Nonlinear self-action of light through biological suspensions

Understanding light interaction with microorganisms is of crucial importance in relation to the development of bio-fuels, solar cells, bio-lasers, as well as of fundamental interest in biophotonics, optofluidics, soft-matter physics, and life science. Despite of significant efforts in the study of optical properties of biological media, it is commonly pictured that light cannot penetrate deeply into biological environments due to their strong scattering loss and weak optical nonlinearity. In this talk, we report on our recent demonstration of robust propagation and enhanced transmission of a light beam over long distances through a biological suspension of marine bacteria. By deliberately altering the host environments, we show dramatic change of nonlinear propagation dynamics of the light beam, while the viability of the microorganisms remains intact. A theoretical model is developed to show that a nonlocal nonlinearity mediated by optical forces acting on the bacteria could explain the observed phenomenon. In particular, we found that the scattering force from light in the forward direction appears to play a pivotal role in forming the “biological” waveguide that is able to sustain needle-like light propagation through the cell susupension. These findings may open up new opportunities in developing bio-soft-matter systems with tunable optical nonlinearities for various applications.