Figure 20.

Schematic principle of the optical stretcher. (a) Two opposing laser fibers emit a non-focused Gaussian beam acting as an optical trap for dielectric objects (cells) with refractive index higher than the surrounding medium. The cell is centered between the laser fibers. (b) At increased laser power, the cell is deformed along the laser axis due to mechanical stress induced at its surface. (c) The stress at the cell surface is caused by a momentum transfer occurring when light with a momentum p₀ is transmitted from a medium with refractive index n₀ into the cell medium with refractive index n₁ > n₀, where it has a momentum p₁ > p₀. Momentum conservation demands that a momentum p_x1 perpendicular away from the surface acts on it. At exit, another momentum transfer p_x2 occurs, again directed perpendicular away from the surface.