One of the most remarkable invention of the last two decades can arguably be the optical trapping where a light beam can trap a microscopic particle suspended in a medium. Optical trapping is the technique where a focused laser beam or a combination of counter propagating laser beams exerts force on the microscopic particle in order to have control over the position of the particle. A particle can also be trapped and manipulated using a single laser beam focused tightly using a high numerical aperture lens. Such a trap is termed as optical tweezers. Interaction between the light beam and the microscopic particle near the focus leads to two kinds of forces, namely the scattering force and the gradient force both arising due to conservation of linear momentum of the photon and the particle. It is the gradient force that results in the useful force to implement an optical tweezer. Realising the growing importance of optical tweezers the inventor of the technique, A Ashkin, was awarded the Nobel prize in Physics in 2018.

Now a days experimentalists find interest in holographic optical tweezers where the laser beam used for trapping is holographically designed. Using holographic means one can implement several high quality optical traps to efficiently manipulate microscopic particles suspended in liquid medium. A realtime user reconfigurable hologram for such a purpose can be implemented using liquid crystal spatial light modulator (LCSLM).

In our laboratory we have realized a holographic optical tweezer using an LCSLM based arrangement. Our experimental setup uses an Olympus inverted microscope with 100x oil immersion objective lens of numerical aperture 1.4 to focus the holographically designed beam. We have observed the trapping of polystyrene beads of size 1 μm in the water using light from a DPSS laser of power 1.15W emitting at 532nm. The movie in Fig.1 shows our holographic optical tweezer in action. As seen when the trap is switched on it creates a strong gradient force and the beads near the focus get accumulated at the trap, indicated by the red circle. We then holographically move the trap from left to right, then from right to left and finally downwards.