Optical Tweezers is an important tool that uses highly focused laser beam to trap and manipulate nanometer and micrometer-sized particles. Arthur Ashkin, the winner of 2018 Nobel Prize in Physics, is considered the father of the optical tweezers. He also later used this technique to trap and manipulate atoms, molecules, biological cells, and other microscopic particles.
Although the theory of optical trapping is quite complex, it can be explained rather simply in two different ways for particles smaller than the wavelength of light and much larger. In the Rayleigh regime in which the size is much smaller than the wavelength of the light, the dielectric particle can be approximated as a simple dipole. In a non-homogeneous electric field, with a gradient, forces are exerted on the dipole. As a result there is a net force directed to the highest intensity point of the field. Read more
Trapping, for particles with a diameter significantly greater than the wavelength of light, can be explained by ray optics. When a ray of light enters and exits the particle with a higher refractive index than the surrounding medium it is both reflected and refracted. Light carries momentum, therefore, when the path of light changes there is a change in momentum as well and due to conservation law an equal but opposite momentum will be imparted on the particle which means that there will be forces acting on the sphere. The net force caused by refraction has a component in the direction of propagation of the beam which is called scattering force and a component perpendicular to the direction of propagation is radial force. If a laser has a Gaussian intensity profile, the sum of radial forces results in the net force called the gradient force, which is directed to the highest intensity point.
A particle with a lower refractive index than the surrounding medium is not trapped by the focused laser. In this case optical tweezers have an opposite effects. The path of the light in the particle changes in the way that the forces which are exerted on the particle are directed from the focus point and therefore the particle is repulsed. For example, air bubbles in water would be repulsed by the laser focus point since air has a lower refractive index than water.
Typical optical tweezers setup requires very expensive and controlled optical tools. However, lab-based test set up can be developed using cheap diode lasers and an optical microscope. In a project, I developed such a set up to trap micro-particles. The project was carried out at the Lund University, Sweden.
Experimental setup
The laser light was collimated and guided through an objective to focus on the sample plane. Several filters were also used to have different intensities of the laser.
Optical tweezers has a wide field of applications including biology, chemistry and physics. Manipulating micrometer sized beads, trapping atoms, sorting cells, measuring forces as weak as a few piconewtons – these are just a few of those applications.
Trapping micro-beads
Using the above mentioned simple setup we can trap micro-particles in a liquid medium.
If the laser power is high enough it will induce thermal damage which may enable cutting or dissecting biological tissues or organism, or even to perform microsurgery.
Laser Dissection
Using the same optical tweezers’ setup, we were able to dissect Drosophila melanogaster (dead) in a liquid medium. In this experiment a red (650 nm) and a blue (405 nm) lasers were used.
By using a simple optical tweezers setup it is possible to perform effective optical surgery with a broad range of biological samples. The images show cutting of vitelline membrane and decapitation of a fruit fly.
