Optical tweezing, sometimes known as optical manipulation or optical trapping, is a technique, which allows the capture, trapping and moving of small particles by means of shaping laser light, typically to a tight focus. In general, small dielectric particles are naturally attracted to the centre of this tightly focused beam. This is where the electric field gradient is highest. Once in the centre of the beam, if small movements are assumed, the particle can be thought of as being attached to a spring with forces acting to pull the particle to the beam centre. As such the particle with follow laser beam movements. Using this technique, particles can be transported through space using only “light”.
This technique is ideal for use in a wide range of laboratories due to the range of particles that can be manipulated. This can extend from single atoms to biological cells. For example, bio-engineering labs have used it to capture virus’ and bacteria allowing spectroscopy and topographical measures of the cell, without interference from binding or holding methods.
Since it can also be used to calculate exerted forces, optical manipulation can be used to measure rheological properties e.g. the elasticity of cell walls and DNA strands and the forces available from molecular motors.
The earliest and simplest optical traps were developed through modification of optical microscopes, the trapping beam was introduced before the objective lens, which acted to focus the beam at the sample plane.
More complex systems can use spatial light modulators to split a laser into multiple beams to create multiple traps, or by manipulation of laser phase move the trapped particle in the axial direction, i.e. into and out of the central plane, defining complex patterns.
For simple traps, the demands on the laser are based around the quality of the beam, i.e. TEM00 with Gaussian power distribution. Any pointing instability from the laser source will cause displacement in the sample position and power fluctuations will have a detrimental effect on the stiffness of the trap. The laser should therefore have a circular beam with high power and pointing stabilities.
The choice of wavelength is also of importance. Most biological and in vivo samples will be damaged by visible light that will cause denaturation of proteins, so IR wavelengths are generally used. There is a sweet spot where the absorption of visible light by proteins and IR light by water is reduced and therefore photodamage is at a minimum. This falls between 750-1200nm with the ideal being ~830-970 nm. This is however a dead spot for lasers with beam characteristics suitable for optical trapping, so most laboratories use 1064 nm.
If biological damage is not an issue, any wavelength that is transmitted through the sample will be suitable, but consideration should be made for absorption of the laser energy and the heat this may cause.