysis is further extended to two dimensions, and it is shown that stimulated Brillouin scattering is primarily excited in the backward direction. The broader backscattering profile for stimulated Raman scattering exhibits a cutoff angle approaching 90° near a “critical” surface at one-fourth the critical density. At this density a strong instability is predicted corresponding to the excitation of Langmuir waves.

Longitudinal Wave Propagation in a Plasma Containing an External Time-Dependent Electric Field

Frequency and damping decrements for high frequency longitudinal oscillations in a plasma containing an external high frequency (Ω ≫ k υ¯_) electric field are obtained by means of a time-asymptotic solution to the linearized Vlasov equation, with an approximate collision term included. If the external field frequency is close to (ωp_2 + 3 k2υ¯_2)1/2, the contribution of the ion motion to Landau damping is significant for small external fields. The threshold field necessary to excite a high frequency instability in a plasma in which the number of a particles in a Debye sphere is very large is obtained. This problem is of interest since laser-induced instabilities may be responsible for an anomalous heating effect in the plasma.

Laser-Induced Instabilities in Raman-Active Media

Laser-induced instabilities in Raman-active media are studied. The stimulated Raman scattering process is found to be important in benzene even for a Stokes conversion rate of a few percent. Growth rates are obtained by numerical analysis, using equations governing the motion of a saddle point.

Linearized Theory of Laser-Induced Instabilities in Liquids and Gases

A macroscopic linearized instability theory is developed to describe a class of instabilities resulting from optical-acoustic coupling of a laser beam in fluids and gases. A phenomenon of particular interest is the initial phase in the development of the high-intensity optical filaments which are observed when an intense laser beam propagates through certain fluids. It is suggested that for certain laser cell geometries and a sufficiently high power flux density, filament formation may be preceded by a breakdown in the mode structure of the incident laser beam as a result of coupling to the eigenmodes of the medium. The interaction mechanisms considered are stimulated Raman scattering, electrostriction, the high-frequency Kerr effect, and thermal-energy deposition, while the response of the laser-fluid system is described by Maxwell's equations combined with the appropriately modified conservation equations from hydrodynamics. On the basis of this model, it is proposed that inhomogeneities in the laser intensity, or in the density and temperature of the fluid, act as sources of instability growth for the induced waves which are generated when the primary and scattered optical waves interfere. The dispersion relation for the problem is derived and a procedure for calculating the growth rates of this instability is outlined. The method is illustrated by detailed computations on carbon disulfide covering a range of laser intensities, and it is shown that the laser-Stokes coupling terms do not significantly affect the initial growth rates. In the case of gases, where the electrostrictive effect can be ignored, analytic expressions for the spatial and temporal gains are derived. Under the assumption that the first-order contributions in the linear theory become important after they have undergone several $e$ foldings, these results indicate---for a power flux density, optical-absorption coefficient product of ${10}^{\ensuremath{-}8}$ MW/${\mathrm{cm}}^{3}$---that mode degeneration is expected to occur in a laser beam which has propagated a distance of the order of a few kilometers through air at a pressure of 1 atm or which has a pulse length of several microseconds.