An Introduction to Optical Tweezers

Optical tweezers is among the key tools in the hands of biophysicists. With this they are unraveling the energetics and kinetic tracts of biologically active single molecules and proteins.Optical Tweezers with the help of light can manipulate small microscopic objects like a single atom. It employs radiation pressure from a highly focused laser beam for trapping and then manipulating very small particles( micron-sized cells) with pico-newton sized forces.

Applications Of Optical Tweezers

The Optical Tweezers are used in a variety of applications. Though there can be a change in the operating principles depending upon models and applications, but the end result would be the same. The basic principle of keeping the particles in a focal plane and their subsequent manipulation is a thread that runs through all the applications. Though primarily their maximum usage is in the biological laboratories, but their usage spreads also to fields of biophysics, nanotechnology. Some of their useful laboratory applications are enumerated here.

• Optical Tweezers can easily aid the differentiation of cells

• They can stretch single polymer chains

• Facilitates the study of laser physics

• Used in laser cooling applications

• Optical Tweezers are used for applying forces in the pN-range and for effectively measuring displacements in the nm range of objects in the size range of 10 nm to 100 mm or more.

• Recent enhancements in the Optical Tweezer technique have made it an important tool in cell biology where it is applied extensively in studying the dynamics of biological systems.

• For studying DNA mechanics, RNA and DNA polymerization, viral packaging mechanics and kinetics.

• Studying of Brownian motion.

• In microbiology, Optical tweezers are used to manipulate single cells like bacteria, sperms (optically assisted in vitro fertilization) or blood cells etc.

How Does an Optical Tweezer Work?

In the basic form of an optical trap a high-quality microscope objective is used for focusing laser beam to a spot in the specimen plane. This spot makes an "optical trap" that holds a small particle at its center. The forces on this particle typically consist of the light scattering as well as gradient forces (formed from interaction of the particle with light). Often Optical Tweezers are made by the modification of a standard optical microscope. There has been a rapid transformation in the technology of Optical Tweezers. From being a simple tool for manipulating micron-sized objects to state -of the art sophisticated instruments, fully computerized for accurately measuring displacements and forces.

The next figure takes a detailed look at the working of an optical trap. It is the transfer of momentum associated with bending light that is the underneath principle behind functioning of optical tweezers. Light carries a momentum proportional to its energy. This momentum is in the direction of propagation. As the direction of light changes, whether by reflection or refraction, there would be a change in the light's momentum.When an object bends the light, thus changing its momentum, then according to the principle of conservation of momentum the object must also undergo an equal and opposite change in momentum. This results in a force that acts on the object.

In a normal Optical Tweezer setup a laser is the source of incoming light. The laser has a "Gaussian intensity

profile". The light at the center of the beam is more bright as compared to the light at the edges. On interaction of the light with a bead, the light rays bent in accordance with the laws of reflection and refraction.The sum total of the forces arising from the rays can be effectively split into two components. The first is the F(scattering), the scattering force, that points towards the incident light (z axes in Fig).

The second is the F(gradient), this is the gradient force that emanates from the gradient of the Gaussian intensity profile. This force in the in x-y plane points towards the center of the beam (in dotted lines of the figure). The gradient force is nothing but a restoring force. It pulls the bead into the center. If the contribution of the refracted rays to the F(scattering) is larger as compared to that of the reflected rays then there is a creation of a restoring force along the z-axis.