**2. Optical trapping and optical tweezers**

Optical tweezer, which employs a tightly focused laser to trap microscopic objects have proven to be a versatile tool for many scientific researches such as in biophysics [21] and was recently recognized with a 2018 Physics Nobel prize. The initial experiment to trap particles with a laser beam used two counter-propagating loosely focused beams to localize the particles at a node of the generated standing wave, and this was reported by Arthur Ashkin and his colleagues in 1970 [22]. Subsequently, a single beam optical tweezer, which employs a tightly focused laser beam to achieve three-dimensional manipulation and trapping of microscale particles was demonstrated in 1986 [23]. Dielectric particles are trapped when the refractive index of the particles is higher than the refractive index of the surrounding medium.

Generally, the optical force is decomposed into two parts, the gradient force and the scattering force. The scattering force is also called radiation pressure and acts in the direction of light propagation. The gradient force is the significant part in single beam optical trapping procedure, because it is oriented perpendicular to the axis and push particles towards a region with higher optical intensity. Thus, the gradient force ensures that a particle is trapped in an optical tweezer. The total optical force induced on a particle can be determined by Maxwell's Stress Tensor method (MST). When the size of the particle is much smaller than the wavelength of trapping light, the particle can be considered as a dipole, so that the dipole approximation can be

#### *Nanomanipulation with Designer Thermoplasmonic Metasurface DOI: http://dx.doi.org/10.5772/intechopen.91880*

applied to simplifying the calculation of the optical force [24]. The detailed derivation of MST and dipole approximation can be obtained in Ref. [25].

In biological applications, target particles range in size from micrometer size scales such as cells and nanometer length scales such as viruses, protein molecules and vesicles. Trapping nanoscale particles is challenging in free-space optical tweezers. There are two main challenges faced by researchers using optical tweezers. The first challenge arises because the value of the gradient force is proportional to the third power of the radius of the particles in the quasi-static limit. Secondly, the optical gradient force is proportional to the gradient of light field intensity, and it is thus limited because the diffraction limit limits the achievable gradient in the light field intensity. One approach explored by Ashkin to increase the trapping stability for nanoscale objects in free-space optical tweezers involves the use of very high optical powers. However, this results in serious damage to the objects being trapped, a process termed opticution.
