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

Abstract

2Controlling light transport in soft-matter systems could be crucial in many and diverse fields of science and technology. For example, in colloidal suspensions, this can be accomplished through optical radiation forces capable of manipulating particle concentration and molecular kinetics at the mesoscopic level 2,7,11,12 . In principle, such optically induced processes can be exploited for initiating and regulating chemical reactions, for sorting different species of nanoparticles, and for influencing diffusion and osmotic pressure effects, to mention a few These issues can be addressed by first considering how the particle polarizability is related to optical nonlinearities that are solely mediated by radiation pressure effects. In general, a particle displays a positive polarizability (PP) whenever its refractive index exceeds that of the background medium, while in the converse case its polarizability is negative (NP). As indicated in several studies Fig. 1(c)], one could synthesize soft-matter systems with a tunable polarizability, optimized nonlinear response, and enhanced light transmission.In this study, we experimentally demonstrate a new class of synthetic colloidal suspensions capable of exhibiting negative polarizabilities. This is accomplished in a stabilized mixture of Polytetrafluoroethylene (PTFE) particles in a glycerin-water solution. We show that by judiciously introducing NP particles in conventional PP colloidal suspensions, the resulting "mixed" polarizability can be fine-tuned, thereby enabling us to modify the nonlinear response of these systems. In particular,we observe robust propagation and up to a fourfold-enhanced transmission of an optical beam when traversing an NP suspension as compared to that in a typical PP suspension. Such light penetration through otherwise strong scattering environment is attributed to the interplay between optical forces and 4 self-activated transparency effects while no thermal effect is involved. Our experimental observations are in agreement with a previously derived thermodynamic model that takes into account the interplay between the optical intensity and the osmotic pressure in the presence of particle-particle interactions 19 .These findings may pave the way towards synthesizing soft-matter systems with customized optical nonlinearities that can enable an intensity-dependent reduction in scattering losses.To understand light-particle dynamics in colloidal dispersions, we first consider the optical gradient force that typically dominates the interaction process. To first order, this component of the optical force is given 2,16 by = ∇ /4, where I is the optical intensity and α is the polarizability of the particle. In the dipole regime, = 3 ( − 1)/( + 2), where V is the particle's volume, = / is a measure of the contrast between the refractive index of the particle and that of the background fluid . Therefore, PP dielectric particles with > ( > 0) will be attracted towards

Two-dimensional bright solitons in dipolar Bose-Einstein condensates with tilted dipoles

“…Last two decades have witnessed intensive investigations, both theoretical and experimental, on multi-dimensional solitons, especially in systems possessing nonlocal nonlinearity (NLNL) [1][2][3] such as Bose-Einstein condensates (BECs) of particles with permanent [4,5] or induced dipole moments [6] (dipolar BECs), photo refractive materials [7], nematic liquid crystals [8,9] and others [10][11][12]. Two-dimensional (2D) optical bright solitons [13] and three-dimensional (3D) light bullets [14] have been reported in non-local media.…”

Optomechanical self-structuring in a cold atomic gas

“…As the lattice is self-organized and not externally imposed, it is not rigid but can have dynamics leading to the possibility to study the propagation of coupled light-density perturbations, correlations of fluctuations in the light and density modes, and, in presence of atomic coherence, corresponding features in quantum transport. The mechanism of self-structuring with optical feedback described here is expected to apply also to the electron density in a plasma in the presence of ponderomotive forces 28 , to 'soft matter' formed from dielectric beads 22,23 , and deformable membranes 29 . Beside periodic patterns, one emerging feature suggested by theoretical simulations is the possibility of localized, soliton-like, density-light structures (single or multiple holes in the density), which can be set and erased by external control pulses and sustained by homogeneous driving only 30 .…”

“…The crucial new ingredient in cold atoms is the existence of macroscopic matter transport processes due to dipole forces, leading to density self-organization without the need for an intrinsic optical nonlinearity (first experimental clues of such an effect in a very low aspect ratio situation were obtained in 9 ). Compared to soft matter systems [21][22][23] cold atoms have the advantage that the dynamics can be studied without viscous damping of motion, allowing for a dissipation free evolution with a Hamiltonian description of the system. It is possible, however, to introduce dissipation in a controlled way, if desired, via optical molasses 6,9,13,14 .…”

Nonlinear self-action of light through biological suspensions

“…In particular, an optical nonlinearity can lead to stable low-loss propagation and deep penetration of light in scattering media such as nanoparticle suspensions, which could be employed to noninvasively initiate and control chemical or mesoscopic kinetic processes, as well as to study living organisms with high-resolution depth-resolved optical imaging [13,14]. Although nonlinear self-trapping of light was demonstrated in colloidal suspensions of stiff nanoparticles [15][16][17], the study of the nonlinear response of biological media has been very limited. In fact, it is commonly believed that light cannot penetrate deeply into biological environments due to strong scattering loss and weak optical nonlinearities.…”

“…The experiments were carried out with a linearly polarized laser beam (λ ¼ 532 nm), which is collimated and then focused inside the medium with an input FWHM about 50 μm [15] and sent through a 4-cm-long glass cuvette filled with either a synthetic seawater medium (ASN-III) alone or with an additional colloidal suspension of Synechococcus cells. The Synechococcus cyanobacterial genus is naturally distributed in high concentration (∼10 3 -10 5 cells=ml) throughout the marine photic zone [18][19][20], and plays a major role in global carbon cycles.…”