Self-focusing Of Light Research Articles

On the relationship between fundamental science and contemporary applied research in the field of high-rate laser Nanowhiskerography

In practice, in various areas of life, there is often a need to use materials that have mutually exclusive requirements. Therefore, the study of the issues related to manufacturing materials in certain states for certain technical applications, and methods of controlling structurally sensitive properties to obtain specified effects, is quite relevant today. A new effect was discovered, namely, the rapid growth of amorphous and composite materials in the form of “coherent” nanowhiskers using laser-induced plasma at temperatures exceeding 4,000oC and high pressures up to 100 thousand atmospheres at a rate reaching 80-100 m/s. This method of growing whiskers is based on fundamental studies of pulsed laser radiation and predictions of Nobel Prize laureates – Ch.H. Townes, A.M. Prokhorov (splitting of a laser beam of above-threshold power) and G.A. Askaryan (effects of self-focusing of light in the condensed state of matter, and sublimation evaporation). The authors show possible practical applications of this method, such as protection of securities, banknotes, and plastic cards, as well as production of a new type of silicon batteries, automotive catalysts, and solar silicon batteries.

Chirality-Enhanced Periodic Self-Focusing of Light in Soft Birefringent Media.

We experimentally and numerically show that chirality can play a major role in the nonlinear optical response of soft birefringent materials, by studying the nonlinear propagation of laser beams in frustrated cholesteric liquid crystal samples. Such beams exhibit a periodic nonlinear response associated with a bouncing pattern for the optical fields, as well as a self-focusing effect enhanced by the chirality of the birefringent material. Our results open new possible designs of nonlinear optical devices with low power consumption and tunable interactions with localized topological solitons.

Expansion of the strongly interacting superfluid Fermi gas: Symmetries and self-similar regimes

We consider an expansion of the strongly interacting superfluid Fermi gas in a vacuum, assuming absence of the trapping potential, in the so-called unitary regime (see, for instance, \cite{pitaevskii2008superfluid}) when the chemical potential $\mu \propto \hbar^2n^{2/3}/m$ where $n$ is the density of the Bose-Einstein condensate of Cooper pairs of fermionic atoms. In low temperatures, $T\to 0$, such expansion can be described in the framework of the Gross-Pitaevskii equation (GPE). Because of the chemical potential dependence on the density, $\sim n^{2/3}$, the GPE has additional symmetries, resulting in the existence of the virial theorem \cite% {vlasov1971averaged}, connecting the mean size of the gas cloud and its Hamiltonian. It leads asymptotically at $t\to\infty$ to the gas cloud expansion, linearly growing in time. We study such asymptotics, and reveal the perfect match between the quasi-classical self-similar solution and the asymptotic expansion of the non-interacting gas. This match is governed by the virial theorem, derived through utilizing the Talanov transformation \cite{talanov1970focusing}, which was first obtained for the stationary self-focusing of light in media with a cubic nonlinearity due to the Kerr effect. In the quasi-classical limit, the equations of motion coincide with 3D hydrodynamics for the perfect monoatomic gas with $\gamma=5/3$. Their self-similar solution describes, on the background of the gas expansion, the angular deformities of the gas shape in the framework of the Ermakov--Ray--Reid type system.

Semiclassical expansion of quantum gases into a vacuum

In the framework of the Gross-Pitaevskii equation, we consider the problem of the expansion of quantum gases into a vacuum. For them, the chemical potential μ has a power-law dependence on the density n with the exponent ν = 2/D, whereD is the space dimension. For gas condensates of Bose atoms as the temperature T → 0, s scattering gives the main contribution to the interaction of atoms in the leading order in the gas parameter. Therefore, the exponent ν = 1 for an arbitrary D. In the three-dimensional case, ν = 2/3 is realized for condensates of Fermi atoms in the so-called unitary limit. For ν = 2/D, the Gross-Pitaevskii equation has an additional symmetry under Talanov transformations of the conformal type, which were first found for the stationary self-focusing of light. A consequence of this symmetry is the virial theorem relating the average size R of an expanding gas cloud to its Hamiltonian. The quantity R asymptotically increases linearly with time as t →∞. In the semiclassical limit, the equations of motion coincide with those of the hydrodynamics of an ideal gas with the adiabatic exponent γ = 1+2/D. In this approximations, self-similar solutions describe angular deformations of the gas cloud against the background of the expanding gas in the framework of equations of the Ermakov-Ray-Reid type.

Influence of strong light beams on the nonlinear refraction and absorption coefficients of transparent materials

We explore the effective nonlinear (NL) optical response of transparent materials illuminated by strong light beams. Two different setups were used to study the influence of the numerical aperture (NA) corresponding to the collecting lens on the measured coefficients in the open- and closed-aperture Z-scan transmittance. For that, we have reproduced experiments in ${{\rm CS}_2}$CS2 at 532 nm in the picosecond regime using the ${D}4\sigma$D4σ-Z-scan method. It is found that measurements of high-order NL coefficients are not possible after a limit defined by the self-focusing of light inside the cell, and that stimulated light scattering is not significant to explain alone the saturation behavior that occurs in the NL refraction measurements. Indications are given for future Z-scan-based experiments that may clarify the boundary conditions to be respected for the measurement.

Diagrammatic approach to nonlinear optical response with application to Weyl semimetals

Nonlinear optical responses are a crucial probe of physical systems including periodic solids. In the absence of electron-electron interactions, they are calculable with standard perturbation theory starting from the band structure of Bloch electrons, but the resulting formulas are often large and unwieldy, involving many energy denominators from intermediate states. This work gives a Feynman diagram approach to calculating non-linear responses. This diagrammatic method is a systematic way to perform perturbation theory, which often offers shorter derivations and also provides a natural interpretation of nonlinear responses in terms of physical processes. Applying this method to second-order responses concisely reproduces formulas for the second-harmonic, shift current. We then apply this method to third-order responses and derive formulas for third-harmonic generation and self-focusing of light, which can be directly applied to tight-binding models. Third-order responses in the semiclasscial regime include a Berry curvature quadrupole term, whose importance is discussed including symmetry considerations and when the Berry curvature quadrupole becomes the leading contribution. The method is applied to compute third-order optical responses for a model Weyl semimetal, where we find a new topological contribution that diverges in a clean material, as well as resonances with a peculiar linear character.

On Demand Spatial Beam Self-Focusing in Hexagonal Multicore Fiber

Combination of the classical effect of light self-focusing and recently emerged multi-core fiber technology offers new opportunities for the spatio-temporal control and manipulation of high-power light radiation. Here we apply genetic algorithm to design a system enabling self-focusing of light in various fiber cores on demand. The proposed concept is general and can be applied and adapted to any multi-core fiber or 2D array of coupled waveguides paving a way for numerous applications.

Light self-focusing in the atmosphere: thin window model.

Ultra-high power (exceeding the self-focusing threshold by more than three orders of magnitude) light beams from ground-based laser systems may find applications in space-debris cleaning. The propagation of such powerful laser beams through the atmosphere reveals many novel interesting features compared to traditional light self-focusing. It is demonstrated here that for the relevant laser parameters, when the thickness of the atmosphere is much shorter than the focusing length (that is, of the orbit scale), the beam transit through the atmosphere in lowest order produces phase distortion only. This means that by using adaptive optics it may be possible to eliminate the impact of self-focusing in the atmosphere on the laser beam. The area of applicability of the proposed “thin window” model is broader than the specific physical problem considered here. For instance, it might find applications in femtosecond laser material processing.

Energy light structures during femtosecond laser radiation filamentation in air. To the 50th anniversary of the first paper about light self-focusing

A class of energy light structures in nonlinear wave physics, so-called diffraction-beam tubes in a self-focusing dissipative medium, has been studied. It is shown that their interaction along a high-power femtosecond laser radiation propagation path produces areas of light energy localization, stable in physical characteristics, i.e., a filament and a post-filamentation channel, as well as an energy-conservative periphery. Stabilization of boundaries of the above areas is provided by additional contraction of a diffractive nature from the side of the periphery. The dependencies of filamentation length of a laser pulse on the power and beam radius and the fact of high intensity conservation in the post-filamentation light channel are interpreted using this concept.

Self-Organization Effects in Photopolymerizable Nanocomposite

The main problem of photolithography is to overcome the diffraction limit. A possibility of overcoming this limit by the application of self-writing effects is discussed. Some of these effects are studied in the present work: the self-focusing of light in a photopolymer with a positive change of the refractive index, photo-induced transportation of nanoparticles, and the threshold of oxygen-based polymerization. The results of applications of these effects to the lithography process are shown.

Formation of elliptically polarized ring-shaped electric field structures on the self-focusing of light in an isotropic medium featuring a spatially disperse nonlinearity

The possibility is demonstrated that the self-focusing of an elliptically polarized beam in a medium featuring a spatially disperse cubic nonlinearity can give rise to several radially symmetric ring-shaped regions of the same sense of rotation (right or left) of the electric-field vector in the cross-section of the beam. The formation dynamics and propagation features of such electric field structures are studied for various parameters of the incident radiation and nonlinear medium.

Microelements with High Aspect Ratio Prepared by Self-Focusing of Light at UV-Curing

The results of deep lithography based on the long-wavelength UV photopolymerization with a light self-focusing effect are presented. The effect of the formation of “bridges” between the nearest photopolymerization areas is revealed. Its sources such as the inhibition of photopolymerization by oxygen due to its diffusion to the exposed area are discussed. The conditions for a high aspect ratio of microelements (up to 100) are determined.

The formation of microstructures as a result of the self-focusing of light in a photopolymer nanocomposite

This paper presents the results of studies of the process of forming microstructures in UV-hardenable monomeric composites. The main effects that determine the parameters of the resulting structures are investigated: the self-focusing of light in a material with a positive sign of the refractive-index change during photopolymerization, shrinkage during photopolymerization, and the photoradical-diffusion rate. The conditions are determined for obtaining microstructures with a format ratio up to 1:50.

First-principles calculations of band structures and dynamic optical properties of CsCdBr3 and RbCdI3∙H2O crystals

We report the band structures as well as frequency-dependent (dynamic) linear and nonlinear optical properties of CsCdBr3 and RbCdI3∙H2O crystals calculated within density functional theory with a nonlocal gradient correction, including the dielectric function, linear and nonlinear refractive indices, frequency doubling, and self-focusing of light. The electronic origin of the optical susceptibilities is described and the critical power of light self-focusing is estimated for these two crystals.

Band-gap control optical Kerr effect and light self-focusing of semiconducting materials CdxHg1−xGa2S4

We report the dispersion of the optical Kerr effect and the wavelength-dependent critical power of light self-focusing for semiconductors CdxHg1−xGa2S4 in view of the calculated results from the density functional theory combined with an anharmonic oscillator model. It is shown that the third-order optical properties are controlled by the band gaps or cation contents of CdxHg1−xGa2S4, and it is indicated that the optical Kerr effect decreases and the critical power of light self-focusing increases as the x contents and band gaps increase for CdxHg1−xGa2S4.

Experimental and theoretical analysis of the self-focusing of light by a photonic crystal lens

We demonstrate the self-focusing of electromagnetic waves in the microwave regime by using a two-dimensional photonic crystal lens with rectangular shape. A detailed study of the band structure shows that the focusing is due to the refractivelike behavior of the photonic crystal with a negative effective index of refraction in the frequency range under study. In contrast to lenses based on homogeneous isotropic materials, it is shown that the symmetry of the mode and the direction of incidence play a fundamental role in the performance of the photonic crystal lens.

Solitary beam formation with partially coherent light in an anisotropic photorefractive medium

We report on the stable self-focusing of light for various degrees of spatial coherence. Since the diffraction of a partially spatial coherent light beam is mainly governed by the transverse correlation width, its self-focusing is generally non-trivial. Here, we systematically demonstrate that partially coherent solitons can be generated in a non-instantaneous nonlinear material when the external parameters are properly adapted to the degree of coherence of the light beam.

Observation of self-focusing of light mediated by cubic nonlinearities in potassium titanyl phosphate

We report our observations of the transient self-narrowing of light beams mediated by dominant Kerr nonlinearities and two-photon absorption in a bulk potassium titanyl phosphate crystal near conditions for second-harmonic generation. Drastic differences between upconversion and downconversion processes are highlighted.

Low confinement factors for suppressed filaments in semiconductor lasers

Filament formation through self-focusing of light in semiconductor lasers leads to beam quality degradation, as well as lifetime and facet degradation. In this paper, we rederive an expression for the growth rate of sinusoidal perturbations, or filaments superimposed on the steady-state field in a semiconductor laser. We note that while the mode gain decreases with the confinement factor in a linear manner, the gain for filaments decreases far more rapidly. This agrees with recent observations of improved beam quality in broad-area semiconductor lasers with lowered confinement factors. Finally, we describe design rules for low-confinement-factor semiconductor lasers offering low-filamentation and greatly improved beam quality.

Self-focusing and its related interactions at very high laser intensities for fast ignition at Osaka University

At the Institute of Laser Engineering, various type of experiments related to fast ignition were performed with the 12-beam laser system GEKKO XII and the newly added 100 TW beams line. Using both X-ray and UV laser probes, drilling via ponderomotive laser light self-focusing was studied to show drilling well into the overdense plasma over a distance of 100 μ m at a self-focused laser intensity of 10 18 W / cm 2 . This type of self-focusing accelerated electrons up to 0.1 to 1 MeV and was also applied to an imploding shell.