Optical tweezers are tools for optically trapping and controlling the movements of particles. They enable precise manipulation and measurement of cells, molecules, nanoparticles, and even single atoms to perform biological experiments such as cell sorting, physics applications such as atom cooling, and advanced manufacturing of opto-electronic devices.
Optical tweezers utilise the fact that photons carry momentum and therefore impart a force on collision with an object. A large photon flux can therefore apply a non-negligible radiation pressure on microscopic particles.
Gradient and scattering forces
When a Gaussian laser beam is made incident on a spherical silica particle and the diameter of the particle is one to two times larger than the wavelength, an approaching photon can either reflect off the initial surface or refract into the material. A ‘scattering force’ and ‘gradient force’ arise as a result of these phenomena, respectively.
An unfocussed Gaussian beam incident on an optical sphere results in a scattering force in the direction of beam propagation, and a gradient force in the direction of the optical axis.
Rayleigh scattering imparts a force on the particle in the direction of beam propagation. This ‘scattering force’ is caused by the transfer of momentum to the particle from the incident laser beam.
The direction of photons change at each refraction, so momentum must be conserved by imparting a ‘gradient force’ on the particle. Rays nearest the centre of the Gaussian beam are more intense, and thus impart a larger force than rays on the edge of the beam. The result is a gradient force, which pulls the particle toward the centre of the beam. No net gradient force is present if the particle is located at the centre of the beam as the light refracts symmetrically.
If the diameter of the particle is much smaller than the wavelength of light, then the ray optics model breaks down. The gradient force has the same effect except that it is caused by the laser’s electric field. This induces a dipole moment in the dielectric material and the result is attraction toward the area of strongest electric field. Dipole moments are also present in the ray optics model, but the effects are negligible.
When the beam is unfocussed, the particle is pushed downstream due to the scattering force and is only trapped laterally. To also trap the particle along the optical axis, the beam must be focussed with a high-NA objective. This is because a converging laser beam can refract in such a way that the resultant gradient force opposes the scattering force and they cancel out.
A highly-focussed Gaussian beam incident on an optical sphere results in a scattering force and a gradient force that cancel each other out at the focus, deeming the particle optically trapped.
Optical tweezers and traps are prominently used in the field of cold atom physics for probing the quantum characteristics of atoms and developing technologies such as atomic clocks and quantum computers.
Manufacturing and materials processing
By individually lifting micron sized objects and moving them around in three-dimensions, optical tweezers are often used as tools for constructing small opto-electronic circuits and lab-on-a-chip systems that perform experiments with single molecules.
A DNA strand that is attached to an optically trapped particle can exert forces that vary according to its tensile strength and molecular motors. A beam deflection, typcially measured by a quadrant PSD, indicates a force exerted by the DNA strand on the particle.
By connecting a DNA strand to two optical particles, silica or otherwise, optical tweezers can be used to investigate its molecular properties. One such particle is held in optical tweezers and the other is fixed to a pipette. The DNA is unravelled by stretching the molecule with the optical tweezers. The influence of molecular motors, such as enzymes that wrap the DNA, can then be investigated by measuring the relatively small force exerted on the trapped particle by the DNA strand as it relaxes. This is accomplished by coupling the exit beam to a position sensing detector (PSD) and measuring its offset from the centre.