Self-trapping Of Light Research Articles

Bremsstrahlung Gamma-Ray Source and Gamma Radiography Based on Laser-Triggered Electron Acceleration in the Regime of Relativistic Self-Trapping of Light

Bremsstrahlung Gamma-Ray Source and Gamma Radiography Based on Laser-Triggered Electron Acceleration in the Regime of Relativistic Self-Trapping of Light

Impact of nonlinear electron transport model on character of light propagation in photorefractive semiconductors

The paper presents an analysis of nonlinear light propagation in photorefractive gallium arsenide. Two methods of nonlinear electron transport modelling have been distinguished, differentiated by the curve representing the dependence of electron drift velocity on the electric field. It has been shown that the phenomena of self-trapping of optical beams and the effect of bending of beam trajectories are possible both within the model for which the electron drift curve does not pass a local minimum, as in the case when the existence of a local minimum is taken into account. Since the model without a local minimum allows the excitation of charge carrier domains oscillations, the impact of this phenomenon on light propagation has been analysed. It has been shown that the oscillations have a limiting effect on the time window in which effective self-trapping of light occurs, and that they can manifest themselves in the form of oscillations in the intensity of the optical beam.

High repetition rate green-pumped supercontinuum generation in calcium fluoride

We compare supercontinuum generation in hbox {CaF}_2 crystal under tight and loose focusing of 150 fs, 515 nm second harmonic pulses from an amplified Yb:KGW laser at a repetition rate of 10 kHz. It is demonstrated that supercontinuum generation geometry applying loose focusing (hbox {NA}=0.004) of the pump beam into a long (25 mm) hbox {CaF}_2 sample is advantageous in terms of supercontinuum spectral extent and durability of damage-free operation of the nonlinear material as compared to a commonly used supercontinuum generation setup which employs tight focusing (hbox {NA}=0.012) into a short (5 mm) sample and to setup which uses tight focusing into a long (25 mm) sample. More specifically, loose focusing into a long sample showed remarkably longer (20 min) damage-free operation of the nonlinear material, which was not translated with respect of the pump beam, while in tight focusing condition the sample is damaged just within 2 min of operation, leading to a complete extinction of the supercontinuum spectrum. The evolution of optical degradation of the nonlinear material in time and its impact to supercontinuum spectrum is studied in terms of filament-induced luminescence due to self-trapped exciton emission and light scattering at the pump wavelength indicating the onset of optical damage. Our findings are supported by the numerical simulations which compare relevant parameters related to filament propagation in tight and loose focusing conditions.

Localized Photon Lasing in a Polaritonic Lattice Landscape

Periodic photonic structures have attracted much interest due to their versatility for controlling light propagation. The dispersion of photon modes in photonic lattices exhibits an energy band structure analogous to the electronic one in crystals. An important consequence of the photonic band structure is the localization of light in a band gap, which can be engineered by breaking the translational symmetry introducing a defect in the lattice. Here, we demonstrate experimentally the localization of light in a two-dimensional periodic system of polaritons confined in a patterned microcavity. We generate a self-trapping of light by optically inducing a local breaking of the strong-coupling regime of excitons to photons. In the photon lasing regime we show the existence of confined modes that can be controlled by the shape of the generated defect. We demonstrate single on-site localization with lasing mode at the edge of the Brillouin zone similar to a gap soliton. Our results pave the way for useful tools to optically control localization and propagation of light towards the generation of microlasers.

Opto-chemo-mechanical transduction in photoresponsive gels elicits switchable self-trapped beams with remote interactions

Next-generation photonics envisions circuitry-free, rapidly reconfigurable systems powered by solitonic beams of self-trapped light and their particlelike interactions. Progress, however, has been limited by the need for reversibly responsive materials that host such nonlinear optical waves. We find that repeatedly switchable self-trapped visible laser beams, which exhibit strong pairwise interactions, can be generated in a photoresponsive hydrogel. Through comprehensive experiments and simulations, we show that the unique nonlinear conditions arise when photoisomerization of spiropyran substituents in pH-responsive poly(acrylamide-co-acrylic acid) hydrogel transduces optical energy into mechanical deformation of the 3D cross-linked hydrogel matrix. A Gaussian beam self-traps when localized isomerization-induced contraction of the hydrogel and expulsion of water generates a transient waveguide, which entraps the optical field and suppresses divergence. The waveguide is erased and reformed within seconds when the optical field is sequentially removed and reintroduced, allowing the self-trapped beam to be rapidly and repeatedly switched on and off at remarkably low powers in the milliwatt regime. Furthermore, this opto-chemo-mechanical transduction of energy mediated by the 3D cross-linked hydrogel network facilitates pairwise interactions between self-trapped beams both in the short range where there is significant overlap of their optical fields, and even in the long range--over separation distances of up to 10 times the beam width--where such overlap is negligible.

Fast self-trapping of light in photorefractive semiconductors

The paper presents an analysis of fast self-trapping of light as part of high-intensity pulse propagation in photorefractive gallium arsenide. Temporal evolution of picosecond pulses has been analysed for various material parameter values, using numerical techniques which take into consideration bipolar carrier transport and the hot-electron effect. A strong laser beam self-bending effect, which can be used for switching optical signals, has been demonstrated, and the conditions of its occurrence have been discussed.

3-D Spiraling Self-Trapped Light Beams in Photochemical Systems.

A pair of visible laser beams self-trap and spiral about each other as they propagate through polymer gels undergoing two different photochemical reactions. When launched into gels that undergo photopolymerization of methacrylate substituents or photo-oxidation of iodide anion, two non-coplanar (skewed) Gaussian beams collide and spiral about each other as they advance through the evolving medium. In the absence of chemical reactions, the linearly polarized beams broaden naturally and propagate along their original, straight-pathed trajectories. By contrast, refractive index gradients generated by the photochemical reactions elicit self-trapping and introduce an attractive interaction between the self-trapped beams. The self-trapped beams spiral about each other when this mutual attraction perfectly counterbalances their original tendency to diverge away from each other. These findings show that the photochemically mediated interactions of incident optical fields within the gel medium impart a helical trajectory and angular velocity to the self-trapped beam pair.

Self-Trapping of Light Using the Pancharatnam-Berry Phase

Since its introduction by Sir Michael Berry in 1984, geometric phase became of fundamental importance in physics, with applications ranging from solid state physics to optics. In optics, Pancharatnam-Berry phase allows the tailoring of optical beams by a local control of their polarization. Here we discuss light propagation in the presence of an intensity-dependent local modulation of the Pancharatnam-Berry phase. The corresponding self-modulation of the wavefront counteracts the natural spreading due to diffraction, i.e., self-focusing takes place. No refractive index variation is associated with the self-focusing: the confinement is uniquely due to a nonlinear spin-orbit interaction. The phenomenon is investigated, both theoretically and experimentally, considering the reorientational nonlinearity in liquid crystals, where light is able to rotate the local optical axis through an intensity-dependent optical torque. Our discoveries pave the way to the investigation of a new family of nonlinear waves featuring a strong interaction between the spin and the orbital degrees of freedom.

Plasmonic resonant nonlinearity and synthetic optical properties in gold nanorod suspensions

We experimentally demonstrate self-trapping of light, as a result of plasmonic resonant optical nonlinearity, in both aqueous and organic (toluene) suspensions of gold nanorods. The threshold power for soliton formation is greatly reduced in toluene as opposed to aqueous suspensions. It is well known that the optical gradient forces are optimized at off-resonance wavelengths at which suspended particles typically exhibit a strong positive (or negative) polarizability. However, surprisingly, as we tune the wavelength of the optical beam from a continuous-wave (CW) laser, we find that the threshold power is reduced by more than threefold at the plasmonic resonance frequency. By analyzing the optical forces and torque acting on the nanorods, we show theoretically that it is possible to align the nanorods inside a soliton waveguide channel into orthogonal orientations by using merely two different laser wavelengths. We perform a series of experiments to examine the transmission of the soliton-forming beam itself, as well as the polarization transmission spectrum of a low-power probe beam guided along the soliton channel. It is found that the expected synthetic anisotropic properties are too subtle to be clearly observed, in large part due to Brownian motion of the solvent molecules and a limited ordering region where the optical field from the self-trapped beam is strong enough to overcome thermodynamic fluctuations. The ability to achieve tunable nonlinearity and nanorod orientations in colloidal nanosuspensions with low-power CW laser beams may lead to interesting applications in all-optical switching and transparent display technologies.

Simulations of Morphology Evolution in Polymer Blends during Light Self-Trapping

Simulations are presented for binary phase morphologies prepared via coupling the self-trapping properties of light with photopolymerization induced phase separation in blends of reactive monomer and inert linear chain polymer. The morphology forming process is simulated based on a spatially varying photopolymerization rate, dictated by self-trapped light, coupled with the Cahn–Hilliard equation that incorporates the free energy of polymer mixing, degree of polymerization, and polymer mobility. Binary phase morphologies form with a structure that spatially correlates to the profile of the self-trapped beam. Attaining this spatial correlation emerges through a balance between the competitive processes entailed in photopolymerization-induced decreases in diffusion mobility and the drive for the blend components to phase separate. The simulations demonstrate the ability for a self-trapped optical beam to direct binary phase morphology along its propagation path. Such studies are important for controlling the...

Control of Morphology in Polymer Blends through Light Self-Trapping: An in Situ Study of Structure Evolution, Reaction Kinetics, and Phase Separation

We report on how polymer morphology is controlled through the self-trapping of transmitted optical beams in photoreactive polymer blends. Self-trapped optical beams, characterized by divergence-free propagation, drives the growth of a congruent arrangement of polymer filaments in the blends. With suitable component weight fractions and exposure intensity, binary phase morphologies form in precisely the same pattern as the beams’ arrangement, thereby producing 2D structures in polymer blend volumes of large depth and area. Morphology evolution and the formation processes were observed by in situ microscopy. In situ confocal Raman measurements of polymer conversion and molecular weight increase along the filament regions reveal that polymerization undergoes autoacceleration, followed by the onset of mixing instability which leads to phase separation. These phenomena begin at the front end of the filament and propagate along its length over the depth of the blend. Control over morphology is discussed with respect to the competitive processes of phase separation and photo-cross-linking.

Light interaction and self-trapping in subwavelength dielectric waveguide arrays

Based on numerical experimental techniques, we demonstrate the possibility to control light by light in nonlinear arrays of subwavelength dielectric parallel waveguides. To reach our goal, we work with waveguides of different cross sections: circular and square. Characteristic dimensions of waveguides are in the order of half of the carrier wavelength, and propagation distances are more than hundreds of wavelengths. We show that with the proper selection of parameters it is possible to obtain light self-trapped in a single waveguide that will allow us to study the optical fields’ interaction in the array. The main result that we discuss here is that we can control the light output position for two input beams. The light behavior is a function of the phase difference and incidence angle, as well as the light power at the input of the array.

Properties of the self-trapped light in an array of subwavelength waveguides

Based in the numerical experiment techniques, we study the propagation dynamics of electromagnetic waves along one-dimensional array of parallel cylindrical subwavelength waveguides, possessing Kerr nonlinearity. By means of the proper selection of the array parameters, as well as the incident radiation properties and applying the Finite Difference Time Domain method, we are able to observe the light self-trapping in one single subwavelength waveguide. Furthermore, the position of the output beam can be controlled by the phase difference and the angle of incidence of the input beams. These results could give the possibility to control light by light, with perspectives in applications to implement integrated optics devices at nanoscale.

Plasmonic Resonant Solitons in Metallic Nanosuspensions

Robust propagation of self-trapped light over distances exceeding 25 diffraction lengths has been demonstrated for the first time in plasmonic nanosuspensions. This phenomenon results from the interplay between optical forces and enhanced polarizability that would have been otherwise impossible in conventional dielectric dispersions. Plasmonic nanostructures such as core-shell particles, nanorods, and spheres are shown to display tunable polarizabilities depending on their size, shape, and composition, as well as the wavelength of illumination. Here we discuss nonlinear light-matter dynamics arising from an effective positive Kerr effect, which in turn allows for deep penetration of long needles of light through dissipative colloidal media. Our findings may open up new possibilities toward synthesizing soft-matter systems with customized optical nonlinearities.

Optical Nonlinearities and Enhanced Light Transmission in Soft-Matter Systems with Tunable Polarizabilities

We demonstrate a new class of synthetic colloidal suspensions capable of exhibiting negative polarizabilities, and observe for the first time robust propagation and enhanced transmission of self-trapped light over long distances that would have been otherwise impossible in conventional suspensions with positive polarizabilities. Such light penetration through the strong scattering environment is attributed to the interplay between optical forces and self-activated transparency effects while no thermal effect is involved. By judiciously mixing colloidal particles of both negative and positive polarizabilities, we show that the resulting nonlinear response of these systems can be fine-tuned. Our experimental observations are in agreement with theoretical analysis based on a thermodynamic model that takes into account particle-particle interactions. These results may open up new opportunities in developing soft-matter systems with engineered optical nonlinearities.

Spontaneous knotting of self-trapped waves

We describe theory and simulations of a spinning optical soliton whose propagation spontaneously excites knotted and linked optical vortices. The nonlinear phase of the self-trapped light beam breaks the wave front into a sequence of optical vortex loops around the soliton, which, through the soliton's orbital angular momentum and spatial twist, tangle on propagation to form links and knots. We anticipate similar spontaneous knot topology to be a universal feature of waves whose phase front is twisted and nonlinearly modulated, including superfluids and trapped matter waves.

Modeling the photochemical kinetics induced by holographic exposures in PQ/PMMA photopolymer material

Phenanthrenequinone (PQ) doped poly(methyl methacrylate) (PMMA) photopolymer material is receiving ever greater attention in the literature due to its attractive properties for applications such as holographic data storage, hybrid optoelectronics, solar concentrators, self-trapping of light, and diffractive optical elements. PQ/PMMA photopolymer material can be used to produce three-dimensional low loss, low shrinkage recordings that are environmentally stable, have high contrast refractive index variations, and can produce high-optical-quality devices. However, in any attempt to further develop the potential of such materials, a more physical and accurate theoretical model has become ever more necessary and important. In this paper, based on a detailed analysis of the photochemical mechanisms present in PQ/PMMA photopolymer during holographic grating formation, a set of rate equations are derived governing the temporal and spatial variations of each associated chemical species concentration. Experimental results are presented, which are then fit using this model. In this way, values for several kinetic parameters are estimated and their significance is discussed.

Light self-trapping in a large cloud of cold atoms

We show that, for a near-resonant propagating beam, a large cloud of cold Rb87 atoms acts as a saturable Kerr medium and produces self-trapping of light. By side fluorescence imaging, we monitor the transverse size of the beam and, depending on the sign of the laser detuning with respect to the atomic transition, we observe self-focusing or self-defocusing, with the waist remaining stationary for an appropriate choice of parameters. We analyze our observations by using numerical simulations based on a simple two-level atom model.

Nematicons Interaction in Chiral Nematic Liquid Crystals

Recently, it has been shown experimentally that due to the reorientational nonlinearity in nematic liquid crystals (NLCs) it is possible to generate self-trapped non-diffractive light beams for relatively low power. Such spatial solitary waves in NLCs are called nematicons. Very interesting and promising properties have nematicons in chiral nematic liquid crystals (ChNLCs), where the incident light propagates perpendicular to the helical axis. In this work we report on the experimental studies of the existence of nematicons in such ChNLCs cells. We show that it is possible to utilize multi-layers for propagation of independent and interacting nematicons.

Saddle solitons: a balance between bi-diffraction and hybrid nonlinearity

We demonstrate self-trapping of light by simultaneously compensating normal and anomalous (saddle-shaped) diffractions with self-focusing and self-defocusing hybrid nonlinearity in optically induced ionic-type photonic lattices. Innovative two-dimensional gap solitons, named "saddle solitons," are established, whose phase and spectrum characteristics are different from all previously observed spatial solitons.