**2. Working principle of OFTs**

OFTs, generally based on a tapered fiber probe, can be fabricated by drawing a commercial single-mode optical fiber through a flame-heating technique. The shape of OFTs tip can be controlled by controlling the heating temperature and the drawing speed. The operation principle of typical OFTs has been detailedly analyzed and described [14]. As schematically shown in **Figure 1a**, an OFT is immersed in water. *D*A means the axial distance of a dispersed particle to the OFTs tip, while *D*T means the transverse distance. With a laser beam launched into the OFTs, particle will be trapped and manipulated by the generated optical force. There two components of the optical force, *i.e.*, gradient force (*F*g) and scattering force (*F*s). *F*g is directed to the region with stronger light intensity and

#### **Figure 1.**

*Principle of a single optical fiber tweezers for trapping of particles [14]. (a) Schematic of particle manipulation by an OFT with light launched. (b) Simulated electric field amplitude (EA) distribution by FDTD method. (c) Calculated optical force exerted on particles along the x direction. (d) Calculated optical force and trapping potential along the y direction.*

#### *Optical Fiber Tweezers for the Assembly of Living Photonic Probes DOI: http://dx.doi.org/10.5772/intechopen.98845*

is responsible to trap the particle, while *F*s is directed along the light propagation and can push particles away from the OFTs tip. When a particle is near the axial axis of the OFTs, it will be trapped to the axis by *F*g. For particle near the OFTs tip, the dominated *F*g can trap the particle to the fiber tip. As the distance to the tip increases, *F*s will become larger than *F*g, and the dominated *F*s will push the particle away from the fiber tip. The electric field amplitude (*E*A) distribution around the OFTs was shown in **Figure 1b**, with a laser beam at a wavelength of 980 nm launched into the fiber probe. It can be seen that the light outputted from the OFTs is firstly focused at the tip and subsequently diverged out in water with a divergence angle of 32°. **Figure 1c** shows the calculated optical force exerted on a 3-μm silica particle along the *x* direction. It can be seen that, near the fiber tip, the force is negative, indicating a trapping force for particles. Therefore, particles near the fiber tip can be trapped by the OFTs. As the distance increases, the force is positive, indicating a driving force for particles. Therefore, particles can be pushed away by the OFTs. **Figure 1d** shows the calculated force and trapping potential in the y direction. It can be seen that the trapping potential on the axis is the smallest, and therefore particles beside the axis can be trapped at the axis. These optical forces enable the trapping capability of OFTs. By simply moving the fiber probe, the trapped particles can be manipulated in a highly flexible manner.
