Optical force-induced nonlinearity and self-guiding of light in human red blood cell suspensions

Osmotic conditions play an important role in the cell properties of human red blood cells(RBCs),which are crucial for the pathological analysis of some blood diseases such as malaria.Over the past decades,numerous efforts have mainly focused on the study of the RBC biomechanical properties that arise from the unique deformability of erythrocytes.Here,we demonstrate nonlinear optical effects from human RBCs suspended in different osmotic solutions.Specifically,we observe self-trapping and scattering-resistant nonlinear propagation of a laser beam through RBC suspensions under all three osmotic conditions,where the strength of the optical nonlinearity increases with osmotic pressure on the cells.This tunable nonlinearity is attributed to optical forces,particularly the forward-scattering and gradient forces.Interestingly,in aged blood samples(with lysed cells),a notably different nonlinear behavior is observed due to the presence of free hemoglobin.We use a theoretical model with an optical force-mediated nonlocal nonlinearity to explain the experimental observations.Our work on light self-guiding through scattering biosoft-matter may introduce new photonic tools for noninvasive biomedical imaging and medical diagnosis.

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.