  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 bio-soft-matter may introduce new photonic tools for noninvasive
biomedical imaging and medical diagnosis.
