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Optical tweezers: Picking things up with light

Physics lessons in school taught us that light is electromagnetic radiation, where electric and magnetic fields oscillate in harmony and travel perpendicular to one another. Over centuries, the properties of light have led to the greatest discoveries—from far-away galaxies to understanding the structure of atoms. What if we put the light rays to do some mechanical work? That’s what optical tweezers—scientific instruments used to manipulate microscopic objects—do!

The invention of optical tweezers is credited to Arthur Ashkin, an American scientist, whose work paved the way for the early development of these tweezers. Today, they are widely used by researchers for a variety of applications—to trap living cells, hold micro particles, stretch membranes and much more. Ashkin was awarded the 2018 Nobel Prize in physics for his contributions to this field. Unlike mechanical tweezers, using light to pick or hold these delicate things do not harm them.

Optical tweezers use light beams, mostly originating through lasers, to work with tiny objects. They operate through the principle of momentum transfer, associated with bending light. Light carries momentum, which is proportional to its energy, in the direction of propagation. When an object bends light, there is a change in its momentum. Since the overall momentum needs to be conserved, the object experiences an equal and opposite change in its momentum, giving rise to a force that pins it.

The direction and magnitude of the electrical and mechanical fields of these light beams can be controlled and used to manipulate objects in its way. When a laser beam is focussed on tiny particles, they are pushed to the centre of the laser beam and pinned there. They can then be guided in the desired direction by controlling the laser.

The use of optical tweezers is not limited to manipulating particles or atoms. They have potential applications in quantum computing, cancer therapeutics and in exploring the quantum mechanics of chemical reactions.

An Optical trap [Image Source]


A fibre-based laser trap capable of holding and precisely orienting living cells. [Image Source]