Why LP21 Mode?
The bending-independent and linear twist dependence of LP21 mode are two very special mode characteristics, offering many advantages for particle capture and rotation. A number of low-order modes have been investigated. Among these LPnm mode, except fundamental Gaussian mode, LP21 mode the lowest order mode that can maintain a stable pattern in propagation, yet provide us with four-lobed tooling bits to allow a convenient delivery of angular momentum to particle for rotation and other reliable manipulations. Some of the other common low-order multi-modes including two spots LP11, circular spot LP02, five spots and other transport modes in an optical fiber are extremely venerable to fiber handling process, a fiber twist and bending will cause coupling of mode energy, resulting in significant change in output light pattern. Instability of these modes makes the selection of modes become very difficult. These phenomena are very easy to observe during the light coupling, causing uncertainties in cell manipulations. But the LP21 mode has a bending independent feature, completely independent of fiber bending in any direction, the output beam pattern will not be affected in the position and shape.
Light intensity distribution of LP21 model has a suitable optical potential well in the center of beam spot. LP21 mode has four spots with axial symmetry, the optical trap is formed as a four jaw mechanical chuck, holding the particles in a potential well very stably during cell translation even in an accelerated motion. It is capable of trapping a chosen cell inside a cell cluster, grouping cells in addition to ordinary cell manipulation including translation and rotation.
Rotation of LP21 speckle gram by a simple fiber twister serves as an excellent control means to manipulate cell in another degree of freedom-axial rotation. The linear twist-rotation characteristic of LP21 mode is confirmed experimentally for a range from 0° all the way to 1200°. Through the comparison between experiments and theories, it was verified that the linear factor is very stable, complementing to optical trapping with a great potential to manipulate cells with more delicate features.