Nonlinear focusing of light with cyanobacteria
Light doesn't have mass, but it has momentum. If light changes direction, something had to make it change. From conservation of momentum, any light refracting through cyanobacteria, which is an interesting bacteria that obtains its energy through photosynthesis and produces oxygen, transfers momentum to the bacteria. We show that free-floating bacteria in cuvette of deionized water are pushed by light into a channel and in turn guide light.
Tug-of-War Optical Tweezers rotate S. Meliloti Bacteria with PodJ+ gene modification
Triangular Tug-of-War Optical tweezers use a diverging beam to trap and manipulate particles. The principle behind optical tweezers is that light refracts through the bacteria in a controlled manner, such that we may move the bacteria. But, shape matters. If the shape of the bacteria is asymmetrical, the optical trap is usually unstable. With the Tug-of-War tweezer design, the beams exert a lateral pulling force and stably trap asymmetric samples. The video shows optical tweezers rotating a mutant bacteria (S. Meliloti with PodJ+ gene modification) from this paper.
Nonlinear focusing of light with red blood cells
Similar to the work with cyanobacteria, red blood cells (RBCs) also focus light. The RBCs form a channel and guide light. Our optics lab at SFSU is investigating the correlation between the cell's shape and the power required to focus the beam. The shape of the RBC change the way the light scatters, which change the optical force acting on it. We expect that the laser may create a temperature gradient, which implies that thermophoresis plays a critical role in the effect. Just as in a room with cold and warm air where the warm air moves to the cold side, the RBCs would move to reach thermal equilibrium with its environment.