Laser tweezers or optical tweezers are devices used to trap particles (whose sizes range from tens of nanometers to tens of micrometers) using a highly focused laser beam. They are able to hold and move microscopic particle with a laser, thus manipulating the properties of the particle in a precise and contactless way. In 1970, Arthur Ashkin initially reported the detection of optical scattering and gradient forces acting on micron-sized particles. Subsequently, Ashkin and colleagues made the first observation of what is now known as an optical tweezer. He has been awarded with the Nobel Prize for physics in 2018 for “optical tweezers and their application to biological systems”.

When a particle is held in air or vacuum without any physical support, it is called optical levitation. The light exerts an attractive or repulsive force (in the order of piconewtons) on the particle depending on the relative refractive index of the particle and the surrounding medium. In order to levitate a particle, the force of levitation must counter the downward force of gravity. This is possible by using a focused high intensity laser beam. Particles (usually transparent dielectric spheres) with size ranges of micrometer such as fused silica spheres, oil or water droplets can be levitated using a focused laser.

Optical tweezers are based on the fact that light has momentum and can exert optical forces on objects. When a laser beam passes through an object, it bends and changes direction (called refraction) and alters its momentum. According to Newton’s third law, the object undergoes an equal and opposite momentum change, a reaction force, for the system to conserve the total momentum.

In a typical optical tweezer, the incoming light originates from a laser beam focused through a microscope objective and focuses on a spot in the sample.

The total forces experienced by the object consist of a scattering force and a gradient force. The scattering force arises when a light beam is scattered by the surface of the object. This scattering produces a net momentum transfer from the photons to the object and causes the bead to be pushed towards the beam propagation. The gradient force results from the intensity profile of the laser beam which acts as an attractive force, drawing the bead towards the region with greater light intensity. In the case of a focused laser beam with a Gaussian intensity profile (a normal distribution), the gradient force pulls the object into the center of the focal plane.

The reason the object stays in the center of the beam is because of the sum of the forces acting upon it. In the center, rays of light refract or scatter through the object the same way on both sides of the vertical plane, which cancels forces from moving the object sideways. If the object drifts to one side, it returns to the center.

Working

Consider a transparent, spherical particle or droplet having a refractive index greater than the surrounding medium. A laser beam with a wavelength much smaller than the radius of the particle is applied on the particle. The beam is sharply focused (usually using a lens) to a spot with a spot size comparable to or smaller than the wavelength.

As mentioned earlier, photons have a linear momentum which changes when the photon changes direction. This happens when it passes through an interface between two media of different refractive indices. Since the total momentum is conserved, the difference in momentum when the photon encounters an interface is transferred to the medium. In other words, when the direction of the photon changes, momentum is imparted to the interface. This rate of change of momentum is experienced as a force by the particle.

Consider the above figure, a photon travelling along the path abcd imparts momentum to the particle at b and c. The resultant force is pq. Similarly, a highly intense photon travelling the path ABCD transfers a momentum at B and C. The resultant is a longer vector PQ. It is clear that the resultant of pq and PQ is towards the region of more intensity. For a laser beam, the more intense region is always at the focal point. This means that the spherical particle is forced to get trapped at the focal point of the laser beam.

Experimental Setup

A schematic diagram of a simple optical trap is shown in the figure below:

A laser beam is first expanded by the lenses L1 and L2, which then passes through the beam splitter BS. The beam splitter reflects the light in to the optical path of the microscope where it is focused by the objective MO into the sample on the stage S. The particle to be trapped is suspended in water on a microscope slide. By carefully choosing L1 and L2, the beam can be made to focus near the image plane of the microscope so that the beam spot and the trapped object can be viewed through the camera. The position of the beam spot can be changed by deflecting the beam and hence the particle can be moved around the slide. 

Applications

Optical tweezers are a powerful and versatile tool in the field of experimental physics and biology. They use the momentum of photons to trap and manipulate microscopic objects, typically ranging from individual atoms to biological cells. Here are some key applications of optical tweezers:

• Biological and Medical Research: 
  • Cell Manipulation: Optical tweezers are widely used to trap, move, and manipulate biological cells. This is valuable for studying cell mechanics, migration, and interactions.
  • Single Molecule Studies: Researchers use optical tweezers to study the mechanical properties of individual biomolecules, such as DNA, proteins, and motor proteins like kinesin and myosin.
• Micro- and Nanoscale Assembly:
  • Microfabrication: Optical tweezers can be used to assemble microscale and nanoscale structures, making them useful in the development of microdevices and nanotechnology applications.
  • Particle Trapping: Researchers can use optical tweezers to trap and arrange nanoparticles, allowing for the controlled assembly of materials at the nanoscale.
• Physics Experiments:
  • Atomic Physics: Optical tweezers can trap and manipulate individual atoms, facilitating studies in quantum optics and quantum information science.
  • Condensed Matter Physics: Optical tweezers are used to trap and manipulate colloidal particles, enabling the study of phase transitions and other phenomena in condensed matter systems.
• Microrheology:
  • Optical tweezers can be employed to measure the mechanical properties of materials at the microscale. This is useful in studying the viscoelastic properties of complex fluids and biological tissues.
• Force Spectroscopy:
  • Optical tweezers allow for the precise measurement of forces exerted on trapped particles. This is particularly useful in studying molecular interactions and measuring forces at the nanoscale.
• Quantum Optics and Information Processing:
  • Optical tweezers play a role in experiments involving quantum information processing, where the manipulation of individual quantum states and particles is essential.
• Colloidal Studies:
  • Optical tweezers are used to study the behavior of colloidal particles, enabling investigations into phase transitions, self-assembly, and other colloidal phenomena.
• Astrophysics:
  • In some experiments, optical tweezers have been used to simulate conditions found in astrophysical environments, such as studying cosmic dust and related phenomena. 

These applications highlight the versatility of optical tweezers across various scientific disciplines, from biology and medicine to physics and materials science. The precise control they offer at the microscopic and nanoscopic levels makes them invaluable tools for researchers exploring a wide range of phenomena.

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