Nonlinear Self-Action of Light through Biological Suspensions

It is commonly thought that biological media cannot exhibit an appreciable nonlinear optical response. We demonstrate, for the first time to our knowledge, a tunable optical nonlinearity in suspensions of cyanobacteria that leads to robust propagation and strong self-action of a light beam. By deliberately altering the host environment of the marine bacteria, we show experimentally that nonlinear interaction can result in either deep penetration or enhanced scattering of light through the bacterial suspension, while the viability of the cells remains intact. A theoretical model is developed to show that a nonlocal nonlinearity mediated by optical forces (including both gradient and forward-scattering forces) acting on the bacteria explains our experimental observations.

Figure 1

Nonlinear self-trapping of light through cyanobacteria in seawater. (a) Side view of normal diffraction of an intense laser beam in seawater, showing no self-action of the beam when no bacteria are present. (b),(c) When Synechococcus cells are suspended in seawater, the beam undergoes linear diffraction or scattering at low power (b) yet it experiences nonlinear self-trapping at high power (c). (d) Side view of the same beam in (c) imaged using autofluorescence of the cells (in red) when the green light is partially blocked, thus indicating survival of the trapped cells under laser illumination. (e)–(g) Corresponding 3D plots of the beam’s normalized intensity profiles after 4 cm of propagation, captured by the CCD camera. (h) Image of the Synechococcus cells taken with an epifluorescence microscope using 100X objective when excited by green light. (i) Transmission percentage measured as a function of input power for two different cell concentrations. (j) Semilogarithmic plot of transmission percentage as a function of cell concentration at a fixed input power of 3 W.

Figure 2

Comparison of two different models to describe nonlinear beam dynamics in biological suspensions. (a),(b) Side view of beam propagation (normalized linear scale) obtained numerically using (a) an optical gradient force only (exponential model) and (b) an optical gradient along with a forward-scattering force (forward-scattering model). (c),(d) Corresponding theoretical distributions of the normalized concentrations of bacterialike particles induced by the respective types of light-particle interactions.

Figure 3

Numerical simulations of nonlinear beam propagation in biological suspensions. (a)–(c) Side view of the laser beam and (d)–(f) corresponding output intensity profiles simulated with parameters obtained from the experimental results shown in Fig. 1. (a),(d) Case of seawater only without suspended particles, exhibiting linear diffraction. (b),(e) Case of a low power beam in the presence of suspended particles, showing linear diffraction with additional broadening due to random scattering. (c),(f) Case of a high power beam in the presence of suspended particles, where nonlinear self-trapping of the beam is achieved.

Figure 4

Enhanced scattering of light through cyanobacteria in glycerol-seawater mixtures. (a) Measured beam size after 4 cm of propagation as a function of input power through Synechococcus suspensions of varying glycerol-seawater ratios. (b)–(e) Side view of the beam taken from auto fluorescence of the cells when excited by laser light at 3 W input power, corresponding to the four samples of cyanobacterial suspensions in (a). (f)–(i) Corresponding transverse intensity profiles taken at the output of the samples.

Figure 5

Impact of environment on force distribution and cell mobility dynamics. Comparison of cell size and force distribution between cyanobacteria in seawater (top row) and in the glycerol-water mixture (bottom row). (a),(d) Schematic illustration of a Synechococcus cell under isotonic (as in seawater) and hypertonic (as in glycerol-water mixture) conditions. Red and orange arrows denote an influx of glycerol and water, respectively, while yellow arrows denote a water outflux. (b),(e) Bright-field images of Synechococcus cells taken when they are in seawater (b), and in a $3:1$ glycerol-water mixture (e), where the average volume of individual cells has decreased by 30%. (c), (f) Schematic force diagrams showing that the cells form a light guide in seawater (c), but behave like a light scatterer in glycerol-rich environments (f). The laser light is represented in green.