First introduced in 1986 at Bell labs, optical tweezers quickly emerged as an indispensable tool that can be used for a variety of different applications in chemistry and biology. By observing that a focused laser beam can be used to manipulate certain objects, Arthur Ashkin pioneered the development of a three-dimensional stable trap based on radiation pressure from a single laser beam, capable of holding and applying forces to micron-sized dielectric particles1. Not long after this initial breakthrough, optical tweezers, or optical traps, were successfully used to physically trap and control viruses, bacteria and single-cells, paving the way for the mechanical and kinetic study of biomolecules at the single-molecule level2-3.

Presently, optical tweezers have been used in a variety of different applications, such as in the study of the interactions between proteins and DNA involved in DNA organization, replication, transcription and repair; the study of the energy landscape of proteins and the kinetics of molecular motors4-7. Simply put, the value of optical tweezers lies in the fact that they can be used to perform experiments to probe the properties of single-molecules by applying forces in the range of picoNewtons and by measuring distance displacements in the range of nanometers.

Working Principle

Stable optical trapping of dielectric particles occurs due to the interaction between light and the object itself. In a fundamental level, the working principle of optical traps is based on the understanding that light carries momentum proportional of its energy and in the direction of propagation. When a laser beam passes through an object and refracts, it bends and changes direction, altering its momentum. For the system to conserve the total momentum according to Newton’s third law, the object undergoes an equal and opposite momentum change, leading to a reaction force acting on the object. Figure 1 illustrates the transfer of photon momentum occurring when a light beam travels through a microsphere.
Laser Trapping of bead - Optical Tweezers

Figure 1: Re-direction of a light path and change of momentum as it passes through a microsphere or “bead” with high index of refraction related to the medium (left). Momentum of equal and opposite force is transferred from the photons to the bead according to Newton’s Law of energy conservation (right).

Laser Traps

Ray optics - trap- optical tweezers

Figure 2: Two light paths passing through a dielectric micron sized bead. Due to the light gradient, the path originating from the center of the beam carries more photons than the light path commencing from the outlines of the beam, resulting to a larger force pulling the bead towards the focal point.

In a typical optical tweezers configuration the incoming light originates from a focused laser beam through a microscope objective and focuses on a spot in the sample. The spot creates a trap able to hold a small dielectric object at place. The total forces experienced by the object, or bead in most experimental settings, consist of a scattering force and a gradient force8. The scattering force arises when an incident light beam is scattered by the surface of the bead.

This scattering produces a net momentum transfer from the light photons to the object and causes the bead to be pushed towards the beam propagation. The gradient force is a result of the intensity profile of the laser beam which acts as an attractive force drawing the bead towards the region of greater light intensity. In the case of a highly focused laser beam with a Gaussian intensity profile, the latter force operates like a restoring force that pulls the object into the center of the focal plane. Figure 2 illustrates how the gradient force restores an off-centered bead towards the center of the focal plane, effectively trapping the object in all dimensions.

Extra Resources

Introduction to Optical Tweezers-Fluorescence Microscopy

A short informative video on a typical workflow of the C-Trap™, providing correlative optical tweezers – confocal fluorescence microscopy.

Optical Traps: an Introduction
by Prof. Carlos Bustamante

The video shows how optical trapping of particles using laser works and explains the working principle behind optical tweezers. The presentation is given by Prof. Carlos Bustamante from UC Berkeley.

C-Trap™ Optical Tweezers – Fluorescence Microscopy: Workflow

This movie shows the workflow of a typical single molecule experiment using optical tweezers coupled with fluorescence microscopy and microfluidics.

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